Sample records for accreting compact stars

  1. Workshop on Physics of Accretion Disks Around Compact and Young Stars

    NASA Technical Reports Server (NTRS)

    Liang, E (Editor); Stepinski, T. F. (Editor)

    1995-01-01

    The purpose of the two-day Workshop on Physics of Accretion Disks Around Compact and Young Stars was to bring together workers on accretion disks in the western Gulf region (Texas and Louisiana). Part 2 presents the workshop program, a list of poster presentations, and a list of workshop participants. Accretion disks are believed to surround many stars. Some of these disks form around compact stars, such as white dwarfs, neutron stars, or black holes that are members of binary systems and reveal themselves as a power source, especially in the x-ray and gamma regions of the spectrum. On the other hand, protostellar disks are believed to be accretion disks associated with young, pre-main-sequence stars and manifest themselves mostly in infrared and radio observations. These disks are considered to be a natural outcome of the star formation process. The focus of this workshop included theory and observations relevant to accretion disks around compact objects and newly forming stars, with the primary purpose of bringing the two communities together for intellectual cross-fertilization. The nature of the workshop was exploratory, to see how much interaction is possible between distinct communities and to better realize the local potential in this subject. A critical workshop activity was identification and documentation of key issues that are of mutual interest to both communities.

  2. Mapping the QCD Phase Transition with Accreting Compact Stars

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Blaschke, D.; Bogoliubov Laboratory for Theoretical Physics, JINR Dubna, Joliot-Curie str. 6, 141980 Dubna; Poghosyan, G.

    2008-10-29

    We discuss an idea for how accreting millisecond pulsars could contribute to the understanding of the QCD phase transition in the high-density nuclear matter equation of state (EoS). It is based on two ingredients, the first one being a ''phase diagram'' of rapidly rotating compact star configurations in the plane of spin frequency and mass, determined with state-of-the-art hybrid equations of state, allowing for a transition to color superconducting quark matter. The second is the study of spin-up and accretion evolution in this phase diagram. We show that the quark matter phase transition leads to a characteristic line in themore » {omega}-M plane, the phase border between neutron stars and hybrid stars with a quark matter core. Along this line a drop in the pulsar's moment of inertia entails a waiting point phenomenon in the accreting millisecond pulsar (AMXP) evolution: most of these objects should therefore be found along the phase border in the {omega}-M plane, which may be viewed as the AMXP analog of the main sequence in the Hertzsprung-Russell diagram for normal stars. In order to prove the existence of a high-density phase transition in the cores of compact stars we need population statistics for AMXPs with sufficiently accurate determination of their masses, spin frequencies and magnetic fields.« less

  3. On Magnetic Dynamos in Thin Accretion Disks around Compact and Young Stars

    NASA Technical Reports Server (NTRS)

    Stepinski, T. F.

    1993-01-01

    A variety of geometrically thin accretion disks commonly associated with such astronomical objects as X-ray binaries, cataclysmic variables, and protostars are likely to be seats of MHD dynamo actions. Thin disk geometry and the particular physical environment make accretion disk dynamos different from stellar, planetary, or even galactic dynamos. We discuss those particular features of disk dynamos with emphasis on the difference between protoplanetary disk dynamos and those associated with compact stars. We then describe normal mode solutions for thin disk dynamos and discuss implications for the dynamical behavior of dynamo-magnetized accretion disks.

  4. Massive star formation by accretion. I. Disc accretion

    NASA Astrophysics Data System (ADS)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.

    2016-01-01

    Context. Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the Hertzsprung-Russell (HR) diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. Aims: We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the HR diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. Methods: We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. We compare them with previously published equivalent models. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the accretion history of most of the intermediate-mass stars. Results: In the numerical computation of the time derivative of the entropy, some treatment leads to an artificial loss of entropy and thus reduces the inflation that the accreting star undergoes along the birthline. In the case of cold disc accretion, the existence of a significant swelling during the accretion phase, which leads to radii ≳ 100 R⊙ and brings the star back to the red part of the HR diagram, depends sensitively on the initial conditions. For an accretion rate of 10-3M⊙ yr-1, only models starting from a core with a significant radiative region evolve back to the red part of the HR diagram. We also obtain that, in order to reproduce the observed upper envelope of pre-MS stars in the HR diagram with an accretion law deduced from the observed mass outflows in ultra-compact HII regions, the fraction of the

  5. Accreting neutron stars, black holes, and degenerate dwarf stars.

    PubMed

    Pines, D

    1980-02-08

    During the past 8 years, extended temporal and broadband spectroscopic studies carried out by x-ray astronomical satellites have led to the identification of specific compact x-ray sources as accreting neutron stars, black holes, and degenerate dwarf stars in close binary systems. Such sources provide a unique opportunity to study matter under extreme conditions not accessible in the terrestrial laboratory. Quantitative theoretical models have been developed which demonstrate that detailed studies of these sources will lead to a greatly increased understanding of dense and superdense hadron matter, hadron superfluidity, high-temperature plasma in superstrong magnetic fields, and physical processes in strong gravitational fields. Through a combination of theory and observation such studies will make possible the determination of the mass, radius, magnetic field, and structure of neutron stars and degenerate dwarf stars and the identification of further candidate black holes, and will contribute appreciably to our understanding of the physics of accretion by compact astronomical objects.

  6. Some topics in the magnetohydrodynamics of accreting magnetic compact objects

    NASA Technical Reports Server (NTRS)

    Aly, J. J.

    1986-01-01

    Magnetic compact objects (neutron stars or white dwarfs) are currently thought to be present in many accreting systems that are releasing large amounts of energy. The magnetic field of the compact star may interact strongly with the accretion flow and play an essential role in the physics of these systems. Some magnetohydrodynamic (MHD) problems that are likely to be relevant in building up self-consistent models of the interaction between the accreting plasma and the star's magnetosphere are addressed in this series of lectures. The basic principles of MHD are first introduced and some important MHD mechanisms (Rayleigh-Taylor and Kelvin-Helmholtz instabilities; reconnection) are discussed, with particular reference to their role in allowing the infalling matter to penetrate the magnetosphere and mix with the field. The structure of a force-free magnetosphere and the possibility of quasistatic momentum and energy transfer between regions linked by field-aligned currents are then studied in some detail. Finally, the structure of axisymmetric accretion flows onto magnetic compact objects is considered.

  7. Evolution of dynamo-generated magnetic fields in accretion disks around compact and young stars

    NASA Technical Reports Server (NTRS)

    Stepinski, Tomasz F.

    1994-01-01

    Geometrically thin, optically thick, turbulent accretion disks are believed to surround many stars. Some of them are the compact components of close binaries, while the others are throught to be T Tauri stars. These accretion disks must be magnetized objects because the accreted matter, whether it comes from the companion star (binaries) or from a collapsing molecular cloud core (single young stars), carries an embedded magnetic field. In addition, most accretion disks are hot and turbulent, thus meeting the condition for the MHD turbulent dynamo to maintain and amplify any seed field magnetic field. In fact, for a disk's magnetic field to persist long enough in comparison with the disk viscous time it must be contemporaneously regenerated because the characteristic diffusion time of a magnetic field is typically much shorter than a disk's viscous time. This is true for most thin accretion disks. Consequently, studying magentic fields in thin disks is usually synonymous with studying magnetic dynamos, a fact that is not commonly recognized in the literature. Progress in studying the structure of many accretion disks was achieved mainly because most disks can be regarded as two-dimensional flows in which vertical and radial structures are largely decoupled. By analogy, in a thin disk, one may expect that vertical and radial structures of the magnetic field are decoupled because the magnetic field diffuses more rapidly to the vertical boundary of the disk than along the radius. Thus, an asymptotic method, called an adiabatic approximation, can be applied to accretion disk dynamo. We can represent the solution to the dynamo equation in the form B = Q(r)b(r,z), where Q(r) describes the field distribution along the radius, while the field distribution across the disk is included in the vector function b, which parametrically depends on r and is normalized by the condition max (b(z)) = 1. The field distribution across the disk is established rapidly, while the radial

  8. Multi-wavelength Observations of Accreting Compact Objects

    NASA Astrophysics Data System (ADS)

    Hernandez Santisteban, Juan Venancio

    2016-11-01

    The study of compact binaries invokes core astrophysical concepts ranging from stellar and sub-stellar atmospheres and interiors, stellar and binary evolution to physics of accretion. All of these systems are hosts to a compact object a white dwarf, neutron star or black hole ???? which produces a wide variety of exotic and energetic phenomena across the full electromagnetic spectrum. In this thesis, I will make use of multi-wavelength observations ranging from far-ultraviolet to nearinfrared in order to investigate two main topics: a) the late evolution of cataclysmic variables, and b) the accreting state of transitional millisecond pulsars. Firstly, I analyse the Very Large Telescope X-Shooter time-resolved spectroscopy of the short orbital period cataclysmic variable, SDSS J1433+1011, in Chapter 2. The wide wavelength coverage allowed me to perform a detailed characterisation of the system, as well as a direct mass measurement of the brown dwarf companion. I show that the donor in SDSS J1433+1011 successfully transitioned from the stellar to sub-stellar regime, as predicted by evolutionary models. Further light-curve modelling allowed me to show that a low albedo as well as a low heat circulation efficiency is present in the atmosphere of the sub-stellar donor. In Chapter 3, I analyse data from large synoptic surveys, such as SDSS and PTF, to search for the predicted population of dead cataclysmic variables. Following the non-detection of dead CVs, I was able to estimate the space density (?0 < 2?10????5 pc????3) of this hidden population via a Monte Carlo simulation of the Galactic CV population. In Chapter 4, I present Hubble Space Telescope ultraviolet observations of the transitional millisecond pulsar PSR J1023+0038, during its latest accretion state. In combination with optical and near-infrared data, I show that a standard accretion disc does not reach the magnetosphere of the neutron star. Instead, the overall spectrum is consistent with a truncated disc

  9. Clumpy wind accretion in supergiant neutron star high mass X-ray binaries

    NASA Astrophysics Data System (ADS)

    Bozzo, E.; Oskinova, L.; Feldmeier, A.; Falanga, M.

    2016-05-01

    The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the nonstationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total timescale of several hours), the transitions of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the nonstationary wind. The X-ray luminosity released by the system is computed at each time step by taking into account the relevant physical processes occurring in the different accretion regimes. Synthetic lightcurves are derived and qualitatively compared with those observed from classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. Although a number of simplifications are assumed in these calculations, we show that taking into account the effects of the centrifugal and magnetic inhibition of accretion significantly reduces the average X-ray luminosity expected for any neutron star wind-fed binary. The present model calculations suggest that long spin periods and stronger magnetic fields are favored in order to reproduce the peculiar behavior of supergiant fast X-ray transients in the X-ray domain.

  10. Accretion Disk Outflows from Compact Object Mergers

    NASA Astrophysics Data System (ADS)

    Metzger, Brian

    Nuclear reactions play a key role in the accretion disks and outflows associated with the merger of binary compact objects and the central engines of gamma-ray bursts and supernovae. The proposed research program will investigate the impact of nucleosynthesis on these events and their observable signatures by means of analytic calculations and numerical simulations. One focus of this research is rapid accretion following the tidal disruption of a white dwarf (WD) by a neutron star (NS) or black hole (BH) binary companion. Tidal disruption shreds the WD into a massive torus composed of C, O, and/or He, which undergoes nuclear reactions and burns to increasingly heavier elements as it flows to smaller radii towards the central compact object. The nuclear energy so released is comparable to that released gravitationally, suggesting that burning could drastically alter the structure and stability of the accretion flow. Axisymmetric hydrodynamic simulations of the evolution of the torus including nuclear burning will be performed to explore issues such as the mass budget of the flow (accretion vs. outflows) and its thermal stability (steady burning and accretion vs. runaway explosion). The mass, velocity, and composition of outflows from the disk will be used in separate radiative transfer calculations to predict the lightcurves and spectra of the 56Ni-decay powered optical transients from WD-NS/WD-BH mergers. The possible connection of such events to recently discovered classes of sub-luminous Type I supernovae will be assessed. The coalescence of NS-NS/NS-BH binaries also results in the formation of a massive torus surrounding a central compact object. Three-dimensional magnetohydrodynamic simulations of the long-term evolution of such accretion disks will be performed, which for the first time follow the effects of weak interactions and the nuclear energy released by Helium recombination. The nucleosynthetic yield of disk outflows will be calculated using a detailed

  11. Probing thermonuclear burning on accreting neutron stars

    NASA Astrophysics Data System (ADS)

    Keek, L.

    2008-12-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes thermonuclear burning in the neutron star envelope, creating carbon and heavier elements. The fusion process may proceed in an unstable manner, resulting in a thermonuclear runaway. Within one second the entire surface is burned, which is observable as a sharp rise in the emitted X-ray flux: a type I X-ray burst. Afterwards the neutron star surface cools down on a timescale of ten to one hundred seconds. During these bursts the surface of an accreting neutron star can be observed directly, which makes them instrumental for studying this type of stars. We have studied rare kinds of X-ray bursts. One such rare burst is the superburst, which lasts a thousand times longer than an ordinary burst. Superbursts are thought to result from the explosive burning of a thick carbon layer, which lies deeper inside the neutron star, close to a layer known as the crust. A prerequisite for the occurrence of a superburst is a high enough temperature, which is set by the temperature of the crust and the heat conductivity of the envelope. The latter is lowered by the presence of heavy elements that are produced during normal X-ray bursts. Using a large set of observations from the Wide Field Camera's onboard the BeppoSAX satellite, we find that, at high accretion rate, sources which do not exhibit normal bursts likely have a longer superburst recurrence time, than the observed superburst recurrence time of one burster. We analyze in detail the first superburst from a transient source, which went into outburst only 55 days before the superburst. Recent models of the neutron star crust predict that this is too small a time to heat the crust sufficiently for superburst ignition, indicating

  12. Gravitational Waves from Accreting Neutron Stars Undergoing Common-envelope Inspiral

    NASA Astrophysics Data System (ADS)

    Holgado, A. Miguel; Ricker, Paul M.; Huerta, E. A.

    2018-04-01

    The common-envelope phase is a likely formation channel for close binary systems containing compact objects. Neutron stars in common envelopes accrete at a fraction of the Bondi–Hoyle–Lyttleton accretion rate, since the stellar envelope is inhomogeneous, but they may still be able to accrete at hypercritical rates (though not enough to become black holes). We show that common-envelope systems consisting of a neutron star with a massive primary may be gravitational-wave (GW) sources detectable in the Advanced LIGO band as far away as the Magellanic Clouds. To characterize their evolution, we perform orbital integrations using 1D models of 12 M ⊙ and 20 M ⊙ primaries, considering the effects of density gradient on the accretion onto the NS and spin evolution. From the range of possible accretion rates relevant to common-envelope evolution, we find that these systems may be louder GW sources than low-mass X-ray binaries like Sco X-1, which are currently the target of directed searches for continuous GWs. We also find that their strain amplitude signal may allow for novel constraints on the orbital separation and inspiral timescale in common envelopes when combined with pre-common-envelope electromagnetic observations.

  13. Gas Accretion and Star Formation Rates

    NASA Astrophysics Data System (ADS)

    Sánchez Almeida, Jorge

    Cosmological numerical simulations of galaxy evolution show that accretion of metal-poor gas from the cosmic web drives the star formation in galaxy disks. Unfortunately, the observational support for this theoretical prediction is still indirect, and modeling and analysis are required to identify hints as actual signs of star formation feeding from metal-poor gas accretion. Thus, a meticulous interpretation of the observations is crucial, and this observational review begins with a simple theoretical description of the physical process and the key ingredients it involves, including the properties of the accreted gas and of the star formation that it induces. A number of observations pointing out the connection between metal-poor gas accretion and star formation are analyzed, specifically, the short gas-consumption time-scale compared to the age of the stellar populations, the fundamental metallicity relationship, the relationship between disk morphology and gas metallicity, the existence of metallicity drops in starbursts of star-forming galaxies, the so-called G dwarf problem, the existence of a minimum metallicity for the star-forming gas in the local universe, the origin of the α-enhanced gas forming stars in the local universe, the metallicity of the quiescent BCDs, and the direct measurements of gas accretion onto galaxies. A final section discusses intrinsic difficulties to obtain direct observational evidence, and points out alternative observational pathways to further consolidate the current ideas.

  14. Self-similar hot accretion flow onto a neutron star

    NASA Astrophysics Data System (ADS)

    Medvedev, Mikhail V.

    2001-10-01

    We present analytical and numerical solutions which describe a hot, viscous, two-temperature accretion flow onto a neutron star or any other compact star with a surface. We assume Coulomb coupling between the protons and electrons, and free-free cooling from the electrons. Outside a thin boundary layer, where the accretion flow meets the star, we show that there is an extended settling region which is well-described by two self-similar solutions: (1) a two-temperature solution which is valid in an inner zone r<=102.5 (r is in Schwarzchild units), and (2) a one-temperature solution at larger radii. In both zones, ρ~r-2, Ω~r-3/2, v~r0, Tp~r-1 in the two-temperature zone, Te~r-1/2. The luminosity of the settling zone arises from the rotational energy of the star as the star is braked by viscosity; hence the luminosity is independent of Ṁ. The settling solution is convectively and viscously stable and is unlikely to have strong winds or outflows. The flow is thermally unstable, but the instability may be stabilized by thermal conduction. The settling solution described here is not advection-dominated, and is thus different from the self-similar ADAF found around black holes. When the spin of the star is small enough, however, the present solution transforms smoothly to a (settling) ADAF. .

  15. Where a Neutron Star's Accretion Disk Ends

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-03-01

    In X-ray binaries that consist of a neutron star and a companion star, gas funnels from the companion into an accretion disk surrounding the neutron star, spiraling around until it is eventually accreted. How do the powerful magnetic fields threading through the neutron star affect this accretion disk? Recent observations provide evidence that they may push the accretion disk away from the neutron stars surface.Truncated DisksTheoretical models have indicated that neutron star accretion disks may not extend all the way in to the surface of a neutron star, but may instead be truncated at a distance. This prediction has been difficult to test observationally, however, due to the challenge of measuring the location of the inner disk edge in neutron-star X-ray binaries.In a new study, however, a team of scientists led by Ashley King (Einstein Fellow at Stanford University) has managed to measure the location of the inner edge of the disk in Aquila X-1, a neutron-star X-ray binary located 17,000 light-years away.Iron line feature detected by Swift (red) and NuSTAR (black). The symmetry of the line is one of the indicators that the disk is located far from the neutron star; if the inner regions of the disk were close to the neutron star, severe relativistic effects would skew the line to be asymmetric. [King et al. 2016]Measurements from ReflectionsKing and collaborators used observations made by NuSTAR and Swift/XRT both X-ray space observatories of Aquila X-1 during the peak of an X-ray outburst. By observing the reflection of Aquila X-1s emission off of the inner regions of the accretion disk, the authors were able to estimate the location of the inner edge of the disk.The authors find that this inner edge sits at ~15 gravitational radii. Since the neutron stars surface is at ~5 gravitational radii, this means that the accretion disk is truncated far from the stars surface. In spite of this truncation, material still manages to cross the gap and accrete onto the

  16. Cooling of Accretion-Heated Neutron Stars

    NASA Astrophysics Data System (ADS)

    Wijnands, Rudy; Degenaar, Nathalie; Page, Dany

    2017-09-01

    We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.

  17. Evidence for Neutron Star Formation from Accretion Induced Collapse of a White Dwarf

    NASA Technical Reports Server (NTRS)

    Paradijis, J. Van; VanDenHeuvel, E. P. J.; Kouveliotou, C.; Fishman, G. J.; Finger, M. H.; Lewin, W. H. G.

    1997-01-01

    The orbital parameters of the recently discovered transient burster/pulsar GRO J1744-28 indicate that this system is a low-mass X-ray binary in an advanced stage of its mass transfer, with several tenths of a solar mass already transferred from the donor to the compact star. All neutron stars known to have accreted such an amount have very weak magnetic fields, and this has led to the idea that the magnetic fields of neutron stars decay as a result of accretion. The observation of a strongly magnetized neutron star in GRO J1744-28 then suggests that this neutron star was formed recently as a result of the collapse of a white dwarf during an earlier stage of the current phase of mass transfer. It is shown that this model can consistently explain the observed characteristics of GRO J1744-28. Attractive progenitors for such an evolution are the luminous supersoft X-ray sources detected with ROSAT.

  18. Star-disc interaction in galactic nuclei: orbits and rates of accreted stars

    NASA Astrophysics Data System (ADS)

    Kennedy, Gareth F.; Meiron, Yohai; Shukirgaliyev, Bekdaulet; Panamarev, Taras; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-07-01

    We examine the effect of an accretion disc on the orbits of stars in the central star cluster surrounding a central massive black hole by performing a suite of 39 high-accuracy direct N-body simulations using state-of-the art software and accelerator hardware, with particle numbers up to 128k. The primary focus is on the accretion rate of stars by the black hole (equivalent to their tidal disruption rate for black holes in the small to medium mass range) and the eccentricity distribution of these stars. Our simulations vary not only the particle number, but disc model (two models examined), spatial resolution at the centre (characterized by the numerical accretion radius) and softening length. The large parameter range and physically realistic modelling allow us for the first time to confidently extrapolate these results to real galactic centres. While in a real galactic centre both particle number and accretion radius differ by a few orders of magnitude from our models, which are constrained by numerical capability, we find that the stellar accretion rate converges for models with N ≥ 32k. The eccentricity distribution of accreted stars, however, does not converge. We find that there are two competing effects at work when improving the resolution: larger particle number leads to a smaller fraction of stars accreted on nearly circular orbits, while higher spatial resolution increases this fraction. We scale our simulations to some nearby galaxies and find that the expected boost in stellar accretion (or tidal disruption, which could be observed as X-ray flares) in the presence of a gas disc is about a factor of 10. Even with this boost, the accretion of mass from stars is still a factor of ˜100 slower than the accretion of gas from the disc. Thus, it seems accretion of stars is not a major contributor to black hole mass growth.

  19. Probing the Environment of Accreting Compact Objects

    NASA Astrophysics Data System (ADS)

    Hanke, Manfred

    2011-04-01

    X-ray binaries are the topic of this thesis. They consist of a compact object -- a black hole or a neutron star -- and an ordinary star, which loses matter to the compact object. The gravitational energy released through this process of mass accretion is largely converted into X-rays. The latter are used in the present work to screen the environment of the compact object. The main focus in the case of a massive star is on its wind, which is not homogeneous, but may display structures in form of temperature and density variations. Since great importance is, in multiple respects, attached to stellar winds in astrophysics, there is large interest in general to understand these structures more thoroughly. In particular for X-ray binaries, whose compact object obtains matter from the wind of its companion star, the state of the wind can decisively influence mass accretion and its related radiation processes. A detailed introduction to the fundamentals of stellar winds, compact objects, accretion and radiation processes in X-ray binaries, as well as to the employed instruments and analysis methods, is given in chapter 1. The focus of this investigation is on Cygnus X-1, a binary system with a black hole and a blue supergiant, which form a persistently very bright X-ray source because of accretion from the stellar wind. It had been known for a long time that this source -- when the black hole is seen through the dense stellar wind -- often displays abrupt absorption events whose origin is suspected to be in clumps in the wind. More detailed physical properties of these clumps and of the wind in general are explored in this work. Observations that were specifically acquired for this study, as well as archival data from different satellite observatories, are analyzed in view of signatures of the wind and its fine structures. These results are presented in chapter 2. In a first part of the analysis, the statistical distribution of the brightness of Cyg X-1, as measured since

  20. Gamma-burst emission from neutron-star accretion

    NASA Technical Reports Server (NTRS)

    Colgate, S. A.; Petschek, A. G.; Sarracino, R.

    1983-01-01

    A model for emission of the hard photons of gamma bursts is presented. The model assumes accretion at nearly the Eddington limited rate onto a neutron star without a magnetic field. Initially soft photons are heated as they are compressed between the accreting matter and the star. A large electric field due to relatively small charge separation is required to drag electrons into the star with the nuclei against the flux of photons leaking out through the accreting matter. The photon number is not increased substantially by Bremsstrahlung or any other process. It is suggested that instability in an accretion disc might provide the infalling matter required.

  1. Spin Evolution of Accreting Young Stars. II. Effect of Accretion-powered Stellar Winds

    NASA Astrophysics Data System (ADS)

    Matt, Sean P.; Pinzón, Giovanni; Greene, Thomas P.; Pudritz, Ralph E.

    2012-01-01

    We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind (APSW). For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the APSW scenario and the two types of disk-locking models (namely, the X-wind and Ghosh & Lamb type models) and identify some remaining theoretical issues for understanding young star spins.

  2. On the illumination of neutron star accretion discs

    NASA Astrophysics Data System (ADS)

    Wilkins, D. R.

    2018-03-01

    The illumination of the accretion disc in a neutron star X-ray binary by X-rays emitted from (or close to) the neutron star surface is explored through general relativistic ray tracing simulations. The applicability of the canonical suite of relativistically broadened emission line models (developed for black holes) to discs around neutron stars is evaluated. These models were found to describe well emission lines from neutron star accretion discs unless the neutron star radius is larger than the innermost stable orbit of the accretion disc at 6 rg or the disc is viewed at high inclination, above 60° where shadowing of the back side of the disc becomes important. Theoretical emissivity profiles were computed for accretion discs illuminated by hotspots on the neutron star surfaces, bands of emission and emission by the entirety of the hot, spherical star surface and in all cases, the emissivity profile of the accretion disc was found to be well represented by a single power law falling off slightly steeper than r-3. Steepening of the emissivity index was found where the emission is close to the disc plane and the disc can appear truncated when illuminated by a hotspot at high latitude. The emissivity profile of the accretion disc in Serpens X-1 was measured and found to be consistent with a single unbroken power law with index q=3.5_{-0.4}^{+0.3}, suggestive of illumination by the boundary layer between the disc and neutron star surface.

  3. Measurements of mass accretion rates in Herbig Ae/Be stars

    NASA Astrophysics Data System (ADS)

    Donehew, Brian

    Herbig Ae/Be stars(HAeBes) are young stellar objects of spectral class F2 through B0, with the central star often surrounded by a circumstellar disk of gas and dust. They are the higher mass analogs to T Tauri stars. The interaction between the star and the disk is not well understood, nor is the disk structure. The central star will often accrete mass from the disk, and the mass accretion rate is an important parameter for modeling the disk structure and evolution. The methods for measuring mass accretion rates of T Tauri stars are generally not applicable to HAeBe stars. As such, reliable measurements of mass accretion rates for HAeBes are rare. Garrison(1978) saw that the Balmer Discontinuity of HAeBes was veiled, and attributed this veiling to accretion luminosity. Building on Garrison(1978) and the work of Muzerolle et al. (2004), I determine the mass accretion rates and accretion luminosities of a large sample of HAeBe stars by measuring the veiling of the Balmer Discontinuity due to the accretion luminosity. Muzerolle et al. (1998) established a strong correlation between the accretion luminosity of T Tauri stars and the luminosity of Br gamma, and this correlation seems to extend to the evolutionary precursors to HAeBes, intermediate T Tauri stars, as well Calvet et al. (2004). I test this correlation for HAeBes and discover that it is valid for HAe stars but not for HBe stars. From examining the HAeBes of my sample from spectral range A3 to B7, there does not seem to be a particular spectral type at which the correlation fails. A few of the late HBe stars are consistent with the correlation, but most of the HBe stars have Br gamma luminosities much larger than what one would expect from the correlation. This suggests that there might be a significant stellar wind component to the Br gamma luminosity for many of the HBe stars. T Tauri stars accrete mass from their disks magnetospherically, in which the strong stellar field of the star truncates the disk at

  4. Early Results from NICER Observations of Accreting Neutron Stars

    NASA Astrophysics Data System (ADS)

    Chakrabarty, Deepto; Ozel, Feryal; Arzoumanian, Zaven; Gendreau, Keith C.; Bult, Peter; Cackett, Ed; Chenevez, Jerome; Fabian, Andy; Guillot, Sebastien; Guver, Tolga; Homan, Jeroen; Keek, Laurens; Lamb, Frederick; Ludlam, Renee; Mahmoodifar, Simin; Markwardt, Craig B.; Miller, Jon M.; Psaltis, Dimitrios; Strohmayer, Tod E.; Wilson-Hodge, Colleen A.; Wolff, Michael T.

    2018-01-01

    The Neutron Star Interior Composition Explorer (NICER) offers significant new capabilities for the study of accreting neuton stars relative to previous X-ray missions including large effective area, low background, and greatly improved low-energy response. The NICER Burst and Accretion Working Group has designed a 2 Ms observation program to study a number of phenomena in accreting neutron stars including type-I X-ray bursts, superbursts, accretion-powered pulsations, quasi-periodic oscillations, and accretion disk reflection spectra. We present some early results from the first six months of the NICER mission.

  5. On the thickness of accretion curtains on magnetized compact objects from analysis of their fast aperiodic time variability.

    NASA Astrophysics Data System (ADS)

    Semena, Andrey

    It is widely accepted that accretion onto magnetized compact objects is channelled to some areas close to magnetic poles of the star. Thickness of this channelled accretion flow intimately depends on details of penetration of highly conducting plasma of the flow to the compact object magnetosphere, i.e. on magnetic diffusivity etc. Until now our knowledge of these plasma properties is scarce. In our work we present our attempts to estimate the thickness of the plasma flow on top of the magnetosphere from observations of accreting intermediate polars (magnetized white dwarfs). We show that properties of aperiodic noise of accreting intermediate polars can be used to put constrains on cooling time of hot plasma, heated in the standing shock wave above the WD surface. Estimates of the cooling time and the mass accretion rate provide us a tool to measure the density of post-shock plasma and the cross-sectional area of the accretion funnel at the WD surface. We have studied aperiodic noise of emission of one of the brightest intermediate polar EX Hya with the help of data in optical and X-ray energy bands. We put an upper limit on the plasma cooling timescale tau <0.2-0.5 sec, on the fractional area of the accretion curtain footprint f < 1.6 × 10(-4) . We show that measurements of accretion column footprints, combined with results of the eclipse mapping, can be used to obtain an upper limit on the penetration depth of the accretion disc plasma at the boundary of the magnetosphere, Delta r / r ≈ 10(-3) If the magnetospheres of accreting neutron stars have similar plasma penetration depths at their boundaries, we predict that footprints of their accretion columns should be very small, with fractional areas < 10(-6) .

  6. SPIN EVOLUTION OF ACCRETING YOUNG STARS. II. EFFECT OF ACCRETION-POWERED STELLAR WINDS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Matt, Sean P.; Pinzon, Giovanni; Greene, Thomas P.

    2012-01-20

    We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effectmore » of a spin-down torque that arises from an accretion-powered stellar wind (APSW). For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the APSW scenario and the two types of disk-locking models (namely, the X-wind and Ghosh and Lamb type models) and identify some remaining theoretical issues for understanding young star spins.« less

  7. Accretion torques in X-ray pulsars

    NASA Technical Reports Server (NTRS)

    Rappaport, S.; Joss, P. C.

    1977-01-01

    An analysis of the accretion process in an X-ray pulsar, whereby angular momentum is transferred to the star and its rotation period is changed, is presented, and an expression for the fractional rate of change of the pulse period in terms of X-ray luminosity and other star parameters is derived. It is shown that observed characteristic spin-up time scales for seven X-ray pulsars strongly support the view that in every source (1) the pulse period reflects the rotation period of a compact object, (2) the accretion is mediated by a disk surrounding the compact object and rotating in the same sense, and (3) the compact object is a neutron star rather than a white dwarf.

  8. Time-dependent Models of Magnetospheric Accretion onto Young Stars

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Robinson, C. E.; Espaillat, C. C.; Owen, J. E.

    Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star’s surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner, and the source of this variability remains an open problem, particularly on the shortest (∼day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the absence of any other source of variability. However, we show that ifmore » the density in the inner disk varies smoothly in time with ∼day-long timescales (e.g., due to turbulence), this complication can lead to the development of shocks in the accretion column. These shocks propagate along the accretion column and ultimately hit the star, leading to rapid, large amplitude changes in the accretion rate. We argue that when these shocks hit the star, the observed time dependence will be a rapid increase in accretion luminosity, followed by a slower decline, and could be an explanation for some of the short-period variability observed in accreting young stars. Our one-dimensional approach bridges previous analytic work to more complicated multi-dimensional simulations and observations.« less

  9. Cyclotron Lines in Accreting Neutron Star Spectra

    NASA Astrophysics Data System (ADS)

    Wilms, Jörn; Schönherr, Gabriele; Schmid, Julia; Dauser, Thomas; Kreykenbohm, Ingo

    2009-05-01

    Cyclotron lines are formed through transitions of electrons between discrete Landau levels in the accretion columns of accreting neutron stars with strong (1012 G) magnetic fields. We summarize recent results on the formation of the spectral continuum of such systems, describe recent advances in the modeling of the lines based on a modification of the commonly used Monte Carlo approach, and discuss new results on the dependence of the measured cyclotron line energy from the luminosity of transient neutron star systems. Finally, we show that Simbol-X will be ideally suited to build and improve the observational database of accreting and strongly magnetized neutron stars.

  10. Observational diagnostics of accretion on young stars and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Stelzer, Beate; Argiroffi, Costanza

    I present a summary of recent observational constraints on the accretion properties of young stars and brown dwarfs with focus on the high-energy emission. In their T Tauri phase young stars assemble a few percent of their mass by accretion from a disk. Various observational signatures of disks around pre-main sequence stars and the ensuing accretion process are found in the IR and optical regime: e.g. excess emission above the stellar photosphere, strong and broad emission lines, optical veiling. At high energies evidence for accretion is less obvious, and the X-ray emission from stars has historically been ascribed to magnetically confined coronal plasmas. While being true for the bulk of the emission, new insight obtained from XMM-Newton and Chandra observations has unveiled contributions from accretion and outflow processes to the X-ray emission from young stars. Their smaller siblings, the brown dwarfs, have been shown to undergo a T Tauri phase on the basis of optical/IR observations of disks and measurements of accretion rates. Most re-cently, first evidence was found for X-rays produced by accretion in a young brown dwarf, complementing the suspected analogy between stars and substellar objects.

  11. On the Maximum Mass of Accreting Primordial Supermassive Stars

    NASA Astrophysics Data System (ADS)

    Woods, T. E.; Heger, Alexander; Whalen, Daniel J.; Haemmerlé, Lionel; Klessen, Ralf S.

    2017-06-01

    Supermassive primordial stars are suspected to be the progenitors of the most massive quasars at z ˜ 6. Previous studies of such stars were either unable to resolve hydrodynamical timescales or considered stars in isolation, not in the extreme accretion flows in which they actually form. Therefore, they could not self-consistently predict their final masses at collapse, or those of the resulting supermassive black hole seeds, but rather invoked comparison to simple polytropic models. Here, we systematically examine the birth, evolution, and collapse of accreting, non-rotating supermassive stars under accretion rates of 0.01-10 M ⊙ yr-1 using the stellar evolution code Kepler. Our approach includes post-Newtonian corrections to the stellar structure and an adaptive nuclear network and can transition to following the hydrodynamic evolution of supermassive stars after they encounter the general relativistic instability. We find that this instability triggers the collapse of the star at masses of 150,000-330,000 M ⊙ for accretion rates of 0.1-10 M ⊙ yr-1, and that the final mass of the star scales roughly logarithmically with the rate. The structure of the star, and thus its stability against collapse, is sensitive to the treatment of convection and the heat content of the outer accreted envelope. Comparison with other codes suggests differences here may lead to small deviations in the evolutionary state of the star as a function of time, that worsen with accretion rate. Since the general relativistic instability leads to the immediate death of these stars, our models place an upper limit on the masses of the first quasars at birth.

  12. Accretion dynamics of EX Lupi in quiescence. The star, the spot, and the accretion column

    NASA Astrophysics Data System (ADS)

    Sicilia-Aguilar, Aurora; Fang, Min; Roccatagliata, Veronica; Collier Cameron, Andrew; Kóspál, Ágnes; Henning, Thomas; Ábrahám, Peter; Sipos, Nikoletta

    2015-08-01

    Context. EX Lupi is a young, accreting M0 star and the prototype of EXor variable stars. Its spectrum is very rich in emission lines, including many metallic lines with narrow and broad components. The presence of a close companion has also been proposed, based on radial velocity signatures. Aims: We use the metallic emission lines to study the accretion structures and to test the companion hypothesis. Methods: We analyse 54 spectra obtained during five years of quiescence time. We study the line profile variability and the radial velocity of the narrow and broad metallic emission lines. We use the velocity signatures of different species with various excitation conditions and their time dependency to track the dynamics associated with accretion. Results: We observe periodic velocity variations in the broad and the narrow line components, consistent with rotational modulation. The modulation is stronger for lines with higher excitation potentials (e.g. He II), which are likely produced in a confined area very close to the accretion shock. Conclusions: We propose that the narrow line components are produced in the post-shock region, while the broad components originate in the more extended, pre-shock material in the accretion column. All the emission lines suffer velocity modulation due to the rotation of the star. The broad components are responsible for the line-dependent veiling observed in EX Lupi. We demonstrate that a rotationally modulated line-dependent veiling can explain the radial velocity signature of the photospheric absorption lines, making the close-in companion hypothesis unnecessary. The accretion structure is locked to the star and very stable during the five years of observations. Not all stars with similar spectral types and accretion rates show the same metallic emission lines, which could be related to differences in temperature and density in their accretion structure(s). The contamination of photospheric signatures by accretion

  13. A Three-dimensional Simulation of a Magnetized Accretion Disk: Fast Funnel Accretion onto a Weakly Magnetized Star

    NASA Astrophysics Data System (ADS)

    Takasao, Shinsuke; Tomida, Kengo; Iwasaki, Kazunari; Suzuki, Takeru K.

    2018-04-01

    We present the results of a global, three-dimensional magnetohydrodynamics simulation of an accretion disk with a rotating, weakly magnetized central star. The disk is threaded by a weak, large-scale poloidal magnetic field, and the central star has no strong stellar magnetosphere initially. Our simulation investigates the structure of the accretion flows from a turbulent accretion disk onto the star. The simulation reveals that fast accretion onto the star at high latitudes occurs even without a stellar magnetosphere. We find that the failed disk wind becomes the fast, high-latitude accretion as a result of angular momentum exchange mediated by magnetic fields well above the disk, where the Lorentz force that decelerates the rotational motion of gas can be comparable to the centrifugal force. Unlike the classical magnetospheric accretion scenario, fast accretion streams are not guided by magnetic fields of the stellar magnetosphere. Nevertheless, the accretion velocity reaches the free-fall velocity at the stellar surface due to the efficient angular momentum loss at a distant place from the star. This study provides a possible explanation why Herbig Ae/Be stars whose magnetic fields are generally not strong enough to form magnetospheres also show indications of fast accretion. A magnetically driven jet is not formed from the disk in our model. The differential rotation cannot generate sufficiently strong magnetic fields for the jet acceleration because the Parker instability interrupts the field amplification.

  14. Constraints on Bygone Nucleosynthesis of Accreting Neutron Stars

    DOE PAGES

    Meisel, Zach; Deibel, Alex

    2017-03-06

    Nuclear burning near the surface of an accreting neutron star produces ashes that, when compressed deeper by further accretion, alter the star’s thermal and compositional structure. Bygone nucleosynthesis can be constrained by the impact of compressed ashes on the thermal relaxation of quiescent neutron star transients. In particular, Urca cooling nuclei pairs in nuclear burning ashes that cool the neutron star crust via neutrino emission from e --capture/β --decay cycles and provide signatures of prior nuclear burning over the ~century timescales it takes to accrete to the e --capture depth of the strongest cooling pairs. By using crust cooling modelsmore » of the accreting neutron star transient MAXI J0556-332, we show that this source likely lacked Type I X-ray bursts and superbursts ≳120 years ago. Reduced nuclear physics uncertainties in rp-process reaction rates and e --capture weak transition strengths for low-lying transitions will improve nucleosynthesis constraints using this technique.« less

  15. Star Formation in Massive Clusters via Bondi Accretion

    NASA Astrophysics Data System (ADS)

    Murray, Norman; Chang, Philip

    2012-02-01

    Essentially all stars form in giant molecular clouds (GMCs). However, inside GMCs, most of the gas does not participate in star formation; rather, denser gas accumulates in clumps in the GMC, with the bulk of the stars in a given GMC forming in a few of the most massive clumps. In the Milky Way, these clumps have masses M cl <~ 5 × 10-2 of the GMC, radii r cl ~ 1 pc, and free-fall times τcl ~ 2 × 105 yr. We show that clumps inside GMCs should accrete at a modified Bondi accretion rate, which depends on clump mass as \\dot{M}_{cl}\\sim M_{cl}^{5/4}. This rate is initially rather slow, usually slower than the initial star formation rate inside the clump (we adopt the common assumption that inside the clump, \\dot{M}_*=\\epsilon _ffM_{cl}/\\tau _{cl}, with epsilonff ≈ 0.017). However, after ~2 GMC free-fall times τGMC, the clump accretion rate accelerates rapidly; formally, the clump can accrete the entire GMC in ~3τGMC. At the same time, the star formation rate accelerates, tracking the Bondi accretion rate. If the GMC is disrupted by feedback from the largest clump, half the stars in that clump form in the final τGMC before the GMC is disrupted. The theory predicts that the distribution of effective star formation rates, measured per GMC free-fall time, is broad, ranging from ~0.001 up to 0.1 or larger and that the mass spectrum of star clusters is flatter than that of clumps, consistent with observations.

  16. On the Maximum Mass of Accreting Primordial Supermassive Stars

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Woods, T. E.; Heger, Alexander; Whalen, Daniel J.

    Supermassive primordial stars are suspected to be the progenitors of the most massive quasars at z ∼ 6. Previous studies of such stars were either unable to resolve hydrodynamical timescales or considered stars in isolation, not in the extreme accretion flows in which they actually form. Therefore, they could not self-consistently predict their final masses at collapse, or those of the resulting supermassive black hole seeds, but rather invoked comparison to simple polytropic models. Here, we systematically examine the birth, evolution, and collapse of accreting, non-rotating supermassive stars under accretion rates of 0.01–10 M {sub ⊙} yr{sup −1} using themore » stellar evolution code Kepler . Our approach includes post-Newtonian corrections to the stellar structure and an adaptive nuclear network and can transition to following the hydrodynamic evolution of supermassive stars after they encounter the general relativistic instability. We find that this instability triggers the collapse of the star at masses of 150,000–330,000 M {sub ⊙} for accretion rates of 0.1–10 M {sub ⊙} yr{sup −1}, and that the final mass of the star scales roughly logarithmically with the rate. The structure of the star, and thus its stability against collapse, is sensitive to the treatment of convection and the heat content of the outer accreted envelope. Comparison with other codes suggests differences here may lead to small deviations in the evolutionary state of the star as a function of time, that worsen with accretion rate. Since the general relativistic instability leads to the immediate death of these stars, our models place an upper limit on the masses of the first quasars at birth.« less

  17. MHD Simulations of Magnetized Stars in the Propeller Regime of Accretion

    NASA Astrophysics Data System (ADS)

    Lii, Patrick; Romanova, Marina; Lovelace, Richard

    2014-01-01

    Accreting magnetized stars may be in the propeller regime of disc accretion in which the angular velocity of the stellar magnetosphere exceeds that of the inner disc. In these systems, the stellar magnetosphere acts as a centrifugal barrier and inhibits matter accretion onto the rapidly rotating star. Instead, the matter accreting through the disc accumulates at the disc-magnetosphere interface where it picks up angular momentum and is ejected from the system as a wide-angled outflow which gradually collimates at larger distances from the star. If the ejection rate is lower than the accretion rate, the matter will accumulate at the boundary faster than it can be ejected; in this case, accretion onto the star proceeds through an episodic accretion instability in which the episodes of matter accumulation are followed by a brief episode of simultaneous ejection and accretion of matter onto the star. In addition to the matter dominated wind component, the propeller outflow also exhibits a well-collimated, magnetically-dominated Poynting jet which transports energy and angular momentum away from the star. The propeller mechanism may explain some of the weakly-collimated jets and winds observed around some T Tauri stars as well as the episodic variability present in their light curves. It may also explain some of the quasi-periodic variability observed in cataclysmic variables, millisecond pulsars and other magnetized stars.

  18. Suppression of accretion on to low-mass Population III stars

    NASA Astrophysics Data System (ADS)

    Johnson, Jarrett L.; Khochfar, Sadegh

    2011-05-01

    Motivated by recent theoretical work suggesting that a substantial fraction of Population (Pop) III stars may have had masses low enough for them to survive to the present day, we consider the role that the accretion of metal-enriched gas may have had in altering their surface composition, thereby disguising them as Pop II stars. We demonstrate that if weak, solar-like winds are launched from low-mass Pop III stars formed in the progenitors of the dark matter halo of the Galaxy, then such stars are likely to avoid significant enrichment via accretion of material from the interstellar medium. We find that at early times accretion is easily prevented if the stars are ejected from the central regions of the haloes in which they form, either by dynamical interactions with more massive Pop III stars or by violent relaxation during halo mergers. While accretion may still take place during passage through sufficiently dense molecular clouds at later times, we find that the probability of such a passage is generally low (≲0.1), assuming that stars have velocities of the order of the maximum circular velocity of their host haloes and accounting for the orbital decay of merging haloes. In turn, due to the higher gas density required for accretion on to stars with higher velocities, we find an even lower probability of accretion (˜10-2) for the subset of Pop III stars formed at z > 10, which are more quickly incorporated into massive haloes than stars formed at lower redshift. While there is no a priori reason to assume that low-mass Pop III stars do not have solar-like winds, without them surface enrichment via accretion is likely to be inevitable. We briefly discuss the implications that our results hold for stellar archaeology.

  19. Accretion Models for Young Neutron Stars

    NASA Astrophysics Data System (ADS)

    Alpar, M. A.

    2003-07-01

    Interaction with possible fallback material, along with the magnetic fields and rotation rates at birth should determine the fates and categories of young neutron stars. This paper addresses some issues related to pure or hybrid accretion models for explaining the properties of young neutron stars.

  20. Accretion from a clumpy massive-star wind in supergiant X-ray binaries

    NASA Astrophysics Data System (ADS)

    El Mellah, I.; Sundqvist, J. O.; Keppens, R.

    2018-04-01

    Supergiant X-ray binaries (SgXB) host a compact object, often a neutron star (NS), orbiting an evolved O/B star. Mass transfer proceeds through the intense line-driven wind of the stellar donor, a fraction of which is captured by the gravitational field of the NS. The subsequent accretion process on to the NS is responsible for the abundant X-ray emission from SgXB. They also display peak-to-peak variability of the X-ray flux by a factor of a few 10-100, along with changes in the hardness ratios possibly due to varying absorption along the line of sight. We use recent radiation-hydrodynamic simulations of inhomogeneities (a.k.a. clumps) in the non-stationary wind of massive hot stars to evaluate their impact on the time-variable accretion process. For this, we run 3D hydrodynamic simulations of the wind in the vicinity of the accretor to investigate the formation of the bow shock and follow the inhomogeneous flow over several spatial orders of magnitude, down to the NS magnetosphere. In particular, we show that the impact of the wind clumps on the time variability of the intrinsic mass accretion rate is severely tempered by the crossing of the shock, compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We also account for the variable absorption due to clumps passing by the line of sight and estimate the final effective variability of the column density and mass accretion rate for different orbital separations. Finally, we compare our results to the most recent analysis of the X-ray flux and the hardness ratio in Vela X-1.

  1. Locating the Accretion Footprint on a Herbig Ae Star: MWC 480

    NASA Technical Reports Server (NTRS)

    Grady, C. A.; Hamaguchi, K.; Schneider, G.; Stecklum, B.; Woodgate, B. E.; McCleary, J. E.; Williger, G. M.; Sitko, M. L.; Menard, F.; Henning, Th.; hide

    2011-01-01

    Accretion is a fundamental process which establishes the dynamics of the protoplanetary disk and the final properties of the forming star. In solar-type stars, the star-disk coupling is determined by the magnetic field structure, which is responsible for funneling material from the disk midplane to higher latitudes on the star. Here, we use pan-chromatic data for the Herbig Ae star MWC 480 to address whether similar processes occur in intermediate-mass stars. MWC 480 has X-ray emission typical of actively accreting Herbig Ae stars, but with 5-9 x more photoelectric absorption than expected from optical and FUV data. We consider 3 sources for the absorption: the disk absorption in a wind or jet, and accretion. While we detect the disk in scattered light in are-analysis of archival HST data. the data are consistent with grazing illumination of the dust disk. We find that MWC 480's disk is stratified, geometrically thin, and is not responsible for the observed photoelectric absorption. MWC 480 drives a bipolar jet, but with a mass loss rate which is low compared to other Herbig Ae stars, where the outflow is more favorably oriented and enhanced photoelectric absorption is not seen. This excludes a jet or wind origin for the enhanced photoelectric absorption. We compare MWC 480's 0 VI emission with other Herbig Ae stars. The distribution of the emission in inclination, and lack of a correlation of profile shape and system inclination excludes equatorially-confined accretion for the FUSE Herbig Ae stars. The photoelectric absorption data further suggest that the accretion footprint on MWC 480 and other Herbig Ae stars is located at high temperate, rather than polar, latitudes. These findings support the presence of funneled accretion in MWC 480 and Herbig Ae stars, strengthening the parallel to T Tauri stars.

  2. Does mass accretion lead to field decay in neutron stars

    NASA Technical Reports Server (NTRS)

    Shibazaki, N.; Murakami, T.; Shaham, Jacob; Nomoto, K.

    1989-01-01

    The recent discovery of cyclotron lines from gamma-ray bursts indicates that the strong magnetic fields of isolated neutron stars might not decay. The possible inverse correlation between the strength of the magnetic field and the mass accreted by the neutron star suggests that mass accretion itself may lead to the decay of the magnetic field. The spin and magnetic field evolution of the neutron star was calculated under the hypothesis of the accretion-induced field decay. It is shown that the calculated results are consistent with the observations of binary and millisecond radio pulsars.

  3. Theory of Bipolar Outflows from Accreting Hot Stars

    NASA Astrophysics Data System (ADS)

    Konigl, A.

    1996-05-01

    There is a growing number of observational indicators for the presence of bipolar outflows in massive, young stellar objects that are still accreting mass as part of their formation process. In particular, there is evidence that the outflows from these objects can attain higher velocities and kinetic luminosities than their lower-mass counterparts. Furthermore, the higher-mass objects appear to smoothly continue the correlation found in T Tauri stars between outflow and accretion signatures, and in several cases there are direct clues to the existence of a disk from optical and infrared spectroscopy. These results suggest that the disk--outflow connection found in low-mass pre--main-sequence stars extends to more massive objects, and that a similar physical mechanism may drive the outflows in both cases. In this presentation, I first critically examine the observational basis for this hypothesis, considering, among other things, the possibility that several low-luminosity outflows might occasionally masquerade as a single flow from a luminous object, and the effects that the radiation field of a hot star could have on the spectroscopic diagnostics of an accretion-driven outflow. I then go on to consider how the commonly invoked centrifugally driven wind models of bipolar outflows in low-mass stars would be affected by the various physical processes (such as photoionization, photoevaporation, radiation pressure, and stellar wind ram pressure) that operate in higher-mass stars. I conclude by mentioning some of the tantalizing questions that one could hope to address as this young field of research continues to develop (for example: is there a high-mass analog of the FU Orionis outburst phenomenon? Could one use observations of progressively more massive, and hence less convective, stars to elucidate the role of stellar magnetic fields in the accretion and outflow processes? Would it be possible to observationally identify massive stars that have reached the main

  4. DIBS independent of accretion in T Tauri stars

    NASA Technical Reports Server (NTRS)

    Ghandour, Louma; Jenniskens, Peter; Hartigan, P.

    1994-01-01

    The examination of high resolution spectra (5200 - 7000 Angstroms) of 36 T Tauri stars ranging in accretion rates was performed. Only the lambda lambda 5780, 5797, and 6613 bands were found detectable to within an equivalent width of 10 micro Angstroms. They are strongest in DG Tau, DR Tau, Dl Tau, and AS 353A. DR Tau was monitored over the course of four years; during this time, the accretion rate varied by a factor of five, but the equivalent widths of the DIB's (Diffuse Interstellar Bands) remained constant. The lack of correlation of the strength of the bands with the accretion rates implies that the bands are not directly produced by UV radiation from the accretion process. The bands have line strengths and ratios characteristic of the diffuse interstellar medium, from which we conclude that the diffuse interstellar bands seen in the spectra of T Tauri stars do not originate in the stars' immediate environment. Instead, they are part of a foreground extinction, probably due to the parent molecular cloud.

  5. Formation and pre-MS Evolution of Massive Stars with Growing Accretion

    NASA Astrophysics Data System (ADS)

    Maeder, A.; Behrend, R.

    2002-10-01

    We briefly describe the three existing scenarios for forming massive stars and emphasize that the arguments often used to reject the accretion scenario for massive stars are misleading. It is usually not accounted for the fact that the turbulent pressure associated to large turbulent velocities in clouds necessarily imply relatively high accretion rates for massive stars. We show the basic difference between the formation of low and high mass stars based on the values of the free fall time and of the Kelvin-Helmholtz timescale, and define the concept of birthline for massive stars. Due to D-burning, the radius and location of the birthline in the HR diagram, as well as the lifetimes are very sensitive to the accretion rate dM/dt(accr). If a form dM/dt(accr) propto A(M/Msun)phi is adopted, the observations in the HR diagram and the lifetimes support a value of A approx 10-5 Msun/yr and a value of phi > 1. Remarkably, such a law is consistent with the relation found by Churchwell and Henning et al. between the outflow rates and the luminosities of ultracompact HII regions, if we assume that a fraction 0.15 to 0.3 of the global inflow is accreted. The above relation implies high dM/dt(accr) approx 10-3 Msun/yr for the most massive stars. The physical possibility of such high dM/dt(accr) is supported by current numerical models. Finally, we give simple analytical arguments in favour of the growth of dM/dt(accr) with the already accreted mass. We also suggest that due to Bondi-Hoyle accretion, the formation of binary stars is largely favoured among massive stars in the accretion scenario.

  6. Massive star formation by accretion. II. Rotation: how to circumvent the angular momentum barrier?

    NASA Astrophysics Data System (ADS)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.; Klessen, R. S.

    2017-06-01

    Context. Rotation plays a key role in the star-formation process, from pre-stellar cores to pre-main-sequence (PMS) objects. Understanding the formation of massive stars requires taking into account the accretion of angular momentum during their PMS phase. Aims: We study the PMS evolution of objects destined to become massive stars by accretion, focusing on the links between the physical conditions of the environment and the rotational properties of young stars. In particular, we look at the physical conditions that allow the production of massive stars by accretion. Methods: We present PMS models computed with a new version of the Geneva Stellar Evolution code self-consistently including accretion and rotation according to various accretion scenarios for mass and angular momentum. We describe the internal distribution of angular momentum in PMS stars accreting at high rates and we show how the various physical conditions impact their internal structures, evolutionary tracks, and rotation velocities during the PMS and the early main sequence. Results: We find that the smooth angular momentum accretion considered in previous studies leads to an angular momentum barrier and does not allow the formation of massive stars by accretion. A braking mechanism is needed in order to circumvent this angular momentum barrier. This mechanism has to be efficient enough to remove more than two thirds of the angular momentum from the inner accretion disc. Due to the weak efficiency of angular momentum transport by shear instability and meridional circulation during the accretion phase, the internal rotation profiles of accreting stars reflect essentially the angular momentum accretion history. As a consequence, careful choice of the angular momentum accretion history allows circumvention of any limitation in mass and velocity, and production of stars of any mass and velocity compatible with structure equations.

  7. Accretion shocks in the laboratory: Design of an experiment to study star formation

    DOE PAGES

    Young, Rachel P.; Kuranz, C. C.; Drake, R. P.; ...

    2017-02-13

    Here, we present the design of a laboratory-astrophysics experiment to study magnetospheric accretion relevant to young, pre-main-sequence stars. Spectra of young stars show evidence of hotspots created when streams of accreting material impact the surface of the star and create shocks. The structures that form during this process are poorly understood, as the surfaces of young stars cannot be spatially resolved. Our experiment would create a scaled "accretion shock" at a major (several kJ) laser facility. The experiment drives a plasma jet (the "accretion stream") into a solid block (the "stellar surface"), in the presence of a parallel magnetic fieldmore » analogous to the star's local field.« less

  8. NuSTAR and XMM-Newton Observations of 1E1743.1-2843: Indications of a Neutron Star LMXB Nature of the Compact Object

    NASA Technical Reports Server (NTRS)

    Lotti, Simone; Natalucci, Lorenzo; Mori, Kaya; Baganoff, Frederick K.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Harrison, Fiona A.; Hong, Jaesub; hide

    2016-01-01

    We report on the results of NuSTAR and XMM-Newton observations of the persistent X-ray source 1E1743.1-2843, located in the Galactic Center region. The source was observed between 2012 September and October by NuSTAR and XMM-Newton, providing almost simultaneous observations in the hard and soft X-ray bands. The high X-ray luminosity points to the presence of an accreting compact object. We analyze the possibilities of this accreting compact object being either a neutron star (NS) or a black hole, and conclude that the joint XMM-Newton and NuSTAR spectrum from 0.3 to 40 keV fits a blackbody spectrum with kT approximately 1.8 keV emitted from a hot spot or an equatorial strip on an NS surface. This spectrum is thermally Comptonized by electrons with kTe approximately 4.6 keV. Accepting this NS hypothesis, we probe the low-mass X-ray binary (LMXB) or high-mass X-ray binary (HMXB) nature of the source. While the lack of Type-I bursts can be explained in the LMXB scenario, the absence of pulsations in the 2 MHz-49 Hz frequency range, the lack of eclipses and of an IR companion, and the lack of a Kaline from neutral or moderately ionized iron strongly disfavor interpreting this source as a HMXB. We therefore conclude that 1E1743.1-2843 is most likely an NS-LMXB located beyond the Galactic Center. There is weak statistical evidence for a soft X-ray excess which may indicate thermal emission from an accretion disk. However, the disk normalization remains unconstrained due to the high hydrogen column density (N(sub H) approximately 1.6 x 10(exp 23) cm(exp -2)).

  9. Accreting Millisecond Pulsars: Neutron Star Masses and Radii

    NASA Technical Reports Server (NTRS)

    Strohmayer, Tod

    2004-01-01

    High amplitude X-ray brightness oscillations during thermonuclear X-ray bursts were discovered with the Rossi X-ray Timing Explorer (RXTE) in early 1996. Spectral and timing evidence strongly supports the conclusion that these oscillations are caused by rotational modulation of the burst emission and that they reveal the spin frequency of neutron stars in low mass X-ray binaries. The recent discovery of X-ray burst oscillations from two accreting millisecond pulsars has confirmed this basic picture and provided a new route to measuring neutron star properties and constraining the dense matter equation of state. I will briefly summarize the current observational understanding of accreting millisecond pulsars, and describe recent attempts to determine the mass and radius of the neutron star in XTE J1814-338.

  10. Radius of the neutron star magnetosphere during disk accretion

    NASA Astrophysics Data System (ADS)

    Filippova, E. V.; Mereminskiy, I. A.; Lutovinov, A. A.; Molkov, S. V.; Tsygankov, S. S.

    2017-11-01

    The dependence of the spin frequency derivative \\dot ν of accreting neutron stars with a strongmagnetic field (X-ray pulsars) on the mass accretion rate (bolometric luminosity, L bol) has been investigated for eight transient pulsars in binary systems with Be stars. Using data from the Fermi/GBM and Swift/BAT telescopes, we have shown that for seven of the eight systems the dependence \\dot ν ( L bol) can be fitted by the model of angular momentum transfer through an accretion disk, which predicts the relation \\dot ν ˜ L 6/7 bol. Hysteresis in the dependence \\dot ν ( L bol) has been confirmed in the system V 0332+53 and has been detected for the first time in the systems KS 1947+300, GRO J1008-57, and 1A 0535+26. Estimates for the radius of the neutron star magnetosphere in all of the investigated systems have been obtained. We show that this quantity varies from pulsar to pulsar and depends strongly on the analytical model and the estimates for the neutron star and binary system parameters.

  11. Panel 1: A pulsating red giant star and a compact, hot white dwarf star orbit each other.

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Panel 1: A pulsating red giant star and a compact, hot white dwarf star orbit each other. Panel 2: The red giant sheds much of its outer layers in a stellar wind. The white dwarf helps concentrate the wind along a thin equatorial plane. The white dwarf accretes some of this escaping gas forming a disk around the itself. Panel 3: When enough gas accumulates on the white dwarf's surface it explodes as a nova outburst. Most of the hot gas forms a pair of expanding bubbles above and below the equatorial disk. Panel 4: A few thousand years after the bubbles expand into space, the white dwarf goes through another nova outburst and makes another pair of bubbles, which form a distinctive hourglass shape.

  12. Determining the properties of accretion-gap neutron stars

    NASA Technical Reports Server (NTRS)

    Kluzniak, Wlodzimierz; Michelson, Peter; Wagoner, Robert V.

    1990-01-01

    If neutron stars have radii as small as has been argued by some, observations of accretion-powered X-rays could verify the existence of innermost stable circular orbits (predicted by general relativity) around weakly magnetized neutron stars. This may be done by detecting X-ray emission from clumps of matter before and after they cross the gap (where matter cannot be supported by rotation) between the inner accretion disk and the stellar surface. Assuming the validity of general relativity, it would then be possible to determine the masses of such neutron stars independently of any knowledge of binary orbital parameters. If an accurate mass determination were already available through any of the methods conventionally used, the new mass determination method proposed here could then be used to quantitatively test strong field effects of gravitational theory.

  13. The Beta Pictoris Phenomenon in A-Shell Stars: Detection of Accreting Gas

    NASA Technical Reports Server (NTRS)

    Grady, C. A.; Perez, Mario R.; Talavera, A.; McCollum, B.; Rawley, L. A.; England, M. N.; Schlegel, M.

    1996-01-01

    We present the results of an expanded survey of A-shell stars using IUE high-dispersion spectra and find accreting, circumstellar gas in the line of sight to nine stars, in addition to the previously identified beta Pic, HR 10, and 131 Tau, which can be followed to between +70 and 100 km/s relative to the star. Two of the program stars, HD 88195 and HD 148283, show variable high-velocity gas. Given the small number of IUE spectra for our program stars, detection of high-velocity, accreting gas in 2/3 of the A-shell stars sampled indicates that accretion is an intrinsic part of the A-shell phenomenon and that beta Pic is not unique among main-sequence A stars in exhibiting such activity. Our program stars, as a group, have smaller column densities of high-velocity gas and smaller near-IR excesses compared with beta Pic. These features are consistent with greater central clearing of a remnant debris disk, compared with beta Pic, and suggest that the majority of field A-shell stars are older than beta Pic.

  14. An ultraluminous X-ray source powered by an accreting neutron star.

    PubMed

    Bachetti, M; Harrison, F A; Walton, D J; Grefenstette, B W; Chakrabarty, D; Fürst, F; Barret, D; Beloborodov, A; Boggs, S E; Christensen, F E; Craig, W W; Fabian, A C; Hailey, C J; Hornschemeier, A; Kaspi, V; Kulkarni, S R; Maccarone, T; Miller, J M; Rana, V; Stern, D; Tendulkar, S P; Tomsick, J; Webb, N A; Zhang, W W

    2014-10-09

    The majority of ultraluminous X-ray sources are point sources that are spatially offset from the nuclei of nearby galaxies and whose X-ray luminosities exceed the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes. Their X-ray luminosities in the 0.5-10 kiloelectronvolt energy band range from 10(39) to 10(41) ergs per second. Because higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit, theoretical models have focused on black hole rather than neutron star systems. The most challenging sources to explain are those at the luminous end of the range (more than 10(40) ergs per second), which require black hole masses of 50-100 times the solar value or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries, or both. Here we report broadband X-ray observations of the nuclear region of the galaxy M82 that reveal pulsations with an average period of 1.37 seconds and a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to an X-ray luminosity in the 3-30 kiloelectronvolt range of 4.9 × 10(39) ergs per second. The pulsating source is spatially coincident with a variable source that can reach an X-ray luminosity in the 0.3-10 kiloelectronvolt range of 1.8 × 10(40) ergs per second. This association implies a luminosity of about 100 times the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects.

  15. SPIN EVOLUTION OF ACCRETING YOUNG STARS. I. EFFECT OF MAGNETIC STAR-DISK COUPLING

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Matt, Sean P.; Greene, Thomas P.; Pinzon, Giovanni

    2010-05-10

    We present a model for the rotational evolution of a young, solar mass star interacting with an accretion disk. The model incorporates a description of the angular momentum transfer between the star and the disk due to a magnetic connection, and includes changes in the star's mass and radius and a decreasing accretion rate. The model also includes, for the first time in a spin evolution model, the opening of the stellar magnetic field lines, as expected to arise from twisting via star-disk differential rotation. In order to isolate the effect that this has on the star-disk interaction torques, wemore » neglect the influence of torques that may arise from open field regions connected to the star or disk. For a range of magnetic field strengths, accretion rates, and initial spin rates, we compute the stellar spin rates of pre-main-sequence stars as they evolve on the Hayashi track to an age of 3 Myr. How much the field opening affects the spin depends on the strength of the coupling of the magnetic field to the disk. For the relatively strong coupling (i.e., high magnetic Reynolds number) expected in real systems, all models predict spin periods of less than {approx}3 days, in the age range of 1-3 Myr. Furthermore, these systems typically do not reach an equilibrium spin rate within 3 Myr, so that the spin at any given time depends upon the choice of initial spin rate. This corroborates earlier suggestions that, in order to explain the full range of observed rotation periods of approximately 1-10 days, additional processes, such as the angular momentum loss from powerful stellar winds, are necessary.« less

  16. NuSTAR and Suzaku X-ray Spectroscopy of NGC 4151: Evidence for Reflection from the Inner Accretion Disk

    NASA Astrophysics Data System (ADS)

    Keck, M. L.; Brenneman, L. W.; Ballantyne, D. R.; Bauer, F.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Dauser, T.; Elvis, M.; Fabian, A. C.; Fuerst, F.; García, J.; Grefenstette, B. W.; Hailey, C. J.; Harrison, F. A.; Madejski, G.; Marinucci, A.; Matt, G.; Reynolds, C. S.; Stern, D.; Walton, D. J.; Zoghbi, A.

    2015-06-01

    We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spin a\\gt 0.9 accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection (IDR) flux variation drives the spectral variability during the observations. We discuss various physical scenarios for the IDR model and we find that a compact corona is consistent with the observed features.

  17. Recent Observational Progress on Accretion Disks Around Compact Objects

    NASA Astrophysics Data System (ADS)

    Miller, Jon M.

    2016-04-01

    Studies of accretion disks around black holes and neutron stars over the last ten years have made remarkable progress. Our understanding of disk evolution as a function of mass accretion rate is pushing toward a consensus on thin/thick disk transitions; an apparent switching between disk-driven outflow modes has emerged; and monitoring observations have revealed complex spectral energy distributions wherein disk reprocessing must be important. Detailed studies of disk winds, in particular, have the potential to reveal the basic physical processes that mediate disk accretion, and to connect with numerical simulations. This talk will review these developments and look ahead to the potential of Astro-H.

  18. A Continuum of Accretion Burst Behavior in Young Stars Observed by K2

    NASA Astrophysics Data System (ADS)

    Cody, Ann Marie; Hillenbrand, Lynne A.; David, Trevor J.; Carpenter, John M.; Everett, Mark E.; Howell, Steve B.

    2017-02-01

    We present 29 likely members of the young ρ Oph or Upper Sco regions of recent star formation that exhibit “accretion burst” type light curves in K2 time series photometry. The bursters were identified by visual examination of their ˜80-day light curves, though all satisfy the M< -0.25 flux asymmetry criterion for burst behavior defined by Cody et al. The burst sources represent ≈9% of cluster members with strong infrared excess indicative of circumstellar material. Higher amplitude burster behavior is correlated with larger inner disk infrared excesses, as inferred from WISE W1-W2 color. The burst sources are also outliers in their large Hα emission equivalent widths. No distinction between bursters and non-bursters is seen in stellar properties such as multiplicity or spectral type. The frequency of bursters is similar between the younger, more compact ρ Oph region, and the older, more dispersed Upper Sco region. The bursts exhibit a range of shapes, amplitudes (˜10%-700%), durations (˜1-10 days), repeat timescales (˜3-80 days), and duty cycles (˜10%-100%). Our results provide important input to models of magnetospheric accretion, in particular, by elucidating the properties of accretion-related variability in the low state between major longer duration events such as EX Lup and FU Ori type accretion outbursts. We demonstrate the broad continuum of accretion burst behavior in young stars—extending the phenomenon to lower amplitudes and shorter timescales than traditionally considered in the theory of pre-main sequence accretion history.

  19. The evidence for clumpy accretion in the Herbig Ae star HR 5999

    NASA Technical Reports Server (NTRS)

    Perez, M. R.; Grady, C. A.; The, P. S.

    1993-01-01

    Analysis of IUE high- and low-dispersion spectra of the young Herbig Ae star HR 5999 (HD 144668) covering 1978-1992 revealed dramatic changes in the Mg II h and k (2795.5, 2802.7 A) emission profiles, changes in the column density and distribution in radial velocity of accreting gas, and flux in the Ly(alpha), O I, and C IV emission lines, which are correlated with the UV excess luminosity. Variability in the spectral type inferred from the UV spectral energy distribution, ranging from A5 IV-III in high state to A7 III in the low state, was also observed. The trend of earlier inferred spectral type with decreasing wavelength and with increasing UV continuum flux has previously been noted as a signature of accretion disks in lower mass pre-main sequence stars (PMS) and in systems undergoing FU Orionis-type outbursts. Our data represent the first detection of similar phenomena in an intermediate mass (M greater than or equal to 2 solar mass) PMS star. Recent IUE spectra show gas accreting toward the star with velocities as high as plus 300 km/s, much as is seen toward beta Pic, and suggest that we also view this system through the debris disk. The absence of UV lines with the rotational broadening expected given the optical data (A7 IV, V sini=180 plus or minus 20 km/s for this system) also suggests that most of the UV light originates in the disk, even in the low continuum state. The dramatic variability in the column density of accreting gas, is consistent with clumpy accretion, such as has been observed toward beta Pic, is a hallmark of accretion onto young stars, and is not restricted to the clearing phase, since detectable amounts of accretion are present for stars with 0.5 Myr less than t(sub age) less than 2.8 Myr. The implications for models of beta Pic and similar systems are briefly discussed.

  20. Compact Stars with Sequential QCD Phase Transitions.

    PubMed

    Alford, Mark; Sedrakian, Armen

    2017-10-20

    Compact stars may contain quark matter in their interiors at densities exceeding several times the nuclear saturation density. We explore models of such compact stars where there are two first-order phase transitions: the first from nuclear matter to a quark-matter phase, followed at a higher density by another first-order transition to a different quark-matter phase [e.g., from the two-flavor color-superconducting (2SC) to the color-flavor-locked (CFL) phase]. We show that this can give rise to two separate branches of hybrid stars, separated from each other and from the nuclear branch by instability regions, and, therefore, to a new family of compact stars, denser than the ordinary hybrid stars. In a range of parameters, one may obtain twin hybrid stars (hybrid stars with the same masses but different radii) and even triplets where three stars, with inner cores of nuclear matter, 2SC matter, and CFL matter, respectively, all have the same mass but different radii.

  1. The Evolution of Compact Binary Star Systems.

    PubMed

    Postnov, Konstantin A; Yungelson, Lev R

    2006-01-01

    We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and BHs are thought to be the primary astrophysical sources of gravitational waves (GWs) within the frequency band of ground-based detectors, while compact binaries of WDs are important sources of GWs at lower frequencies to be covered by space interferometers (LISA). Major uncertainties in the current understanding of properties of NSs and BHs most relevant to the GW studies are discussed, including the treatment of the natal kicks which compact stellar remnants acquire during the core collapse of massive stars and the common envelope phase of binary evolution. We discuss the coalescence rates of binary NSs and BHs and prospects for their detections, the formation and evolution of binary WDs and their observational manifestations. Special attention is given to AM CVn-stars - compact binaries in which the Roche lobe is filled by another WD or a low-mass partially degenerate helium-star, as these stars are thought to be the best LISA verification binary GW sources.

  2. CONSTRAINTS ON THE NEUTRON STAR AND INNER ACCRETION FLOW IN SERPENS X-1 USING NuSTAR

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Miller, J. M.; Parker, M. L.; Fabian, A. C.

    2013-12-10

    We report on an observation of the neutron star low-mass X-ray binary Serpens X-1, made with NuSTAR. The extraordinary sensitivity afforded by NuSTAR facilitated the detection of a clear, robust, relativistic Fe K emission line from the inner disk. A relativistic profile is required over a single Gaussian line from any charge state of Fe at the 5σ level of confidence, and any two Gaussians of equal width at the same confidence. The Compton back-scattering ''hump'' peaking in the 10-20 keV band is detected for the first time in a neutron star X-ray binary. Fits with relativistically blurred disk reflection modelsmore » suggest that the disk likely extends close to the innermost stable circular orbit (ISCO) or stellar surface. The best-fit blurred reflection models constrain the gravitational redshift from the stellar surface to be z {sub NS} ≥ 0.16. The data are broadly compatible with the disk extending to the ISCO; in that case, z {sub NS} ≥ 0.22 and R {sub NS} ≤ 12.6 km (assuming M {sub NS} = 1.4 M {sub ☉} and a = 0, where a = cJ/GM {sup 2}). If the star is as large or larger than its ISCO, or if the effective reflecting disk leaks across the ISCO to the surface, the redshift constraints become measurements. We discuss our results in the context of efforts to measure fundamental properties of neutron stars, and models for accretion onto compact objects.« less

  3. X-shooter study of accretion in Chamaeleon I. II. A steeper increase of accretion with stellar mass for very low-mass stars?

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Testi, L.; Herczeg, G. J.; Pascucci, I.; Alcalá, J. M.; Natta, A.; Antoniucci, S.; Fedele, D.; Mulders, G. D.; Henning, T.; Mohanty, S.; Prusti, T.; Rigliaco, E.

    2017-08-01

    The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star-forming region carried out using spectra taken with the ESO VLT/X-shooter spectrograph. The sample is nearly complete down to stellar masses (M⋆) 0.1 M⊙ for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broadband flux-calibrated medium resolution spectrum. The correlation between accretion luminosity to stellar luminosity, and of mass accretion rate to stellar mass in the logarithmic plane yields slopes of 1.9 ± 0.1 and 2.3 ± 0.3, respectively. These slopes and the accretion rates are consistent with previous results in various star-forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity lower than 0.45 L⊙ and for stellar masses lower than 0.3 M⊙ is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane that are empty of objects: one region at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second region is located just above the observational limits imposed by chromospheric emission, at M⋆ 0.3 - 0.4 M⊙. These are typical masses where photoevaporation is known to be effective. The mass accretion rates of this region are 10-10M⊙/yr, which is compatible with the value expected for photoevaporation to rapidly dissipate the inner disk. This work is

  4. Magnetically Controlled Spasmodic Accretion during Star Formation. II. Results

    NASA Astrophysics Data System (ADS)

    Tassis, Konstantinos; Mouschovias, Telemachos Ch.

    2005-01-01

    The problem of the late accretion phase of the evolution of an axisymmetric, isothermal magnetic disk surrounding a forming star has been formulated in a companion paper. The ``central sink approximation'' is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 1011 cm-3 and radii smaller than a few AU. Only the electrons are assumed to be attached to the magnetic field lines, and the effects of both negatively and positively charged grains are accounted for. After a mass of 0.1 Msolar accumulates in the central cell (forming star), a series of magnetically driven outflows and associated outward-propagating shocks form in a quasi-periodic fashion. As a result, mass accretion onto the protostar occurs in magnetically controlled bursts. We refer to this process as spasmodic accretion. The shocks propagate outward with supermagnetosonic speeds. The period of dissipation and revival of the outflow decreases in time, as the mass accumulated in the central sink increases. We evaluate the contribution of ambipolar diffusion to the resolution of the magnetic flux problem of star formation during the accretion phase, and we find it to be very significant albeit not sufficient to resolve the entire problem yet. Ohmic dissipation is completely negligible in the disk during this phase of the evolution. The protostellar disk is found to be stable against interchange-like instabilities, despite the fact that the mass-to-flux ratio has temporary local maxima.

  5. Oscillations of Accretion Disks in Cataclysmic Variable Stars

    NASA Astrophysics Data System (ADS)

    Osaki, Y.

    2013-12-01

    The disk instability model for the outbursts of dwarf novae is reviewed, with particular attention given to the superoutburst of SU UMa stars. Two intrinsic instabilities in accretion disks of dwarf novae are known; the thermal instability and the tidal instability. The thermal-tidal instability model (abbreviated the TTI model), which combines these two instabilities, was first proposed in 1989 by Osaki (1989) to explain the superoutburst phenomenon of SU UMa stars. Recent Kepler observations of one SU UMa star, V1504 Cyg, have dramatically demonstrated that the superoutburst phenomenon of the SU UMa stars is explained by the thermal-tidal instability model.

  6. CSI 2264: Accretion process in classical T Tauri stars in the young cluster NGC 2264

    NASA Astrophysics Data System (ADS)

    Sousa, A. P.; Alencar, S. H. P.; Bouvier, J.; Stauffer, J.; Venuti, L.; Hillenbrand, L.; Cody, A. M.; Teixeira, P. S.; Guimarães, M. M.; McGinnis, P. T.; Rebull, L.; Flaccomio, E.; Fürész, G.; Micela, G.; Gameiro, J. F.

    2016-02-01

    Context. NGC 2264 is a young stellar cluster (~3 Myr) with hundreds of low-mass accreting stars that allow a detailed analysis of the accretion process taking place in the pre-main sequence. Aims: Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars to the physical processes acting in the stellar and circumstellar environment, within a few stellar radii from the star. Methods: NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and photometric and spectroscopic observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several accretion diagnostics and disk parameters. Results: The morphology of the CoRoT light curve reflects the evolution of the accretion process and of the inner disk region. Accretion burst stars present high mass-accretion rates and optically thick inner disks. AA Tau-like systems, whose light curves are dominated by circumstellar dust obscuration, show intermediate mass-accretion rates and are located in the transition of thick to anemic disks. Classical T Tauri stars with spot-like light curves correspond mostly to systems with a low mass-accretion rate and low mid-IR excess. About 30% of the classical T Tauri stars observed in the 2008 and 2011 CoRoT runs changed their light-curve morphology. Transitions from AA Tau-like and spot-like to aperiodic light curves and vice versa were common. The analysis of the Hα emission line variability of 58 accreting stars showed that 8 presented a periodicity that in a few cases was coincident with the photometric period. The blue and red wings of the Hα line profiles often do not correlate with each other, indicating that they are strongly influenced by different physical processes. Classical T Tauri stars have a dynamic stellar and circumstellar environment that can be explained by magnetospheric

  7. Roto-chemical heating in a neutron star with fall-back disc accretion

    NASA Astrophysics Data System (ADS)

    Wei, Wei; Liu, Xi-Wei; Zheng, Xiao-Ping

    2018-07-01

    Recent research on the classical pulsar B0950+08 demonstrates that the explanation of its high surface temperature by roto-chemical heating encounters some difficulties. We assume that there is a fall-back disc around the newborn neutron star, which originates from the supernova ejecta and influences the spin and magnetic evolution of the star. By taking into account disc accretion and magnetic field evolution simultaneously, the effect of the fall-back disc accretion process on the roto-chemical heating in the neutron star is studied. The results show that there are two roto-chemical deviation phases (spin-up deviation and spin-down deviation), but that only the spin-down deviation leads to heating. The specific cooling curve depends on the accretion disc mass, the initial magnetic field and the magnetic field decay rate. Most importantly, the observations of surface temperature, magnetic field strength and spin period of the classical pulsar B0950+08 are well explained by the accretion roto-chemical heating model. The fall-back accretion process is important in roto-chemical heating for explanations of classical pulsars with high temperature. Given the absence of any evidence of fall-back accretion on to B0950+08, our study is purely hypothetical.

  8. Time-dependent spherically symmetric accretion onto compact X-ray sources

    NASA Technical Reports Server (NTRS)

    Cowie, L. L.; Ostriker, J. P.; Stark, A. A.

    1978-01-01

    Analytical arguments and a numerical hydrodynamic code are used to investigate spherically symmetric accretion onto a compact object, in an attempt to provide some insight into gas flows heated by an outgoing X-ray flux. It is shown that preheating of spherically symmetric accretion flows by energetic radiation from an X-ray source results in time-dependent behavior for a much wider range of source parameters than was determined previously and that there are two distinct types of instability. The results are compared with observations of X-ray bursters and transients as well as with theories on quasars and active galactic nuclei that involve quasi-spherically symmetric accretion onto massive black holes. Models based on spherically symmetric accretion are found to be inconsistent with observations of bursters and transients.

  9. Modeling of charged anisotropic compact stars in general relativity

    NASA Astrophysics Data System (ADS)

    Dayanandan, Baiju; Maurya, S. K.; T, Smitha T.

    2017-06-01

    A charged compact star model has been determined for anisotropic fluid distribution. We have solved the Einstein-Maxwell field equations to construct the charged compact star model by using the radial pressure, the metric function e^{λ} and the electric charge function. The generic charged anisotropic solution is verified by exploring different physical conditions like causality condition, mass-radius relation and stability of the solution (via the adiabatic index, TOV equations and the Herrera cracking concept). It is observed that the present charged anisotropic compact star model is compatible with the star PSR 1937+21. Moreover, we also presented the EOS ρ = f(p) for the present charged compact star model.

  10. The Structure of a Quasi-Keplerian Accretion Disk around Magnetized Stars

    NASA Astrophysics Data System (ADS)

    Habumugisha, Isaac; Jurua, Edward; Tessema, Solomon B.; Simon, Anguma K.

    2018-06-01

    In this paper, we present the complete structure of a quasi-Keplerian thin accretion disk with an internal dynamo around a magnetized neutron star. We assume a full quasi-Keplerian disk with the azimuthal velocity deviating from the Keplerian fashion by a factor of ξ (0 < ξ < 2). In our approach, we vertically integrate the radial component of the momentum equation to obtain the radial pressure gradient equation for a thin quasi-Keplerian accretion disk. Our results show that, at large radial distance, the accretion disk behaves in a Keplerian fashion. However, close to the neutron star, pressure gradient force (PGF) largely modifies the disk structure, resulting into sudden dynamical changes in the accretion disk. The corotation radius is shifted inward (outward) for ξ > 1 (for ξ < 1), and the position of the inner edge with respect to the new corotation radius is also relocated accordingly, as compared to the Keplerian model. The resulting PGF torque couples with viscous torque (when ξ < 1) to provide a spin-down torque and a spin-up torque (when ξ > 1) while in the advective state. Therefore, neglecting the PGF, as has been the case in previous models, is a glaring omission. Our result has the potential to explain the observable dynamic consequences of accretion disks around magnetized neutron stars.

  11. Sleuthing the Isolated Compact Stars

    NASA Astrophysics Data System (ADS)

    Drake, J. J.

    2004-08-01

    In the early 1990's, isolated thermally-emitting neutron stars accreting from the interstellar medium were predicted to show up in their thousands in the ROSAT soft X-ray all-sky survey. The glut of sources would provide unprecedented opportunities for probing the equation of state of ultra-dense matter. Only seven objects have been firmly identified to date. The reasons for this discrepency are discussed and recent high resolution X-ray spectroscopic observations of these objects are described. Spectra of the brightest of the isolated neutron star candidates, RX J1856.5-3754, continue to present interpretational difficulties for current neutron star model atmospheres and alternative models are briefly discussed. RX J1856.5-3754 remains a valid quark star candidate.

  12. Cooling of Compact Stars with Nucleon Superfluidity and Quark Superconductivity

    NASA Astrophysics Data System (ADS)

    Noda, Tsuneo; Hashimoto, Masa-aki; Yasutake, Nobutoshi; Maruyama, Toshiki; Tatsumi, Toshitaka

    We show a cooling scenario of compact stars to satisfy recent observations of compact stars. The central density of compact stars can exceed the nuclear density, and it is considered that many hadronic phases appear at such a density. It is discussed that neutron superfluidity (1S0 for lower density, and 3P2 for higher density) and proton superfluidity/superconductivity (1S0) appears in all compact stars. And some "Exotic" states are considered to appear in compact stars, such as meson condensation, hyperon mixing, deconfinement of quarks and quark colour superconductivity. These exotic states appear at the density region above the threshold densities of each state. We demonstrate the thermal evolution of isolated compact stars, adopting the effects of nucleon superfluidity and quark colour superconductivity. We assume large gap energy (Δ > 10 MeV) for colour superconducting quark phase, and include the effects of nucleon superfluidity with parametrised models. We simulate the cooling history of compact stars, and shows that the heavier star does not always cool faster than lighter one, which is determined by the parameters of neutron 3P2 superfluidity.

  13. An X-ray outburst from the rapidly accreting young star that illuminates McNeil's nebula.

    PubMed

    Kastner, J H; Richmond, M; Grosso, N; Weintraub, D A; Simon, T; Frank, A; Hamaguchi, K; Ozawa, H; Henden, A

    2004-07-22

    Young, low-mass stars are luminous X-ray sources whose powerful X-ray flares may exert a profound influence over the process of planet formation. The origin of the X-ray emission is uncertain. Although many (or perhaps most) recently formed, low-mass stars emit X-rays as a consequence of solar-like coronal activity, it has also been suggested that X-ray emission may be a direct result of mass accretion onto the forming star. Here we report X-ray imaging spectroscopy observations which reveal a factor approximately 50 increase in the X-ray flux from a young star that is at present undergoing a spectacular optical/infrared outburst (this star illuminates McNeil's nebula). The outburst seems to be due to the sudden onset of a phase of rapid accretion. The coincidence of a surge in X-ray brightness with the optical/infrared eruption demonstrates that strongly enhanced high-energy emission from young stars can occur as a consequence of high accretion rates. We suggest that such accretion-enhanced X-ray emission from erupting young stars may be short-lived, because intense star-disk magnetospheric interactions are quenched rapidly by the subsequent flood of new material onto the star.

  14. The Interplay of Star formation and Accretion in the Local Universe

    NASA Astrophysics Data System (ADS)

    Green, Paul

    2010-09-01

    Galaxy evolution and supermassive black hole growth are closely linked, but the inter-relationships between active accretion and star formation, AGN outflows, and host morphological trends remain poorly understood. We propose to study an unprecedented sample of 615 low redshift SDSS galaxies and AGN detected in archival Chandra fields. We will measure diverse optical and X-ray spectroscopic properties spanning the artificial galaxy/AGN divide, and provide detailed results of our model fitting. We highlight tests of (1) an evolutionary sequence from star-forming through AGN to passive galaxy modes (2) narrow line Sy1 galaxies and new parallels between the accretion modes of AGN and stellar mass X-ray binaries and (3) the relationship of host morphology and mergers to accretion.

  15. Strange matter in compact stars

    NASA Astrophysics Data System (ADS)

    Klähn, Thomas; Blaschke, David B.

    2018-02-01

    We discuss possible scenarios for the existence of strange matter in compact stars. The appearance of hyperons leads to a hyperon puzzle in ab-initio approaches based on effective baryon-baryon potentials but is not a severe problem in relativistic mean field models. In general, the puzzle can be resolved in a natural way if hadronic matter gets stiffened at supersaturation densities, an effect based on the quark Pauli quenching between hadrons. We explain the conflict between the necessity to implement dynamical chiral symmetry breaking into a model description and the conditions for the appearance of absolutely stable strange quark matter that require both, approximately masslessness of quarks and a mechanism of confinement. The role of strangeness in compact stars (hadronic or quark matter realizations) remains unsettled. It is not excluded that strangeness plays no role in compact stars at all. To answer the question whether the case of absolutely stable strange quark matter can be excluded on theoretical grounds requires an understanding of dense matter that we have not yet reached.

  16. The Evolution of Compact Binary Star Systems.

    PubMed

    Postnov, Konstantin A; Yungelson, Lev R

    2014-01-01

    We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Mergings of compact-star binaries are expected to be the most important sources for forthcoming gravitational-wave (GW) astronomy. In the first part of the review, we discuss observational manifestations of close binaries with NS and/or BH components and their merger rate, crucial points in the formation and evolution of compact stars in binary systems, including the treatment of the natal kicks, which NSs and BHs acquire during the core collapse of massive stars and the common envelope phase of binary evolution, which are most relevant to the merging rates of NS-NS, NS-BH and BH-BH binaries. The second part of the review is devoted mainly to the formation and evolution of binary WDs and their observational manifestations, including their role as progenitors of cosmologically-important thermonuclear SN Ia. We also consider AM CVn-stars, which are thought to be the best verification binary GW sources for future low-frequency GW space interferometers.

  17. Wind accretion and formation of disk structures in symbiotic binary systems

    NASA Astrophysics Data System (ADS)

    de Val-Borro, M.; Karovska, M.; Sasselov, D. D.; Stone, J. M.

    2015-05-01

    We investigate gravitationally focused wind accretion in binary systems consisting of an evolved star with a gaseous envelope and a compact accreting companion. We study the mass accretion and formation of an accretion disk around the secondary caused by the strong wind from the primary late-type component using global 2D and 3D hydrodynamic numerical simulations. In particular, the dependence of the mass accretion rate on the mass loss rate, wind temperature and orbital parameters of the system is considered. For a typical slow and massive wind from an evolved star the mass transfer through a focused wind results in rapid infall onto the secondary. A stream flow is created between the stars with accretion rates of a 2--10% percent of the mass loss from the primary. This mechanism could be an important method for explaining periodic modulations in the accretion rates for a broad range of interacting binary systems and fueling of a large population of X-ray binary systems. We test the plausibility of these accretion flows indicated by the simulations by comparing with observations of the symbiotic variable system CH Cyg.

  18. Galactic Halo Stars in Phase Space: A Hint of Satellite Accretion?

    NASA Astrophysics Data System (ADS)

    Brook, Chris B.; Kawata, Daisuke; Gibson, Brad K.; Flynn, Chris

    2003-03-01

    The present-day chemical and dynamical properties of the Milky Way bear the imprint of the Galaxy's formation and evolutionary history. One of the most enduring and critical debates surrounding Galactic evolution is that regarding the competition between ``satellite accretion'' and ``monolithic collapse'' the apparent strong correlation between orbital eccentricity and metallicity of halo stars was originally used as supporting evidence for the latter. While modern-day unbiased samples no longer support the claims for a significant correlation, recent evidence has been presented by Chiba & Beers for the existence of a minor population of high-eccentricity metal-deficient halo stars. It has been suggested that these stars represent the signature of a rapid (if minor) collapse phase in the Galaxy's history. Employing velocity and integrals of motion phase-space projections of these stars, coupled with a series of N-body/smoothed particle hydrodynamic chemodynamical simulations, we suggest that an alternative mechanism for creating such stars may be the recent accretion of a polar orbit dwarf galaxy.

  19. On the origin of jets from disc-accreting magnetized stars

    NASA Astrophysics Data System (ADS)

    Lovelace, Richard V. E.; Romanova, Marina M.; Lii, Patrick; Dyda, Sergei

    2014-09-01

    A brief review of the origin of jets from disc-accreting rotating magnetized stars is given. In most models, the interior of the disc is characterized by a turbulent viscosity and magnetic diffusivity ("alpha" discs) whereas the coronal region outside the disc is treated using ideal magnetohydrodynamics (MHD). Extensive MHD simulations have established the occurrence of long-lasting outflows in the case of both slowly and rapidly rotating stars. (1) Slowly rotating stars exhibit a new type of outflow, conical winds. Conical winds are generated when stellar magnetic flux is bunched up by the inward motion of the accretion disc. Near their region of origin, the winds have a thin conical shell shape with half opening angle of ˜30°. At large distances, their toroidal magnetic field collimates the outflow forming current carrying, matter dominated jets. These winds are predominantly magnetically and not centrifugally driven. About 10-30% of the disc matter from the inner disc is launched in the conical wind. Conical winds may be responsible for episodic as well as long lasting outflows in different types of stars. (2) Rapidly rotating stars in the "propeller regime" exhibit two-component outflows. One component is similar to the matter dominated conical wind, where a large fraction of the disc matter may be ejected in this regime. The second component is a high-velocity, low-density magnetically dominated axial jet where matter flows along the open polar field lines of the star. The axial jet has a mass flux of about 10% that of the conical wind, but its energy flux, due to the Poynting flux, can be as large as for the conical wind. The jet's magnetically dominated angular momentum flux causes the star to spin down rapidly. Propeller-driven outflows may be responsible for protostellar jets and their rapid spin-down. When the artificial requirement of symmetry about the equatorial plane is dropped, the conical winds are found to come alternately from one side of the

  20. ULXs from Accreting Neutron Stars: the Light Cylinder, the Stellar Surface, and Everything in Between

    NASA Astrophysics Data System (ADS)

    Parfrey, K.; Tchekhovskoy, A.

    2017-10-01

    I will present results from the first relativistic MHD simulations of accretion onto magnetized neutron stars, performed in general relativity in the Kerr spacetime. The accretion flow is geometrically thick with a relativistic-gas equation of state, appropriate for super-Eddington systems. Four regimes are recovered, in order of increasing stellar magnetic field strength (equivalently, decreasing mass accretion rate): (a) crushing of the stellar magnetosphere and direct accretion; (b) magnetically channeled accretion onto the stellar poles; (c) the propeller state, where material enters through the light cylinder but is prevented from accreting by the centrifugal barrier; (d) almost perfect exclusion of the accretion flow from the light cylinder by the pulsar's electromagnetic wind. A Poynting-flux-dominated relativistic jet, powered by stellar rotation, is produced when the intruding plasma succeeds in opening the pulsar's previously closed magnetic field lines. I will demonstrate the effect of changing the relative orientation of the stellar dipole and the large-scale magnetic field in the accreting plasma, and discuss our results in the context of the neutron-star-powered ULXs, as well as the transitional millisecond X-ray/radio pulsars and jet-launching neutron-star X-ray binaries.

  1. Self-similar Hot Accretion Flow onto a Neutron Star

    NASA Astrophysics Data System (ADS)

    Medvedev, Mikhail V.; Narayan, Ramesh

    2001-06-01

    We consider hot, two-temperature, viscous accretion onto a rotating, unmagnetized neutron star. We assume Coulomb coupling between the protons and electrons, as well as free-free cooling from the electrons. We show that the accretion flow has an extended settling region that can be described by means of two analytical self-similar solutions: a two-temperature solution that is valid in an inner zone, r<~102.5, where r is the radius in Schwarzschild units; and a one-temperature solution that is valid in an outer zone, r>~102.5. In both zones the density varies as ρ~r-2 and the angular velocity as Ω~r-3/2. We solve the flow equations numerically and confirm that the analytical solutions are accurate. Except for the radial velocity, all gas properties in the self-similar settling zone, such as density, angular velocity, temperature, luminosity, and angular momentum flux, are independent of the mass accretion rate; these quantities do depend sensitively on the spin of the neutron star. The angular momentum flux is outward under most conditions; therefore, the central star is nearly always spun down. The luminosity of the settling zone arises from the rotational energy that is released as the star is braked by viscosity, and the contribution from gravity is small; hence, the radiative efficiency, η=Lacc/Mc2, is arbitrarily large at low M. For reasonable values of the gas adiabatic index γ, the Bernoulli parameter is negative; therefore, in the absence of dynamically important magnetic fields, a strong outflow or wind is not expected. The flow is also convectively stable but may be thermally unstable. The described solution is not advection dominated; however, when the spin of the star is small enough, the flow transforms smoothly to an advection-dominated branch of solution.

  2. The Orbital Period of the SU Ursae Majoris Star EK Trianguli Australis and Evidence for Ring-Like Accretion Disks in Long-Supercycle Length SU Ursae Majoris Stars

    NASA Astrophysics Data System (ADS)

    Mennickent, Ronald E.; Arenas, Jose

    1998-06-01

    An orbital period of 0.06288(5) d has been found from a radial velocity study of the Hα emission line. In addition, we have detected an extra line emitting source located ~ 80(deg) apart from the vector joining the secondary--primary centers, as measured in the opposite sense to the binary rotational motion. This is not the expected location for the hotspot in dwarf novae. This anomaly could be removed by assuming a line emission lagging behind the white dwarf binary motion. In addition, we have estimated line emissivity (~ r(-alpha ) ) and disk radius (R equiv r_in/r_out) for 8 SU UMa stars. Most stars fit alpha = 1.8 +/- 0.1 but AK Cnc and WZ Sge strongly deviate from the mean; their emission line shapes can be explained assuming a post-outburst accretion disk mostly emitting close to the white dwarf (AK Cnc) and a ring-like disk (WZ Sge). In addition, we have found a tendency of long-supercycle length SU UMa stars to show very compact (large R; probably ring-like) accretion disks. If the supercycle length were basically controlled by the mass transfer rate (dot {M}), the inner disk radius would be a function of dot {M}. A white dwarf magnetic field ~ 5000 G is required to fit the truncation radius with the magnetosphere radius of SU UMa stars.

  3. Well behaved anisotropic compact star models in general relativity

    NASA Astrophysics Data System (ADS)

    Jasim, M. K.; Maurya, S. K.; Gupta, Y. K.; Dayanandan, B.

    2016-11-01

    Anisotropic compact star models have been constructed by assuming a particular form of a metric function e^{λ}. We solved the Einstein field equations for determining the metric function e^{ν}. For this purpose we have assumed a physically valid expression of radial pressure (pr). The obtained anisotropic compact star model is representing the realistic compact objects such as PSR 1937 +21. We have done an extensive study about physical parameters for anisotropic models and found that these parameters are well behaved throughout inside the star. Along with these we have also determined the equation of state for compact star which gives the radial pressure is purely the function of density i.e. pr=f(ρ).

  4. Spectral energy distributions of T Tauri stars - Disk flaring and limits on accretion

    NASA Technical Reports Server (NTRS)

    Kenyon, S. J.; Hartmann, L.

    1987-01-01

    The Adams et al. (1987) conclusion that much of the IR excess emission in the spectral energy distribution of T Tauri stars arises from reprocessing of stellar radiation by a dusty circumstellar disk is presently supported by analyses conducted in light of various models of these stars' spectra. A low mass reprocessing disk can, however, produce these spectra as well as a massive accretion disk. The detection of possible boundary layer radiation in the optical and near-UV regions poses the strongest limits on accretion rates. Disk accretion in the T Tauri phase does not significantly modify stellar evolution.

  5. Star-formation rate in compact star-forming galaxies

    NASA Astrophysics Data System (ADS)

    Izotova, I. Y.; Izotov, Y. I.

    2018-03-01

    We use the data for the Hβ emission-line, far-ultraviolet (FUV) and mid-infrared 22 μm continuum luminosities to estimate star formation rates < SFR > averaged over the galaxy lifetime for a sample of about 14000 bursting compact star-forming galaxies (CSFGs) selected from the Data Release 12 (DR12) of the Sloan Digital Sky Survey (SDSS). The average coefficient linking < SFR > and the star formation rate SFR0 derived from the Hβ luminosity at zero starburst age is found to be 0.04. We compare < SFR > s with some commonly used SFRs which are derived adopting a continuous star formation during a period of {˜} 100 Myr, and find that the latter ones are 2-3 times higher. It is shown that the relations between SFRs derived using a geometric mean of two star-formation indicators in the UV and IR ranges and reduced to zero starburst age have considerably lower dispersion compared to those with single star-formation indicators. We suggest that our relations for < SFR > determination are more appropriate for CSFGs because they take into account a proper temporal evolution of their luminosities. On the other hand, we show that commonly used SFR relations can be applied for approximate estimation within a factor of {˜} 2 of the < SFR > averaged over the lifetime of the bursting compact galaxy.

  6. Laboratory unraveling of matter accretion in young stars

    PubMed Central

    Revet, Guilhem; Chen, Sophia N.; Bonito, Rosaria; Khiar, Benjamin; Filippov, Evgeny; Argiroffi, Costanza; Higginson, Drew P.; Orlando, Salvatore; Béard, Jérôme; Blecher, Marius; Borghesi, Marco; Burdonov, Konstantin; Khaghani, Dimitri; Naughton, Kealan; Pépin, Henri; Portugall, Oliver; Riquier, Raphael; Rodriguez, Rafael; Ryazantsev, Sergei N.; Yu. Skobelev, Igor; Soloviev, Alexander; Willi, Oswald; Pikuz, Sergey; Ciardi, Andrea; Fuchs, Julien

    2017-01-01

    Accretion dynamics in the formation of young stars is still a matter of debate because of limitations in observations and modeling. Through scaled laboratory experiments of collimated plasma accretion onto a solid in the presence of a magnetic field, we open a first window on this phenomenon by tracking, with spatial and temporal resolution, the dynamics of the system and simultaneously measuring multiband emissions. We observe in these experiments that matter, upon impact, is ejected laterally from the solid surface and then refocused by the magnetic field toward the incoming stream. This ejected matter forms a plasma shell that envelops the shocked core, reducing escaped x-ray emission. This finding demonstrates one possible structure reconciling current discrepancies between mass accretion rates derived from x-ray and optical observations, respectively. PMID:29109974

  7. Laboratory unraveling of matter accretion in young stars

    DOE PAGES

    Revet, Guilhem; Chen, Sophia N.; Bonito, Rosaria; ...

    2017-11-01

    Accretion dynamics in the formation of young stars is still a matter of debate because of limitations in observations and modeling. Through scaled laboratory experiments of collimated plasma accretion onto a solid in the presence of a magnetic field, we open a first window on this phenomenon by tracking, with spatial and temporal resolution, the dynamics of the system and simultaneously measuring multiband emissions. Here, we observe in these experiments that matter, upon impact, is ejected laterally from the solid surface and then refocused by the magnetic field toward the incoming stream. This ejected matter forms a plasma shell thatmore » envelops the shocked core, reducing escaped x-ray emission. Our finding demonstrates one possible structure reconciling current discrepancies between mass accretion rates derived from x-ray and optical observations, respectively.« less

  8. Laboratory unraveling of matter accretion in young stars.

    PubMed

    Revet, Guilhem; Chen, Sophia N; Bonito, Rosaria; Khiar, Benjamin; Filippov, Evgeny; Argiroffi, Costanza; Higginson, Drew P; Orlando, Salvatore; Béard, Jérôme; Blecher, Marius; Borghesi, Marco; Burdonov, Konstantin; Khaghani, Dimitri; Naughton, Kealan; Pépin, Henri; Portugall, Oliver; Riquier, Raphael; Rodriguez, Rafael; Ryazantsev, Sergei N; Yu Skobelev, Igor; Soloviev, Alexander; Willi, Oswald; Pikuz, Sergey; Ciardi, Andrea; Fuchs, Julien

    2017-11-01

    Accretion dynamics in the formation of young stars is still a matter of debate because of limitations in observations and modeling. Through scaled laboratory experiments of collimated plasma accretion onto a solid in the presence of a magnetic field, we open a first window on this phenomenon by tracking, with spatial and temporal resolution, the dynamics of the system and simultaneously measuring multiband emissions. We observe in these experiments that matter, upon impact, is ejected laterally from the solid surface and then refocused by the magnetic field toward the incoming stream. This ejected matter forms a plasma shell that envelops the shocked core, reducing escaped x-ray emission. This finding demonstrates one possible structure reconciling current discrepancies between mass accretion rates derived from x-ray and optical observations, respectively.

  9. Laboratory unraveling of matter accretion in young stars

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Revet, Guilhem; Chen, Sophia N.; Bonito, Rosaria

    Accretion dynamics in the formation of young stars is still a matter of debate because of limitations in observations and modeling. Through scaled laboratory experiments of collimated plasma accretion onto a solid in the presence of a magnetic field, we open a first window on this phenomenon by tracking, with spatial and temporal resolution, the dynamics of the system and simultaneously measuring multiband emissions. Here, we observe in these experiments that matter, upon impact, is ejected laterally from the solid surface and then refocused by the magnetic field toward the incoming stream. This ejected matter forms a plasma shell thatmore » envelops the shocked core, reducing escaped x-ray emission. Our finding demonstrates one possible structure reconciling current discrepancies between mass accretion rates derived from x-ray and optical observations, respectively.« less

  10. Compact stars

    NASA Astrophysics Data System (ADS)

    Estevez-Delgado, Gabino; Estevez-Delgado, Joaquin

    2018-05-01

    An analysis and construction is presented for a stellar model characterized by two parameters (w, n) associated with the compactness ratio and anisotropy, respectively. The reliability range for the parameter w ≤ 1.97981225149 corresponds with a compactness ratio u ≤ 0.2644959374, the density and pressures are positive, regular and monotonic decrescent functions, the radial and tangential speed of sound are lower than the light speed, moreover, than the plausible stability. The behavior of the speeds of sound are determinate for the anisotropy parameter n, admitting a subinterval where the speeds are monotonic crescent functions and other where we have monotonic decrescent functions for the same speeds, both cases describing a compact object that is also potentially stable. In the bigger value for the observational mass M = 2.05 M⊙ and radii R = 12.957 Km for the star PSR J0348+0432, the model indicates that the maximum central density ρc = 1.283820319 × 1018 Kg/m3 corresponds to the maximum value of the anisotropy parameter and the radial and tangential speed of the sound are monotonic decrescent functions.

  11. Accreting Neutron Star and Black Hole Binaries with NICER

    NASA Astrophysics Data System (ADS)

    Chakrabarty, Deepto

    2018-01-01

    The NICER mission on the International Space Station has significant new capabilities for the study of accreting neutron stars and blackholes, including large effective area, low background, and excellent low-energy X-ray response. Both the NICER Burst and Accretion Working Group and the Observatory Science Working Group have designed observing programs that probe various aspects of accretion physics. I will present some early results from the first six months of the NICER mission, including observations of the black hole transients MAXI J1535-571 and GX 339-4, the high-mass X-ray binary pulsars GRO J1008-57 and Swift J02436+6124, and the X-ray burster 4U 1820-30.

  12. Role of pressure anisotropy on relativistic compact stars

    NASA Astrophysics Data System (ADS)

    Maurya, S. K.; Banerjee, Ayan; Hansraj, Sudan

    2018-02-01

    We investigate a compact spherically symmetric relativistic body with anisotropic particle pressure profiles. The distribution possesses characteristics relevant to modeling compact stars within the framework of general relativity. For this purpose, we consider a spatial metric potential of Korkina and Orlyanskii [Ukr. Phys. J. 36, 885 (1991)] type in order to solve the Einstein field equations. An additional prescription we make is that the pressure anisotropy parameter takes the functional form proposed by Lake [Phys. Rev. D 67, 104015 (2003), 10.1103/PhysRevD.67.104015]. Specifying these two geometric quantities allows for further analysis to be carried out in determining unknown constants and obtaining a limit of the mass-radius diagram, which adequately describes compact strange star candidates like Her X-1 and SMC X-1. Using the anisotropic Tolman-Oppenheimer-Volkoff equations, we explore the hydrostatic equilibrium and the stability of such compact objects. Then, we investigate other physical features of this model, such as the energy conditions, speeds of sound, and compactness of the star, in detail and show that our results satisfy all the required elementary conditions for a physically acceptable stellar model. The results obtained are useful in analyzing the stability of other anisotropic compact objects like white dwarfs, neutron stars, and gravastars.

  13. Study of magnetized accretion flow with variable Γ equation of state

    NASA Astrophysics Data System (ADS)

    Singh, Kuldeep; Chattopadhyay, Indranil

    2018-05-01

    We present here the solutions of magnetized accretion flow on to a compact object with hard surface such as neutron stars. The magnetic field of the central star is assumed dipolar and the magnetic axis is assumed to be aligned with the rotation axis of the star. We have used an equation of state for the accreting fluid in which the adiabatic index is dependent on temperature and composition of the flow. We have also included cooling processes like bremsstrahlung and cyclotron processes in the accretion flow. We found all possible accretion solutions. All accretion solutions terminate with a shock very near to the star surface and the height of this primary shock does not vary much with either the spin period or the Bernoulli parameter of the flow, although the strength of the shock may vary with the period. For moderately rotating central star, there is possible formation of multiple sonic points in the flow and therefore, a second shock far away from the star surface may also form. However, the second shock is much weaker than the primary one near the surface. We found that if rotation period is below a certain value (P*), then multiple critical points or multiple shocks are not possible and P* depends upon the composition of the flow. We also found that cooling effect dominates after the shock and that the cyclotron and the bremsstrahlung cooling processes should be considered to obtain a consistent accretion solution.

  14. Numerical Simulations of Wind Accretion in Symbiotic Binaries

    NASA Astrophysics Data System (ADS)

    de Val-Borro, M.; Karovska, M.; Sasselov, D.

    2009-08-01

    the mass loss from the AGB star. Our simulations of gravitationally focused wind accretion in symbiotic binaries show the formation of stream flows and enhanced accretion rates onto the compact component. We conclude that mass transfer through a focused wind is an important mechanism in wind accreting interacting binaries and can have a significant impact on the evolution of the binary itself and the individual components.

  15. Star Clusters within FIRE

    NASA Astrophysics Data System (ADS)

    Perez, Adrianna; Moreno, Jorge; Naiman, Jill; Ramirez-Ruiz, Enrico; Hopkins, Philip F.

    2017-01-01

    In this work, we analyze the environments surrounding star clusters of simulated merging galaxies. Our framework employs Feedback In Realistic Environments (FIRE) model (Hopkins et al., 2014). The FIRE project is a high resolution cosmological simulation that resolves star forming regions and incorporates stellar feedback in a physically realistic way. The project focuses on analyzing the properties of the star clusters formed in merging galaxies. The locations of these star clusters are identified with astrodendro.py, a publicly available dendrogram algorithm. Once star cluster properties are extracted, they will be used to create a sub-grid (smaller than the resolution scale of FIRE) of gas confinement in these clusters. Then, we can examine how the star clusters interact with these available gas reservoirs (either by accreting this mass or blowing it out via feedback), which will determine many properties of the cluster (star formation history, compact object accretion, etc). These simulations will further our understanding of star formation within stellar clusters during galaxy evolution. In the future, we aim to enhance sub-grid prescriptions for feedback specific to processes within star clusters; such as, interaction with stellar winds and gas accretion onto black holes and neutron stars.

  16. Accretion and Structure in the SW Sextantis Stars

    NASA Astrophysics Data System (ADS)

    Hoard, Donald Wayne

    1998-09-01

    The SW Sextantis stars are cataclysmic variables (CVs) sharing properties that set them apart from other CVs. These include: strong He II λ4686 emission, velocity curves implying asymmetric disk emission, and variable line profiles displaying a transient absorption feature at specific orbital phases. A number of mechanisms have been proposed to explain these characteristics including (non-disk) circumstellar material, a bipolar disk wind, a white dwarf magnetic field, and coherent accretion stream overflow across the disk, but none has been completely satisfying. I present the results of new photometric and spectroscopic observations of seven SW Sex stars, including Doppler tomogram mapping of emission regions in these systems. These observations, along with recent advances in simulations of accretion disks, suggest a scenario in which the accretion stream undergoes a violent impact with the disk edge. Depending on the mass transfer rate in the stream, the impact site will either cool efficiently (low M) and allow substantial material to flow directly over the disk, or will cool inefficiently (high M) and form a prominent bright spot at the impact site with hot stream material swept 'downstream' along the disk edge. In the former case, non-axisymmetric vertical structure develops in the disk at the terminus of the stream overflow (accounting for absorption seen at φapprox0.5), while in the latter case vertical structure is built up along the disk edge (accounting for absorption at φapprox0.8). The absorption feature phasing in different SW Sex stars implies M decreases as P orb decreases (as expected during CV evolution), but it is not clear whether normal CV evolution can drive changes in M rapidly enough to generate the onset of the SW Sex phenomenon in the narrow range of orbital period they occupy (P orb=3[-]4 hr). I present a gallery of new and archived IUE spectra of the SW Sex stars that display the typically strong UV resonant scattering lines seen in

  17. A neutron star with a carbon atmosphere in the Cassiopeia A supernova remnant.

    PubMed

    Ho, Wynn C G; Heinke, Craig O

    2009-11-05

    The surface of hot neutron stars is covered by a thin atmosphere. If there is accretion after neutron-star formation, the atmosphere could be composed of light elements (H or He); if no accretion takes place or if thermonuclear reactions occur after accretion, heavy elements (for example, Fe) are expected. Despite detailed searches, observations have been unable to confirm the atmospheric composition of isolated neutron stars. Here we report an analysis of archival observations of the compact X-ray source in the centre of the Cassiopeia A supernova remnant. We show that a carbon atmosphere neutron star (with low magnetic field) produces a good fit to the spectrum. Our emission model, in contrast with others, implies an emission size consistent with theoretical predictions for the radius of neutron stars. This result suggests that there is nuclear burning in the surface layers and also identifies the compact source as a very young ( approximately 330-year-old) neutron star.

  18. General Relativistic Non-radial Oscillations of Compact Stars

    NASA Astrophysics Data System (ADS)

    Hall, Zack, II; Jaikumar, Prashanth

    2017-01-01

    Currently, we lack a means of identifying the type of matter at the core of compact stars, but in the future, we may be able to use gravitational wave signals produced by fluid oscillations inside compact stars to discover new phases of dense matter. To this end, we study the fluid perturbations inside compact stars such as Neutron Stars and Strange Quark Stars, focusing on modes that couple to gravitational waves. Using a modern equation of state for quark matter that incorporates interactions at moderately high densities, we implement an efficient computational scheme to solve the oscillation equations in the framework of General Relativity, and determine the complex eigenfrequencies that describe the oscillation and damping of the non-radial fluid modes. We discuss the significance of our results for future detection of these modes through gravitational waves. This work is supported in part by the CSULB Graduate Research Fellowship and by the National Science Foundation NSF PHY-1608959.

  19. A Strong Shallow Heat Source in the Accreting Neutron Star MAXI J0556-332

    NASA Astrophysics Data System (ADS)

    Deibel, Alex; Cumming, Andrew; Brown, Edward F.; Page, Dany

    2015-08-01

    An accretion outburst in an X-ray transient deposits material onto the neutron star primary; this accumulation of matter induces reactions in the neutron star’s crust. During the accretion outburst these reactions heat the crust out of thermal equilibrium with the core. When accretion halts, the crust cools to its long-term equilibrium temperature on observable timescales. Here we examine the accreting neutron star transient MAXI J0556-332, which is the hottest transient, at the start of quiescence, observed to date. Models of the quiescent light curve require a large deposition of heat in the shallow outer crust from an unknown source. The additional heat injected is ≈4-10 MeV per accreted nucleon; when the observed decline in accretion rate at the end of the outburst is accounted for, the required heating increases to ≈6-16 MeV. This shallow heating is still required to fit the light curve even after taking into account a second accretion episode, uncertainties in distance, and different surface gravities. The amount of shallow heating is larger than that inferred for other neutron star transients and is larger than can be supplied by nuclear reactions or compositionally driven convection; but it is consistent with stored mechanical energy in the accretion disk. The high crust temperature ({T}b≳ {10}9 {{K}}) makes its cooling behavior in quiescence largely independent of the crust composition and envelope properties, so that future observations will probe the gravity of the source. Fits to the light curve disfavor the presence of Urca cooling pairs in the crust.

  20. The magnetic nature of disk accretion onto black holes.

    PubMed

    Miller, Jon M; Raymond, John; Fabian, Andy; Steeghs, Danny; Homan, Jeroen; Reynolds, Chris; van der Klis, Michiel; Wijnands, Rudy

    2006-06-22

    Although disk accretion onto compact objects-white dwarfs, neutron stars and black holes-is central to much of high-energy astrophysics, the mechanisms that enable this process have remained observationally difficult to determine. Accretion disks must transfer angular momentum in order for matter to travel radially inward onto the compact object. Internal viscosity from magnetic processes and disk winds can both in principle transfer angular momentum, but hitherto we lacked evidence that either occurs. Here we report that an X-ray-absorbing wind discovered in an observation of the stellar-mass black hole binary GRO J1655 - 40 (ref. 6) must be powered by a magnetic process that can also drive accretion through the disk. Detailed spectral analysis and modelling of the wind shows that it can only be powered by pressure generated by magnetic viscosity internal to the disk or magnetocentrifugal forces. This result demonstrates that disk accretion onto black holes is a fundamentally magnetic process.

  1. Star formation with disc accretion and rotation. I. Stars between 2 and 22 M⊙ at solar metallicity

    NASA Astrophysics Data System (ADS)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.

    2013-09-01

    Context. The way angular momentum is built up in stars during their formation process may have an impact on their further evolution. Aims: In the framework of the cold disc accretion scenario, we study how angular momentum builds up inside the star during its formation for the first time and what the consequences are for its evolution on the main sequence (MS). Methods: Computation begins from a hydrostatic core on the Hayashi line of 0.7 M⊙ at solar metallicity (Z = 0.014) rotating as a solid body. Accretion rates depending on the luminosity of the accreting object are considered, which vary between 1.5 × 10-5 and 1.7 × 10-3 M⊙ yr-1. The accreted matter is assumed to have an angular velocity equal to that of the outer layer of the accreting star. Models are computed for a mass-range on the zero-age main sequence (ZAMS) between 2 and 22 M⊙. Results: We study how the internal and surface velocities vary as a function of time during the accretion phase and the evolution towards the ZAMS. Stellar models, whose evolution has been followed along the pre-MS phase, are found to exhibit a shallow gradient of angular velocity on the ZAMS. Typically, the 6 M⊙ model has a core that rotates 50% faster than the surface on the ZAMS. The degree of differential rotation on the ZAMS decreases when the mass increases (for a fixed value of vZAMS/vcrit). The MS evolution of our models with a pre-MS accreting phase show no significant differences with respect to those of corresponding models computed from the ZAMS with an initial solid-body rotation. Interestingly, there exists a maximum surface velocity that can be reached through the present scenario of formation for masses on the ZAMS larger than 8 M⊙. Typically, only stars with surface velocities on the ZAMS lower than about 45% of the critical velocity can be formed for 14 M⊙ models. Reaching higher velocities would require starting from cores that rotate above the critical limit. We find that this upper velocity

  2. Accretion Rates for T Tauri Stars Using Nearly Simultaneous Ultraviolet and Optical Spectra

    NASA Astrophysics Data System (ADS)

    Ingleby, Laura; Calvet, Nuria; Herczeg, Gregory; Blaty, Alex; Walter, Frederick; Ardila, David; Alexander, Richard; Edwards, Suzan; Espaillat, Catherine; Gregory, Scott G.; Hillenbrand, Lynne; Brown, Alexander

    2013-04-01

    We analyze the accretion properties of 21 low-mass T Tauri stars using a data set of contemporaneous near-UV (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph and the ground-based Small and Medium Aperture Research Telescope System, a unique data set because of the nearly simultaneous broad wavelength coverage. Our data set includes accreting T Tauri stars in Taurus, Chamaeleon I, η Chamaeleon, and the TW Hydra Association. For each source we calculate the accretion rate (\\dot{M}) by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high-density, low filling factor accretion spots coexist with low-density, high filling factor spots. By fitting the UV and optical spectra with multiple accretion components, we can explain excesses which have been observed in the near-IR. Comparing our estimates of \\dot{M} to previous estimates, we find some discrepancies; however, they may be accounted for when considering assumptions for the amount of extinction and variability in optical spectra. Therefore, we confirm many previous estimates of the accretion rate. Finally, we measure emission line luminosities from the same spectra used for the \\dot{M} estimates, to produce correlations between accretion indicators (Hβ, Ca II K, C II], and Mg II) and accretion properties obtained simultaneously.

  3. General Relativistic Radiation MHD Simulations of Supercritical Accretion onto a Magnetized Neutron Star: Modeling of Ultraluminous X-Ray Pulsars

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Takahashi, Hiroyuki R.; Ohsuga, Ken, E-mail: takahashi@cfca.jp, E-mail: ken.ohsuga@nao.ac.jp

    By performing 2.5-dimensional general relativistic radiation magnetohydrodynamic simulations, we demonstrate supercritical accretion onto a non-rotating, magnetized neutron star, where the magnetic field strength of dipole fields is 10{sup 10} G on the star surface. We found the supercritical accretion flow consists of two parts: the accretion columns and the truncated accretion disk. The supercritical accretion disk, which appears far from the neutron star, is truncated at around ≃3 R {sub *} ( R {sub *} = 10{sup 6} cm is the neutron star radius), where the magnetic pressure via the dipole magnetic fields balances with the radiation pressure of themore » disks. The angular momentum of the disk around the truncation radius is effectively transported inward through magnetic torque by dipole fields, inducing the spin up of a neutron star. The evaluated spin-up rate, ∼−10{sup −11} s s{sup −1}, is consistent with the recent observations of the ultraluminous X-ray pulsars. Within the truncation radius, the gas falls onto a neutron star along the dipole fields, which results in a formation of accretion columns onto the northern and southern hemispheres. The net accretion rate and the luminosity of the column are ≃66 L {sub Edd}/ c {sup 2} and ≲10 L {sub Edd}, where L {sub Edd} is the Eddington luminosity and c is the light speed. Our simulations support a hypothesis whereby the ultraluminous X-ray pulsars are powered by the supercritical accretion onto the magnetized neutron stars.« less

  4. Temperature profiles of accretion discs around rapidly rotating strange stars in general relativity: A comparison with neutron stars

    NASA Astrophysics Data System (ADS)

    Bhattacharyya, S.; Thampan, A. V.; Bombaci, I.

    2001-06-01

    We compute the temperature profiles of accretion discs around rapidly rotating strange stars, using constant gravitational mass equilibrium sequences of these objects, considering the full effect of general relativity. Beyond a certain critical value of stellar angular momentum (J), we observe the radius (r_orb) of the innermost stable circular orbit (ISCO) to increase with J (a property seen neither in rotating black holes nor in rotating neutron stars). The reason for this is traced to the crucial dependence of dr_orb/dJ on the rate of change of the radial gradient of the Keplerian angular velocity at r_orb with respect to J. The structure parameters and temperature profiles obtained are compared with those of neutron stars, as an attempt to provide signatures for distinguishing between the two. We show that when the full gamut of strange star equation of state models, with varying degrees of stiffness are considered, there exists a substantial overlap in properties of both neutron stars and strange stars. However, applying accretion disc model constraints to rule out stiff strange star equation of state models, we notice that neutron stars and strange stars exclusively occupy certain parameter spaces. This result implies the possibility of distinguishing these objects from each other by sensitive observations through future X-ray detectors.

  5. Ultra high energy gamma rays, cosmic rays and neutrinos from accreting degenerate stars

    NASA Technical Reports Server (NTRS)

    Brecher, K.; Chanmugam, G.

    1985-01-01

    Super-Eddington accretion for a recently proposed unipolar induction model of cosmic ray acceleration in accreting binary star systems containing magnetic white dwarfs or neutron stars is considered. For sufficiently high accretion rates and low magnetic fields, the model can account for: (1) acceleration of cosmic ray nuclei up to energies of 10 to the 19th power eV; (2) production of more or less normal solar cosmic ray composition; (3) the bulk of cosmic rays observed with energies above 1 TeV, and probably even down to somewhat lower energies as well; and (4) possibly the observed antiproton cosmic ray flux. It can also account for the high ultra high energy (UHE) gamma ray flux observed from several accreting binary systems (including Cygnus X-3), while allowing the possibility of an even higher neutrino flux from these sources, with L sub nu/L sub gamma is approximately 100.

  6. Initial data for high-compactness black hole-neutron star binaries

    NASA Astrophysics Data System (ADS)

    Henriksson, Katherine; Foucart, François; Kidder, Lawrence E.; Teukolsky, Saul A.

    2016-05-01

    For highly compact neutron stars, constructing numerical initial data for black hole-neutron star binary evolutions is very difficult. We describe improvements to an earlier method that enable it to handle these more challenging cases. These improvements were found by invoking a general relaxation principle that may be helpful in improving robustness in other initial data solvers. We examine the case of a 6:1 mass ratio system in inspiral close to merger, where the star is governed by a polytropic {{Γ }}=2, an SLy, or an LS220 equation of state (EOS). In particular, we are able to obtain a solution with a realistic LS220 EOS for a star with compactness 0.26 and mass 1.98 M ⊙, which is representative of the highest reliably determined neutron star masses. For the SLy EOS, we can obtain solutions with a comparable compactness of 0.25, while for a family of polytropic equations of state, we obtain solutions with compactness up to 0.21, the largest compactness that is stable in this family. These compactness values are significantly higher than any previously published results.

  7. Accretion dynamics and polarized X-ray emission of magnetized neutron stars

    NASA Technical Reports Server (NTRS)

    Arons, Jonathan

    1991-01-01

    The basic ideas of accretion onto magnetized neutron stars are outlined. These are applied to a simple model of the structure of the plasma mound sitting at the magnetic poles of such a star, in which upward diffusion of photons is balanced by their downward advection. This steady flow model of the plasma's dynamical state is used to compute the emission of polarized X-raysfrom the optically thick, birefringent medium. The linear polarization of the continuum radiation emerging from the quasi-static mound is found to be as much as 40 percent at some rotation phases, but is insensitive to the geometry of the accretion flow. The role of the accretion shock, whose detailed polarimetric and spectral characteristics have yet to be calculated, is emphasized as the final determinant of the properties of the emerging X-rays. Some results describing the fully time dependent dynamics of the flow are also presented. In particular, steady flow onto a neutron star is shown to exhibit formation of 'photon bubbles', regions of greatly reduced plasma density filled with radiation which form and rise on millisecond time scale. The possible role of these complex structures in the flow for the formation of the emergent spectrum is briefly outlined.

  8. Gravitational Waves from Fallback Accretion onto Neutron Stars

    NASA Astrophysics Data System (ADS)

    Piro, Anthony L.; Thrane, Eric

    2012-12-01

    Massive stars generally end their lives as neutron stars (NSs) or black holes (BHs), with NS formation typically occurring at the low-mass end and collapse to a BH more likely at the high-mass end. In an intermediate regime, with a mass range that depends on the uncertain details of rotation and mass loss during the star's life, an NS is initially formed, which then experiences fallback accretion and collapse to a BH. The electromagnetic consequence of such an event is not clear. Depending on the progenitor's structure, possibilities range from a long gamma-ray burst to a Type II supernova (which may or may not be jet powered) to a collapse with a weak electromagnetic signature. Gravitational waves (GWs) provide the exciting opportunity to peer through the envelope of a dying massive star and directly probe what is occurring inside. We explore whether fallback onto young NSs can be detected by ground-based interferometers. When the incoming material has sufficient angular momentum to form a disk, the accretion spins up the NS sufficiently to produce non-axisymmetric instabilities and gravitational radiation at frequencies of ~700-2400 Hz for ~30-3000 s until collapse to a BH occurs. Using a realistic excess cross-power search algorithm, we show that such events are detectable by Advanced LIGO out to ≈17 Mpc. From the rate of nearby core-collapse supernovae in the past five years, we estimate that there will be ~1-2 events each year that are worth checking for fallback GWs. The observation of these unique GW signatures coincident with electromagnetic detections would identify the transient events that are associated with this channel of BH formation, while providing information about the protoneutron star progenitor.

  9. Social stars: Modeling the interactive lives of stars in dense clusters and binary systems in the era of time domain astronomy

    NASA Astrophysics Data System (ADS)

    MacLeod, Morgan Elowe

    This thesis uses computational modeling to study of phases of dramatic interaction that intersperse stellar lifetimes. In galactic centers stars trace dangerously wandering orbits dictated by the combined gravitational force of a central, supermassive black hole and all of the surrounding stars. In binary systems, stars' evolution -- which causes their radii to increase substantially -- can bring initially non-interacting systems into contact. Moments of strong stellar interaction transform stars, their subsequent evolution, and the stellar environments they inhabit. In tidal disruption events, a star is partially or completely destroyed as tidal forces from a supermassive black hole overwhelm the star's self gravity. A portion of the stellar debris falls back to the black hole powering a luminous flare as it accretes. This thesis studies the relative event rates and properties of tidal disruption events for stars across the stellar evolutionary spectrum. Tidal disruptions of giant stars occur with high specific frequency; these objects' extended envelopes make them vulnerable to disruption. More-compact white dwarf stars are tidally disrupted relatively rarely. Their transients are also of very different duration and luminosity. Giant star disruptions power accretion flares with timescales of tens to hundreds of years; white dwarf disruption flares take hours to days. White dwarf tidal interactions can additionally trigger thermonuclear burning and lead to transients with signatures similar to type I supernovae. In binary star systems, a phase of hydrodynamic interaction called a common envelope episode occurs when one star evolves to swallow its companion. Dragged by the surrounding gas, the companion star spirals through the envelope to tighter orbits. This thesis studies accretion and flow morphologies during this phase. Density gradients across the gravitationally-focussed material lead to a strong angular momentum barrier to accretion during common envelope

  10. Simultaneous Spectral and Timing Observations of Accreting Neuron Stars

    NASA Technical Reports Server (NTRS)

    Kaaret, P.; West, Donald K. (Technical Monitor)

    2001-01-01

    The goal of this proposal was to perform simultaneous x-ray spectral and millisecond timing observations of accreting neutron stars to further our understanding of their accretion dynamics and in the hope of using these systems as probes of the physics of strong gravitational fields. Observations of the neutron star binaries 4U0614+091, 4U1728-34, 4U1820-30, and Cyg X-2 were carried out with RXTE and BeppoSAX, ASCA, and Chandra (not all simultaneously). In addition, archival data were analyzed for 4U0614+091 and 4U1820-30. This investigation led to publication of three papers in peer-reviewed journals. These are listed below. In addition, the results were presented at several meetings including the two poster presentations listed below. Dr. Santina Piraino visited SAO for 4 months during 2000 to collaborate on analysis of the data from NAG5-8408 and NAG5-9104.

  11. Probing crustal structures from neutron star compactness

    NASA Astrophysics Data System (ADS)

    Sotani, Hajime; Iida, Kei; Oyamatsu, Kazuhiro

    2017-10-01

    With various sets of the parameters that characterize the equation of state (EOS) of nuclear matter, we systematically examine the thickness of a neutron star crust and of the pasta phases contained therein. Then, with respect to the thickness of the phase of spherical nuclei, the thickness of the cylindrical phase and the crust thickness, we successfully derive fitting formulas that express the ratio of each thickness to the star's radius as a function of the star's compactness, the incompressibility of symmetric nuclear matter and the density dependence of the symmetry energy. In particular, we find that the thickness of the phase of spherical nuclei has such a strong dependence on the stellar compactness as the crust thickness, but both of them show a much weaker dependence on the EOS parameters. Thus, via determination of the compactness, the thickness of the phase of spherical nuclei as well as the crust thickness can be constrained reasonably, even if the EOS parameters remain to be well-determined.

  12. Accretion onto neutron stars with the presence of a double layer

    NASA Technical Reports Server (NTRS)

    Williams, A. C.; Weisskopf, M. C.; Elsner, R. F.; Darbro, W.; Sutherland, P. G.

    1986-01-01

    It is known from laboratory experiments that double layers can form in plasmas, usually in the presence of an electric current. It is argued that a double layer may be present in the accretion column of a neutron star in a binary system. It is suggested that the double layer may be the predominant deceleration mechanism for the accreting ions, especially for sources with X-ray luminosities of less than about 10 to the 37th erg/s. Previous models have involved either a collisionless shock or an assumed gradual deceleration of the accreting ions to thermalize the energy of the infalling matter.

  13. Accretion onto neutron stars with the presence of a double layer

    NASA Technical Reports Server (NTRS)

    Williams, A. C.; Weisskopf, M. C.; Elsner, R. F.; Darbro, W.; Sutherland, P. G.

    1987-01-01

    It is known, from laboratory experiments, that double layers will form in plasmas, usually in the presence of an electric current. It is argued that a double layer may be present in the accretion column of a neutron star in a binary system. It is suggested that the double layer may be the predominant deceleration mechanism for the accreting ions, especially for sources with X-ray luminosities of less than about 10 to the 37th erg/s. Previous models have involved either a collisionless shock or an assumed gradual deceleration of the accreting ions to thermalize the energy of the infalling matter.

  14. SUSTAINING STAR FORMATION RATES IN SPIRAL GALAXIES: SUPERNOVA-DRIVEN TURBULENT ACCRETION DISK MODELS APPLIED TO THINGS GALAXIES

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Vollmer, Bernd; Leroy, Adam K., E-mail: bvollmer@astro.u-strasbg.fr

    2011-01-15

    Gas disks of spiral galaxies can be described as clumpy accretion disks without a coupling of viscosity to the actual thermal state of the gas. The model description of a turbulent disk consisting of emerging and spreading clumps contains free parameters, which can be constrained by observations of molecular gas, atomic gas, and the star formation rate for individual galaxies. Radial profiles of 18 nearby spiral galaxies from THINGS, HERACLES, SINGS, and GALEX data are used to compare the observed star formation efficiency, molecular fraction, and velocity dispersion to the model. The observed radially decreasing velocity dispersion can be reproducedmore » by the model. In the framework of this model, the decrease in the inner disk is due to the stellar mass distribution which dominates the gravitational potential. Introducing a radial break in the star formation efficiency into the model improves the fits significantly. This change in the star formation regime is realized by replacing the free-fall time in the prescription of the star formation rate with the molecule formation timescale. Depending on the star formation prescription, the break radius is located near the transition region between the molecular-gas-dominated and atomic-gas-dominated parts of the galactic disk or closer to the optical radius. It is found that only less massive galaxies (log M(M{sub sun}) {approx}< 10) can balance gas loss via star formation by radial gas accretion within the disk. These galaxies can thus access their gas reservoirs with large angular momentum. On the other hand, the star formation of massive galaxies is determined by the external gas mass accretion rate from a putative spherical halo of ionized gas or from satellite accretion. In the absence of this external accretion, star formation slowly exhausts the gas within the optical disk within the star formation timescale.« less

  15. Sustaining Star Formation Rates in Spiral Galaxies Supernova-driven Turbulent Accretion Disk Models Applied to THINGS Galaxies

    NASA Astrophysics Data System (ADS)

    Vollmer, Bernd; Leroy, Adam K.

    2011-01-01

    Gas disks of spiral galaxies can be described as clumpy accretion disks without a coupling of viscosity to the actual thermal state of the gas. The model description of a turbulent disk consisting of emerging and spreading clumps contains free parameters, which can be constrained by observations of molecular gas, atomic gas, and the star formation rate for individual galaxies. Radial profiles of 18 nearby spiral galaxies from THINGS, HERACLES, SINGS, and GALEX data are used to compare the observed star formation efficiency, molecular fraction, and velocity dispersion to the model. The observed radially decreasing velocity dispersion can be reproduced by the model. In the framework of this model, the decrease in the inner disk is due to the stellar mass distribution which dominates the gravitational potential. Introducing a radial break in the star formation efficiency into the model improves the fits significantly. This change in the star formation regime is realized by replacing the free-fall time in the prescription of the star formation rate with the molecule formation timescale. Depending on the star formation prescription, the break radius is located near the transition region between the molecular-gas-dominated and atomic-gas-dominated parts of the galactic disk or closer to the optical radius. It is found that only less massive galaxies (log M(M ⊙) <~ 10) can balance gas loss via star formation by radial gas accretion within the disk. These galaxies can thus access their gas reservoirs with large angular momentum. On the other hand, the star formation of massive galaxies is determined by the external gas mass accretion rate from a putative spherical halo of ionized gas or from satellite accretion. In the absence of this external accretion, star formation slowly exhausts the gas within the optical disk within the star formation timescale.

  16. Accreting Planets in the Habitable Zones of M-Stars Are Too Hot to Retain Liquid Water

    NASA Astrophysics Data System (ADS)

    Ramirez, R. M.; Kopparapu, R. K.; Kasting, J. F.

    2014-12-01

    Previous studies1,2 have shown that young accreting planets in the habitable zones (HZ) of pre-main sequence M-stars face major dynamical hurdles in both the retention and acquisition of volatiles. High collision rates with other bodies, short planetary formation timescales, and inefficient radial mixing are among the major problems encountered. However, another equally-important concern is the high temperatures predicted within the circumstellar disk, greatly hindering volatile delivery. We use a 1-D radiative-convective climate model to demonstrate that the fluxes received by accreting planets orbiting late K-M stars exceed the runaway greenhouse threshold. Given that M-stars are disproportionately brighter in their pre main-sequence lifetimes as compared to Sun-like stars (i.e. G-class insolation), planets orbiting M-stars are especially susceptible to the runaway, with intensity and duration increasing for cooler M-stars. Thus, accreting planetesimals in the HZs of M-stars could be too hot to maintain liquid water on their surfaces. In contrast, accreting planets located at Earth's distance (or farther) from a pre-main sequence solar analogue (i.e. G2 spectral class) receive stellar fluxes well below that of the runaway point. Our results suggest that future missions and surveys can improve their prospects of finding alien life by targeting HZ planets orbiting Sun-like stars. Moreover, our findings support recent claims that Venus may have lost its water during accretion3. REFERENCES1. Lissauer, Jack J. "Planets formed in habitable zones of M dwarf stars probably are deficient in volatiles." The Astrophysical Journal Letters 660.2 (2007): L149. 2. Raymond, Sean N., John Scalo, and Victoria S. Meadows. "A decreased probability of habitable planet formation around low-mass stars." The Astrophysical Journal 669.1 (2007): 606. 3. Hamano, Keiko, Yutaka Abe, and Hidenori Genda. "Emergence of two types of terrestrial planet on solidification of magma ocean." Nature

  17. Accretion Disks around Young Stars: An Observational Perspective

    NASA Astrophysics Data System (ADS)

    Ménard, F.; Bertout, C.

    Accretion disks are pivotal elements in the formation and early evolution of solar-like stars. On top of supplying the raw material, their internal conditions also regulate the formation of planets. Their study therefore holds the key to solve this long standing mystery: how did our Solar System form? This chapter focuses on observational studies of the circumstellar environment, and in particular of circumstellar disks, associated with pre-main sequence solar-like stars. The direct measurement of disk parameters poses an obvious challenge: at the distance of the typical star forming regions ( e.g. 140 pc for Taurus), a planetary system like ours (with diameter simeq50 AU out to Pluto, but excluding the Kuiper belt which could extend much farther out) subtends only 0.35''. Yet its surface brightness is low in comparison to the bright central star and high angular and high contrast imaging techniques are required if one hopes to resolve and measure these protoplanetary disks. Fortunately, capable instruments providing 0.1'' resolution or better and high contrast have been available for just about 10 years now. They are covering a large part of the electromagnetic spectrum, from the UV/Optical with HST and the near-infrared from ground-based adaptive optics systems, to the millimetric range with long-baseline radio interferometers. It is therefore not surprising that our knowledge of the structure of the disks surrounding low-mass stars has made a gigantic leap forward in the last decade. In the following pages we will attempt to describe, in a historical perpective, the road that led to the idea that most solar-like stars are surrounded by an accretion disk at one point in their early life and how, nowadays, their structural and physical parameters can be estimated from direct observations. We will follow by a short discussion of a few of the constraints available regarding the evolution and dissipation of these disks. This last topic is particularly relevant today

  18. Neutron and antineutron production in accretion onto compact objects

    NASA Technical Reports Server (NTRS)

    Dermer, C. D.; Ramaty, R.

    1986-01-01

    Nuclear reactions in the hot accretion plasma surrounding a collapsed star are a source of neutrons, primarily through spallation and pion-producing reactions, and antineutrons, principally through the reaction p+p yields p+p+n+anti-n. We calculate spectra of neutrons and antineutrons produced by a variety of nonthermal energetic particle distributions in which the target particles are either at rest or in motion. If only neutral particles are free to escape the interaction site, a component of the proton and antiproton fluxes in the cosmic radiation results from the neutrons and antineutrons which leave the accretion plasma and subsequently decay in the interstellar medium. This additional antiproton component could account for the enhanced flux of antiprotons in the cosmic radiation, compared to values expected from the standard leaky-box model of cosmic-ray propagation and confinement. Moreover, the low-energy antiproton flux measured by Buffington et al. (1981) could result from target-particle motion in the accretion plasma. This model for the origin of antiprotons predicts a narrow 2.223 MeV line which could be observable.

  19. The origin of ultra-compact binaries

    NASA Technical Reports Server (NTRS)

    Hachisu, Izumi; Miyaji, Shigeki; Saio, Hideyuki

    1987-01-01

    The origin of ultra-compact binaries composed of a neutron star and a low-mass (about 0.06 solar mass) white dwarf is considered. Taking account of the systemic losses of mass and angular momentum, it was found that a serious difficulty exists in the scenarios which involve tidal captures of a normal star (a main sequence star or a red giant) by a neutron star. This difficulty can be avoided if a red giant star is captured by a massive white dwarf (M is approx. greater than 1.2 solar masses), which becomes a neutron star through the accretion induced collapse.

  20. A method for measuring vertical accretion, elevation, and compaction of soft, shallow-water sediments

    USGS Publications Warehouse

    Cahoon, D.R.; Marin, P.E.; Black, B.K.; Lynch, J.C.

    2000-01-01

    High-resolution measures of vertical accretion, elevation, and compaction of shallow-water sediments are fundamental to understanding the processes that control elevation change and the mechanisms of progradation (e.g., development of mudflats and intertidal wetlands) in coastal systems. Yet, measurements of elevation by traditional survey methods often are of low accuracy because of the compressible nature of the substrates. Nor do they provide measures of vertical accretion or sediment compaction. This paper evaluates the use in shallow-water systems of an approach designed to measure these variables in vegetated wetlands. The approach employs simultaneous measures of elevation from temporary benchmarks using a sedimentation-erosion table (SET) and vertical accretion from marker horizons with sediment cores collected with a cryogenic coring apparatus. The measures are made with a level of resolution sufficient to distinguish between the influence of surface and subsurface processes on elevation, thus providing quantitative estimates of shallow subsidence. The SET-marker horizon approach was evaluated on a developing splay created by an artificial crevasse of a distributary in the Mississippi River delta. The approach provided high-resolution measures of vertical accretion (48.3 ' 2.0 cm.) and elevation (36.7 ' 1.6 cm) over a 4-year period, with the difference between the two indicating the amount of shallow subsidence. In addition, by laying new marker horizons in later years, the approach provided rates not only of shallow subsidence (3.9 ' 0.5 cm y-1) but also compaction of newly deposited seiments (2.1 ' 0.6 cm y-1) and compaction of underlying sediments (1.8 ' 2.0 cm y-1 ) over a two-year period. Hence, the SET-marker horizon approach has widespread applicability in both emergent wetland and shallow water environments for providing high resolution measures of the processes controlling elevation change.

  1. Young Stellar Objects in Lynds 1641: Disks, Accretion, and Star Formation History

    NASA Astrophysics Data System (ADS)

    Fang, Min; Kim, Jinyoung Serena; van Boekel, Roy; Sicilia-Aguilar, Aurora; Henning, Thomas; Flaherty, Kevin

    2013-07-01

    We investigate the young stellar objects (YSOs) in the Lynds 1641 (L1641) cloud using multi-wavelength data including Spitzer, WISE, the Two Micron All Sky Survey, and XMM covering ~1390 YSOs across a range of evolutionary stages. In addition, we targeted a sub-sample of YSOs for optical spectroscopy with the MMT/Hectospec and the MMT/Hectochelle. We use these data, along with archival photometric data, to derive spectral types, extinction values, masses, ages, and accretion rates. We obtain a disk fraction of ~50% in L1641. The disk frequency is almost constant as a function of stellar mass with a slight peak at log (M */M ⊙) ≈ -0.25. The analysis of multi-epoch spectroscopic data indicates that the accretion variability of YSOs cannot explain the two orders of magnitude of scatter for YSOs with similar masses. Forty-six new transition disk (TD) objects are confirmed in this work, and we find that the fraction of accreting TDs is lower than for optically thick disks (40%-45% versus 77%-79%, respectively). We confirm our previous result that the accreting TDs have a median accretion rate similar to normal optically thick disks. We confirm that two star formation modes (isolated versus clustered) exist in L1641. We find that the diskless YSOs are statistically older than the YSOs with optically thick disks and the TD objects have a median age that is intermediate between those of the other two populations. We tentatively study the star formation history in L1641 based on the age distribution and find that star formation started to be active 2-3 Myr ago.

  2. An ultra-relativistic outflow from a neutron star accreting gas from a companion.

    PubMed

    Fender, Rob; Wu, Kinwah; Johnston, Helen; Tzioumis, Tasso; Jonker, Peter; Spencer, Ralph; Van Der Klis, Michiel

    2004-01-15

    Collimated relativistic outflows-also known as jets-are amongst the most energetic phenomena in the Universe. They are associated with supermassive black holes in distant active galactic nuclei, accreting stellar-mass black holes and neutron stars in binary systems and are believed to be responsible for gamma-ray bursts. The physics of these jets, however, remains something of a mystery in that their bulk velocities, compositions and energetics remain poorly determined. Here we report the discovery of an ultra-relativistic outflow from a neutron star accreting gas within a binary stellar system. The velocity of the outflow is comparable to the fastest-moving flows observed from active galactic nuclei, and its strength is modulated by the rate of accretion of material onto the neutron star. Shocks are energized further downstream in the flow, which are themselves moving at mildly relativistic bulk velocities and are the sites of the observed synchrotron emission from the jet. We conclude that the generation of highly relativistic outflows does not require properties that are unique to black holes, such as an event horizon.

  3. Stable accretion from a cold disc in highly magnetized neutron stars

    NASA Astrophysics Data System (ADS)

    Tsygankov, S. S.; Mushtukov, A. A.; Suleimanov, V. F.; Doroshenko, V.; Abolmasov, P. K.; Lutovinov, A. A.; Poutanen, J.

    2017-11-01

    Aims: The aim of this paper is to investigate the transition of a strongly magnetized neutron star into the accretion regime with very low accretion rate. Methods: For this purpose, we monitored the Be-transient X-ray pulsar GRO J1008-57 throughout a full orbital cycle. The current observational campaign was performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV) between two subsequent Type I outbursts in January and September 2016. Results: The expected transition to the propeller regime was not observed. However, transitions between different regimes of accretion were detected. In particular, after an outburst, the source entered a stable accretion state characterised by an accretion rate of 1014-1015 g s-1. We associate this state with accretion from a cold (low-ionised) disc of temperature below 6500 K. We argue that a transition to this accretion regime should be observed in all X-ray pulsars that have a certain combination of the rotation frequency and magnetic field strength. The proposed model of accretion from a cold disc is able to explain several puzzling observational properties of X-ray pulsars.

  4. Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem

    NASA Astrophysics Data System (ADS)

    Sahai, R.; Sánchez Contreras, C.; Mangan, A. S.; Sanz-Forcada, J.; Muthumariappan, C.; Claussen, M. J.

    2018-06-01

    Binarity is believed to dramatically affect the history and geometry of mass loss in AGB and post-AGB stars, but observational evidence of binarity is sorely lacking. As part of a project to search for hot binary companions to cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to be a source of strong and variable UV and X-ray emission. Here we report UV spectroscopic observations of Y Gem obtained with the Hubble Space Telescope that show strong flickering in the UV continuum on timescales of ≲20 s, characteristic of an active accretion disk. Several UV lines with P-Cygni-type profiles from species such as Si IV and C IV are also observed, with emission and absorption features that are red- and blueshifted by velocities of ∼500 {km} {{{s}}}-1 from the systemic velocity. Our model for these (and previous) observations is that material from the primary star is gravitationally captured by a companion, producing a hot accretion disk. The latter powers a fast outflow that produces blueshifted features due to the absorption of UV continuum emitted by the disk, whereas the redshifted emission features arise in heated infalling material from the primary. The outflow velocities support a previous inference by Sahai et al. that Y Gem’s companion is a low-mass main-sequence star. Blackbody fitting of the UV continuum implies an accretion luminosity of about 13 L ⊙, and thus a mass-accretion rate >5 × 10‑7 M ⊙ yr‑1 we infer that Roche-lobe overflow is the most likely binary accretion mode for Y Gem.

  5. TIME-DEPENDENT, COMPOSITIONALLY DRIVEN CONVECTION IN THE OCEANS OF ACCRETING NEUTRON STARS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Medin, Zach; Cumming, Andrew, E-mail: zmedin@lanl.gov, E-mail: cumming@physics.mcgill.ca

    2015-03-20

    We discuss the effect of convection driven by chemical separation at the ocean-crust boundary of accreting neutron stars. We extend the steady-state results of Medin and Cumming to transient accretors, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. During accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. During quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layersmore » hot. We find that this leads to a sharp drop in surface emission at around a week followed by a gradual recovery as cooling becomes dominated by the crust. Such a dip should be observable in the light curves of these neutron star transients, if enough data is taken at a few days to a month after the end of accretion. If such a dip is definitively observed, it will provide strong constraints on the chemical composition of the ocean and outer crust.« less

  6. Correlating non-linear properties with spectral states of RXTE data: possible observational evidences for four different accretion modes around compact objects

    NASA Astrophysics Data System (ADS)

    Adegoke, Oluwashina; Dhang, Prasun; Mukhopadhyay, Banibrata; Ramadevi, M. C.; Bhattacharya, Debbijoy

    2018-05-01

    By analysing the time series of RXTE/PCA data, the non-linear variabilities of compact sources have been repeatedly established. Depending on the variation in temporal classes, compact sources exhibit different non-linear features. Sometimes they show low correlation/fractal dimension, but in other classes or intervals of time they exhibit stochastic nature. This could be because the accretion flow around a compact object is a non-linear general relativistic system involving magnetohydrodynamics. However, the more conventional way of addressing a compact source is the analysis of its spectral state. Therefore, the question arises: What is the connection of non-linearity to the underlying spectral properties of the flow when the non-linear properties are related to the associated transport mechanisms describing the geometry of the flow? This work is aimed at addressing this question. Based on the connection between observed spectral and non-linear (time series) properties of two X-ray binaries: GRS 1915+105 and Sco X-1, we attempt to diagnose the underlying accretion modes of the sources in terms of known accretion classes, namely, Keplerian disc, slim disc, advection dominated accretion flow and general advective accretion flow. We explore the possible transition of the sources from one accretion mode to others with time. We further argue that the accretion rate must play an important role in transition between these modes.

  7. Compact Stars in Eddington-inspired Born-Infeld Gravity and General Relativity

    NASA Astrophysics Data System (ADS)

    Sham, Yu Hin

    In this thesis we apply the Eddington inspired Born-Infeld (EiBI) gravity to study the structure and the properties of compact stars. The hydrostatic equilibrium structure of compact stars characterized by different equations of state (EOSs) is considered and it is found that EiBI gravity can lead to different new features that are not found in standard general relativity (GR). A unified framework to study radial perturbations and the stability of compact stars in this theory is also developed. As in the GR case, the frequency- square of the fundamental oscillation mode vanishes for the maximum mass stellar configuration. Also, the oscillation modes depend on the parameter kappa introduced in EiBI gravity and the dependence is stronger for higher-order modes. We also discover that EiBI gravity imposes certain constraints on the EOSs that allow physical stable equilibrium states of compact stars to exist. However, such constraints are unphysical as the validity of an EOS should be independent of the theory of gravity, hinting that EiBI gravity needs to be modified. On the other hand, we demonstrate that two universal relations of compact stars, namely the I-Love-Q relation, which relates the moment of intertia, the tidal Love number and the quadrupole moment of compact stars, and the f-I relation, which links the f-mode oscillation frequency and the moment of inertia of compact stars together, still hold in EiBI gravity within the observational bounds of kappa. The origin of the two universal relations is then studied and it is found that a stiff EOS at the core of the compact star guarantees the universality. The two universal relations are further extended and universal relations relating the multipolar f-mode oscillation frequency and the corresponding multipolar tidal Love number, which can be derived analytically in the Newtonian limit for stars with sufficiently stiff EOSs, are found.

  8. Tidal deformations of compact stars with crystalline quark matter

    NASA Astrophysics Data System (ADS)

    Lau, S. Y.; Leung, P. T.; Lin, L.-M.

    2017-05-01

    We study the tidal deformability of bare quark stars and hybrid compact stars composed of a quark-matter core in general relativity, assuming that the deconfined quark matter exists in a crystalline color superconducting phase. We find that taking the elastic property of crystalline quark matter into account in the calculation of the tidal deformability can break the universal I-Love relation discovered for fluid compact stars, which connects the moment of inertia and tidal deformability. Our result suggests that measurements of the moment of inertia and tidal deformability can in principle be used to test the existence of solid quark stars, despite our ignorance of the high-density equation of state. Assuming that the moment of inertia can be measured to 10% level, one can then distinguish a 1.4 (1 ) M⊙ solid quark star described by our quark-matter equation of state model with a gap parameter Δ =25 MeV from a fluid compact star if the tidal deformability can be measured to about 10% (45%) level. On the other hand, we find that the nuclear matter fluid envelope of a hybrid star can screen out the effect of the solid core significantly so that the resulting I-Love relation for hybrid stars still agrees with the universal relation for fluid stars to about 1% level.

  9. Long-orbital-period Prepolars Containing Early K-type Donor Stars. Bottleneck Accretion Mechanism in Action

    NASA Astrophysics Data System (ADS)

    Tovmassian, G.; González–Buitrago, D.; Zharikov, S.; Reichart, D. E.; Haislip, J. B.; Ivarsen, K. M.; LaCluyze, A. P.; Moore, J. P.; Miroshnichenko, A. S.

    2016-03-01

    We studied two objects identified as cataclysmic variables (CVs) with periods exceeding the natural boundary for Roche-lobe-filling zero-age main sequence (ZAMS) secondary stars. We present observational results for V1082 Sgr with a 20.82 hr orbital period, an object that shows a low luminosity state when its flux is totally dominated by a chromospherically active K star with no signs of ongoing accretion. Frequent accretion shutoffs, together with characteristics of emission lines in a high state, indicate that this binary system is probably detached, and the accretion of matter on the magnetic white dwarf takes place through stellar wind from the active donor star via coupled magnetic fields. Its observational characteristics are surprisingly similar to V479 And, a 14.5 hr binary system. They both have early K-type stars as donor stars. We argue that, similar to the shorter-period prepolars containing M dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar wind from the K star to be captured and channeled through the bottleneck connecting the two stars onto the white dwarf’s magnetic pole, mimicking a magnetic CV. Hence, they become interactive binaries before they reach contact. This will help to explain an unexpected lack of systems possessing white dwarfs with strong magnetic fields among detached white+red dwarf systems.

  10. The Eating Habits of Milky Way Mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

    DOE PAGES

    Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

    2016-04-01

    In this paper, we study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M vir ~ 10 12.1 M ⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M star ~ 10 8–10 10M ⊙. Halos with more quiescent accretion histories tendmore » to have lower mass progenitors (10 8–10 9 M ⊙), and lower overall accreted stellar masses. Ultra-faint mass (M star < 10 5 M ⊙) dwarfs contribute a negligible amount (<<1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (~2%–5%) of the very metal-poor stars with [Fe/H] < -2. Dwarfs with masses 10 5 < M star/M ⊙ < 10 8 provide a substantial amount of the very metal-poor stellar material (~40%–80%), and even relatively metal-rich dwarfs with M star > 10 8 M ⊙ can contribute a considerable fraction (~20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. In conclusion, we suggest that the MW could be a "transient fossil"; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.« less

  11. Magnetospheric accretion models for T Tauri stars. 1: Balmer line profiles without rotation

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee; Hewett, Robert; Calvet, Nuria

    1994-01-01

    We argue that the strong emission lines of T Tauri stars are generally produced in infalling envelopes. Simple models of infall constrained to a dipolar magnetic field geometry explain many peculiarities of observed line profiles that are difficult, if not impossible, to reproduce with wind models. Radiative transfer effects explain why certain lines can appear quite symmetric while other lines simultaneously exhibit inverse P Cygni profiles, without recourse to complicated velocity fields. The success of the infall models in accounting for qualitative features of observed line profiles supports the proposal that stellar magnetospheres disrupt disk accretion in T Tauri stars, that true boundary layers are not usually present in T Tauri stars, and that the observed 'blue veiling' emission arises from the base of the magnetospheric accretion column.

  12. FUV Emission from AGB Stars: Modeling Accretion Activity Associated with a Binary Companion

    NASA Technical Reports Server (NTRS)

    Stevens, Alyx Catherine; Sahai, Raghvendra

    2012-01-01

    It is widely believed that the late stages of evolution for Asymptotic Giant Branch (AGB) stars are influenced by the presence of binary companions. Unfortunately, there is a lack of direct observational evidence of binarity. However, more recently, strong indirect evidence comes from the discovery of UV emission in a subsample of these objects (fuvAGB stars). AGB stars are comparatively cool objects (< or =3000 K), thus their fluxes falls off drastically for wavelengths 3000 Angstroms and shorter. Therefore, ultraviolet observations offer an important, new technique for detecting the binary companions and/or associated accretion activity. We develop new models of UV emission from fuvAGB stars constrained by GALEX photometry and spectroscopy of these objects. We compare the GALEX UV grism spectra of the AGB M7 star EY Hya to predictions using the spectral synthesis code Cloudy, specifically investigating the ultraviolet wavelength range (1344-2831 Angstroms). We investigate models composed of contributions from a photoionized "hot spot" due to accretion activity around the companion, and "chromospheric" emission from collisionally ionized plasma, to fit the UV observations.

  13. Constraints on the Neutron Star and Inner Accretion Flow in Serpens X-1 Using Nustar

    NASA Technical Reports Server (NTRS)

    Miller, J. M.; Parker, M. L.; Fuerst, F.; Bachetti, M.; Barret, D.; Grefenstette, B. W.; Tendulkar, S.; Harrison, F. A.; Boggs, S. E.; Chakrabarty, D.; hide

    2013-01-01

    We report on an observation of the neutron star low-mass X-ray binary Serpens X-1, made with NuSTAR. The extraordinary sensitivity afforded by NuSTAR facilitated the detection of a clear, robust, relativistic Fe K emission line from the inner disk. A relativistic profile is required over a single Gaussian line from any charge state of Fe at the 5 sigma level of confidence, and any two Gaussians of equal width at the same confidence. The Compton back-scattering "hump" peaking in the 10-20 keV band is detected for the first time in a neutron star X-ray binary. Fits with relativistically blurred disk reflection models suggest that the disk likely extends close to the innermost stable circular orbit (ISCO) or stellar surface. The best-fit blurred reflection models constrain the gravitational redshift from the stellar surface to be ZnS (is) greater than 0.16. The data are broadly compatible with the disk extending to the ISCO; in that case,ZnS(is) greater than 0.22 and RNS (is) less than12.6 km (assuming MnS = 1.4 solar mass and a = 0, where a = cJ/GM2). If the star is as large or larger than its ISCO, or if the effective reflecting disk leaks across the ISCO to the surface, the redshift constraints become measurements. We discuss our results in the context of efforts to measure fundamental properties of neutron stars, and models for accretion onto compact objects.

  14. A Search for X-Ray Evidence of a Compact Companion to the Unusual Wolf-Rayet Star HD 50896 (EZ CMa)

    NASA Technical Reports Server (NTRS)

    Skinner, Stephen L.; Itoh, Masayuki; Nagase, Fumiaki

    1998-01-01

    We analyze results of a approx.25 ksec ASCA X-ray observation of the unusual Wolf-Rayet star HD 50896 (= EZ CMa). This WN5 star shows optical and ultraviolet variability at a 3.766 day period, which has been interpreted as a possible signature of a compact companion. Our objective was to search for evidence of hard X-rays (greater than or equal to 5 keV) which could be present if the WN5 wind is accreting onto a compact object. The ASCA spectra are dominated by emission below 5 keV and show no significant emission in the harder 5-10 keV range. Weak emission lines are present, and the X-rays arise in an optically thin plasma which spans a range of temperatures from less than or equal to 0.4 keV up to at least approx. 2 keV. Excess X-ray absorption above the interstellar value is present, but the column density is no larger than N(sub H) approx. 10(exp 22)/sq cm. The absorption-corrected X-ray luminosity L(sub x)(0.5 - 10 keV) = 10(exp 32.85) erg/s gives L(sub x)/ L(sub bol) approx. 10(exp -6), a value that is typical of WN stars. No X-ray variability was detected. Our main conclusion is that the X-ray properties of HD 50896 are inconsistent with the behavior expected for wind accretion onto a neutron star or black hole companion. Alternative models based on wind shocks can explain most aspects of the X-ray behavior, and we argue that the hotter plasma near approx. 2 keV could be due to the WR wind shocking onto a normal (nondegenerate) companion.

  15. I-Love-Q relations: from compact stars to black holes

    NASA Astrophysics Data System (ADS)

    Yagi, Kent; Yunes, Nicolás

    2016-05-01

    The relations between most observables associated with a compact star, such as the mass and radius of a neutron star or a quark star, typically depend strongly on their unknown internal structure. The recently discovered I-Love-Q relations (between the moment of inertia, the tidal deformability and the quadrupole moment) are however approximately insensitive to this structure. These relations become exact for stationary black holes (BHs) in General Relativity as shown by the no-hair theorems, mainly because BHs are vacuum solutions with event horizons. In this paper, we take the first steps toward studying how the approximate I-Love-Q relations become exact in the limit as compact stars become BHs. To do so, we consider a toy model for compact stars, i.e. incompressible stars with anisotropic pressure, which allows us to model an equilibrium sequence of stars with ever increasing compactness that approaches the BH limit arbitrarily closely. We numerically construct such a sequence in the slow-rotation and in the small-tide approximations by extending the Hartle-Thorne formalism, and then extract the I-Love-Q trio from the asymptotic behavior of the metric tensor at spatial infinity. We find that the I-Love-Q relations approach the BH limit in a nontrivial way, with the quadrupole moment and the tidal deformability changing sign as the compactness and the amount of anisotropy are increased. Through a generalization of Maclaurin spheroids to anisotropic stars, we show that the multipole moments also change sign in the Newtonian limit as the amount of anisotropy is increased because the star becomes prolate. We also prove analytically that the stellar moment of inertia reaches the BH limit as the compactness reaches a critical BH value in the strongly anisotropic limit. Modeling the BH limit through a sequence of anisotropic stars, however, can fail when considering other theories of gravity. We calculate the scalar dipole charge and the moment of inertia in a

  16. CSI 2264: Simultaneous optical and X-ray variability in pre-main sequence stars. I. Time resolved X-ray spectral analysis during optical dips and accretion bursts in stars with disks

    NASA Astrophysics Data System (ADS)

    Guarcello, M. G.; Flaccomio, E.; Micela, G.; Argiroffi, C.; Sciortino, S.; Venuti, L.; Stauffer, J.; Rebull, L.; Cody, A. M.

    2017-06-01

    Context. Pre-main sequence stars are variable sources. The main mechanisms responsible for their variability are variable extinction, unsteady accretion, and rotational modulation of both hot and dark photospheric spots and X-ray-active regions. In stars with disks, this variability is related to the morphology of the inner circumstellar region (≤0.1 AU) and that of the photosphere and corona, all impossible to be spatially resolved with present-day techniques. This has been the main motivation for the Coordinated Synoptic Investigation of NGC 2264, a set of simultaneous observations of NGC 2264 with 15 different telescopes. Aims: In this paper, we focus on the stars with disks. We analyze the X-ray spectral properties extracted during optical bursts and dips in order to unveil the nature of these phenomena. Stars without disks are studied in a companion paper. Methods: We analyze simultaneous CoRoT and Chandra/ACIS-I observations to search for coherent optical and X-ray flux variability in stars with disks. Then, stars are analyzed in two different samples. In stars with variable extinction, we look for a simultaneous increase of optical extinction and X-ray absorption during the optical dips; in stars with accretion bursts, we search for soft X-ray emission and increasing X-ray absorption during the bursts. Results: We find evidence for coherent optical and X-ray flux variability among the stars with variable extinction. In 9 of the 24 stars with optical dips, we observe a simultaneous increase of X-ray absorption and optical extinction. In seven dips, it is possible to calculate the NH/AV ratio in order to infer the composition of the obscuring material. In 5 of the 20 stars with optical accretion bursts, we observe increasing soft X-ray emission during the bursts that we associate to the emission of accreting gas. It is not surprising that these properties are not observed in all the stars with dips and bursts, since favorable geometric configurations are

  17. The Effect of Quantum Fluctuations in Compact Star Observables

    NASA Astrophysics Data System (ADS)

    Pósfay, P.; Barnaföldi, G. G.; Jakovác, A.

    2018-05-01

    Astrophysical measurements regarding compact stars are just ahead of a big evolution jump, since the NICER experiment deployed on ISS on 2017 June 14. This will provide soon data that would enable the determination of compact star radius with less than 10% error. This can be further constrained by the new observation of gravitational waves originated from merging neutron stars, GW170817. This poses new challenges to nuclear models aiming to explain the structure of super dense nuclear matter found in neutron stars. Detailed studies of the QCD phase diagram show the importance of bosonic quantum fluctuations in the cold dense matter equation of state. Here we used a demonstrative model with one bosonic and one fermionic degree of freedom coupled by Yukawa coupling, we show the effect of bosonic quantum fluctuations on compact star observables such as mass, radius, and compactness. We have also calculated the difference in the value of compressibility which is caused by quantum fluctuations. The above-mentioned quantities are calculated in the mean field, one-loop, and in high order many loop approximation. The results show that the magnitude of these effects is in the range of 4-5%, which place it into the region where modern measurements may detect it. This forms a base for further investigations that how these results carry over to more complicated models.

  18. Veiling and Accretion Around the Young Binary Stars S and VV Corona Australis

    NASA Astrophysics Data System (ADS)

    Sullivan, Kendall; Prato, Lisa; Avilez, Ian

    2018-01-01

    S CrA and VV CrA are two young binary star systems with separations of 170 AU and 250 AU, respectively, in the southern star-forming region Corona Australis. The spectral types of the four stars in these two systems are similar, approximately K7 to M1, hence the stellar masses are also similar. The study of young stars just emerging from their natal cloud cores at the very limits of observability allows us to probe the extreme environments in which planet formation begins to occur. Stars in this early evolutionary stage can have circumstellar or circumbinary disks, and sometimes remnants of the envelopes which surrounded them during the protostellar stage. Envelopes accrete onto disks and disks in turn accrete onto the central stars, triggering elevated continuum emission, line emission, outflows, and stellar winds. This violent stage marks the onset of the epoch of planet formation. Using high-resolution near-infrared, H-band spectroscopy from the Keck II telescope using the NIRSPEC instrument over 4-6 epochs, we are probing the chaotic environment surrounding the four stars in these systems. We determine the spectral types for VV CrA A and B for the first time, and examine the variable veiling and emission occurring around each of these stars. This research was supported in part by NSF grants AST-1461200 and AST-1313399.

  19. Accretion disk dynamics in X-ray binaries

    NASA Astrophysics Data System (ADS)

    Peris, Charith Srian

    Accreting X-ray binaries consist of a normal star which orbits a compact object with the former transferring matter onto the later via an accretion disk. These accretion disks emit radiation across the entire electromagnetic spectrum. This thesis exploits two regions of the spectrum, exploring the (1) inner disk regions of an accreting black hole binary, GRS1915+105, using X-ray spectral analysis and (2) the outer accretion disks of a set of neutron star and black hole binaries using Doppler Tomography applied on optical observations. X-ray spectral analysis of black hole binary GRS1915+105: GRS1915+105 stands out as an exceptional black hole primarily due to the wild variability exhibited by about half of its X-ray observations. This study focused on the steady X-ray observations of the source, which were found to exhibit significant curvature in the harder coronal component within the RXTE/PCA band-pass. The roughly constant inner-disk radius seen in a majority of the steady-soft observations is strongly reminiscent of canonical soft state black-hole binaries. Remarkably, the steady-hard observations show the presence of growing truncation in the inner-disk. A majority of the steady observations of GRS1915+105 map to the states observed in canonical black hole binaries which suggests that within the complexity of this source is a simpler underlying basis of states. Optical tomography of X-ray binary systems: Doppler tomography was applied to the strong line features present in the optical spectra of X-ray binaries in order to determine the geometric structure of the systems' emitting regions. The point where the accretion stream hits the disk, also referred to as the "hotspot'', is clearly identified in the neutron star system V691 CrA and the black hole system Nova Muscae 1991. Evidence for stream-disk overflows exist in both systems, consistent with relatively high accretion rates. In contrast, V926 Sco does not show evidence for the presence of a hotspot which

  20. Relativistic compact stars with charged anisotropic matter

    NASA Astrophysics Data System (ADS)

    Maurya, S. K.; Banerjee, Ayan; Channuie, Phongpichit

    2018-05-01

    In this article, we perform a detailed theoretical analysis of new exact solutions with anisotropic fluid distribution of matter for compact objects subject to hydrostatic equilibrium. We present a family solution to the Einstein-Maxwell equations describing a spherically symmetric, static distribution of a fluid with pressure anisotropy. We implement an embedding class one condition to obtain a relation between the metric functions. We generalize the properties of a spherical star with hydrostatic equilibrium using the generalised Tolman-Oppenheimer-Volkoff (TOV) equation. We match the interior solution to an exterior Reissner-Nordström one, and study the energy conditions, speed of sound, and mass-radius relation of the star. We also show that the obtained solutions are compatible with observational data for the compact object Her X-1. Regarding our results, the physical behaviour of the present model may serve for the modeling of ultra compact objects.

  1. Gravitationally Focused Dark Matter around Compact Stars

    NASA Astrophysics Data System (ADS)

    Bromley, Benjamin C.

    2011-12-01

    If dark matter self-annihilates then it may produce an observable signal when its density is high. The details depend on the intrinsic properties of dark matter and how it clusters in space. For example, the density profile of some dark matter candidates may rise steeply enough toward the Galactic Center that self-annihilation may produce detectable γ-ray emission. Here, we discuss the possibility that an annihilation signal arises near a compact object (e.g., neutron star or black hole) even when the density of dark matter in the neighborhood of the object is uniform. Gravitational focusing produces a local enhancement of density with a profile that falls off approximately as the inverse square-root of distance from the compact star. While geometric dilution may overwhelm the annihilation signal from this local enhancement, magnetic fields tied to the compact object can increase the signal's contrast relative to the background.

  2. Radio outburst from a massive (proto)star. When accretion turns into ejection

    NASA Astrophysics Data System (ADS)

    Cesaroni, R.; Moscadelli, L.; Neri, R.; Sanna, A.; Caratti o Garatti, A.; Eisloffel, J.; Stecklum, B.; Ray, T.; Walmsley, C. M.

    2018-05-01

    Context. Recent observations of the massive young stellar object S255 NIRS 3 have revealed a large increase in both methanol maser flux density and IR emission, which have been interpreted as the result of an accretion outburst, possibly due to instabilities in a circumstellar disk. This indicates that this type of accretion event could be common in young/forming early-type stars and in their lower mass siblings, and supports the idea that accretion onto the star may occur in a non-continuous way. Aims: As accretion and ejection are believed to be tightly associated phenomena, we wanted to confirm the accretion interpretation of the outburst in S255 NIRS 3 by detecting the corresponding burst of the associated thermal jet. Methods: We monitored the radio continuum emission from S255 NIRS 3 at four bands using the Karl G. Jansky Very Large Array. The millimetre continuum emission was also observed with both the Northern Extended Millimeter Array of IRAM and the Atacama Large Millimeter/Submillimeter Array. Results: We have detected an exponential increase in the radio flux density from 6 to 45 GHz starting right after July 10, 2016, namely 13 months after the estimated onset of the IR outburst. This is the first ever detection of a radio burst associated with an IR accretion outburst from a young stellar object. The flux density at all observed centimetre bands can be reproduced with a simple expanding jet model. At millimetre wavelengths we infer a marginal flux increase with respect to the literature values and we show this is due to free-free emission from the radio jet. Conclusions: Our model fits indicate a significant increase in the jet opening angle and ionized mass loss rate with time. For the first time, we can estimate the ionization fraction in the jet and conclude that this must be low (<14%), lending strong support to the idea that the neutral component is dominant in thermal jets. Our findings strongly suggest that recurrent accretion + ejection

  3. Near-ultraviolet Excess in Slowly Accreting T Tauri Stars: Limits Imposed by Chromospheric Emission

    NASA Astrophysics Data System (ADS)

    Ingleby, Laura; Calvet, Nuria; Bergin, Edwin; Herczeg, Gregory; Brown, Alexander; Alexander, Richard; Edwards, Suzan; Espaillat, Catherine; France, Kevin; Gregory, Scott G.; Hillenbrand, Lynne; Roueff, Evelyne; Valenti, Jeff; Walter, Frederick; Johns-Krull, Christopher; Brown, Joanna; Linsky, Jeffrey; McClure, Melissa; Ardila, David; Abgrall, Hervé; Bethell, Thomas; Hussain, Gaitee; Yang, Hao

    2011-12-01

    Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the ~5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph on the Hubble Space Telescope, as well as optical and near-infrared spectroscopic observations. The two stars have similar levels of near-UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of Hα and He I λ10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g., U-band excess luminosity or the Ca II triplet line luminosity, are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 × 10-10 M ⊙ yr-1 for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near-infrared, indicating that substantial dust remains. Fluorescent H2 emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

  4. Disk Accretion in the 10 Myr Old T Tauri Stars TW Hydrae and Hen 3-600A.

    PubMed

    Muzerolle; Calvet; Briceño; Hartmann; Hillenbrand

    2000-05-20

    We have found that two members of the TW Hydrae association, TW Hydrae and Hen 3-600A, are still actively accreting, based on the ballistic infall signature of their broad Halpha emission profiles. We present the first quantitative analysis of accretion in these objects and conclude that the same accretion mechanisms which operate in the well-studied 1 Myr old T Tauri stars can and do occur in older (10 Myr) stars. We derive the first estimates of the disk mass accretion rate in TW Hya and Hen 3-600A, which are 1-2 orders of magnitude lower than the average rates in 1 Myr old objects. The decrease in accretion rates over 10 Myr, as well as the low fraction of TW Hya association objects still accreting, points to significant disk evolution, possibly linked to planet formation. Given the multiplicity of the Hen 3-600 system and the large UV excess of TW Hya, our results show that accretion disks can be surprisingly long lived in spite of the presence of companions and significant UV ionizing flux.

  5. Disk-accreting magnetic neutron stars as high-energy particle accelerators

    NASA Technical Reports Server (NTRS)

    Hamilton, Russell J.; Lamb, Frederick K.; Miller, M. Coleman

    1994-01-01

    Interaction of an accretion disk with the magnetic field of a neutron star produces large electromotive forces, which drive large conduction currents in the disk-magnetosphere-star circuit. Here we argue that such large conduction currents will cause microscopic and macroscopic instabilities in the magnetosphere. If the minimum plasma density in the magnetosphere is relatively low is less than or aproximately 10(exp 9)/cu cm, current-driven micro-instabilities may cause relativistic double layers to form, producing voltage differences in excess of 10(exp 12) V and accelerating charged particles to very high energies. If instead the plasma density is higher (is greater than or approximately = 10(exp 9)/cu cm, twisting of the stellar magnetic field is likely to cause magnetic field reconnection. This reconnection will be relativistic, accelerating plasma in the magnetosphere to relativistic speeds and a small fraction of particles to very high energies. Interaction of these high-energy particles with X-rays, gamma-rays, and accreting plasma may produce detectable high-energy radiation.

  6. Optical veiling, disk accretion, and the evolution of T Tauri stars

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Hartmann, L.W.; Kenyon, S.J.

    1990-01-01

    High-resolution spectra of 31 K7-M1 T Tauri stars (TTs) in the Taurus-Auriga molecular cloud demonstrate that most of these objects exhibit substantial excess emission at 5200 A. Extrapolations of these data consistent with low-resolution spectrophotometry indicate that the extra emission is comparable to the stellar luminosity in many cases. If this continuum emission arises in the boundary layers of accreting disks, more than about 30 percent of all TTs may be accreting material at a rate which is sufficiently rapid to alter their evolution from standard Hayashi tracks. It is estimated that roughly 10 percent of the final stellar massmore » is accreted in the TT phase. This amount of material is comparable to the minimum gravitationally unstable disk mass estimated by Larson and it is speculated that the TT phase represents the final stages of disk accretion driven by gravitational instabilities. 40 refs.« less

  7. The Dripping Handrail Model: Transient Chaos in Accretion Systems

    NASA Technical Reports Server (NTRS)

    Young, Karl; Scargle, Jeffrey D.; Cuzzi, Jeffrey (Technical Monitor)

    1995-01-01

    We define and study a simple dynamical model for accretion systems, the "dripping handrail" (DHR). The time evolution of this spatially extended system is a mixture of periodic and apparently random (but actually deterministic) behavior. The nature of this mixture depends on the values of its physical parameters - the accretion rate, diffusion coefficient, and density threshold. The aperiodic component is a special kind of deterministic chaos called transient chaos. The model can simultaneously exhibit both the quasiperiodic oscillations (QPO) and very low frequency noise (VLFN) that characterize the power spectra of fluctuations of several classes of accretion systems in astronomy. For this reason, our model may be relevant to many such astrophysical systems, including binary stars with accretion onto a compact object - white dwarf, neutron star, or black hole - as well as active galactic nuclei. We describe the systematics of the DHR's temporal behavior, by exploring its physical parameter space using several diagnostics: power spectra, wavelet "scalegrams," and Lyapunov exponents. In addition, we note that for large accretion rates the DHR has periodic modes; the effective pulse shapes for these modes - evaluated by folding the time series at the known period - bear a resemblance to the similarly- determined shapes for some x-ray pulsars. The pulsing observed in some of these systems may be such periodic-mode accretion, and not due to pure rotation as in the standard pulsar model.

  8. Multiphase environment of compact galactic nuclei: the role of the nuclear star cluster

    NASA Astrophysics Data System (ADS)

    Różańska, A.; Kunneriath, D.; Czerny, B.; Adhikari, T. P.; Karas, V.

    2017-01-01

    We study the conditions for the onset of thermal instability in the innermost regions of compact galactic nuclei, where the properties of the interstellar environment are governed by the interplay of quasi-spherical accretion on to a supermassive black hole (SMBH) and the heating/cooling processes of gas in a dense nuclear star cluster (NSC). Stellar winds are the source of material for radiatively inefficient (quasi-spherical, non-magnetized) inflow/outflow on to the central SMBH, where a stagnation point develops within the Bondi-type accretion. We study the local thermal equilibrium to determine the parameter space that allows cold and hot phases in mutual contact to co-exist. We include the effects of mechanical heating by stellar winds and radiative cooling/heating by the ambient field of the dense star cluster. We consider two examples: the NSC in the Milky Way central region (including the gaseous mini-spiral of Sgr A*), and the ultracompact dwarf galaxy M60-UCD1. We find that the two systems behave in different ways because they are placed in different areas of parameter space in the instability diagram: gas temperature versus dynamical ionization parameter. In the case of Sgr A*, stellar heating prevents the spontaneous formation of cold clouds. The plasma from stellar winds joins the hot X-ray emitting phase and forms an outflow. In M60-UCD1, our model predicts spontaneous formation of cold clouds in the inner part of the galaxy. These cold clouds may survive since the cooling time-scale is shorter than the inflow/outflow time-scale.

  9. Manifestations of dynamo driven large-scale magnetic field in accretion disks of compact objects

    NASA Technical Reports Server (NTRS)

    Chagelishvili, G. D.; Chanishvili, R. G.; Lominadze, J. G.; Sokhadze, Z. A.

    1991-01-01

    A turbulent dynamo nonlinear theory of turbulence was developed that shows that in the compact objects of accretion disks, the generated large-scale magnetic field (when the generation takes place) has a practically toroidal configuration. Its energy density can be much higher than turbulent pulsations energy density, and it becomes comparable with the thermal energy density of the medium. On this basis, the manifestations to which the large-scale magnetic field can lead at the accretion onto black holes and gravimagnetic rotators, respectively, are presented.

  10. Net Reaction Rate and Neutrino Cooling Rate for the Urca Process in Departure from Chemical Equilibrium in the Crust of Fast-accreting Neutron Stars

    NASA Astrophysics Data System (ADS)

    Wang, Wei-Hua; Huang, Xi; Zheng, Xiao-Ping

    We discuss the effect of compression on Urca shells in the ocean and crust of accreting neutron stars, especially in superbursting sources. We find that Urca shells may be deviated from chemical equilibrium in neutron stars which accrete at several tenths of the local Eddington accretion rate. The deviation depends on the energy threshold of the parent and daughter nuclei, the transition strength, the temperature, and the local accretion rate. In a typical crust model of accreting neutron stars, the chemical departures range from a few tenths of kBT to tens of kBT for various Urca pairs. If the Urca shell can exist in crusts of accreting neutron stars, compression may enhance the net neutrino cooling rate by a factor of about 1-2 relative to the neutrino emissivity in chemical equilibrium. For some cases, such as Urca pairs with small energy thresholds and/or weak transition strength, the large chemical departure may result in net heating rather than cooling, although the released heat can be small. Strong Urca pairs in the deep crust are hard to be deviated even in neutron stars accreting at the local Eddington accretion rate.

  11. Simultaneous Spectral and Timing Observations of Accreting Neuron Stars

    NASA Technical Reports Server (NTRS)

    Kaaret, P.; Oliversen, Ronald J. (Technical Monitor)

    2002-01-01

    The goal of this proposal is to perform simultaneous x-ray spectral and millisecond timing observations of accreting neutron stars to further our understanding of their accretion dynamics and in the hope of using these systems as probes of the physics of strong gravitational fields. NAG5-9104 is the successor grant to NAG5-8408. Observations using the Rossi X-Ray Timing Explorer (RXTE) and BeppoSAX were performed of 4U1702-429, 4U1735-44, and Cyg X-2. Unfortunately, only a small fraction of the approved observing time was obtained for the first two targets and the data are of limited scientific value. Data analysis has been completed on the observations of Cyg X-2. We discovered a correlation between the frequency of the horizontal branch oscillations (HBO) and a soft, thermal component of the x-ray spectrum likely associated with emission from the accretion disk. This correlation may place constraints on models of the oscillations. A paper based on these results appeared in the Astrophysical Journal.

  12. Collisions in Compact Star Clusters.

    NASA Astrophysics Data System (ADS)

    Portegies Zwart, S. F.

    The high stellar densities in young compact star clusters, such as the star cluster R136 in the 30 Doradus region, may lead to a large number of stellar collisions. Such collisions were recently found to be much more frequent than previous estimates. The number of collisions scales with the number of stars for clusters with the same initial relaxation time. These collisions take place in a few million years. The collision products may finally collapse into massive black holes. The fraction of the total mass in the star cluster which ends up in a single massive object scales with the total mass of the cluster and its relaxation time. This mass fraction is rather constant, within a factor two or so. Wild extrapolation from the relatively small masses of the studied systems to the cores of galactic nuclei may indicate that the massive black holes in these systems have formed in a similar way.

  13. Focused Wind Mass Accretion in Mira AB

    NASA Astrophysics Data System (ADS)

    Karovska, Margarita; de Val-Borro, M.; Hack, W.; Raymond, J.; Sasselov, D.; Lee, N. P.

    2011-05-01

    At a distance of about only 100pc, Mira AB is the nearest symbiotic system containing an Asymptotic Giant Branch (AGB) star (Mira A), and a compact accreting companion (Mira B) at about 0.5" from Mira A. Symbiotic systems are interacting binaries with a key evolutionary importance as potential progenitors of a fraction of asymmetric Planetary Nebulae, and SN type Ia, cosmological distance indicators. The region of interaction has been studied using high-angular resolution, multiwavelength observations ranging from radio to X-ray wavelengths. Our results, including high-angular resolution Chandra imaging, show a "bridge" between Mira A and Mira B, indicating gravitational focusing of the Mira A wind, whereby components exchange matter directly in addition to the wind accretion. We carried out a study using 2-D hydrodynamical models of focused wind mass accretion to determine the region of wind acceleration and the characteristics of the accretion in Mira AB. We highlight some of our results and discuss the impact on our understanding of accretion processes in symbiotic systems and other detached and semidetached interacting systems.

  14. Shrinking galaxy disks with fountain-driven accretion from the halo

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Elmegreen, Bruce G.; Struck, Curtis; Hunter, Deidre A., E-mail: bge@watson.ibm.com, E-mail: curt@iastate.edu, E-mail: dah@lowell.edu

    2014-12-01

    Star formation in most galaxies requires cosmic gas accretion because the gas consumption time is short compared to the Hubble time. This accretion presumably comes from a combination of infalling satellite debris, cold flows, and condensation of hot halo gas at the cool disk interface, perhaps aided by a galactic fountain. In general, the accretion will have a different specific angular momentum than the part of the disk that receives it, even if the gas comes from the nearby halo. The gas disk then expands or shrinks over time. Here we show that condensation of halo gas at a ratemore » proportional to the star formation rate in the fountain model will preserve an initial shape, such as an exponential, with a shrinking scale length, leaving behind a stellar disk with a slightly steeper profile of younger stars near the center. This process is slow for most galaxies, producing imperceptible radial speeds, and it may be dominated by other torques, but it could be important for blue compact dwarfs, which tend to have large, irregular gas reservoirs and steep blue profiles in their inner stellar disks.« less

  15. The Interaction Between Accretion from the Interstellar Medium and Accretion from the Evolved Binary Component in Barium Stars

    NASA Astrophysics Data System (ADS)

    Jeong, Yeuncheol; Yushchenko, Alexander V.; Doikov, Dmytry N.

    2018-03-01

    The reanalysis of the previously published abundance pattern of mild barium star HD202109 (ζ Cyg) and the chemical compositions of 129 thin disk barium stars facilitated the search for possible correlations of different stellar parameters with second ionization potentials of chemical elements. Results show that three valuable correlations exist in the atmospheres of barium stars. The first is the relationship between relative abundances and second ionization potentials. The second is the age dependence of mean correlation coefficients of relative abundances vs. second ionization potentials, and the third one is the changes in correlation coefficients of relative abundances vs. second ionization potentials as a function of stellar spatial velocities and overabundances of s-process elements. These findings demonstrate the possibility of hydrogen and helium accretion from the interstellar medium on the atmospheres of barium stars.

  16. Asymmetric MHD outflows/jets from accreting T Tauri stars

    NASA Astrophysics Data System (ADS)

    Dyda, S.; Lovelace, R. V. E.; Ustyugova, G. V.; Lii, P. S.; Romanova, M. M.; Koldoba, A. V.

    2015-06-01

    Observations of jets from young stellar objects reveal the asymmetric outflows from some sources. A large set of 2.5D magnetohydrodynamic simulations was carried out for axisymmetric viscous/diffusive disc accretion to rotating magnetized stars for the purpose of assessing the conditions where the outflows are asymmetric relative to the equatorial plane. We consider initial magnetic fields that are symmetric about the equatorial plane and consist of a radially distributed field threading the disc (disc field) and a stellar dipole field. (1) For pure disc-fields the symmetry or asymmetry of the outflows is affected by the mid-plane plasma β of the disc. For discs with small plasma β, outflows are symmetric to within 10 per cent over time-scales of hundreds of inner disc orbits. For higher β discs, the coupling of the upper and lower coronal plasmas is broken, and quasi-periodic field motion leads to asymmetric episodic outflows. (2) Accreting stars with a stellar dipole field and no disc-field exhibit episodic, two component outflows - a magnetospheric wind and an inner disc wind. Both are characterized by similar velocity profiles but the magnetospheric wind has densities ≳ 10 times that of the disc wind. (3) Adding a disc field parallel to the stellar dipole field enhances the magnetospheric winds but suppresses the disc wind. (4) Adding a disc field which is antiparallel to the stellar dipole field in the disc suppresses the magnetospheric and disc winds. Our simulations reproduce some key features of observations of asymmetric outflows of T Tauri stars.

  17. LAMP: the long-term accretion monitoring programme of T Tauri stars in Chamaeleon I

    NASA Astrophysics Data System (ADS)

    Costigan, G.; Scholz, A.; Stelzer, B.; Ray, T.; Vink, J. S.; Mohanty, S.

    2012-12-01

    We present the results of a variability study of accreting young stellar objects in the Chameleon I star-forming region, based on ˜300 high-resolution optical spectra from the Fibre Large Area Multi-Element Spectrograph (FLAMES) at the European Southern Observatory (ESO) Very Large Telescope (VLT). 25 objects with spectral types from G2-M5.75 were observed 12 times over the course of 15 months. Using the emission lines Hα (6562.81 Å) and Ca II (8662.1 Å) as accretion indicators, we found 10 accreting and 15 non-accreting objects. We derived accretion rates for all accretors in the sample using the Hα equivalent width, Hα 10 per cent width and Ca II (8662.1 Å) equivalent width. We found that the Hα equivalent widths of accretors varied by ˜7-100 Å over the 15-month period. This corresponds to a mean amplitude of variations in the derived accretion rate of ˜0.37 dex. The amplitudes of variations in the derived accretion rate from Ca II equivalent width were ˜0.83 dex and those from Hα 10 per cent width were ˜1.11 dex. Based on the large amplitudes of variations in accretion rate derived from the Hα 10 per cent width with respect to the other diagnostics, we do not consider it to be a reliable accretion rate estimator. Assuming the variations in Hα and Ca II equivalent width accretion rates to be closer to the true value, these suggest that the spread that was found around the accretion rate to stellar-mass relation is not due to the variability of individual objects on time-scales of weeks to ˜1 year. From these variations, we can also infer that the accretion rates are stable within <0.37 dex over time-scales of less than 15 months. A major portion of the accretion variability was found to occur over periods shorter than the shortest time-scales in our observations, 8-25 days, which are comparable with the rotation periods of these young stellar objects. This could be an indication that what we are probing is spatial structure in the accretion flows

  18. Spectroscopic diagnostics of UV power and accretion in T Tauri stars

    NASA Astrophysics Data System (ADS)

    Brooks, D. H.; Costa, V. M.

    2003-02-01

    It is known that in the upper atmospheres of the Sun and some late-type stars there is a systematic relationship between the optically thin total radiated power and the power emitted by single spectral lines. Using recently derived emission-measure distributions from IUE spectra for BP Tau, CV Cha, RY Tau, RU Lupi and GW Ori, we demonstrate that this is also true for classical T Tauri stars (CTTSs). As in the solar case it is found that the CIV resonance doublet at 1548 Å is also the most accurate indicator of the total radiated power from the atmospheres of CTTSs. Since the total radiated-power density in CTTSs exceeds that of the Sun by over three orders of magnitude we derive new analytic expressions that can be used to estimate the values for these stars. We also discuss the implications of these results with regard to the influence or absence of accretion in this sample of stars and suggest that the method can be used to infer properties of the geometrical structure of the emission regions. As a demonstration case we also use archived HST-GHRS data to estimate the total radiative losses in the UV emitting region of BP Tau. We find values of 4.57 × 109 erg cm-2 s-1 and 5.11 × 1032 erg s-1 dependent on the geometry of the emission region. These results are several orders of magnitude larger than would be expected if the UV emission came primarily from an atmosphere covered in solar-like active regions and are closer to values associated with solar flares. They lead to luminosity estimates of 0.07 and 0.13 Lsolar, respectively, which are in broad agreement with results obtained from theoretical accretion shock models. Taken together they suggest that accretion may well be the dominant contributor to the UV emission in BP Tau.

  19. Constraining the initial conditions and final outcomes of accretion processes around young stars and supermassive black holes

    NASA Astrophysics Data System (ADS)

    Stone, Jordan M.

    2015-04-01

    In this thesis I discuss probes of small spatial scales around young stars and protostars and around the supermassive black hole at the galactic center. I begin by describing adaptive optics-fed infrared spectroscopic studies of nascent and newborn binary systems. Binary star formation is a significant mode of star formation that could be responsible for the production of a majority of the galactic stellar population. Better characterization of the binary formation mechanism is important for better understanding many facets of astronomy, from proper estimates of the content of unresolved populations, to stellar evolution and feedback, to planet formation. My work revealed episodic accretion onto the more massive component of the pre-main sequence binary system UY Aur. I also showed changes in the accretion onto the less massive component, revealing contradictory indications of the change in accretion rate when considering disk-based and shock-based tracers. I suggested two scenarios to explain the inconsistency. First, increased accretion should alter the disk structure, puffing it up. This change could obscure the accretion shock onto the central star if the disk is highly inclined. Second, if accretion through the disk is impeded before it makes it all the way onto the central star, then increased disk tracers of accretion would not be accompanied by increased shock tracers. In this case mass must be piling up at some radius in the disk, possibly supplying the material for planet formation or a future burst of accretion. My next project focused on characterizing the atmospheres of very low-mass companions to nearby young stars. Whether these objects form in an extension of the binary-star formation mechanism to very low masses or they form via a different process is an open question. Different accretion histories should result in different atmospheric composition, which can be constrained with spectroscopy. I showed that 3--4mum spectra of a sample of these

  20. On the Observability of Individual Population III Stars and Their Stellar-mass Black Hole Accretion Disks through Cluster Caustic Transits

    NASA Astrophysics Data System (ADS)

    Windhorst, Rogier A.; Timmes, F. X.; Wyithe, J. Stuart B.; Alpaslan, Mehmet; Andrews, Stephen K.; Coe, Daniel; Diego, Jose M.; Dijkstra, Mark; Driver, Simon P.; Kelly, Patrick L.; Kim, Duho

    2018-02-01

    We summarize panchromatic Extragalactic Background Light data to place upper limits on the integrated near-infrared surface brightness (SB) that may come from Population III stars and possible accretion disks around their stellar-mass black holes (BHs) in the epoch of First Light, broadly taken from z ≃ 7–17. Theoretical predictions and recent near-infrared power spectra provide tighter constraints on their sky signal. We outline the physical properties of zero-metallicity Population III stars from MESA stellar evolution models through helium depletion and of BH accretion disks at z≳ 7. We assume that second-generation non-zero-metallicity stars can form at higher multiplicity, so that BH accretion disks may be fed by Roche-lobe overflow from lower-mass companions. We use these near-infrared SB constraints to calculate the number of caustic transits behind lensing clusters that the James Webb Space Telescope and the next-generation ground-based telescopes may observe for both Population III stars and their BH accretion disks. Typical caustic magnifications can be μ ≃ {10}4{--}{10}5, with rise times of hours and decline times of ≲ 1 year for cluster transverse velocities of {v}T≲ 1000 km s‑1. Microlensing by intracluster-medium objects can modify transit magnifications but lengthen visibility times. Depending on BH masses, accretion-disk radii, and feeding efficiencies, stellar-mass BH accretion-disk caustic transits could outnumber those from Population III stars. To observe Population III caustic transits directly may require monitoring 3–30 lensing clusters to {AB}≲ 29 mag over a decade.

  1. Three-dimensional hydrodynamical models of wind and outburst-related accretion in symbiotic systems

    NASA Astrophysics Data System (ADS)

    de Val-Borro, M.; Karovska, M.; Sasselov, D. D.; Stone, J. M.

    2017-07-01

    Gravitationally focused wind accretion in binary systems consisting of an evolved star with a gaseous envelope and a compact accreting companion is a possible mechanism to explain mass transfer in symbiotic binaries. We study the mass accretion around the secondary caused by the strong wind from the primary late-type component using global three-dimensional hydrodynamic numerical simulations during quiescence and outburst stages. In particular, the dependence of the mass accretion rate on the mass-loss rate, wind parameters and phases of wind outburst development is considered. For a typical wind from an asymptotic giant branch star with a mass-loss rate of 10-6 M⊙ yr-1 and wind speeds of 20-50 km s-1, the mass transfer through a focused wind results in efficient infall on to the secondary. Accretion rates on to the secondary of 5-20 per cent of the mass-loss from the primary are obtained during quiescence and outburst periods where the wind velocity and mass-loss rates are varied, about 20-50 per cent larger than in the standard Bondi-Hoyle-Lyttleton approximation. This mechanism could be an important method for explaining observed accretion luminosities and periodic modulations in the accretion rates for a broad range of interacting binary systems.

  2. Accreting neutron stars by QFT

    NASA Astrophysics Data System (ADS)

    Chen, Shao-Guang

    the negative charge from ionosphere electrons again rotate, thereby come into being the solar basal magnetic field. The solar surface plasma with additional electrons get the dynamic balance between the upwards force of stable positive charge distribution in the solar upside gas and the downwards force of the vacuum net nuν _{0} flux pressure (solar gravity). When the Jupiter enter into the connecting line of the Sun and the center of the Galaxy, the pressure (solar gravity) observed from earth will weaken because of the Jupiter stop (shield) the net nuν _{0} flux which shoot to Sun from the center of Galaxy. The dynamic balance of forces on the solar surface plasma at once is broken and the plasma will upwards eject as the solar wind with redundant negative charge. At the same time, the solar surface remain a cavity as a sunspot whorl with the positive electric potential relative to around plasma. The whorl is caused by the reaction of plasma eject front and upwards with the different velocity at different latitude of solar rotation, it leads to the cavity around in the downwards and backwards helix movement. The solar rotation more slow, when the cavity is filled by around plasma in the reverse turn direction and return to carry-over negative charge, the Jupiter at front had been produced a new cavity carry-over positive charge, so we had observe the sunspot pair with different whorl directions and different magnetic polarity. Jupiter possess half mass of all planets in solar system, its action to stop net nuν _{0} flux is primary, so that Jupiter’s period of 11.8 sidereal years accord basically with the period of sunspot eruptions. In my paper ‘Nonlinear superposition of strong gravitational field of compact stars’(E15-0039-08), according to QFT it is deduced that: let q is a positive shielding coefficient, 1- q show the gravity weaken degree, the earth (104 km) as a obstructing layer q = 4.6*10 (-10) . A spherical shell of neutron star as obstructing

  3. Constraining Accreting Binary Populations in Normal Galaxies

    NASA Astrophysics Data System (ADS)

    Lehmer, Bret; Hornschemeier, A.; Basu-Zych, A.; Fragos, T.; Jenkins, L.; Kalogera, V.; Ptak, A.; Tzanavaris, P.; Zezas, A.

    2011-01-01

    X-ray emission from accreting binary systems (X-ray binaries) uniquely probe the binary phase of stellar evolution and the formation of compact objects such as neutron stars and black holes. A detailed understanding of X-ray binary systems is needed to provide physical insight into the formation and evolution of the stars involved, as well as the demographics of interesting binary remnants, such as millisecond pulsars and gravitational wave sources. Our program makes wide use of Chandra observations and complementary multiwavelength data sets (through, e.g., the Spitzer Infrared Nearby Galaxies Survey [SINGS] and the Great Observatories Origins Deep Survey [GOODS]), as well as super-computing facilities, to provide: (1) improved calibrations for correlations between X-ray binary emission and physical properties (e.g., star-formation rate and stellar mass) for galaxies in the local Universe; (2) new physical constraints on accreting binary processes (e.g., common-envelope phase and mass transfer) through the fitting of X-ray binary synthesis models to observed local galaxy X-ray binary luminosity functions; (3) observational and model constraints on the X-ray evolution of normal galaxies over the last 90% of cosmic history (since z 4) from the Chandra Deep Field surveys and accreting binary synthesis models; and (4) predictions for deeper observations from forthcoming generations of X-ray telesopes (e.g., IXO, WFXT, and Gen-X) to provide a science driver for these missions. In this talk, we highlight the details of our program and discuss recent results.

  4. MAXI J1957+032: An Accreting Neutron Star Possibly in a Triple System

    NASA Astrophysics Data System (ADS)

    Ravi, V.

    2017-12-01

    I present an optical characterization of the Galactic X-ray transient source MAXI J1957+032. This system flares by a factor of ≳104 every few hundred days, with each flare lasting ∼5 days. I identify its quiescent counterpart to be a late-K/early-M dwarf star at a distance of 5 ± 2 kpc. This implies that the peak 0.5{--}10 {keV} luminosity of the system is {10}36.4+/- 0.4 erg s‑1. As found by Mata Sanchez et al. the outburst properties of MAXI J1957+032 are most consistent with the sample of accreting millisecond pulsars. However, the low inferred accretion rate, and the lack of evidence for a hydrogen-rich accretion flow, are difficult to reconcile with the late-K/early-M dwarf counterpart being the mass donor. Instead, the observations are best described by a low-mass hydrogen- and possibly helium-poor mass donor, such as a carbon–oxygen white dwarf, forming a tight interacting binary with a neutron star. The observed main-sequence counterpart would then likely be in a wide orbit around the inner binary.

  5. Cooling of hypernuclear compact stars

    NASA Astrophysics Data System (ADS)

    Raduta, Adriana R.; Sedrakian, Armen; Weber, Fridolin

    2018-04-01

    We study the thermal evolution of hypernuclear compact stars constructed from covariant density functional theory of hypernuclear matter and parametrizations which produce sequences of stars containing two-solar-mass objects. For the input in the simulations, we solve the Bardeen-Cooper-Schrieffer gap equations in the hyperonic sector and obtain the gaps in the spectra of Λ, Ξ0, and Ξ- hyperons. For the models with masses M/M⊙ ≥ 1.5 the neutrino cooling is dominated by hyperonic direct Urca processes in general. In the low-mass stars the (Λp) plus leptons channel is the dominant direct Urca process, whereas for more massive stars the purely hyperonic channels (Σ-Λ) and (Ξ-Λ) are dominant. Hyperonic pairing strongly suppresses the processes on Ξ-s and to a lesser degree on Λs. We find that intermediate-mass 1.5 ≤ M/M⊙ ≤ 1.8 models have surface temperatures which lie within the range inferred from thermally emitting neutron stars, if the hyperonic pairing is taken into account. Most massive models with M/M⊙ ≃ 2 may cool very fast via the direct Urca process through the (Λp) channel because they develop inner cores where the S-wave pairing of Λs and proton is absent.

  6. Analyzing the Spectra of Accreting X-Ray Pulsars

    NASA Astrophysics Data System (ADS)

    Wolff, Michael

    This proposal seeks funding for the analysis of accretion-powered X-ray pulsar spectra from NASA/ HEASARC archived X-ray data. Spectral modeling of accreting X-ray pulsars can tell us a great deal about the physical conditions in and near high mass X-ray binary systems. Such systems have accretion flows where plasma is initially channeled from an accretion disk by the strong neutron star magnetic field, eventually falling onto the magnetic polar cap of the neutron star compact object. Many of these accreting X-ray pulsars have X-ray spectra that consist of broad power-law continua with superposed cyclotron resonant scattering features indicating magnetic field strengths above 10^12 G. The energies of these cyclotron line features have recently been shown to vary with X-ray luminosity in a number of sources such as Her X-1 and V 0332+53, a phenomenon not well understood. Another recent development is the relatively new analytic model for the spectral continuum formation in accretion-powered pulsar systems developed by Becker & Wolff. In their formalism the accretion flows are assumed to go through radiation- dominated radiative shocks and settle onto the neutron star surface. The radiation field consists of strongly Comptonized bremsstrahlung emission from the entire plasma, Comptonized cyclotron emission from the de-excitations of Landau-excited electrons in the neutron star magnetic field, and Comptonized black-body emission from a thermal mound near the neutron star surface. We seek to develop the data analysis tools to apply this model framework to the X-ray data from a wide set of sources to make progress characterizing the basic accretion properties (e.g., magnetic field strength, plasma temperatures, polar cap size, accretion rate per unit area, dominance of bulk vs. thermal Comptonization) as well as understanding the variations of the cyclotron line energies with X-ray luminosity. The three major goals of our proposed work are as follows: In the first year

  7. Long-term Accretion Variations of the Magnetic Cataclysmic Variable Star QQ Vulpecula

    NASA Astrophysics Data System (ADS)

    Cooper Rose, Sanaea; Kafka, Stella; Jorgenson, Regina; Carr, Derrick; Childs, Francesca; Christenson, Holly; Karim, Md. Tanveer; Konchady, Tarini; Walker, Gary E.; Honeycutt, R. K.

    2017-01-01

    Magnetic cataclysmic variable stars have brightness variations that repeat with each revolution of the two stars about the center of mass of the system. However, in the case of QQ Vulpecula (QQ Vul), this brightness variation pattern changes in the long term. This study makes use of two decades worth of data from the Roboscope Telescope as well as data from the American Association of Variable Star Observers (AAVSO) database to examine the long-term evolution of QQ Vul’s phase curves. Nightly observations using the Maria Mitchell Association's Vestal and Loines Observatories supplemented this analysis by clarifying short-term brightness variation. The long-term data was divided into four commonly observed behavioral types ranging from a double peaked curve of ~15.5 magnitude to a ~15.0 magnitude curve that had a primary minimum and a slow, linear rise in brightness in place of the secondary minimum. The nightly data kept within the confines of these categories, though the secondary minimum in the nightly data never vanished. No periodicity was found in the long-term variations. The model often invoked to explain the double peaked curve consists of single pole accretion in which a partial self-eclipse causes the secondary minimum and cyclotron beaming causes the primary minimum. However, the long-term variation may indicate a changing accretion rate, which may manifest itself in changes to the shape, size, or location of the accretion spot on the white dwarf such that it lessens or removes the secondary minimum. This project was supported by the NSF REU grant AST-1358980, the Massachusetts Space Grant, and the Nantucket Maria Mitchell Association.

  8. Interaction between bosonic dark matter and stars

    NASA Astrophysics Data System (ADS)

    Brito, Richard; Cardoso, Vitor; Macedo, Caio F. B.; Okawa, Hirotada; Palenzuela, Carlos

    2016-02-01

    We provide a detailed analysis of how bosonic dark matter "condensates" interact with compact stars, extending significantly the results of a recent Letter [1]. We focus on bosonic fields with mass mB , such as axions, axion-like candidates and hidden photons. Self-gravitating bosonic fields generically form "breathing" configurations, where both the spacetime geometry and the field oscillate, and can interact and cluster at the center of stars. We construct stellar configurations formed by a perfect fluid and a bosonic condensate, and which may describe the late stages of dark matter accretion onto stars, in dark-matter-rich environments. These composite stars oscillate at a frequency which is a multiple of f =2.5 ×1014(mBc2/eV ) Hz . Using perturbative analysis and numerical relativity techniques, we show that these stars are generically stable, and we provide criteria for instability. Our results also indicate that the growth of the dark matter core is halted close to the Chandrasekhar limit. We thus dispel a myth concerning dark matter accretion by stars: dark matter accretion does not necessarily lead to the destruction of the star, nor to collapse to a black hole. Finally, we argue that stars with long-lived bosonic cores may also develop in other theories with effective mass couplings, such as (massless) scalar-tensor theories.

  9. Connection between jets, winds and accretion in T Tauri stars. The X-shooter view

    NASA Astrophysics Data System (ADS)

    Nisini, B.; Antoniucci, S.; Alcalá, J. M.; Giannini, T.; Manara, C. F.; Natta, A.; Fedele, D.; Biazzo, K.

    2018-01-01

    Mass loss from jets and winds is a key ingredient in the evolution of accretion discs in young stars. While slow winds have been recently extensively studied in T Tauri stars, little investigation has been devoted on the occurrence of high velocity jets and on how the two mass-loss phenomena are connected with each other, and with the disc mass accretion rates. In this framework, we have analysed the [O I]6300 Å line in a sample of 131 young stars with discs in the Lupus, Chamaeleon and σ Orionis star forming regions. The stars were observed with the X-shooter spectrograph at the Very Large Telescope and have mass accretion rates spanning from 10-12 to 10-7M⊙ yr-1. The line profile was deconvolved into a low velocity component (LVC, | Vr | < 40 km s-1) and a high velocity component (HVC, | Vr | > 40 km s-1), originating from slow winds and high velocity jets, respectively. The LVC is by far the most frequent component, with a detection rate of 77%, while only 30% of sources have a HVC. The fraction of HVC detections slightly increases (i.e. 39%) in the sub-sample of stronger accretors (i.e. with log (Lacc/L⊙) >-3). The [O I]6300 Å luminosity of both the LVC and HVC, when detected, correlates with stellar and accretion parameters of the central sources (i.e. L∗, M∗, Lacc, Ṁacc), with similar slopes for the two components. The line luminosity correlates better (i.e. has a lower dispersion) with the accretion luminosity than with the stellar luminosity or stellar mass. We suggest that accretion is the main drivers for the line excitation and that MHD disc-winds are at the origin of both components. In the sub-sample of Lupus sources observed with ALMA a relationship is found between the HVC peak velocity and the outer disc inclination angle, as expected if the HVC traces jets ejected perpendicularly to the disc plane. Mass ejection rates (Ṁjet) measured from the detected HVC [O I]6300 Å line luminosity span from 10-13 to 10-7M⊙ yr-1. The

  10. Gravitational radiation emitted when a mass falls onto a compact star.

    NASA Astrophysics Data System (ADS)

    Borelli, A.

    1997-03-01

    The authors study the energy spectrum related to the axial perturbations of a compact star when a particle falls spiralling onto it. They find that both slowly-damped quasi-normal modes and strongly damped w-modes are excited, and that a part of the energy in the process is associated to these w-modes. A substantial difference between the energy spectra of compact stars and black holes is shown.

  11. Bondi Accretion and the Problem of the Missing Isolated Neutron Stars

    NASA Technical Reports Server (NTRS)

    Perna, Rosalba; Narayan, Ramesh; Rybicki, George; Stella, Luigi; Treves, Aldo

    2003-01-01

    A large number of neutron stars (NSs), approximately 10(exp 9), populate the Galaxy, but only a tiny fraction of them is observable during the short radio pulsar lifetime. The majority of these isolated NSs, too cold to be detectable by their own thermal emission, should be visible in X-rays as a result of accretion from the interstellar medium. The ROSAT All-Sky Survey has, however, shown that such accreting isolated NSs are very elusive: only a few tentative candidates have been identified, contrary to theoretical predictions that up to several thousand should be seen. We suggest that the fundamental reason for this discrepancy lies in the use of the standard Bondi formula to estimate the accretion rates. We compute the expected source counts using updated estimates of the pulsar velocity distribution, realistic hydrogen atmosphere spectra, and a modified expression for the Bondi accretion rate, as suggested by recent MHD simulations and supported by direct observations in the case of accretion around supermassive black holes in nearby galaxies and in our own. We find that, whereas the inclusion of atmospheric spectra partly compensates for the reduction in the counts due to the higher mean velocities of the new distribution, the modified Bondi formula dramatically suppresses the source counts. The new predictions are consistent with a null detection at the ROSAT sensitivity.

  12. Is an Accreting Binary Black Hole Precursor Driving the Ionisation Structure and its Kinematics in the Carafe?

    NASA Astrophysics Data System (ADS)

    Shastri, Prajval

    2017-09-01

    We seek to test the hypothesis that radiatively efficient accretion onto the central supermassive black holes (SMBHs) of two merging galaxies drive the emission-line structure and kinematics that we see in the ROSAT-detected Carafe. We have confirmed the presence of two compact sources with LINER-type spectra, which coincide with two compact radio sources that we detect. We have obtained the emission-line structure and kinematics of the Carafe with an optical IFU mosaic. We demonstrate that the proposed 35ksec ACIS imaging will yield both the soft and hard X-ray photons that we need to definitively distinguish between the following hypotheses: that the driver of the system is a pair of accreting SMBH, or that the hot extended gas in the Carafe is shock-excited by two compact star bursts.

  13. Stationary bound-state massive scalar field configurations supported by spherically symmetric compact reflecting stars

    NASA Astrophysics Data System (ADS)

    Hod, Shahar

    2017-12-01

    It has recently been demonstrated that asymptotically flat neutral reflecting stars are characterized by an intriguing no-hair property. In particular, it has been proved that these horizonless compact objects cannot support spatially regular static matter configurations made of scalar (spin-0) fields, vector (spin-1) fields and tensor (spin-2) fields. In the present paper we shall explicitly prove that spherically symmetric compact reflecting stars can support stationary (rather than static) bound-state massive scalar fields in their exterior spacetime regions. To this end, we solve analytically the Klein-Gordon wave equation for a linearized scalar field of mass μ and proper frequency ω in the curved background of a spherically symmetric compact reflecting star of mass M and radius R_{ {s}}. It is proved that the regime of existence of these stationary composed star-field configurations is characterized by the simple inequalities 1-2M/R_{ {s}}<(ω /μ )^2<1. Interestingly, in the regime M/R_{ {s}}≪ 1 of weakly self-gravitating stars we derive a remarkably compact analytical equation for the discrete spectrum {ω (M,R_{ {s}},μ )}^{n=∞}_{n=1} of resonant oscillation frequencies which characterize the stationary composed compact-reflecting-star-linearized-massive-scalar-field configurations. Finally, we verify the accuracy of the analytically derived resonance formula of the composed star-field configurations with direct numerical computations.

  14. CSI 2264: Characterizing Accretion-burst Dominated Light Curves for Young Stars in NGC 2264

    NASA Astrophysics Data System (ADS)

    Stauffer, John; Cody, Ann Marie; Baglin, Annie; Alencar, Silvia; Rebull, Luisa; Hillenbrand, Lynne A.; Venuti, Laura; Turner, Neal J.; Carpenter, John; Plavchan, Peter; Findeisen, Krzysztof; Carey, Sean; Terebey, Susan; Morales-Calderón, María; Bouvier, Jerome; Micela, Giusi; Flaccomio, Ettore; Song, Inseok; Gutermuth, Rob; Hartmann, Lee; Calvet, Nuria; Whitney, Barbara; Barrado, David; Vrba, Frederick J.; Covey, Kevin; Herbst, William; Furesz, Gabor; Aigrain, Suzanne; Favata, Fabio

    2014-04-01

    Based on more than four weeks of continuous high-cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA's Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high-quality, multi-wavelength light curves for young stellar objects whose optical variability is dominated by short-duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief—several hours to one day—brightenings at optical and near-infrared wavelengths with amplitudes generally in the range of 5%-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a 30 day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u - g versus g - r color-color diagram with the largest UV excesses. These stars also have large Hα equivalent widths, and either centrally peaked, lumpy Hα emission profiles or profiles with blueshifted absorption dips associated with disk or stellar winds. Light curves of this type have been predicted for stars whose accretion is dominated by Rayleigh-Taylor instabilities at the boundary between their magnetosphere and inner circumstellar disk, or where magneto-rotational instabilities modulate the accretion rate from the inner disk. Among the stars with the largest UV excesses or largest Hα equivalent widths, light curves with this type of variability greatly outnumber light curves with relatively smooth sinusoidal variations associated with long-lived hot spots. We provide quantitative statistics for the average duration and strength of the accretion bursts and for the fraction of the accretion luminosity associated with these bursts. Based on data from the Spitzer and CoRoT missions, as well as the Canada-France-Hawaii Telescope (CFHT) MegaCam CCD, and the European Southern Observatory Very Large Telescope, Paranal Chile, under

  15. AN ULTRA-LOW-MASS AND SMALL-RADIUS COMPACT OBJECT IN 4U 1746-37?

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Li, Zhaosheng; Qu, Zhijie; Guo, Yanjun

    Photospheric radius expansion (PRE) bursts have already been used to constrain the masses and radii of neutron stars. RXTE observed three PRE bursts in 4U 1746-37, all with low touchdown fluxes. We discuss here the possibility of a low-mass neutron star in 4U 1746-37 because the Eddington luminosity depends on stellar mass. With typical values of hydrogen mass fraction and color correction factor, a Monte Carlo simulation was applied to constrain the mass and radius of a neutron star in 4U 1746-37. 4U 1746-37 has a high inclination angle. Two geometric effects, the reflection of the far-side accretion disk andmore » the obscuration of the near-side accretion disk, have also been included in the mass and radius constraints of 4U 1746-37. If the reflection of the far-side accretion disk is accounted for, a low-mass compact object (mass of 0.41 ± 0.14 M {sub ☉} and radius of 8.73 ± 1.54 km at 68% confidence) exists in 4U 1746-37. If another effect operated, 4U 1746-37 may contain an ultra-low-mass and small-radius object (M = 0.21 ± 0.06 M {sub ☉}, R = 6.26 ± 0.99 km at 68% confidence). Combining all possibilities, the mass of 4U 1746-37 is 0.41{sub −0.30}{sup +0.70} M{sub ⊙} at 99.7% confidence. For such low-mass neutron stars, it could be reproduced by a self-bound compact star, i.e., a quark star or quark-cluster star.« less

  16. Scalar field configurations supported by charged compact reflecting stars in a curved spacetime

    NASA Astrophysics Data System (ADS)

    Peng, Yan

    2018-05-01

    We study the system of static scalar fields coupled to charged compact reflecting stars through both analytical and numerical methods. We enclose the star in a box and our solutions are related to cases without box boundaries when putting the box far away from the star. We provide bottom and upper bounds for the radius of the scalar hairy compact reflecting star. We obtain numerical scalar hairy star solutions satisfying boundary conditions and find that the radius of the hairy star in a box is continuous in a range, which is very different from cases without box boundaries where the radius is discrete in the range. We also examine effects of the star charge and mass on the largest radius.

  17. REVIEWS OF TOPICAL PROBLEMS: The nature of accretion disks of close binary stars: overreflection instability and developed turbulence

    NASA Astrophysics Data System (ADS)

    Fridman, A. M.; Bisikalo, D. V.

    2008-06-01

    The current status of the physics of accretion disks in close binary stars is reviewed, with an emphasis on the hydrodynamic overreflection instability, which is a factor leading to the accretion disk turbulence. The estimated turbulent viscosity coefficients are in good agreement with observations and explain the high angular momentum transfer rate and the measured accretion rate. Based on the observations, a power-law spectrum for the developed turbulence is obtained.

  18. Supercritical Accretion onto a Non-magnetized Neutron Star: Why is it Feasible?

    NASA Astrophysics Data System (ADS)

    Takahashi, Hiroyuki R.; Mineshige, Shin; Ohsuga, Ken

    2018-01-01

    To understand why supercritical accretion is feasible onto a neutron star (NS), we carefully examine the accretion flow dynamics by 2.5-dimensional general relativistic radiation magnetohydrodynamic (RMHD) simulations, comparing the cases of accretion onto a non-magnetized NS and that onto a black hole (BH). Supercritical BH accretion is relatively easy, since BHs can swallow excess radiation energy, so that radiation flux can be inward in its vicinity. This mechanism can never work for an NS, which has a solid surface. In fact, we find that the radiation force is always outward. Instead, we found significant reduction in the mass accretion rate due to strong radiation-pressure-driven outflow. The radiation flux F rad is self-regulated such that the radiation force balances with the sum of gravity and centrifugal forces. Even when the radiation energy density greatly exceeds that expected from the Eddington luminosity {E}{rad}≃ {F}{rad}τ /c> {10}2{L}{Edd}/(4π {r}2c), the radiation flux is always kept below a certain value, which makes it possible not to blow all the gas away from the disk. These effects make supercritical accretion feasible. We also find that a settling region, where accretion is significantly decelerated by a radiation cushion, is formed around the NS surface. In the settling region, the radiation temperature and mass density roughly follow {T}{rad}\\propto {r}-1 and ρ \\propto {r}-3, respectively. No settling region appears around the BH, so matter can be directly swallowed by the BH with supersonic speed.

  19. Study of isotropic compact stars in f(R,T,R_{μν}T^{μν}) gravity

    NASA Astrophysics Data System (ADS)

    Sharif, M.; Waseem, Arfa

    2016-06-01

    In this paper, we investigate physical behavior and stability of compact stars filled with isotropic fluid in f(R,T,R_{μν}T^{μν}) gravity. We consider the static spherically symmetric spacetime and choose the simplest model of this gravity, i.e., R+α R_{μν}T^{μν} . To examine the basic features of compact stars like Her X-1, SAX J 1808.4-3658 and 4U 1820-30, we apply analytic solutions of Krori and Barua metric using the mass-radius relation. We study the behavior of effective energy density, pressure, equation of state parameter and energy conditions in the interior of compact stars. We also explore the stability criteria of compact stars via the speed of sound. It is concluded that all the energy conditions are satisfied and the compact stars are found to be stable at the boundary for this particular model.

  20. High-mass Star Formation through Filamentary Collapse and Clump-fed Accretion in G22

    NASA Astrophysics Data System (ADS)

    Yuan, Jinghua; Li, Jin-Zeng; Wu, Yuefang; Ellingsen, Simon P.; Henkel, Christian; Wang, Ke; Liu, Tie; Liu, Hong-Li; Zavagno, Annie; Ren, Zhiyuan; Huang, Ya-Fang

    2018-01-01

    How mass is accumulated from cloud-scale down to individual stars is a key open question in understanding high-mass star formation. Here, we present the mass accumulation process in a hub-filament cloud G22 that is composed of four supercritical filaments. Velocity gradients detected along three filaments indicate that they are collapsing with a total mass infall rate of about 440 M ⊙ Myr‑1, suggesting the hub mass would be doubled in six free-fall times, adding up to ∼2 Myr. A fraction of the masses in the central clumps C1 and C2 can be accounted for through large-scale filamentary collapse. Ubiquitous blue profiles in HCO+ (3–2) and 13CO (3–2) spectra suggest a clump-scale collapse scenario in the most massive and densest clump C1. The estimated infall velocity and mass infall rate are 0.31 km s‑1 and 7.2 × 10‑4 M ⊙ yr‑1, respectively. In clump C1, a hot molecular core (SMA1) is revealed by the Submillimeter Array observations and an outflow-driving high-mass protostar is located at the center of SMA1. The mass of the protostar is estimated to be 11–15 M ⊙ and it is still growing with an accretion rate of 7 × 10‑5 M ⊙ yr‑1. The coexistent infall in filaments, clump C1, and the central hot core in G22 suggests that pre-assembled mass reservoirs (i.e., high-mass starless cores) may not be required to form high-mass stars. In the course of high-mass star formation, the central protostar, the core, and the clump can simultaneously grow in mass via core-fed/disk accretion, clump-fed accretion, and filamentary/cloud collapse.

  1. The Evolution of the Accretion Disk Around 4U 1820-30 During a Superburst

    NASA Technical Reports Server (NTRS)

    Ballantyne, D. R.; Strohmayer, T. E.

    2004-01-01

    Accretion from a disk onto a collapsed, relativistic star - a neutron star or black hole - is the mechanism widely believed to be responsible for the emission from compact X-ray binaries. Because of the extreme spatial resolution required, it is not yet possible to directly observe the evolution or dynamics of the inner parts of the accretion disk where general relativistic effects are dominant. Here, we use the bright X-ray emission from a superburst on the surface of the neutron star 4U 1820-30 as a spotlight to illuminate the disk surface. The X-rays cause iron atoms in the disk t o fluoresce, allowing a determination of the ionization state, covering factor and inner radius of the disk over the course of the burst. The time-resolved spectral fitting shows that the inner region of the disk is disrupted by the burst, possibly being heated into a thicker, more tenuous flow, before recovering its previous form in approximately 1000 s. This marks the first instance that the evolution of the inner regions of an accretion disk has been observed in real-time.

  2. Laboratory analogue of a supersonic accretion column in a binary star system.

    PubMed

    Cross, J E; Gregori, G; Foster, J M; Graham, P; Bonnet-Bidaud, J-M; Busschaert, C; Charpentier, N; Danson, C N; Doyle, H W; Drake, R P; Fyrth, J; Gumbrell, E T; Koenig, M; Krauland, C; Kuranz, C C; Loupias, B; Michaut, C; Mouchet, M; Patankar, S; Skidmore, J; Spindloe, C; Tubman, E R; Woolsey, N; Yurchak, R; Falize, É

    2016-06-13

    Astrophysical flows exhibit rich behaviour resulting from the interplay of different forms of energy-gravitational, thermal, magnetic and radiative. For magnetic cataclysmic variable stars, material from a late, main sequence star is pulled onto a highly magnetized (B>10 MG) white dwarf. The magnetic field is sufficiently large to direct the flow as an accretion column onto the poles of the white dwarf, a star subclass known as AM Herculis. A stationary radiative shock is expected to form 100-1,000 km above the surface of the white dwarf, far too small to be resolved with current telescopes. Here we report the results of a laboratory experiment showing the evolution of a reverse shock when both ionization and radiative losses are important. We find that the stand-off position of the shock agrees with radiation hydrodynamic simulations and is consistent, when scaled to AM Herculis star systems, with theoretical predictions.

  3. On the Observability of Individual Population III Stars and Their Stellar-mass Black Hole Accretion Disks through Cluster Caustic Transits

    NASA Astrophysics Data System (ADS)

    Windhorst, Rogier A.; Wyithe, Stuart; Alpaslan, Mehmet; Timmes, F. X.; Andrews, Stephen K.; Kim, Duho; Kelly, Patrick; Coe, Dan A.; Diego, Jose M.; Driver, Simon P.; Dijkstra, Mark

    2018-06-01

    We summarize panchromatic Extragalactic Background Light data to place upper limits on the integrated near-IR surface brightness (SB) that may come from Population III stars and possible accretion disks around their stellar-mass black holes (BHs) in the epoch of First Light, broadly taken from z=7-17.We outline the physical properties of zero-metallicity Population III stars from MESA stellar evolution models through helium depletion and of BH accretion disks at z>7. We assume that second-generation non-zero-metallicity stars can form at higher multiplicity, so that BH accretion disks may be fed by Roche-lobe overflow from lower-mass companions.We use these near-infrared SB constraints to calculate the number of caustic transits behind lensing clusters that the James Webb Space Telescope and the next-generation ground-based telescopes may observe for both Population III stars and their BH accretion disks. Typical caustic magnifications can be 10^4-10^5x, with rise times of hours and decline times of z~<1 year for cluster transverse velocities of v_T<~1000 km/s.Microlensing by intracluster-medium objects can modify transit magnifications but lengthen visibility times. Depending on BH masses, accretion-disk radii, and feeding efficiencies, stellar-mass BH accretion-disk caustic transits could outnumber those from Population III stars. To observe Population III caustic transits directly may require monitoring 3-30 lensing clusters to AB<29 mag over a decade (see Windhorst et al. 2018, ApJS, 234, 41; astro-ph/1801.03584).This work was supported by NASA JWST Interdisciplinary Scientist grants NAG5-12460, NX14AN10G, and 80NSSC18K0200, NASA Theoretical and Computational Astrophysics Networks grant NNX14AB53G, NSF Software Infrastructure for Sustained Innovation grant 1339600, NSF Physics Frontier Center JINA-CEE grant PHY-1430152, Australian Research Council projects AYA2015-64508-P, AYA2012-39475-C02-01, and Ministerio de Economia y Competitividad of Spain Consolider

  4. Nuclear Reactions in the Crusts of Accreting Neutron Stars

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Lau, Rita; Beard, Mary; Gupta, Sanjib S.

    X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density ismore » $$\\rho =2\\times {10}^{12}\\,{\\rm{g}}\\,{\\mathrm{cm}}^{-3}$$ using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond $$\\rho =2\\times {10}^{12}\\,{\\rm{g}}\\,{\\mathrm{cm}}^{-3}$$. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A ≥ 102 nuclei, where the inner crust remains impure with an impurity parameter of Q imp ≈ 20 owing to the N = 82 shell closure. In agreement with previous work, we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. As a result, this work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.« less

  5. Nuclear Reactions in the Crusts of Accreting Neutron Stars

    NASA Astrophysics Data System (ADS)

    Lau, R.; Beard, M.; Gupta, S. S.; Schatz, H.; Afanasjev, A. V.; Brown, E. F.; Deibel, A.; Gasques, L. R.; Hitt, G. W.; Hix, W. R.; Keek, L.; Möller, P.; Shternin, P. S.; Steiner, A. W.; Wiescher, M.; Xu, Y.

    2018-05-01

    X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density is ρ =2× {10}12 {{g}} {cm}}-3 using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond ρ =2× {10}12 {{g}} {cm}}-3. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A ≥ 102 nuclei, where the inner crust remains impure with an impurity parameter of Q imp ≈ 20 owing to the N = 82 shell closure. In agreement with previous work, we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. This work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.

  6. Nuclear Reactions in the Crusts of Accreting Neutron Stars

    DOE PAGES

    Lau, Rita; Beard, Mary; Gupta, Sanjib S.; ...

    2018-05-24

    X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density ismore » $$\\rho =2\\times {10}^{12}\\,{\\rm{g}}\\,{\\mathrm{cm}}^{-3}$$ using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond $$\\rho =2\\times {10}^{12}\\,{\\rm{g}}\\,{\\mathrm{cm}}^{-3}$$. This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A ≥ 102 nuclei, where the inner crust remains impure with an impurity parameter of Q imp ≈ 20 owing to the N = 82 shell closure. In agreement with previous work, we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. As a result, this work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.« less

  7. Formation of Compact Ellipticals in the merging star cluster scenario

    NASA Astrophysics Data System (ADS)

    Urrutia Zapata, Fernanda Cecilia; Theory and star formation group

    2018-01-01

    In the last years, extended old stellar clusters have been observed. They are like globular clusters (GCs) but with larger sizes(a limit of Re=10 pc is currently seen as reasonable). These extended objects (EOs) cover a huge range of mass. Objects at the low mass end with masses comparable to normal globular clusters are called extended clusters or faint fuzzies Larsen & Brodie (2000) and objects at the high-mass end are called ultra compact dwarf galaxies (UCDs). Ultra compact dwarf galaxies are compact object with luminositys above the brigtest known GCs. UCDs are more compact than typical dwarf galaxies but with comparable luminosities. Usually, a lower mass limit of 2 × 10^6 Solar masses is applied.Fellhauer & Kroupa (2002a,b) demostrated that object like ECs, FFs and UCDs can be the remnants of the merger of star clusters complexes, this scenario is called the Merging Star Cluster Scenario. Amore concise study was performed by Bruens et al. (2009, 2011).Our work tries to explain the formation of compact elliptical(cE). These objects are a comparatively rare class of spheroidal galaxies, possessing very small Re and high central surface brightnesses (Faber 1973). cEs have the same parameters as extended objects but they are slightly larger than 100 pc and the luminosities are in the range of -11 to -12 Mag.The standard formation sceanrio of these systems proposes a galaxy origin. CEs are the result of tidal stripping and truncation of nucleated larger systems. Or they could be a natural extension of the class of elliptical galaxies to lower luminosities and smaller sizes.We want to propose a completely new formation scenario for cEs. In our project we try to model cEs in a similar way that UCDs using the merging star cluster scenario extended to much higher masses and sizes. We think that in the early Universe we might have produced sufficiently strong star bursts to form cluster complexes which merge into cEs. So far it is observationally unknown if cEs are

  8. Disk Accretion and the Stellar Birthline

    NASA Astrophysics Data System (ADS)

    Hartmann, Lee; Cassen, Patrick; Kenyon, Scott J.

    1997-02-01

    We present a simplified analysis of some effects of disk accretion on the early evolution of fully convective, low-mass pre-main-sequence stars. Our analysis builds on the previous seminal work of Stahler, but it differs in that the accretion of material occurs over a small area of the stellar surface, such as through a disk or magnetospheric accretion column, so that most of the stellar photosphere is free to radiate to space. This boundary condition is similar to the limiting case considered by Palla & Stahler for intermediate-mass stars. We argue that for a wide variety of disk mass accretion rates, material will be added to the star with relatively small amounts of thermal energy. Protostellar evolution calculated assuming this ``low-temperature'' limit of accretion generally follows the results of Stahler because of the thermostatic nature of deuterium fusion, which prevents protostars from contracting below a ``birthline'' in the H-R diagram. Our calculated protostellar radii tend to fall below Stahler's at higher masses; the additional energy loss from the stellar photosphere in the case of disk accretion tends to make the protostar contract. The low-temperature disk accretion evolutionary tracks never fall below the deuterium-fusion birthline until the internal deuterium is depleted, but protostellar tracks can lie above the birthline in the H-R diagram if the initial radius of the protostellar core is large enough or if rapid disk accretion (such as might occur during FU Ori outbursts) adds significant amounts of thermal energy to the star. These possibilities cannot be ruled out by either theoretical arguments or observational constraints at present, so that individual protostars might evolve along a multiplicity of birthlines with a modest range of luminosity at a given mass. Our results indicate that there are large uncertainties in assigning ages for the youngest stars from H-R diagram positions, given the uncertainty in birthline positions. Our

  9. Nonradial oscillation modes of compact stars with a crust

    NASA Astrophysics Data System (ADS)

    Flores, Cesar Vásquez; Hall, Zack B.; Jaikumar, Prashanth

    2017-12-01

    Oscillation modes of isolated compact stars can, in principle, be a fingerprint of the equation of state (EoS) of dense matter. We study the non-radial high-frequency l =2 spheroidal modes of neutron stars and strange quark stars, adopting a two-component model (core and crust) for these two types of stars. Using perturbed fluid equations in the relativistic Cowling approximation, we explore the effect of a strangelet or hadronic crust on the oscillation modes of strange stars. The results differ from the case of neutron stars with a crust. In comparison to fluid-only configurations, we find that a solid crust on top of a neutron star increases the p -mode frequency slightly with little effect on the f -mode frequency, whereas for strange stars, a strangelet crust on top of a quark core significantly increases the f -mode frequency with little effect on the p -mode frequency.

  10. Reconstruction of the accretion disk in six cataclysmic variable stars

    NASA Astrophysics Data System (ADS)

    Rutten, R. G. M.; van Paradijs, J.; Tinbergen, J.

    1992-07-01

    The maximum-entropy eclipse-mapping algorithm is used to reconstruct images of the accretion disks of the novalike variable stars RW Tri, UX UMa, SW Sex, LX Ser, V 1315 Aql, and V363 Aur. The 2D disk intensity maps deduced from the light curves reveal the size of the disk and its radial intensity dependence. Black-body temperature maps deduced from the intensity maps at different wavelengths show that the disks in RW Tri, UX UMa, and V363 Aur have a radial temperature dependence which closely matches the fundamental theoretical run of the effective temperature with radial distance from disk center: T(eff) varies as R exp -3/4. The system V1315 Aql and SW Sex exhibit a much flatter run of T(R) in the inner region of the disk, while LX Ser appears to hold a position in between these two extremes. The consequences of these results for accretion disk models are also discussed.

  11. Binary and ternary ionic compounds in the outer crust of accreted neutron stars

    NASA Astrophysics Data System (ADS)

    Chamel, N.

    2017-12-01

    The outer crust of an accreted neutron star is thought to contain a large distribution of different nuclear species resulting from the burying of ashes of X-ray bursts and superbursts. By analysing the stability of multicomponent Coulomb crystals against phase separation, it is found that various binary and ternary ionic compounds could be formed.

  12. A key factor to the spin parameter of uniformly rotating compact stars: crust structure

    NASA Astrophysics Data System (ADS)

    Qi, Bin; Zhang, Nai-Bo; Sun, Bao-Yuan; Wang, Shou-Yu; Gao, Jian-Hua

    2016-04-01

    We study the dimensionless spin parameter j ≡ cJ/(GM2) of different kinds of uniformly rotating compact stars, including traditional neutron stars, hyperonic neutron stars and hybrid stars, based on relativistic mean field theory and the MIT bag model. It is found that jmax ˜ 0.7, which had been suggested in traditional neutron stars, is sustained for hyperonic neutron stars and hybrid stars with M > 0.5 M⊙. Not the interior but rather the crust structure of the stars is a key factor to determine jmax for three kinds of selected compact stars. Furthermore, a universal formula j = 0.63(f/fK) - 0.42(f/fK)2 + 0.48(f/fK)3 is suggested to determine the spin parameter at any rotational frequency f smaller than the Keplerian frequency fK.

  13. Extremes of the jet–accretion power relation of blazars, as explored by NuSTAR

    DOE PAGES

    Sbarrato, T.; Ghisellini, G.; Tagliaferri, G.; ...

    2016-07-18

    Hard X-ray observations are crucial to study the non-thermal jet emission from high-redshift, powerful blazars. We observed two bright z > 2 flat spectrum radio quasars (FSRQs) in hard X-rays to explore the details of their relativistic jets and their possible variability. S5 0014+81 (at z = 3.366) and B0222+185 (at z=2.690) have been observed twice by the Nuclear Spectroscopic Telescope Array (NuSTAR) simultaneously with Swift/XRT, showing different variability behaviors. We found that NuSTAR is instrumental to explore the variability of powerful high-redshift blazars, even when no gamma-ray emission is detected. The two sources have proven to have respectively themore » most luminous accretion disk and the most powerful jet among known blazars. Furthermore, thanks to these properties, they are located at the extreme end of the jet-accretion disk relation previously found for gamma-ray detected blazars, to which they are consistent.« less

  14. Extremes of the jet–accretion power relation of blazars, as explored by NuSTAR

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Sbarrato, T.; Ghisellini, G.; Tagliaferri, G.

    Hard X-ray observations are crucial to study the non-thermal jet emission from high-redshift, powerful blazars. We observed two bright z > 2 flat spectrum radio quasars (FSRQs) in hard X-rays to explore the details of their relativistic jets and their possible variability. S5 0014+81 (at z = 3.366) and B0222+185 (at z=2.690) have been observed twice by the Nuclear Spectroscopic Telescope Array (NuSTAR) simultaneously with Swift/XRT, showing different variability behaviors. We found that NuSTAR is instrumental to explore the variability of powerful high-redshift blazars, even when no gamma-ray emission is detected. The two sources have proven to have respectively themore » most luminous accretion disk and the most powerful jet among known blazars. Furthermore, thanks to these properties, they are located at the extreme end of the jet-accretion disk relation previously found for gamma-ray detected blazars, to which they are consistent.« less

  15. Low Gas Fractions Connect Compact Star-Forming Galaxies to their z~2 Quiescent Descendants

    NASA Astrophysics Data System (ADS)

    Spilker, Justin; Bezanson, Rachel; Marrone, Daniel P.; Weiner, Benjamin J.; Whitaker, Katherine E.; Williams, Christina C.

    2017-01-01

    Early quiescent galaxies at z ~ 2 are known to be remarkably compact compared to their nearby counterparts. Possible progenitors of these systems include galaxies that are structurally similar, but are still rapidly forming stars. I will present Karl G. Jansky Very Large Array (VLA) observations of the CO(1-0) line towards three such compact, star-forming galaxies at z ~ 2.3, significantly detecting one. The VLA observations indicate baryonic gas fractions 5 times lower and gas depletion times 10 times shorter than normal, extended massive star-forming galaxies at these redshifts. At their current star formation rates, all three objects will deplete their gas reservoirs within 100Myr. These objects are among the most gas-poor objects observed at z > 2 and are outliers from standard gas scaling relations, a result which remains true regardless of assumptions about the CO-H2 conversion factor. Our observations are consistent with the idea that compact, star-forming galaxies are in a rapid state of transition to quiescence in tandem with the build-up of the z ~ 2 quenched population. In the detected compact galaxy, we see no evidence of rotation or that the CO-emitting gas is spatially extended relative to the stellar light. This casts doubt on recent suggestions that the gas in these compact galaxies is rotating and significantly extended compared to the stars. Instead, we suggest that, at least for this object, the gas is centrally concentrated, and only traces a small fraction of the total galaxy dynamical mass. I will conclude by discussing my ongoing efforts to characterize the gas and star forming properties of this unusual population of galaxies.

  16. Numerical Solution of the Radiative Transfer Equation: X-Ray Spectral Formation from Cylindrical Accretion onto a Magnetized Neutron Star

    NASA Technical Reports Server (NTRS)

    Fairnelli, R.; Ceccobello, C.; Romano, P.; Titarchuk, L.

    2011-01-01

    Predicting the emerging X-ray spectra in several astrophysical objects is of great importance, in particular when the observational data are compared with theoretical models. This requires developing numerical routines for the solution of the radiative transfer equation according to the expected physical conditions of the systems under study. Aims. We have developed an algorithm solving the radiative transfer equation in the Fokker-Planck approximation when both thermal and bulk Comptonization take place. The algorithm is essentially a relaxation method, where stable solutions are obtained when the system has reached its steady-state equilibrium. Methods. We obtained the solution of the radiative transfer equation in the two-dimensional domain defined by the photon energy E and optical depth of the system pi using finite-differences for the partial derivatives, and imposing specific boundary conditions for the solutions. We treated the case of cylindrical accretion onto a magnetized neutron star. Results. We considered a blackbody seed spectrum of photons with exponential distribution across the accretion column and for an accretion where the velocity reaches its maximum at the stellar surface and at the top of the accretion column, respectively. In both cases higher values of the electron temperature and of the optical depth pi produce flatter and harder spectra. Other parameters contributing to the spectral formation are the steepness of the vertical velocity profile, the albedo at the star surface, and the radius of the accretion column. The latter parameter modifies the emerging spectra in a specular way for the two assumed accretion profiles. Conclusions. The algorithm has been implemented in the XPEC package for X-ray fitting and is specifically dedicated to the physical framework of accretion at the polar cap of a neutron star with a high magnetic field (approx > 10(exp 12) G). This latter case is expected to be of typical accreting systems such as X

  17. NuSTAR Discovery of a Cyclotron Line in the Accreting X-Ray Pulsar IGR J16393-4643

    NASA Technical Reports Server (NTRS)

    Bodaghee, Arash; Tomsick, John A.; Fornasini, Francesca M.; Krivonos, Roman; Stern, Daniel; Mori, Kaya; Rahoui, Farid; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; hide

    2016-01-01

    The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was observed by the Nuclear Spectroscope Telescope Array in the 3-79 keV energy band for a net exposure time of 50 ks. We present the results of this observation which enabled the discovery of a cyclotron resonant scattering feature with a centroid energy of -29.3(sup +1.1)(sub -1.3) keV. This allowed us to measure the magnetic field strength of the neutron star for the first time: B = (2.5 +/- 0.1) x 10(exp 12) G. The known pulsation period is now observed at 904.0+/- 0.1 s. Since 2006, the neutron star has undergone a long-term spin-up trend at a rate of P= -2 x 10(exp -8) s s(exp -1) (-0.6 s per year, or a frequency derivative of v = 3 x 10(exp -14) Hz s(exp -1)). In the power density spectrum, a break appears at the pulse frequency which separates the zero slope at low frequency from the steeper slope at high frequency. This addition of angular momentum to the neutron star could be due to the accretion of a quasi-spherical wind, or it could be caused by the transient appearance of a prograde accretion disk that is nearly in corotation with the neutron star whose magnetospheric radius is around 2 x 10(exp 8) cm.

  18. Laboratory simulation of photoionized plasma among astronomical compact objects

    NASA Astrophysics Data System (ADS)

    Fujioka, Shinsuke; Yamamoto, Norimasa; Wang, Feilu; Salzmann, David; Li, Yutong; Rhee, Yong-Joo; Nishimura, Hiroaki; Takabe, Hideaki; Mima, Kunioki

    2008-11-01

    X-ray line emission with several-keV of photon energy was observed from photoionized accreting clouds, for example CYGNUS X-3 and VELA X-1, those are exposed by hard x-ray continuum from the compact objects, such as neutron stars, black holes, or white dwarfs, although accreting clouds are thermally cold. The x-ray continuum-induced line emission gives a good insight to the accreting clouds. We will present a novel laboratory simulation of the photoionized plasma under well-characterized conditions by using high-power laser facility. Blackbody radiator with 500-eV of temperature, as a miniature of a hot compact object, was created.Silicon (Si) plasma with 30-eV of electron temperature was produced in the vicinity of the 0.5-keV blackbody radiator. Line emissions of lithium- and helium-like Si ions was clearly observed around 2-keV of photon-energy from the thermally cold Si plasma, this result is hardly interpreted without consideration of the photoionization. Atomic kinetics code reveals importance of inner-shell ionization directly caused by incoming hard x-rays.

  19. Hard X-ray spectra of neutron stars and black hole candidates

    NASA Technical Reports Server (NTRS)

    Durouchoux, P.; Mahoney, W.; Clenet, Y.; Ling, J.; Wallyn, P.; Wheaton, W.; Corbet, S.; Chapuis, C.

    1997-01-01

    The hard X-ray behavior of several X-ray binary systems containing a neutron star or a black hole candidate is analyzed in an attempt to determine the specific signature of these categories of compact objects. Limiting the consideration to two subclasses of neutron stars, Atoll sources and non-pulsating Z sources, it appears that only the Atoll sources have a spectral behavior similar to black holes. It is proposed that Atoll sources are weakly magnetized neutron stars, whereas Z sources are small radius moderate magnetized neutron stars. Large magnetic fields funnel the accreting matter, thus preventing spherical accretion and free fall if the neutron star radius is smaller than the last stable accreting orbit. Weak magnetic fields do not have this effect, and blackbody soft photons from the stellar surface are upscattered on the relativistic infalling matter, leading to excess hard X-rays. This excess is visible in two of the observed Atoll sources and in the spectrum of a black hole candidate. In the case of a Z source, a lack of photons was remarked, providing a possible signature to distinguish between these classes of objects.

  20. High energy neutrinos and gamma-ray emission from supernovae in compact star clusters

    NASA Astrophysics Data System (ADS)

    Bykov, A. M.; Ellison, D. C.; Gladilin, P. E.; Osipov, S. M.

    2017-01-01

    Compact clusters of young massive stars are observed in the Milky Way and in starburst galaxies. The compact clusters with multiple powerful winds of young massive stars and supernova shocks are favorable sites for high-energy particle acceleration. We argue that expanding young supernova (SN) shells in compact stellar clusters can be very efficient PeV CR accelerators. At a stage when a supernova shock is colliding with collective fast winds from massive stars in a compact cluster the Fermi mechanism allows particle acceleration to energies well above the standard limits of diffusive shock acceleration in an isolated SNR. The energy spectrum of protons in such an accelerator is a hard power-law with a broad spectral upturn above TeV before a break at multi-PeV energies, providing a large energy flux in the high-energy end of the spectrum. The acceleration stage in the colliding shock flow lasts for a few hundred years after the supernova explosion producing high-energy CRs that escape the accelerator and diffuse through the ambient matter producing γ-rays and neutrinos in inelastic nuclear collisions. In starburst galaxies a sizeable fraction of core collapse supernovae is expected to occur in compact star clusters and therefore their high energy gamma-ray and neutrino spectra in the PeV energy regime may differ strongly from that of our Galaxy. To test the model with individual sources we briefly discuss the recent H.E.S.S. detections of gamma-rays from two potential candidate sources, Westerlund 1 and HESS J1806-204 in the Milky Way. We argue that this model of compact star clusters, with typical parameters, could produce a neutrino flux sufficient to explain a fraction of the recently detected IceCube South Pole Observatory neutrinos.

  1. Winds from the S-Star Cluster Reduce the Accretion Rate onto Sgr A*

    NASA Astrophysics Data System (ADS)

    Yusef-Zadeh, Farhad; Wardle, M.; Roberts, D. A; Haggard, Daryl; Lacy, John H.; Royster, Marc; Cotton, William D.

    2014-06-01

    High-resolution radio continuum images of the region within a few arcseconds of Sgr A* at wavelengths of 7 and 12 mm show three new radio structures. One is a 2-3'' hollow teardrop-shaped structure centered on Sgr A*. Highly blue-shifted [NeII] and [FeIII] line emission is detected along the boundary of this teardrop-shaped bubble, ~2.2'' south of Sgr A*. The second structure is a faint, incomplete ring surrounding Sgr A* with typical surface brightness at 7 mm of ~0.1 mJy per ~0.04'' x 0.08'' beam. This partial ring coincides with the outer boundary of the S-star cluster which consists of ~30 B dwarfs orbiting within 1'' of Sgr A*. Lastly, on a scale of ~20'' to the N of Sgr A*, a balloon-shaped structure is detected.We interpret that the new morphological and kinematic structures result from the dynamical effects of a combined cluster wind. This wind is created at a rate ~3 x 10^{-5} solar mass per year by the merging of individual stellar winds from the B stars in the S-star cluster. What is significant about this interpretation is that the expanding wind excludes the shocked winds from O and WR stars in the central parsec of the Galaxy. Meanwhile Sgr A* accretes material from within the S cluster at a rate less than or equal 3 x 10^{-7} solar mass per year, thus explaining the low luminosity of Sgr A* without the ejection of a large fraction of the accreted material.

  2. Formation of new stellar populations from gas accreted by massive young star clusters.

    PubMed

    Li, Chengyuan; de Grijs, Richard; Deng, Licai; Geller, Aaron M; Xin, Yu; Hu, Yi; Faucher-Giguère, Claude-André

    2016-01-28

    Stars in clusters are thought to form in a single burst from a common progenitor cloud of molecular gas. However, massive, old 'globular' clusters--those with ages greater than ten billion years and masses several hundred thousand times that of the Sun--often harbour multiple stellar populations, indicating that more than one star-forming event occurred during their lifetimes. Colliding stellar winds from late-stage, asymptotic-giant-branch stars are often suggested to be triggers of second-generation star formation. For this to occur, the initial cluster masses need to be greater than a few million solar masses. Here we report observations of three massive relatively young star clusters (1-2 billion years old) in the Magellanic Clouds that show clear evidence of burst-like star formation that occurred a few hundred million years after their initial formation era. We show that such clusters could have accreted sufficient gas to form new stars if they had orbited in their host galaxies' gaseous disks throughout the period between their initial formation and the more recent bursts of star formation. This process may eventually give rise to the ubiquitous multiple stellar populations in globular clusters.

  3. Weak and Compact Radio Emission in Early High-Mass Star Forming Regions

    NASA Astrophysics Data System (ADS)

    Rosero, Viviana; P. Hofner, M. Claussen, S. Kurtz, R. Cesaroni, E. D. Araya, C. Carrasco-González, L. F. Rodríguez, K. M. Menten, F. Wyrowski, L. Loinard, S. P. Ellingsen

    2018-01-01

    High-mass protostars are difficult to detect: they have short evolutionary timescales, they tend to be located at large distances, and they are usually embedded within complicated cluster environments. In this work, we aimed to identify and analyze candidates at the earliest stages of high-mass star formation, where only low-level (< 1 mJy) radio emission is expected. We used the Karl G. Jansky Very Large Array to achieve one of the most sensitive (image RMS < 3 -- 10 μJy/beam) centimeter continuum surveys towards high-mass star forming regions to date, with observations at 1.3 and 6 cm and an angular resolution < 0.5". The sample is composed of cold molecular clumps with and without infrared sources (CMC--IRs and CMCs, respectively) and hot molecular cores (HMCs), covering a wide range of parameters such as bolometric luminosity and distance. We detected 70 radio continuum sources that are associated with dust clumps, most of which are weak and compact. We detected centimeter wavelength sources in 100% of our HMCs, which is a higher fraction than previously expected and suggests that radio continuum may be detectable at weak levels in all HMCs. The lack of radio detections for some objects in the sample (including most CMCs) contributes strong evidence that these are prestellar clumps, providing interesting constraints and ideal follow up candidates for studies of the earliest stages of high-mass stars. Our results show further evidence for an evolutionary sequence in the formation of high-mass stars, from starless cores (i.e., CMCs) to relatively more evolved ones (i.e., HMCs). Many of our detections have morphologies and other observational parameters that resemble collimated ionized jets, which is highly relevant for recent theoretical models based on core accretion that predict that the first stages of ionization from high-mass stars are in the form of jets. Additionally, we found that properties of ionized jets from low and high-mass stars are extremely well

  4. A model for neutrino emission from nuclear accretion disks

    NASA Astrophysics Data System (ADS)

    Deaton, Michael

    2015-04-01

    Compact object mergers involving at least one neutron star can produce short-lived black hole accretion engines. Over tens to hundreds of milliseconds such an engine consumes a disk of hot, nuclear-density fluid, and drives changes to its surrounding environment through luminous emission of neutrinos. The neutrino emission may drive an ultrarelativistic jet, may peel off the disk's outer layers as a wind, may irradiate those winds or other forms of ejecta and thereby change their composition, may change the composition and thermodynamic state of the disk itself, and may oscillate in its flavor content. We present the full spatial-, angular-, and energy-dependence of the neutrino distribution function around a realistic model of a nuclear accretion disk, to inform future explorations of these types of behaviors. Spectral Einstein Code (SpEC).

  5. HI properties and star formation history of a fly-by pair of blue compact dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Kim, Jinhyub; Chung, Aeree; Wong, O. Ivy; Lee, Bumhyun; Sung, Eon-Chang; Staveley-Smith, Lister

    2017-09-01

    A fly-by interaction has been suggested to be one of the major explanations for enhanced star formation in blue compact dwarf (BCD) galaxies, yet no direct evidence for this scenario has been found to date. In the Hi Parkes all-sky survey (HIPASS), ESO 435-IG 020 and ESO 435-G 016, a BCD pair were found in a common, extended gas envelope of atomic hydrogen, providing an ideal case to test the hypothesis that the starburst in BCDs can be indeed triggered by a fly-by interaction. Using high-resolution data from the Australia Telescope Compact Array (ATCA), we investigated Hi properties and the spectral energy distribution (SED) of the BCD pair to study their interaction and star formation histories. The high-resolution Hi data of both BCDs reveal a number of peculiarities, which are suggestive of tidal perturbation. Meanwhile, 40% of the HIPASS flux is not accounted for in the ATCA observations with no Hi gas bridge found between the two BCDs. Intriguingly, in the residual of the HIPASS and the ATCA data, 10% of the missing flux appears to be located between the two BCDs. While the SED-based age of the most dominant young stellar population is old enough to have originated from the interaction with any neighbors (including the other of the two BCDs), the most recent star formation activity traced by strong Hα emission in ESO 435-IG 020 and the shear motion of gas in ESO 435-G 016, suggest a more recent or current tidal interaction. Based on these and the residual emission between the HIPASS and the ATCA data, we propose an interaction between the two BCDs as the origin of their recently enhanced star formation activity. The shear motion on the gas disk, potentially with re-accretion of the stripped gas, could be responsible for the active star formation in this BCD pair. The reduced datacube (FITS file) is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/605/A54

  6. Super-Eddington accretion on to the neutron star NGC 7793 P13: Broad-band X-ray spectroscopy and ultraluminous X-ray sources

    NASA Astrophysics Data System (ADS)

    Walton, D. J.; Fürst, F.; Harrison, F. A.; Stern, D.; Bachetti, M.; Barret, D.; Brightman, M.; Fabian, A. C.; Middleton, M. J.; Ptak, A.; Tao, L.

    2018-02-01

    We present a detailed, broad-band X-ray spectral analysis of the ultraluminous X-ray source (ULX) pulsar NGC 7793 P13, a known super-Eddington source, utilizing data from the XMM-Newton, NuSTAR and Chandra observatories. The broad-band XMM-Newton+NuSTAR spectrum of P13 is qualitatively similar to the rest of the ULX sample with broad-band coverage, suggesting that additional ULXs in the known population may host neutron star accretors. Through time-averaged, phase-resolved and multi-epoch studies, we find that two non-pulsed thermal blackbody components with temperatures ∼0.5 and 1.5 keV are required to fit the data below 10 keV, in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column. The characteristic radii of the thermal components appear to be comparable, and are too large to be associated with the neutron star itself, so the need for two components likely indicates the accretion flow outside the magnetosphere is complex. We suggest a scenario in which the thick inner disc expected for super-Eddington accretion begins to form, but is terminated by the neutron star's magnetic field soon after its onset, implying a limit of B ≲ 6 × 1012 G for the dipolar component of the central neutron star's magnetic field. Evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ULXs. Finally, we examine the spectrum exhibited by P13 during one of its unusual 'off' states. These data require both a hard power-law component, suggesting residual accretion on to the neutron star, and emission from a thermal plasma, which we argue is likely associated with the P13 system.

  7. Constraining black holes with light boson hair and boson stars using epicyclic frequencies and quasiperiodic oscillations

    NASA Astrophysics Data System (ADS)

    Franchini, Nicola; Pani, Paolo; Maselli, Andrea; Gualtieri, Leonardo; Herdeiro, Carlos A. R.; Radu, Eugen; Ferrari, Valeria

    2017-06-01

    Light bosonic fields are ubiquitous in extensions of the Standard Model. Even when minimally coupled to gravity, these fields might evade the assumptions of the black-hole no-hair theorems and give rise to spinning black holes which can be drastically different from the Kerr metric. Furthermore, they allow for self-gravitating compact solitons, known as (scalar or Proca) boson stars. The quasiperiodic oscillations (QPOs) observed in the x-ray flux emitted by accreting compact objects carry information about the strong-field region, thus providing a powerful tool to constrain deviations from Kerr's geometry and to search for exotic compact objects. By using the relativistic precession model as a proxy to interpret the QPOs in terms of geodesic frequencies, we investigate how the QPO frequencies could be used to test the no-hair theorem and the existence of light bosonic fields near accreting compact objects. We show that a detection of two QPO triplets with current sensitivity can already constrain these models and that the future eXTP mission or a LOFT-like mission can set very stringent constraints on black holes with bosonic hair and on (scalar or Proca) boson stars. The peculiar geodesic structure of compact scalar/Proca boson stars implies that these objects can easily be ruled out as alternative models for x-ray source GRO J1655-40.

  8. On the nature of the symbiotic star BF Cygni

    NASA Technical Reports Server (NTRS)

    Mikolajewska, J.; Mikolajewski, M.; Kenyon, S. J.

    1989-01-01

    Optical and ultraviolet spectroscopy of the symbiotic binary BF Cyg obtained during 1979-1988 is discussed. This system consists of a low-mass M5 giant filling about 50 percent of its tidal volume and a hot, luminous compact object similar to the central star of a planetary nebula. The binary is embedded in an asymmetric nebula which includes a small, high-density region and an extended region of lower density. The larger nebula is formed by a slow wind ejected by the cool component and ionized by the hot star, while the more compact nebula is material expelled by the hot component in the form of a bipolar wind. The analysis indicates that disk accretion is essential to maintain the nuclear burning shell of the hot star.

  9. Effect of non-stationary accretion on spectral state transitions: An example of a persistent neutron star LMXB 4U1636–536

    NASA Astrophysics Data System (ADS)

    Zhang, Hui; Yu, Wen-Fei

    2018-03-01

    Observations of black hole and neutron star X-ray binaries show that the luminosity of the hard-to-soft state transition is usually higher than that of the soft-to-hard state transition, indicating additional parameters other than mass accretion rate are required to interpret spectral state transitions. It has been found in some individual black hole or neutron star soft X-ray transients that the luminosity corresponding to the hard-to-soft state transition is positively correlated with the peak luminosity of the following soft state. In this work, we report the discovery of the same correlation in the single persistent neutron star low mass X-ray binary (LMXB) 4U 1636–536 based on data from the All Sky Monitor (ASM) on board RXTE, the Gas Slit Camera (GSC) on board MAXI and the Burst Alert Telescope (BAT) on board Swift. We also found such a positive correlation holds in this persistent neutron star LMXB in a luminosity range spanning about a factor of four. Our results indicate that non-stationary accretion also plays an important role in driving X-ray spectral state transitions in persistent accreting systems with small accretion flares, which is much less dramatic compared with the bright outbursts seen in many Galactic LMXB transients.

  10. High-Precision Studies of Compact Variable Stars

    NASA Astrophysics Data System (ADS)

    Bloemen, Steven

    2014-10-01

    This book, which is a reworked and updated version of Steven Bloemen's original PhD thesis, reports on several high-precision studies of compact variable stars. Its strength lies in the large variety of observational, theoretical and instrumentation techniques that are presented and used and paves the way towards new and detailed asteroseismic applications of single and binary subdwarf stars. Close binary stars are studied using high cadence spectroscopic datasets collected with state of the art electron multiplying CCDs and analysed using Doppler tomography visualization techniques. The work touches upon instrumentation, presenting the calibration of a new fast, multi-colour camera installed at the Mercator Telescope on La Palma. The thesis also includes theoretical work on the computation of the temperature range in which stellar oscillations can be driven in subdwarf B-stars. Finally, the highlight of the thesis is the measurement of velocities of stars using only photometric data from NASA's Kepler satellite. Doppler beaming causes stars to appear slightly brighter when they move towards us in their orbits, and this subtle effect can be seen in Kepler's brightness measurements. The thesis presents the first validation of such velocity measurements using independent spectroscopic measurements. Since the detection and validation of this Doppler beaming effect, it has been used in tens of studies to detect and characterize binary star systems, which are key calibrators in stellar astronomy.

  11. An Accretion Model for Anomalous X-Ray Pulsars

    NASA Astrophysics Data System (ADS)

    Chatterjee, Pinaki; Hernquist, Lars; Narayan, Ramesh

    2000-05-01

    We present a model for the anomalous X-ray pulsars (AXPs) in which the emission is powered by accretion from a fossil disk, established from matter falling back onto the neutron star following its birth. The time-dependent accretion drives the neutron star toward a ``tracking'' solution in which the rotation period of the star increases slowly, in tandem with the declining accretion rate. For appropriate choices of disk mass, neutron star magnetic field strength, and initial spin period, we demonstrate that a rapidly rotating neutron star can be spun down to periods characteristic of AXPs on timescales comparable to the estimated ages of these sources. In other cases, accretion onto the neutron star switches off after a short time and the star becomes an ordinary radio pulsar. Thus, in our picture, radio pulsars and AXPs are drawn from the same underlying population, in contrast to the situation in models involving neutron stars with ultrastrong magnetic fields, which require a new population of stars with very different properties.

  12. Jet-induced star formation by accreting black holes: impact on stellar, galaxy, and cosmic evolution

    NASA Astrophysics Data System (ADS)

    Mirabel, Igor Felix

    2016-07-01

    Evidence that relativistic jets trigger star formation along their axis has been found associated to low redshift and high redshift accreting supermassive black holes. However, the physical processes by which jet-cloud interaction may trigger star formation has so far not been elucidated. To gain insight into this potentially important star formation mechanism during reionization, when microquasars were form prolifically before AGN, our international team is carrying out a muliwavelength study of a microquasar jet-induced star formation region in the Milky Way using data from space missions (Chandra, Integral, ISO, Herschel) and from the ground (at cm and mm wavelengths with the VLA and IRAM, and IR with Gemini and VLT). I will show that this relative nearby star forming region is an ideal laboratory to test models of jet-induced star formation elsewhere in the universe.

  13. NuSTAR constraints on coronal cutoffs in Swift-BAT selected Seyfert 1 AGN

    NASA Astrophysics Data System (ADS)

    Kamraj, Nikita; Harrison, Fiona; Balokovic, Mislav; Brightman, Murray; Stern, Daniel

    2017-08-01

    The continuum X-ray emission from Active Galactic Nuclei (AGN) is believed to originate in a hot, compact corona above the accretion disk. Compton upscattering of UV photons from the inner accretion disk by coronal electrons produces a power law X-ray continuum with a cutoff at energies determined by the electron temperature. The NuSTAR observatory, with its high sensitivity in hard X-rays, has enabled detailed broadband modeling of the X-ray spectra of AGN, thereby allowing tight constraints to be placed on the high-energy cutoff of the X-ray continuum. Recent detections of low cutoff energies in Seyfert 1 AGN in the NuSTAR band have motivated us to pursue a systematic search for low cutoff candidates in Swift-BAT detected Seyfert 1 AGN that have been observed with NuSTAR. We use our constraints on the cutoff energy to map out the location of these sources on the compactness - temperature diagram for AGN coronae, and discuss the implications of low cutoff energies for the cooling and thermalization mechanisms in the corona.

  14. Dark matter, neutron stars, and strange quark matter.

    PubMed

    Perez-Garcia, M Angeles; Silk, Joseph; Stone, Jirina R

    2010-10-01

    We show that self-annihilating weakly interacting massive particle (WIMP) dark matter accreted onto neutron stars may provide a mechanism to seed compact objects with long-lived lumps of strange quark matter, or strangelets, for WIMP masses above a few GeV. This effect may trigger a conversion of most of the star into a strange star. We use an energy estimate for the long-lived strangelet based on the Fermi-gas model combined with the MIT bag model to set a new limit on the possible values of the WIMP mass that can be especially relevant for subdominant species of massive neutralinos.

  15. A new anisotropic compact star model having Matese & Whitman mass function

    NASA Astrophysics Data System (ADS)

    Bhar, Piyali; Ratanpal, B. S.

    2016-07-01

    Present paper proposed a new singularity free model of anisotropic compact star. The Einstein field equations are solved in closed form by utilizing Matese & Whitman mass function. The model parameters ρ, pr and pt all are well behaved inside the stellar interior and our model satisfies all the required conditions to be physically acceptable. The model given in the present work is compatible with observational data of compact objects like SAX J 1808.4-3658 (SS1), SAX J 1808.4-3658 (SS2) and 4U 1820-30. A particular model of 4U 1820-30 is studied in detail and found that it satisfies all the condition needed for physically acceptable model. The present work is the generalization of Sharma and Ratanpal (Int. J. Mod. Phys. D 22:1350074, 2013) model for compact stars admitting quadratic equation of state.

  16. Unsteady Plasma Ejections from Hollow Accretion Columns of Galactic Neutron Stars as a Trigger for Gamma-Ray Bursts

    NASA Astrophysics Data System (ADS)

    Gvaramadze, V. V.

    1995-09-01

    We propose a model of gamma-ray bursts (GRBs) based on close Galactic neutron stars with accretion disks. We outline a simple mechanism of unsteady plasma ejections during episodic accretion events. The relative kinetic energy of ejected blobs can be converted into gamma-rays by internal shocks. The beaming of gamma-ray emission can be responsible for the observed isotropic angular distribution of GRBs.

  17. Three-dimensional simulations of the interaction between the nova ejecta, accretion disk, and companion star

    NASA Astrophysics Data System (ADS)

    Figueira, Joana; José, Jordi; García-Berro, Enrique; Campbell, Simon W.; García-Senz, Domingo; Mohamed, Shazrene

    2018-05-01

    Context. Classical novae are thermonuclear explosions hosted by accreting white dwarfs in stellar binary systems. Material piles up on top of the white dwarf star under mildly degenerate conditions, driving a thermonuclear runaway. The energy released by the suite of nuclear processes operating at the envelope, mostly proton-capture reactions and β+-decays, heats the material up to peak temperatures ranging from 100 to 400 MK. In these events, about 10-3-10-7 M⊙, enriched in CNO and, sometimes, other intermediate-mass elements (e.g., Ne, Na, Mg, and Al) are ejected into the interstellar medium. Aims: To date, most of the efforts undertaken in the modeling of classical nova outbursts have focused on the early stages of the explosion and ejection, ignoring the interaction of the ejecta, first with the accretion disk orbiting the white dwarf and ultimately with the secondary star. Methods: A suite of 3D, smoothed-particle hydrodynamics (SPH) simulations of the interaction between the nova ejecta, accretion disk, and stellar companion were performed to fill this gap; these simulations were aimed at testing the influence of the model parameters—that is, the mass and velocity of the ejecta, mass and the geometry of the accretion disk—on the dynamical and chemical properties of the system. Results: We discuss the conditions that lead to the disruption of the accretion disk and to mass loss from the binary system. In addition, we discuss the likelihood of chemical contamination of the stellar secondary induced by the impact with the nova ejecta and its potential effect on the next nova cycle. Movies showing the full evolution of several models are available online at http://https://www.aanda.org and at http://www.fen.upc.edu/users/jjose/Downloads.html

  18. Identifying the donor star of the most extreme ULX pulsar

    NASA Astrophysics Data System (ADS)

    Heida, Marianne

    2017-08-01

    Ultraluminous X-ray sources (ULXs) were once among the most promising candidates for long sought after intermediate-mass black holes, owing to their high X-ray luminosities (>10^39 erg/s) and off-nuclear positions. NGC 5907 ULX-1 was a prime example, and since it regularly reaches 10^41 erg/s it was thought to harbour a black hole with a mass of at least 500 solar masses. But in an astonishing discovery, the source was found to exhibit pulsations in the X-rays on second-timescales, revealing it to be a pulsar powered by accretion onto a neutron star with only 1.4 solar masses. This discovery challenges every known theory of accretion onto a compact object, which in this object exceeds the Eddington limit by a factor of 500. It requires us to imagine extreme departures from known accretion theory and/or binary evolution scenarios. The fuel source should be a massive companion star in order to sustain the required mass accretion rate, however X-ray timing favors a low-mass star. With the ability to detect a massive star, a short HST/WFC3 NIR observation would solve this mystery. A detection of a supergiant donor would open the path to future dynamical mass measurements with JWST, while a non-detection would prove that this extreme ULX pulsar contains a low-mass donor star, forcing us to consider new evolutionary formation channels.

  19. NICER Eyes on Bursting Stars

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2018-03-01

    What happens to a neutron stars accretion disk when its surface briefly explodes? A new instrument recently deployed at the International Space Station (ISS) is now watching bursts from neutron stars and reporting back.Deploying a New X-Ray MissionLaunch of NICER aboard a Falcon 9 rocket in June 2017. [NASA/Tony Gray]In early June of 2017, a SpaceX Dragon capsule on a Falcon 9 rocket launched on a resupply mission to the ISS. The pressurized interior of the Dragon contained the usual manifest of crew supplies, spacewalk equipment, and vehicle hardware. But the unpressurized trunk of the capsule held something a little different: the Neutron star Interior Composition Explorer (NICER).In the two weeks following launch, NICER was extracted from the SpaceX Dragon capsule and installed on the ISS. And by the end of the month, the instrument was already collecting its first data set: observations of a bright X-ray burst from Aql X-1, a neutron star accreting matter from a low-mass binary companion.Impact of BurstsNICERs goal is to provide a new view of neutron-star physics at X-ray energies of 0.212 keV a window that allows us to explore bursts of energy that neutron stars sometimes emit from their surfaces.Artists impression of an X-ray binary, in which a compact object accretes material from a companion star. [ESA/NASA/Felix Mirabel]In X-ray burster systems, hydrogen- and helium-rich material from a low-mass companion star piles up in an accretion disk around the neutron star. This material slowly funnels onto the neutron stars surface, forming a layer that gravitationally compresses and eventually becomes so dense and hot that runaway nuclear fusion ignites.Within seconds, the layer of material is burned up, producing a burst of emission from the neutron star that outshines even the inner regions of the hot accretion disk. Then more material funnels onto the neutron star and the process begins again.Though we have a good picture of the physics that causes these bursts

  20. Neutron stars and millisecond pulsars from accretion-induced collapse in globular clusters

    NASA Technical Reports Server (NTRS)

    Bailyn, Charles D.; Grindlay, Jonathan E.

    1990-01-01

    This paper examines the limits on the number of millisecond pulsars which could be formed in globular clusters by the generally accepted scenario (in which a neutron star is created by the supernova of an initially massive star and subsequently captures a companion to form a low-mass X-ray binary which eventually becomes a millisecond pulsar). It is found that, while the number of observed low-mass X-ray binaries can be adequately explained in this way, the reasonable assumption that the pulsar luminosity function in clusters extends below the current observational limits down to the luminosity of the faintest millisecond pulsars in the field suggests a cluster population of millisecond pulsars which is substantially larger than the standard model can produce. Alleviating this problem by postulating much shorter lifetimes for the X-ray binaries requires massive star populations sufficiently large that the mass loss resulting from their evolution would be likely to unbind the cluster. It is argued that neutron star formation in globular clusters by accretion-induced collapse of white dwarfs may resolve the discrepancy in birthrates.

  1. [Nucleosynthesis, Rotation and Magnetism in Accreting Neutron Stars

    NASA Technical Reports Server (NTRS)

    Bildsten, Lars

    2004-01-01

    This is my final report on the NASA ATP grant on nucleosynthesis, rotation and magnetism in accreting neutron stars (NAG5-8658). In my last two reports, I summarized the science that I have accomplished, which covered a large range of topics. For this report, I want to point out the graduate students that were partially supported on this grant and where they are now. Andrew Cumming is an Assistant Professor of Physics at McGill University, Greg Ushomirsky is a researcher at MIT s Lincoln Laboratories, Dean Townsley is a postdoctoral researcher at Univ. of Chicago, Chris Deloye is a postdoctoral researcher at Northwestern University. The other two students, Phil Chang and Tony Piro, are still at UCSB and will be completing their PhD s in Summer 05 and Summer 06.

  2. Magnetically gated accretion in an accreting 'non-magnetic' white dwarf.

    PubMed

    Scaringi, S; Maccarone, T J; D'Angelo, C; Knigge, C; Groot, P J

    2017-12-13

    White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15 per cent of these binaries, the magnetic field of the white dwarf is strong enough (at 10 6 gauss or more) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as 'non-magnetic', because until now there has been no evidence that they have a magnetic field that is strong enough to affect the accretion dynamics. Here we report an analysis of archival optical observations of the 'non-magnetic' accreting white dwarf in the binary system MV Lyrae, whose light curve displays quasi-periodic bursts of about 30 minutes duration roughly every 2 hours. The timescale and amplitude of these bursts indicate the presence of an unstable, magnetically regulated accretion mode, which in turn implies the existence of magnetically gated accretion, in which disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyrae of between 2 × 10 4 gauss and 1 × 10 5 gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cycles have been identified.

  3. General relativistic spectra of accretion discs around rapidly rotating neutron stars: effect of light bending

    NASA Astrophysics Data System (ADS)

    Bhattacharyya, Sudip; Bhattacharya, Dipankar; Thampan, Arun V.

    2001-08-01

    We present computed spectra, as seen by a distant observer, from the accretion disc around a rapidly rotating neutron star. Our calculations are carried out in a fully general relativistic framework, with an exact treatment of rotation. We take into account the Doppler shift, gravitational redshift and light-bending effects in order to compute the observed spectrum. We find that light bending significantly modifies the high-energy part of the spectrum. Computed spectra for slowly rotating neutron stars are also presented. These results would be important for modelling the observed X-ray spectra of low-mass X-ray binaries containing fast-spinning neutron stars.

  4. Giant Planets around FGK Stars Probably Form through Core Accretion

    NASA Astrophysics Data System (ADS)

    Wang, Wei; Wang, Liang; Li, Xiang; Chen, Yuqin; Zhao, Gang

    2018-06-01

    We present a statistical study of the planet–metallicity (P–M) correlation by comparing the 744 stars with candidate planets (SWPs) in the Kepler field that have been observed with LAMOST, and a sample of distance-independent, fake “twin” stars in the Kepler field with no planet reported (CKSNPs) yet. With well-defined and carefully selected large samples, we find for the first time a turnoff P–M correlation of Δ[Fe/H]SWPs–SNPs, which on average increases from ∼0.00 ± 0.03 dex to 0.06 ± 0.03 dex, and to 0.12 ± 0.03 for stars with Earth-, Neptune-, and Jupiter-sized planets successively, and then declines to ∼‑0.01 ± 0.03 dex for more massive planets or brown dwarfs. Moreover, the percentage of those systems with positive Δ[Fe/H] has the same turnoff pattern. We also find that FG-type stars follow this general trend, but K-type stars are different. Moderate metal enhancement (∼0.1–0.2 dex) for K-type stars with planets of radii between 2 and 4 R ⊕, compared to CKSNPs is observed, which indicates much higher metallicities are required for Super-Earths and Neptune-sized planets to form around K-type stars. We point out that the P–M correlation is actually metallicity-dependent, i.e., the correlation is positive at solar and supersolar metallicities, and negative at subsolar metallicities. No steady increase of Δ[Fe/H] against planet sizes is observed for rocky planets, excluding the pollution scenario as a major mechanism for the P–M correlation. All these clues suggest that giant planets probably form differently from rocky planets or more massive planets/brown dwarfs, and the core accretion scenario is highly favored, and high metallicity is a prerequisite for massive planets to form.

  5. Magnetocentrifugally Driven Flows from Young Stars and Disks. IV. The Accretion Funnel and Dead Zone

    NASA Astrophysics Data System (ADS)

    Ostriker, Eve C.; Shu, Frank H.

    1995-07-01

    We formulate the time-steady, axisymmetric problem of stellar magnetospheric inflow of gas from a surrounding accretion disk. The computational domain is bounded on the outside by a surface of given shape containing the open field lines associated with an induced disk wind. The mechanism for this wind has been investigated in previous publications in this journal. Our zeroth-order solution incorporates an acceptable accounting of the pressure balance between the magnetic field lines loaded with accreting gas (funnel flow) and those empty of matter (dead zone). In comparison with previous models, our funnel-flow/dead-zone solution has the following novel features: (1) Because of a natural tendency for the trapped stellar magnetic flux to pinch toward the corotation radius Rx (X-point of the effective potential), most of the interesting magnetohydrodynamics is initiated within a small neighborhood of Rx (X-region), where the Keplerian angular speed of rotation in the disk equals the spin rate of the star. (2) Unimpeded funnel flow from the inner portion of the X-region to the star can occur when the amount of trapped magnetic flux equals or exceeds 1.5 times the unperturbed dipole flux that would lie outside Rx in the absence of an accretion disk. (3). Near the equatorial plane, radial infall from the X-point is terminated at a "kink" point Rk = 0.74Rx that deflects the flow away from the midplane, mediating thereby between the field topology imposed by a magnetic fan of trapped flux at Rx and the geometry of a strong stellar dipole. (4) The excess angular momentum of accretion that would otherwise spin up the star rapidly is deposited by the magnetic torques of the funnel flow into the inner portion of the X-region of the disk. (5) An induced disk wind arises in the outer portion of the .X-region, where the stellar field lines have been blown open, and removes whatever excess angular momentum that viscous torques do not transport to the outer disk. (6) The interface

  6. Production of the entire range of r-process nuclides by black hole accretion disc outflows from neutron star mergers

    NASA Astrophysics Data System (ADS)

    Wu, Meng-Ru; Fernández, Rodrigo; Martínez-Pinedo, Gabriel; Metzger, Brian D.

    2016-12-01

    We consider r-process nucleosynthesis in outflows from black hole accretion discs formed in double neutron star and neutron star-black hole mergers. These outflows, powered by angular momentum transport processes and nuclear recombination, represent an important - and in some cases dominant - contribution to the total mass ejected by the merger. Here we calculate the nucleosynthesis yields from disc outflows using thermodynamic trajectories from hydrodynamic simulations, coupled to a nuclear reaction network. We find that outflows produce a robust abundance pattern around the second r-process peak (mass number A ˜ 130), independent of model parameters, with significant production of A < 130 nuclei. This implies that dynamical ejecta with high electron fraction may not be required to explain the observed abundances of r-process elements in metal poor stars. Disc outflows reach the third peak (A ˜ 195) in most of our simulations, although the amounts produced depend sensitively on the disc viscosity, initial mass or entropy of the torus, and nuclear physics inputs. Some of our models produce an abundance spike at A = 132 that is absent in the Solar system r-process distribution. The spike arises from convection in the disc and depends on the treatment of nuclear heating in the simulations. We conclude that disc outflows provide an important - and perhaps dominant - contribution to the r-process yields of compact binary mergers, and hence must be included when assessing the contribution of these systems to the inventory of r-process elements in the Galaxy.

  7. Quasispherical subsonic accretion in X-ray pulsars

    NASA Astrophysics Data System (ADS)

    Shakura, Nikolai I.; Postnov, Konstantin A.; Kochetkova, A. Yu; Hjalmarsdotter, L.

    2013-04-01

    A theoretical model is considered for quasispherical subsonic accretion onto slowly rotating magnetized neutron stars. In this regime, the accreting matter settles down subsonically onto the rotating magnetosphere, forming an extended quasistatic shell. Angular momentum transfer in the shell occurs via large-scale convective motions resulting, for observed pulsars, in an almost iso-angular-momentum \\omega \\sim 1/R^2 rotation law inside the shell. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instabilities, with allowance for cooling. A settling accretion regime is possible for moderate accretion rates \\dot M \\lesssim \\dot M_* \\simeq 4\\times 10^{16} g s ^{-1}. At higher accretion rates, a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and the accretion becomes highly nonstationary. Observations of spin-up/spin-down rates of quasispherically wind accreting equilibrium X-ray pulsars with known orbital periods (e.g., GX 301-2 and Vela X-1) enable us to determine the main dimensionless parameters of the model, as well as to estimate surface magnetic field of the neutron star. For equilibrium pulsars, the independent measurements of the neutron star magnetic field allow for an estimate of the stellar wind velocity of the optical companion without using complicated spectroscopic measurements. For nonequilibrium pulsars, a maximum value is shown to exist for the spin-down rate of the accreting neutron star. From observations of the spin-down rate and the X-ray luminosity in such pulsars (e.g., GX 1+4, SXP 1062, and 4U 2206+54), a lower limit can be put on the neutron star magnetic field, which in all cases turns out to be close to the standard value and which agrees with cyclotron line measurements. Furthermore, both explains the spin-up/spin-down of the pulsar frequency on large time-scales and also accounts for the irregular short

  8. Local Swift-BAT active galactic nuclei prefer circumnuclear star formation

    NASA Astrophysics Data System (ADS)

    Lutz, D.; Shimizu, T.; Davies, R. I.; Herrera-Camus, R.; Sturm, E.; Tacconi, L. J.; Veilleux, S.

    2018-01-01

    We use Herschel data to analyze the size of the far-infrared 70 μm emission for z < 0.06 local samples of 277 hosts of Swift-BAT selected active galactic nuclei (AGN), and 515 comparison galaxies that are not detected by BAT. For modest far-infrared luminosities 8.5 compact distribution of star formation (and hence gas) in the AGN hosts, but compact AGN heated dust may contribute in some extremely AGN dominated systems. Our findings are in support of an AGN-host coevolution where accretion onto the central black hole and star formation are fed from the same gas reservoir, with more efficient black hole feeding if that reservoir is more concentrated. The significant scatter in the far-infrared sizes emphasizes that we are mostly probing spatial scales much larger than those of actual accretion, and that rapid accretion variations can smear the distinction between the AGN and comparison categories. Large samples are hence needed to detect structural differences that favor feeding of the black hole. No size difference between AGN host and comparison galaxies is observed at higher far-infrared luminosities log(LFIR [L⊙]) > 10.5 (star formation rates ≳6 M⊙ yr-1), possibly because these are typically reached in more compact regions. Full Table A.1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/609/A9

  9. Metallicity inhomogeneities in local star-forming galaxies as a sign of recent metal-poor gas accretion

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Sánchez Almeida, J.; Morales-Luis, A. B.; Muñoz-Tuñón, C.

    2014-03-01

    We measure the oxygen metallicity of the ionized gas along the major axis of seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14, show ≅0.5 dex metallicity decrements in inner regions with enhanced star formation activity. This behavior is similar to the metallicity drop observed in a number of local tadpole galaxies by Sánchez Almeida et al., and was interpreted as showing early stages of assembling in disk galaxies, with the star formation sustained by external metal-poor gas accretion. The agreement with tadpoles has several implications. (1) It proves that galaxies other than the local tadpoles present the samemore » unusual metallicity pattern. (2) Our metallicity inhomogeneities were inferred using the direct method, thus discarding systematic errors usually attributed to other methods. (3) Taken together with the tadpole data, our findings suggest a threshold around one-tenth the solar value for the metallicity drops to show up. Although galaxies with clear metallicity drops are rare, the physical mechanism responsible for them may sustain a significant part of the star formation activity in the local universe. We argue that the star formation dependence of the mass-metallicity relationship, as well as other general properties followed by most local disk galaxies, is naturally interpreted as side effects of pristine gas infall. Alternatives to the metal-poor gas accretion are examined as well.« less

  10. Evolution of the accretion structure of the compact object in the symbiotic binary BF Cygni during outburst in 2009-2014

    NASA Astrophysics Data System (ADS)

    Tomov, N. A.; Tomova, M. T.; Bisikalo, D. V.

    2017-12-01

    The eclipsing symbiotic binary BF Cyg has had five orbital minima during its last optical outburst after 2006. The second minimum is much shallower than the first one and after that the minimum get deeper again. We determined the parameters of the accretion structure surrounding the compact object in two minima and traced its evolution until 2014. Moreover, we analysed the continuum of the system in the region of the UBVRCIC photometric bands to derive the parameters of its components at two times orbital maximum and calculated the mass-loss rate of the compact object. The results obtained allow us to conclude about the mechanism of fading of the optical light of the system until 2014. These results show that the optical flux of the outbursted compact object decreases because of "contraction" of its observed photosphere (pseudophotosphere) which, on its side, is due to increase of the velocity of its stellar wind, and the optical flux of the circumbinary nebula decreases mainly because of reduction of its mean density, which, on its side, is due to destruction of the accretion structure.

  11. Narrow-band Imagery with the Goddard Fabry-Perot: Probing the Epoch of Active Accretion for PMS Stars

    NASA Technical Reports Server (NTRS)

    Woodgate, Bruce E.; Grady, C.; Endres, M.; Williger, G.

    2006-01-01

    The STIS coronagraphic imaging sample of I'MS stars was surveyed with the Goddard Fabry-Perot (GFP) interferometer to determine what fraction of the stars drive jets, whether there is any difference in behavior for a group of intermediate-mass stars as compared with T Tauri stars, and to search for evolutionary effects. Compared to broad band imaging, the FGP achieves an emission-line nebulosity-to-star contrast gain of between 500 and 3000. To date, we have detected jets associated with classical T Tauri stars spanning a factor of 280 in mass accretion rate in approximately 50% of the STIS coronagraphic imaging sample. We also detected jets or Herbig-HARO knots associated with 5 Herbig Ae stars, all younger than 8 Myr, for a detection fraction which is smaller than the T Tauri survey.

  12. Jets in black-hole and neutron-star X-ray binaries

    NASA Astrophysics Data System (ADS)

    Kylafis, Nikolaos

    2016-07-01

    Jets have been observed from both neutron-star and black-hole X-ray binaries. There are many similarities between the two and a few differences. I will offer a physical explanation of the formation and destruction of jets from compact objects and I will discuss the similarities and differences in the two types. The basic concept in the physical explanation is the Cosmic Battery, the mechanism that creates the required magnetic field for the jet ejection. The Cosmic Battery operates efficiently in accretion flows consisting of an inner hot flow and an outer thin accretion disk, independently of the nature of the compact object. It is therefore natural to always expect a jet in the right part of a spectral hardness - luminosity diagram and to never expect a jet in the left part. As a consequence, most of the phenomenology of an outburst can be explained with only one parameter, the mass accretion rate.

  13. Gamma-ray bursts from accreting black holes in neutron star mergers

    NASA Astrophysics Data System (ADS)

    Ruffert, M.; Janka, H.-Th.

    1999-04-01

    By means of three-dimensional hydrodynamic simulations with a Eulerian PPM code we investigate the formation and the properties of the accretion torus around the stellar mass black hole which we assume to originate from the remnant of a neutron star merger within the dynamical time scale of a few milliseconds. The simulations are performed with four nested cartesian grids which allow for both a good resolution near the central black hole and a large computational volume. They include the use of a physical equation of state as well as the neutrino emission from the hot matter of the torus. The gravity of the black hole is described with a Newtonian and alternatively with a Paczyński-Wiita potential. In a post-processing step, we evaluate our models for the energy deposition by nu bar nu annihilation around the accretion torus. We find that the torus formed after neutron star merging has a mass between several 10(-2} M_{sun) and a few 10(-1} M_{sun) with maximum densities around 10(12) g cm(-3) and maximum temperatures of about 10 MeV (entropies around 5 k_B per nucleon). Correspondingly, the neutrino emission is huge with a total luminosity near 10(53) erg s(-1) . Neutrino-antineutrino annihilation deposits energy in the vicinity of the torus at a rate of (3-5)x 10(50) erg s(-1) . It is most efficient near the rotation axis where 10 to 30% of this energy or up to a total of 10(49) erg are dumped within an estimated emission period of 0.02-0.1 s in a region with a low integral baryonic mass of about 10(-5} M_{sun) . This baryon pollution is still dangerously high, and the estimated maximum relativistic Lorentz factors Gamma -1 are around unity. The conversion of neutrino energy into a pair plasma, however, is sufficiently powerful to blow out the baryons along the axis so that a clean funnel should be produced within only milliseconds. Our models show that accretion on the black hole formed after neutron star merging can yield enough energy by nu bar nu annihilation

  14. Magnetically gated accretion in an accreting ‘non-magnetic’ white dwarf

    NASA Astrophysics Data System (ADS)

    Scaringi, S.; Maccarone, T. J.; D’Angelo, C.; Knigge, C.; Groot, P. J.

    2017-12-01

    White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15 per cent of these binaries, the magnetic field of the white dwarf is strong enough (at 106 gauss or more) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as ‘non-magnetic’, because until now there has been no evidence that they have a magnetic field that is strong enough to affect the accretion dynamics. Here we report an analysis of archival optical observations of the ‘non-magnetic’ accreting white dwarf in the binary system MV Lyrae, whose light curve displays quasi-periodic bursts of about 30 minutes duration roughly every 2 hours. The timescale and amplitude of these bursts indicate the presence of an unstable, magnetically regulated accretion mode, which in turn implies the existence of magnetically gated accretion, in which disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyrae of between 2 × 104 gauss and 1 × 105 gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cycles have been identified.

  15. The optical counterpart to the new accreting pulsar Swift J0243.6+6124 is a Be star

    NASA Astrophysics Data System (ADS)

    Kouroubatzakis, K.; Reig, P.; Andrews, J.; ), A. Zezas

    2017-10-01

    We report optical spectroscopic observations of the optical counterpart to the 9.87-s accreting neutron star transient Swift J0243.6+6124 (ATel#10809, ATel#10812) from the 1.3-m telescope of the Skinakas Observatory (Greece).

  16. Accretion onto CO White Dwarfs using MESA

    NASA Astrophysics Data System (ADS)

    Feng, Wanda; Starrfield, Sumner

    2018-06-01

    The nature of type Ia Supernovae (SNe Ia) progenitor systems and their underlying mechanism are not well understood. There are two competing progenitor scenarios: the single-degenerate scenario wherein a white dwarf (WD) star accretes material from a companion star, reaching the Chandrasekhar mass limit; and, the double-degenerate scenario wherein two WDs merge. In this study, we investigate the single-degenerate scenario by accretion onto carbon-oxygen (CO) WDs using the Modules for Experiments in Stellar Astrophysics (MESA). We vary the WD mass, composition of the accreting material, and accretion rate in our models. Mixing between the accreted material and the WD core is informed by multidimensional studies that suggest occurance after thermonuclear runaway (TNR) ensues. We compare the accretion of solar composition material onto CO WDs with the accretion of mixed solar and core material after TNR. As many of our models eject less material than accreted, our study supports that accretion onto CO WDs is a feasible channel for SNe I progenitors.

  17. Accreting Compact Object at the Center of the Supernova Remnant RCW 103.

    NASA Astrophysics Data System (ADS)

    Sanwal, D.; Garmire, G. P.; Garmire, A.; Pavlov, G. G.; Mignani, R.

    2002-05-01

    We observed the radio-quiet central compact object of the supernova remnant RCW 103 with the Chandra ACIS during 13.8 hours on 2002 March 3, when the source was in high state, with a time-averaged flux of 8*E-12 erg cm-2 s-1 in the 0.5--8.0 keV band. The complex light curve of the source shows a period of about 6.4 hours and two partial eclipses or dips per period, separated by 180o in phase. The variability of the source proves that it is powered by accretion, likely from a low-mass companion in a binary system. Deep near-IR observations of the source with VLT suggest a potential counterpart of the compact object about 2'' from the nominal Chandra position. The magnitudes of the potential counterpart are J ≈ 22.3, H ≈ 19.6, and Ks ≈ 18.5, with an uncertainty of about 0.5 mag. We will discuss possible interpretations of the observational results. This work was partially supported by NASA grants NAS8-01128 and NAG5-10865.

  18. The unusual gamma-ray burst GRB 101225A explained as a minor body falling onto a neutron star.

    PubMed

    Campana, S; Lodato, G; D'Avanzo, P; Panagia, N; Rossi, E M; Della Valle, M; Tagliaferri, G; Antonelli, L A; Covino, S; Ghirlanda, G; Ghisellini, G; Melandri, A; Pian, E; Salvaterra, R; Cusumano, G; D'Elia, V; Fugazza, D; Palazzi, E; Sbarufatti, B; Vergani, S D

    2011-11-30

    The tidal disruption of a solar-mass star around a supermassive black hole has been extensively studied analytically and numerically. In these events, the star develops into an elongated banana-shaped structure. After completing an eccentric orbit, the bound debris falls into the black hole, forming an accretion disk and emitting radiation. The same process may occur on planetary scales if a minor body passes too close to its star. In the Solar System, comets fall directly into our Sun or onto planets. If the star is a compact object, the minor body can become tidally disrupted. Indeed, one of the first mechanisms invoked to produce strong gamma-ray emission involved accretion of comets onto neutron stars in our Galaxy. Here we report that the peculiarities of the 'Christmas' gamma-ray burst (GRB 101225A) can be explained by a tidal disruption event of a minor body around an isolated Galactic neutron star. This would indicate either that minor bodies can be captured by compact stellar remnants more frequently than occurs in the Solar System or that minor-body formation is relatively easy around millisecond radio pulsars. A peculiar supernova associated with a gamma-ray burst provides an alternative explanation.

  19. ULX spectra revisited: Accreting, highly magnetized neutron stars as the engines of ultraluminous X-ray sources

    NASA Astrophysics Data System (ADS)

    Koliopanos, Filippos; Vasilopoulos, Georgios; Godet, Olivier; Bachetti, Matteo; Webb, Natalie A.; Barret, Didier

    2017-12-01

    Aims: In light of recent discoveries of pulsating ultraluminous X-ray sources (ULXs) and recently introduced theoretical schemes that propose neutron stars (NSs) as the central engines of ULXs, we revisit the spectra of eighteen well known ULXs, in search of indications that favour this newly emerging hypothesis. Methods: We examine the spectra from high-quality XMM-Newton and NuSTAR observations. We use a combination of elementary black body and multicolour disk black body (MCD) models, to diagnose the predictions of classic and novel theoretical models of accretion onto NSs. We re-interpret the well established spectral characteristics of ULXs in terms of accretion onto lowly or highly magnetised NSs, and explore the resulting parameter space for consistency. Results: We confirm the previously noted presence of the low-energy (≲6 keV) spectral rollover and argue that it could be interpreted as due to thermal emission. The spectra are well described by a double thermal model consisting of a "hot" (≳1 keV) and a "cool" (≲0.7 keV) multicolour black body (MCB). Under the assumption that the "cool" MCD emission originates in a disk truncated at the neutron star magnetosphere, we find that all ULXs in our sample are consistent with accretion onto a highly magnetised (B ≳ 1012 G) neutron star. We note a strong correlation between the strength of the magnetic field, the temperature of the "hot" thermal component and the total unabsorbed luminosity. Examination of the NuSTAR data supports this interpretation and also confirms the presence of a weak, high-energy (≳15 keV) tail, most likely the result of modification of the MCB emission by inverse Compton scattering. We also note that the apparent high-energy tail, may simply be the result of mismodelling of MCB emission with an atypical temperature (T) versus radius (r) gradient, using a standard MCD model with a fixed gradient of T r-0.75. Conclusions: We have offered a new and robust physical interpretation for

  20. Variability of FUV Emission Line in Classical T Tauri Stars as a Diagnostic for Disc Accretion

    NASA Astrophysics Data System (ADS)

    Ramkumar, B.; Johns-Krull, C. M.

    2005-12-01

    We present our results of FUV emission line variability studies done on four classical T Tauri stars. We have used the IUE Final Archive spectra of pre-main sequence stars to analyze the sample of four stars BP Tau, DR Tau, RU Lup and RY Tau where each of these low-resolution (R ˜6 Å) spectra was observed in the IUE short-wavelength band pass (1150--1980Å). Given a broad time line of multiple observations being available from the IUE Final archive, an intrinsic variability study has been possible with this sample. Our results indicate that the transition region lines \\ion{Si}{4} and \\ion{C}{4}, produced near the accretion shocks at ˜105 K, have a strong correlation between them in all four stars except DR Tau. We also observe a strong correlation between \\ion{C}{4} & \\ion{He}{2} on our entire sample with a correlation coefficient of 0.549 (false alarm probability = 7.9 x 10-2) or higher. In addition, \\ion{He}{2} correlates with the molecular hydrogen (1503Å) line in all but RU Lup. If the \\ion{He}{2} lines are produced because of X-ray ionization then the observed molecular hydrogen emission is indeed controlled by X-ray ionization and therefore \\ion{He}{2} could serve as an X-ray proxy for future studies. Also, our correlation results strengthen the fact that \\ion{C}{4} is a good predictor of \\ion{Si}{4} and have a common origin i.e. in accretion shocks in the star formation process.

  1. X-Ray Emission from "Uranium" Stars

    NASA Technical Reports Server (NTRS)

    Schlegel, Eric; Mushotzky, Richard (Technical Monitor)

    2005-01-01

    The project aims to secure XMM observations of two targets with extremely low abundances of the majority of heavy elements (e.g., log[Fe/H] $\\sim$-4), but that show absorption lines of uranium. The presence of an r-process element such as uranium requires a binary star system in which the companion underwent a supernova explosion. A binary star system raises the distinct possibility of the existence of a compact object, most likely a neutron star, in the binary, assuming it survived the supernova blast. The presence of a compact object then suggests X-ray emission if sufficient matter accretes to the compact object. The observations were completed less than one year ago following a series of reobservations to correct for significant flaring that occurred during the original observations. The ROSAT all-sky survey was used to report on the initial assessment of X-ray emission from these objects; only upper limits were reported. These upper limits were used to justify the XMM observing time, but with the expectation that upper limits would merely be pushed lower. The data analysis hinges critically on the quality and degree of precision with which the background is handled. During the past year, I have spent some time learning the ins and outs of XMM data analysis. In the coming year, I can apply that learning to the analysis of the 'uranium' stars.

  2. On the radiation beaming of bright X-ray pulsars and constraints on neutron star mass-radius relation

    NASA Astrophysics Data System (ADS)

    Mushtukov, Alexander A.; Verhagen, Patrick A.; Tsygankov, Sergey S.; van der Klis, Michiel; Lutovinov, Alexander A.; Larchenkova, Tatiana I.

    2018-03-01

    The luminosity of accreting magnetized neutron stars can largely exceed the Eddington value due to appearance of accretion columns. The height of the columns can be comparable to the neutron star radius. The columns produce the X-rays detected by the observer directly and illuminate the stellar surface, which reprocesses the X-rays and causes additional component of the observed flux. The geometry of the column and the illuminated part of the surface determine the radiation beaming. Curved space-time affects the angular flux distribution. We construct a simple model of the beam patterns formed by direct and reflected flux from the column. We take into account the possibility of appearance of accretion columns, whose height is comparable to the neutron star radius. We argue that depending on the compactness of the star, the flux from the column can be either strongly amplified due to gravitational lensing, or significantly reduced due to column eclipse by the star. The eclipses of high accretion columns result in specific features in pulse profiles. Their detection can put constraints on the neutron star radius. We speculate that column eclipses are observed in X-ray pulsar V 0332+53, leading us to the conclusion of large neutron star radius in this system (˜15 km if M ˜ 1.4 M⊙). We point out that the beam pattern can be strongly affected by scattering in the accretion channel at high luminosity, which has to be taken into account in the models reproducing the pulse profiles.

  3. Low Gas Fractions Connect Compact Star-forming Galaxies to Their z ~ 2 Quiescent Descendants

    NASA Astrophysics Data System (ADS)

    Spilker, Justin S.; Bezanson, Rachel; Marrone, Daniel P.; Weiner, Benjamin J.; Whitaker, Katherine E.; Williams, Christina C.

    2016-11-01

    Early quiescent galaxies at z˜ 2 are known to be remarkably compact compared to their nearby counterparts. Possible progenitors of these systems include galaxies that are structurally similar, but are still rapidly forming stars. Here, we present Karl G. Jansky Very Large Array (VLA) observations of the CO(1-0) line toward three such compact, star-forming galaxies (SFGs) at z˜ 2.3, significantly detecting one. The VLA observations indicate baryonic gas fractions ≳ 5 times lower and gas depletion timescales ≳ 10 times shorter than normal, extended massive SFGs at these redshifts. At their current star formation rates, all three objects will deplete their gas reservoirs within 100 Myr. These objects are among the most gas-poor objects observed at z\\gt 2, and are outliers from standard gas scaling relations, a result that remains true regardless of assumptions about the CO-H2 conversion factor. Our observations are consistent with the idea that compact, SFGs are in a rapid state of transition to quiescence in tandem with the buildup of the z˜ 2 quenched population. In the detected compact galaxy, we see no evidence of rotation or that the CO-emitting gas is spatially extended relative to the stellar light. This casts doubt on recent suggestions that the gas in these compact galaxies is rotating and significantly extended compared to the stars. Instead, we suggest that, at least for this object, the gas is centrally concentrated, and only traces a small fraction of the total galaxy dynamical mass.

  4. Multipolar universal relations between f -mode frequency and tidal deformability of compact stars

    NASA Astrophysics Data System (ADS)

    Chan, T. K.; Sham, Y.-H.; Leung, P. T.; Lin, L.-M.

    2014-12-01

    Though individual stellar parameters of compact stars usually demonstrate obvious dependence on the equation of state (EOS), EOS-insensitive universal formulas relating these parameters remarkably exist. In the present paper, we explore the interrelationship between two such formulas, namely the f -I relation connecting the f -mode quadrupole oscillation frequency ω2 and the moment of inertia I , and the I -Love-Q relations relating I , the quadrupole tidal deformability λ2, and the quadrupole moment Q , which have been proposed by Lau, Leung, and Lin [Astrophys. J. 714, 1234 (2010)] and Yagi and Yunes [Science 341, 365 (2013)], respectively. A relativistic universal relation between ωl and λl with the same angular momentum l =2 ,3 ,… , the so-called "diagonal f -Love relation" that holds for realistic compact stars and stiff polytropic stars, is unveiled here. An in-depth investigation in the Newtonian limit is further carried out to pinpoint its underlying physical mechanism and hence leads to a unified f -I -Love relation. We reach the conclusion that these EOS-insensitive formulas stem from a common physical origin—compact stars can be considered as quasiincompressible when they react to slow time variations introduced by f -mode oscillations, tidal forces and rotations.

  5. Studies of compact objects with Einstein - Review and prospects

    NASA Technical Reports Server (NTRS)

    Grindlay, Jonathan E.

    1990-01-01

    X-ray images and spectra of a wide range of systems containing compact objects were obtained with the Einstein X-ray Observatory. Accreting white dwarfs, neutron stars and black holes were observed in binary systems in the Galaxy, and new constraints were derived for their formation, nature and evolution. Massive black holes were studied in active galactic nuclei, and X-ray spectra (and evolution) of AGN have led to a new model for the diffuse X-ray background.

  6. Symbiotic stars

    NASA Technical Reports Server (NTRS)

    Kafatos, M.; Michalitsianos, A. G.

    1984-01-01

    The physical characteristics of symbiotic star systems are discussed, based on a review of recent observational data. A model of a symbiotic star system is presented which illustrates how a cool red-giant star is embedded in a nebula whose atoms are ionized by the energetic radiation from its hot compact companion. UV outbursts from symbiotic systems are explained by two principal models: an accretion-disk-outburst model which describes how material expelled from the tenuous envelope of the red giant forms an inwardly-spiralling disk around the hot companion, and a thermonuclear-outburst model in which the companion is specifically a white dwarf which superheats the material expelled from the red giant to the point where thermonuclear reactions occur and radiation is emitted. It is suspected that the evolutionary course of binary systems is predetermined by the initial mass and angular momentum of the gas cloud within which binary stars are born. Since red giants and Mira variables are thought to be stars with a mass of one or two solar mass, it is believed that the original cloud from which a symbiotic system is formed can consist of no more than a few solar masses of gas.

  7. Evolution of the magnetized, neutrino-cooled accretion disk in the aftermath of a black hole-neutron star binary merger

    NASA Astrophysics Data System (ADS)

    Hossein Nouri, Fatemeh; Duez, Matthew D.; Foucart, Francois; Deaton, M. Brett; Haas, Roland; Haddadi, Milad; Kidder, Lawrence E.; Ott, Christian D.; Pfeiffer, Harald P.; Scheel, Mark A.; Szilagyi, Bela

    2018-04-01

    Black hole-torus systems from compact binary mergers are possible engines for gamma-ray bursts (GRBs). During the early evolution of the postmerger remnant, the state of the torus is determined by a combination of neutrino cooling and magnetically driven heating processes, so realistic models must include both effects. In this paper, we study the postmerger evolution of a magnetized black hole-neutron star binary system using the Spectral Einstein Code (SpEC) from an initial postmerger state provided by previous numerical relativity simulations. We use a finite-temperature nuclear equation of state and incorporate neutrino effects in a leakage approximation. To achieve the needed accuracy, we introduce improvements to SpEC's implementation of general-relativistic magnetohydrodynamics (MHD), including the use of cubed-sphere multipatch grids and an improved method for dealing with supersonic accretion flows where primitive variable recovery is difficult. We find that a seed magnetic field triggers a sustained source of heating, but its thermal effects are largely cancelled by the accretion and spreading of the torus from MHD-related angular momentum transport. The neutrino luminosity peaks at the start of the simulation, and then drops significantly over the first 20 ms but in roughly the same way for magnetized and nonmagnetized disks. The heating rate and disk's luminosity decrease much more slowly thereafter. These features of the evolution are insensitive to grid structure and resolution, formulation of the MHD equations, and seed field strength, although turbulent effects are not fully converged.

  8. Central Compact Objects: some of them could be spinning up?

    NASA Astrophysics Data System (ADS)

    Benli, O.; Ertan, Ü.

    2018-05-01

    Among confirmed central compact objects (CCOs), only three sources have measured period and period derivatives. We have investigated possible evolutionary paths of these three CCOs in the fallback disc model. The model can account for the individual X-ray luminosities and rotational properties of the sources consistently with their estimated supernova ages. For these sources, reasonable model curves can be obtained with dipole field strengths ˜ a few × 109 G on the surface of the star. The model curves indicate that these CCOs were in the spin-up state in the early phase of evolution. The spin-down starts, while accretion is going on, at a time t ˜ 103 - 104 yr depending on the current accretion rate, period and the magnetic dipole moment of the star. This implies that some of the CCOs with relatively long periods, weak dipole fields and high X-ray luminosities could be strong candidates to show spin-up behavior if they indeed evolve with fallback discs.

  9. BOOK REVIEW: Rotation and Accretion Powered Pulsars

    NASA Astrophysics Data System (ADS)

    Kaspi, V. M.

    2008-03-01

    Pulsar astrophysics has come a long way in the 40 years since the discovery of the first pulsar by Bell and Hewish. From humble beginnings as bits of 'scruff' on the Cambridge University group's chart recorder paper, the field of pulsars has blossomed into a major area of mainstream astrophysics, with an unparalleled diversity of astrophysical applications. These range from Nobel-celebrated testing of general relativity in the strong-field regime to constraining the equation-of-state of ultradense matter; from probing the winds of massive stars to globular cluster evolution. Previous notable books on the subject of pulsars have tended to focus on some particular topic in the field. The classic text Pulsars by Manchester and Taylor (1977 San Francisco, CA: Freeman) targeted almost exclusively rotation-powered radio pulsars, while the Mészáros book High-Energy Radiation from Magnetized Neutron Stars (1992 Chicago, IL: University of Chicago Press) considered both rotation- and accretion-powered neutron stars, but focused on their radiation at x-ray energies and above. The recent book Neutron Stars 1 by Haensel et al (2007 Berlin: Springer) considers only the equation of state and neutron-star structure. Into this context appears Rotation and Accretion Powered Pulsars, by Pranab Ghosh. In contrast to other books, here the author takes an encyclopedic approach and attempts to synthesize practically all of the major aspects of the two main types of neutron star. This is ambitious. The only comparable undertaking is the useful but more elementary Lyne and Graham-Smith text Pulsar Astronomy (1998 Cambridge: Cambridge University Press), or Compact Stellar X-ray Sources (eds Lewin and van der Klis, 2006 Cambridge: Cambridge University Press), an anthology of technical review articles that also includes black hole topics. Rotation and Accretion Powered Pulsars thus fills a clear void in the field, providing a readable, graduate-level book that covers nearly everything you

  10. Compact stars in Eddington-inspired Born-Infeld gravity: Anomalies associated with phase transitions

    NASA Astrophysics Data System (ADS)

    Sham, Y.-H.; Leung, P. T.; Lin, L.-M.

    2013-03-01

    We study how generic phase transitions taking place in compact stars constructed in the framework of the Eddington-inspired Born-Infeld (EiBI) gravity can lead to anomalous behavior of these stars. For the case with first-order phase transitions, compact stars in EiBI gravity with a positive coupling parameter κ exhibit a finite region with constant pressure, which is absent in general relativity. However, for the case with a negative κ, an equilibrium stellar configuration cannot be constructed. Hence EiBI gravity seems to impose stricter constraints on the microphysics of stellar matter. Besides, in the presence of spatial discontinuities in the sound speed cs due to phase transitions, the Ricci scalar is spatially discontinuous and contains δ-function singularities proportional to the jump in cs2 acquired in the associated phase transition.

  11. Compact stars in the non-minimally coupled electromagnetic fields to gravity

    NASA Astrophysics Data System (ADS)

    Sert, Özcan

    2018-03-01

    We investigate the gravitational models with the non-minimal Y(R)F^2 coupled electromagnetic fields to gravity, in order to describe charged compact stars, where Y( R) denotes a function of the Ricci curvature scalar R and F^2 denotes the Maxwell invariant term. We determine two parameter family of exact spherically symmetric static solutions and the corresponding non-minimal model without assuming any relation between energy density of matter and pressure. We give the mass-radius, electric charge-radius ratios and surface gravitational redshift which are obtained by the boundary conditions. We reach a wide range of possibilities for the parameters k and α in these solutions. Lastly we show that the models can describe the compact stars even in the more simple case α =3.

  12. A POSSIBLE SIGNATURE OF LENSE-THIRRING PRECESSION IN DIPPING AND ECLIPSING NEUTRON-STAR LOW-MASS X-RAY BINARIES

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Homan, Jeroen, E-mail: jeroen@space.mit.edu

    2012-12-01

    Relativistic Lense-Thirring precession of a tilted inner accretion disk around a compact object has been proposed as a mechanism for low-frequency ({approx}0.01-70 Hz) quasi-periodic oscillations (QPOs) in the light curves of X-ray binaries. A substantial misalignment angle ({approx}15 Degree-Sign -20 Degree-Sign ) between the inner-disk rotation axis and the compact-object spin axis is required for the effects of this precession to produce observable modulations in the X-ray light curve. A consequence of this misalignment is that in high-inclination X-ray binaries the precessing inner disk will quasi-periodically intercept our line of sight to the compact object. In the case of neutron-starmore » systems, this should have a significant observational effect, since a large fraction of the accretion energy is released on or near the neutron-star surface. In this Letter, I suggest that this specific effect of Lense-Thirring precession may already have been observed as {approx}1 Hz QPOs in several dipping/eclipsing neutron-star X-ray binaries.« less

  13. N-Body Simulations of Planetary Accretion Around M Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Ogihara, Masahiro; Ida, Shigeru

    2009-07-01

    We have investigated planetary accretion from planetesimals in terrestrial planet regions inside the ice line around M dwarf stars through N-body simulations including tidal interactions with disk gas. Because of low luminosity of M dwarfs, habitable zones (HZs) are located in inner regions (~0.1 AU). In the close-in HZ, type-I migration and the orbital decay induced by eccentricity damping are efficient according to the high disk gas density in the small orbital radii. Since the orbital decay is terminated around the disk inner edge and the disk edge is close to the HZ, the protoplanets accumulated near the disk edge affect formation of planets in the HZ. Ice lines are also in relatively inner regions at ~0.3 AU. Due to the small orbital radii, icy protoplanets accrete rapidly and undergo type-I migration before disk depletion. The rapid orbital decay, the proximity of the disk inner edge, and large amount of inflow of icy protoplanets are characteristic in planetary accretion in terrestrial planet regions around M dwarfs. In the case of full efficiency of type-I migration predicted by the linear theory, we found that protoplanets that migrate to the vicinity of the host star undergo close scatterings and collisions, and four to six planets eventually remain in mutual mean-motion resonances and their orbits have small eccentricities (lsim0.01) and they are stable both before and after disk gas decays. In the case of slow migration, the resonant capture is so efficient that densely packed ~40 small protoplanets remain in mutual mean-motion resonances. In this case, they start orbit crossing, after the disk gas decays and eccentricity damping due to tidal interaction with gas is no more effective. Through merging of the protoplanets, several planets in widely separated non-resonant orbits with relatively large eccentricities (~0.05) are formed. Thus, the final orbital configurations (separations, resonant or non-resonant, eccentricity, and distribution) of the

  14. An Observational Study of Accretion Dynamics in Short-Period Pre-Main Sequence Binaries

    NASA Astrophysics Data System (ADS)

    Tofflemire, Benjamin; Mathieu, Robert; Herczeg, Greg; Johns-Krull, Christopher; Akeson, Rachel; Ciardi, David

    2018-01-01

    Over the past thirty years, a detailed picture of star formation has emerged that highlights the importance of the interaction between a pre-main sequence (pre-MS) star and its protoplanetary disk. The properties of an emergent star, the lifetime of a protoplanetary disk, and the formation of planets are all, in part, determined by this star-disk interaction. Many stars, however, form in binary or higher-order systems where orbital dynamics are capable of fundamentally altering this star-disk interaction. Orbital resonances, especially in short-period systems, are capable of clearing the central region of a protoplanetary disk, leaving the possibility for three stable accretion disks: a circumstellar disk around each star and a circumbinary disk. In this model, accretion onto the stars is predicted to proceed in periodic streams that form at the inner edge of the circumbinary disk, cross the dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars themselves. This pulsed-accretion paradigm predicts bursts of accretion that are periodic with the orbital period, where the duration, amplitude, location in orbital phase, and which star if preferentially fed, all depend on the orbital parameters. To test these predictions, we have carried out intensive observational campaigns combining time-series, optical and near-infrared photometry with time-series, optical spectroscopy. These data are capable of monitoring the stellar accretion rate, the properties of warm circumstellar dust, and the kinematics of accretion flows, all as a function of orbital phase. In our sample of 9 pre-MS binaries with diverse orbital parameters, we search for evidence of periodic accretion events and seek to determine the role orbital parameters have on the characteristics of accretion events. Two results from our campaign will be highlighted: 1) the detection of periodic pulsed accretion events in DQ Tau and TWA 3A, and 2) evidence that the TWA 3A primary is the

  15. The Spatial Distribution of Resolved Young Stars in Blue Compact Dwarf Galaxies

    NASA Astrophysics Data System (ADS)

    Murphy, K.; Crone, M. M.

    2002-12-01

    We present the first results from a survey of the distribution of resolved young stars in Blue Compact Dwarf Galaxies. In order to identify the dominant physical processes driving star formation in these puzzling galaxies, we use a multi-scale cluster-finding algorithm to quantify the characteristic scales and properties of star-forming regions, from sizes smaller than 10 pc up to the size of each entire galaxy. This project was partially funded by the Lubin Chair at Skidmore College.

  16. A GMOS-N IFU study of the central H II region in the blue compact dwarf galaxy NGC 4449: kinematics, nebular metallicity and star formation

    NASA Astrophysics Data System (ADS)

    Kumari, Nimisha; James, Bethan L.; Irwin, Mike J.

    2017-10-01

    We use integral field spectroscopic (IFS) observations from the Gemini Multi-Object Spectrograph North (GMOS-N) to study the central H II region in a nearby blue compact dwarf (BCD) galaxy NGC 4449. The IFS data enable us to explore the variation of physical and chemical conditions of the star-forming region and the surrounding gas on spatial scales as small as 5.5 pc. Our kinematical analysis shows possible signatures of shock ionization and shell structures in the surroundings of the star-forming region. The metallicity maps of the region, created using direct Te and indirect strong line methods (R23, O3N2 and N2), do not show any chemical variation. From the integrated spectrum of the central H II region, we find a metallicity of 12 + log(O/H) = 7.88 ± 0.14 ({˜ }0.15^{+0.06}_{-0.04} Z⊙) using the direct method. Comparing the central H II region metallicity derived here with those of H II regions throughout this galaxy from previous studies, we find evidence of increasing metallicity with distance from the central nucleus. Such chemical inhomogeneities can be due to several mechanisms, including gas loss via supernova blowout, galactic winds or metal-poor gas accretion. However, we find that the localized area of decreased metallicity aligns spatially with the peak of star-forming activity in the galaxy, suggesting that gas accretion may be at play here. Spatially resolved IFS data for the entire galaxy are required to confirm the metallicity inhomogeneity found in this study and determine its possible cause.

  17. Star-disk interaction in Herbig Ae/Be stars

    NASA Astrophysics Data System (ADS)

    Speights, Christa Marie

    2012-09-01

    The question of the mechanism of certain types of stars is important. Classical T Tauri (CTTS) stars accrete magnetospherically, and Herbig Ae/Be stars (higher-mass analogs to CTTS) are thought to also accrete magnetospherically, but the source of a kG magnetic field is unknown, since these stars have radiative interiors. For magnetospheric accretion, an equation has been derived (Hartmann, 2001) which relates the truncation radius, stellar radius, stellar mass, mass accretion rate and magnetic field strength. Currently the magnetic field of Herbig stars is known to be somewhere between 0.1 kG and 10 kG. One goal of this research is to further constrain the magnetic field. In order to do that, I use the magnetospheric accretion equation. For CTTS, all of the variables used in the equation can be measured, so I gather this data from the literature and test the equation and find that it is consistent. Then I apply the equation to Herbig Ae stars and find that the error introduced from using random inclinations is too large to lower the current upper limit of the magnetic field range. If Herbig Ae stars are higher-mass analogs to CTTS, then they should have a similar magnetic field distribution. I compare the calculated Herbig Ae magnetic field distribution to several typical magnetic field distributions using the Kolmogorov-Smirnov test, and find that the data distribution does not match any of the distributions used. This means that Herbig Ae stars do not have well ordered kG fields like CTTS.

  18. Constraining the Accretion Geometry of the Intermediate Polar EX Hya Using NuSTAR, Swift, and Chandra Observations

    NASA Astrophysics Data System (ADS)

    Luna, G. J. M.; Mukai, K.; Orio, M.; Zemko, P.

    2018-01-01

    In magnetically accreting white dwarfs, the height above the white dwarf surface where the standing shock is formed is intimately related with the accretion rate and the white dwarf mass. However, it is difficult to measure. We obtained new data with NuSTAR and Swift that, together with archival Chandra data, allow us to constrain the height of the shock in the intermediate polar EX Hya. We conclude that the shock has to form at least at a distance of about one white dwarf radius from the surface in order to explain the weak Fe Kα 6.4 keV line, the absence of a reflection hump in the high-energy continuum, and the energy dependence of the white dwarf spin pulsed fraction. Additionally, the NuSTAR data allowed us to measure the true, uncontaminated hard X-ray (12-40 keV) flux, whose measurement was contaminated by the nearby galaxy cluster Abell 3528 in non-imaging X-ray instruments.

  19. Quasi-spherical accretion in High Mass X-ray Binaries

    NASA Astrophysics Data System (ADS)

    Postnov, Konstantin

    2016-07-01

    Quasi-spherical accreion onto magnetized neutron stars from stellar winds in high-mass X-ray binaries is discussed. Depending on the X-ray luminosity of the neutron star, the accretion can proceed in two regimes (modes): at L_x ≳ 4× 10^{36} erg/s, Compton cooling of accreting matter near magnetosphere leads to a supersonic (Bondi) accretion, while at smaller X-ray luminosity the Compton cooling is ineffective, and subsonic settling accretion regime sets in. In this regime, a hot convective shell is formed around the magnetosphere, and the plasma entry rate into magnetosphere is controlled by less effective radiative plasma cooling. The shell mediates the angular momentum transfer from/to the neutron star magnetosphere. Observational evidences for the different accretion regimes in slowly rotating X-ray pulsars with moderate and low X-ray luminosity, as well as possible manifestations of non-stationary quasi-spherical settling accretion due to the magnetospheric shell instability in Supergiant Fast X-ray Transients will be presented.

  20. An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate.

    PubMed

    Gilfanov, Marat; Bogdán, Akos

    2010-02-18

    There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 10(7) years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of approximately 30-50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate.

  1. Accretion Processes in Cosmic Sources

    NASA Astrophysics Data System (ADS)

    2016-10-01

    Accretion is a universal phenomenon that takes place in the vast majority of astrophysical objects. The progress of ground-based and space-borne observational facilities has resulted in the great amount of information on various accreting astrophysical objects, collected within the last decades. The accretion is accompanied by the process of extensive energy release that takes place on the surface of an accreting object and in various gaseous envelopes, accretion disk, jets and other elements of the flow pattern. The results of observations inspired the intensive development of accretion theory, which, in turn, enabled us to study unique properties of accreting objects and physical conditions in the surrounding environment. One of the most interesting outcomes of this intensive study is the fact that accretion processes are, in a sense, self-similar on various spatial scales from planetary systems to galaxies. This fact gives us new opportunities to investigate objects that, by various reasons, are not available for direct study. Cataclysmic variable stars are unique natural laboratories where one can conduct the detailed observational study of accretion processes and accretion disks. This is the main reason why several participants and a few members of the Organizing Committee of the conference "The Golden Age of Cataclysmic Variables and Related Objects - III" (September 7-12, 2015, Palermo, Italy) have decided to hold a special conference, focused on accretion processes, as a branch of that series. Main topics: Young Stellar Objects, protoplanetary discs, exoplanets in binary stars Accretion on white dwarfs (Cataclysmic variables and related objects) Accretion on neutron stars (X-ray Binary Systems and related objects) Accretion on black holes (stellar BH and AGN) The workshop will include a few 35-minute general review talks to introduce the current problems, and 20-minute talks to discuss new experimental and theoretical results. A series of 15-minute talks

  2. The primary role of the SW Sextantis stars in the evolution of cataclysmic variables

    NASA Astrophysics Data System (ADS)

    Torres, Manuel; Gaensicke, Boris; Rodriguez-Gil, Pablo; Long, Knox; Marsh, Tom; Steeghs, Danny; Munoz-Darias, Teodoro; Shahbaz, Tariq; Schmidtobreick, Linda; Schreiber, Matthias

    2009-02-01

    SW Sextantis stars are a relatively large group of cataclysmic variables (CVs) which plays a fundamental role in our understanding of CV structure and evolution. Very little is known about the properties of their accreting white dwarfs and their donor stars, as the stellar components are usually outshone by an extremely bright accretion flow. Consequently, a proper assesment of their evolutionary state is illusionary. We are monitoring the brightness of a number of SW Sex stars and request here Gemini/GMOS-N ToO time to obtain orbital phase-resolved spectroscopy if one of them enters a low state, since this is the only opportunity for studying the stellar components individually. These data will be used to accurately measure the binary parameters, white dwarf temperature, and distance to the system for a SW Sex star for the first time. The measured stellar masses and radii will especially be a precious input to the theory of compact binary evolution as a whole.

  3. A numerical investigation of wind accretion in persistent supergiant X-ray binaries - I. Structure of the flow at the orbital scale

    NASA Astrophysics Data System (ADS)

    El Mellah, I.; Casse, F.

    2017-05-01

    Classical supergiant X-ray binaries host a neutron star orbiting a supergiant OB star and display persistent X-ray luminosities of 1035-1037 erg s-1. The stellar wind from the massive companion is believed to be the main source of matter accreted by the compact object. With this first paper, we introduce a ballistic model to evaluate the influence of the orbital effects on the structure of the accelerating winds that participate to the accretion process. Thanks to the parametrization we retained the numerical pipeline we designed, we can investigate the supersonic flow and the subsequent observables as a function of a reduced set of characteristic numbers and scales. We show that the shape of the permanent flow is entirely determined by the mass ratio, the filling factor, the Eddington factor and the α-force multiplier that drives the stellar wind acceleration. Provided scales such as the orbital period are known, we can trace back the observables to evaluate the mass accretion rates, the accretion mechanism, the shearing of the inflow and the stellar parameters. We discuss the likelihood of wind-formed accretion discs around the accretors in each case and confront our model to three persistent supergiant X-ray binaries (Vela X-1, IGR J18027-2016, XTE J1855-026).

  4. Understanding the Accretion Engine in Pre-main Sequence Stars

    NASA Astrophysics Data System (ADS)

    Gómez de Castro, Ana I.

    2009-05-01

    Planetary systems are angular momentum reservoirs generated during star formation as a result of the joint action of gravity and angular momentum conservation. The accretion process drives to the generation of powerful engines able to drive the optical jets and the molecular outflows. A fraction of the engine energy is released into heating the circumstellar plasma to temperatures between 3000 K to 10 MK depending on the plasma location and density. There are very important unsolved problems concerning the nature of the engine, its evolution and its impact in the chemical evolution of the disk. Of special relevance is the understanding of the shear layer between the stellar photosphere and the disk; this layer controls a significant fraction of the magnetic field building up and the subsequent dissipative processes ought to be studied in the UV.

  5. Hydrodynamic simulations of stellar wind disruption by a compact X-ray source

    NASA Technical Reports Server (NTRS)

    Blondin, John M.; Kallman, Timothy R.; Fryxell, Bruce A.; Taam, Ronald E.

    1990-01-01

    This paper presents two-dimensional numerical simulations of the gas flow in the orbital plane of a massive X-ray binary system, in which the mass accretion is fueled by a radiation-driven wind from an early-type companion star. These simulations are used to examine the role of the compact object (either a neutron star or a black hole) in disturbing the radiatively accelerating wind of the OB companion, with an emphasis on understanding the origin of the observed soft X-ray photoelectric absorption seen at late orbital phases in these systems. On the basis of these simulations, it is suggested that the phase-dependent photoelectric absorption seen in several of these systems can be explained by dense filaments of compressend gas formed in the nonsteady accreation bow shock and wake of the compact object.

  6. NUSTAR and Suzaku x-ray spectroscopy of NGC 4151: Evidence for reflection from the inner accretion disk

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Keck, M. L.; Brenneman, L. W.; Ballantyne, D. R.

    We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spinmore » $$a\\gt 0.9$$ accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection (IDR) flux variation drives the spectral variability during the observations. We discuss various physical scenarios for the IDR model and we find that a compact corona is consistent with the observed features.« less

  7. Properties of Massive Stars in Primitive Galaxies

    NASA Technical Reports Server (NTRS)

    Heap, Sara

    2012-01-01

    According to R. Dave, the phases of galaxy formation are distinguished by their halo mass and governing feedback mechanism. Galaxies in the birth phase (our "primitive galaxies") have a low halo mass (M<10(exp 9) Msun); and star formation is affected by photoionizing radiation of massive stars. In contrast, galaxies in the growth phase (e.g. Lyman Break galaxies) are more massive (M=10(exp 9)-10(exp 12) Msun); star formation is fueled by cold accretion but modulated by strong outflows from massive stars. I Zw 18 is a local blue, compact dwarf galaxy that meets the requirements for a birth-phase galaxy: halo mass <10(exp 9) Msun, strong photo ionizing radiation, no galactic outflow, and very low metallicity, log(O/H)=7.2. We will describe the properties of massive stars in I Zw 18 based on analysis of ultraviolet spectra obtained with HST.

  8. EPISODIC ACCRETION AT EARLY STAGES OF EVOLUTION OF LOW-MASS STARS AND BROWN DWARFS: A SOLUTION FOR THE OBSERVED LUMINOSITY SPREAD IN H-R DIAGRAMS?

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Baraffe, I.; Chabrier, G.; Gallardo, J.

    2009-09-01

    We present evolutionary models for young low-mass stars and brown dwarfs taking into account episodic phases of accretion at early stages of the evolution, a scenario supported by recent large surveys of embedded protostars. An evolution including short episodes of vigorous accretion followed by longer quiescent phases can explain the observed luminosity spread in H-R diagrams of star-forming regions at ages of a few Myr, for objects ranging from a few Jupiter masses to a few tenths of a solar mass. The gravitational contraction of these accreting objects strongly departs from the standard Hayashi track at constant T{sub eff}. Themore » best agreement with the observed luminosity scatter is obtained if most of the accretion shock energy is radiated away. The obtained luminosity spread at 1 Myr in the H-R diagram is equivalent to what can be misinterpreted as an {approx}10 Myr age spread for non-accreting objects. We also predict a significant spread in radius at a given T{sub eff}, as suggested by recent observations. These calculations bear important consequences for our understanding of star formation and early stages of evolution and on the determination of the initial mass function for young ({<=} a few Myr) clusters. Our results also show that the concept of a stellar birthline for low-mass objects has no valid support.« less

  9. Magnetic field strength of a neutron-star-powered ultraluminous X-ray source

    NASA Astrophysics Data System (ADS)

    Brightman, M.; Harrison, F. A.; Fürst, F.; Middleton, M. J.; Walton, D. J.; Stern, D.; Fabian, A. C.; Heida, M.; Barret, D.; Bachetti, M.

    2018-04-01

    Ultraluminous X-ray sources (ULXs) are bright X-ray sources in nearby galaxies not associated with the central supermassive black hole. Their luminosities imply they are powered by either an extreme accretion rate onto a compact stellar remnant, or an intermediate mass ( 100-105M⊙) black hole1. Recently detected coherent pulsations coming from three bright ULXs2-5 demonstrate that some of these sources are powered by accretion onto a neutron star, implying accretion rates significantly in excess of the Eddington limit, a high degree of geometric beaming, or both. The physical challenges associated with the high implied accretion rates can be mitigated if the neutron star surface field is very high (1014 G)6, since this suppresses the electron scattering cross-section, reducing the radiation pressure that chokes off accretion for high luminosities. Surface magnetic field strengths can be determined through cyclotron resonance scattering features7,8 produced by the transition of charged particles between quantized Landau levels. Here, we present the detection at a significance of 3.8σ of an absorption line at 4.5 keV in the Chandra spectrum of a ULX in M51. This feature is likely to be a cyclotron resonance scattering feature produced by the strong magnetic field of a neutron star. Assuming scattering off electrons, the magnetic field strength is implied to be 1011 G, while protons would imply a magnetic field of B 1015 G.

  10. The Ultraviolet and Infrared Star Formation Rates of Compact Group Galaxies: An Expanded Sample

    NASA Technical Reports Server (NTRS)

    Lenkic, Laura; Tzanavaris, Panayiotis; Gallagher, Sarah C.; Desjardins, Tyler D.; Walker, Lisa May; Johnson, Kelsey E.; Fedotov, Konstantin; Charlton, Jane; Cardiff, Ann H.; Durell, Pat R.

    2016-01-01

    Compact groups of galaxies provide insight into the role of low-mass, dense environments in galaxy evolution because the low velocity dispersions and close proximity of galaxy members result in frequent interactions that take place over extended time-scales. We expand the census of star formation in compact group galaxies by Tzanavaris et al. (2010) and collaborators with Swift UVOT, Spitzer IRAC and MIPS 24 m photometry of a sample of 183 galaxies in 46 compact groups. After correcting luminosities for the contribution from old stellar populations, we estimate the dust-unobscured star formation rate (SFRUV) using the UVOT uvw2 photometry. Similarly, we use the MIPS 24 m photometry to estimate the component of the SFR that is obscured by dust (SFRIR). We find that galaxies which are MIR-active (MIR-red), also have bluer UV colours, higher specific SFRs, and tend to lie in Hi-rich groups, while galaxies that are MIR-inactive (MIR-blue) have redder UV colours, lower specific SFRs, and tend to lie in Hi-poor groups. We find the SFRs to be continuously distributed with a peak at about 1 M yr1, indicating this might be the most common value in compact groups. In contrast, the specific SFR distribution is bimodal, and there is a clear distinction between star-forming and quiescent galaxies. Overall, our results suggest that the specific SFR is the best tracer of gas depletion and galaxy evolution in compact groups.

  11. Accretion disks around neutron and strange stars in R+aR2 gravity

    NASA Astrophysics Data System (ADS)

    Staykov, Kalin V.; Doneva, Daniela D.; Yazadjiev, Stoytcho S.

    2016-08-01

    We study the electromagnetic spectrum of accretion disks around neutron and strange stars in R+aR2 gravity. Both static and rapidly rotating models are investigated. The results are compared with the General Relativistic results. We found difference between the results in both theories of about 50% for the electromagnetic flux and about 20% in the luminosity for models with equal mass and angular velocity in both theories. The observed differences are much lower for models rotating with Keplerian velocity and with equal masses.

  12. Accretion onto a charged Kiselev black hole

    NASA Astrophysics Data System (ADS)

    Abbas, G.; Ditta, A.

    2018-04-01

    Accretion of matter onto a compact is one of the interesting astrophysical processes. Here, we study the accretion of matter onto a charged Kiselev black hole. The problem of static and spherically symmetric accretion of a polytropic fluid is explored for the analytic solution of equations of motion. We have investigated the necessary conditions for existence of the critical flow points and the mass accretion rate. Finally, we discuss the polytropic gas accretion in detail. It has been found that in the accretion process the quintessence and charge parameters play a dominant role.

  13. The vertical structure and stability of accretion disks surrounding black holes and neutron stars

    NASA Technical Reports Server (NTRS)

    Milsom, J. A.; Chen, Xingming; Taam, Ronald E.

    1994-01-01

    The structure and stability of the inner regions of accretion disks surrounding neutron stars and black holes have been investigated. Within the framework of the alpha viscosity prescription for optically thick disks, we assume the viscous stress scales with gas pressure only, and the alpha parameter, which is less than or equal to unity, is formulated as alpha(sub 0)(h/r)(exp n), where h is the local scale height and n and alpha(sub 0) are constants. We neglect advective energy transport associated with radial motions and construct the vertical structure of the disks by assuming a Keplerian rotation law and local hydrostatic and thermal equilibrium. The vertical structures have been calculated with and without convective energy transport, and it has been demonstrated that convection is important especially for mass accretion rates, M-dot, greater than about 0.1 times the Eddington value, M-dot(sub Edd). Although the efficiency of convection is not high, convection significantly modifies the vertical structure of the disk (as compared with a purely radiative model) and leads to lower temperatures at a given M-dot. The results show that the disk can be locally unstable and that for n greater than or = 0.75, an S-shaped relation can exist between M-dot and the column density, sigma, at a given radius. While the lower stable branch (derivative of M-dot/derivative of sigma greater than 0) and middle unstable branch (derivative of M-dot/derivative of sigma less than 0) represent structures for which the gas and radiation pressure dominate respectively, the stable upper branch (derivative of M-dot/derivative of sigma greater than 0) is a consequence of the saturation of alpha. This saturation of alpha can occur for large alpha(sub 0) and at M-dot less than or = M-dot(sub Edd). The instability is found to occur at higher mass accretion rates for neutron stars than for black holes. In particular, the disk is locally unstable for M-dot greater than or = 0.5 M-dot(sub Edd

  14. Bondi-Hoyle accretion in an isothermal magnetized plasma

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Lee, Aaron T.; McKee, Christopher F.; Klein, Richard I.

    2014-03-01

    In regions of star formation, protostars and newborn stars will accrete mass from their natal clouds. These clouds are threaded by magnetic fields with a strength characterized by the plasma β—the ratio of thermal and magnetic pressures. Observations show that molecular clouds have β ≲ 1, so magnetic fields have the potential to play a significant role in the accretion process. We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion onto a uniformly moving point particle from a uniform, non-self-gravitating, isothermal gas. We consider gas moving with sonic Mach numbersmore » of up to M≈45; magnetic fields that are either parallel, perpendicular, or oriented 45° to the flow; and β as low as 0.01. Our simulations utilize adaptive mesh refinement in order to obtain high spatial resolution where it is needed; this also allows the boundaries to be far from the accreting object to avoid unphysical effects arising from boundary conditions. Additionally, we show that our results are independent of our exact prescription for accreting mass in the sink particle. We give simple expressions for the steady-state accretion rate as a function of β and M for the parallel and perpendicular orientations. Using typical molecular cloud values of M∼5 and β ∼ 0.04 from the literature, our fits suggest that a 0.4 M {sub ☉} star accretes ∼4 × 10{sup –9} M {sub ☉} yr{sup –1}, almost a factor of two less than accretion rates predicted by hydrodynamic models. This disparity can grow to orders of magnitude for stronger fields and lower Mach numbers. We also discuss the applicability of these accretion rates versus accretion rates expected from gravitational collapse, and under what conditions a steady state is possible. The reduction in the accretion rate in a magnetized medium leads to an increase in the time required to form stars in competitive accretion models, making such models less efficient than

  15. Radiative GRMHD simulations of accretion and outflow in non-magnetized neutron stars and ultraluminous X-ray sources

    NASA Astrophysics Data System (ADS)

    Abarca, David; Kluźniak, Wlodek; Sądowski, Aleksander

    2018-06-01

    We run two GRRMHD simulations of super-Eddington accretion disks around a black hole and a non-magnetized, non-rotating neutron star. The neutron star was modeled using a reflective inner boundary condition. We observe the formation of a transition layer in the inner region of the disk in the neutron star simulation which leads to a larger mass outflow rate and a lower radiative luminosity over the black hole case. Sphereization of the flow leads to an observable luminosity at infinity around the Eddington value when viewed from all directions for the neutron star case, contrasting to the black hole case where collimation of the emission leads to observable luminosities about an order of magnitude higher when observed along the disk axis. We find the outflow to be optically thick to scattering, which would lead to the obscuring of any neutron star pulsations observed in corresponding ULXs.

  16. UV And X-Ray Emission from Impacts of Fragmented Accretion Streams on Classical T Tauri Stars

    NASA Astrophysics Data System (ADS)

    Colombo, Salvatore; Orlando, Salvatore; Peres, Giovanni; Argiroffi, Costanza; Reale, Fabio

    2016-07-01

    According to the magnetoshperic accretion scenario, during their evo- lution, Classical T Tauri stars accrete material from their circumstellar disk. The accretion process is regulated by the stellar magnetic eld and produces hot and dense post-shocks on the stellar surface as a result of impacts of the downfalling material. The impact regions are expected to strongly radiate in UV and X-rays. Several lines of evidence support the magnetospheric accretion scenario, especially in optical and infrared bands. However several points still remain unclear as, for instance,where the complex-pro le UV lines originate, or whether and how UV and X-ray emission is produced in the same shock region. The analysis of a large solar eruption has shown that EUV excesses might be e ectively produced by the impact of dense fragments onto the stellar surface. Since a steady accretion stream does not reprouce observations, in this work we investi- gate the e ects of a fragmented accretion stream on the uxes and pro les of C IV and O VIII emission lines. To this end we model the impact of a fragmented accretion stream onto the chromosphere of a CTTS with 2D axysimmetric magneto-hydrodynamic simulations. Our model takes into account of the gravity, the stellar magnetic eld, the thermal conduction and the radiative cooling from an optically thin plasma. From the model results, we synthesize the UV and X-ray emission including the e ect of Doppler shift along the line of sight. We nd that a fragmented accretion stream produces complex pro les of UV emission lines which consists of multiple components with di erent Doppler shifts. Our model predicts line pro les that are consistent with those observed and explain their origin as due to the stream fragmentation.

  17. Radial oscillations and stability of compact stars in Eddington-inspired Born-Infeld gravity

    NASA Astrophysics Data System (ADS)

    Sham, Y.-H.; Lin, L.-M.; Leung, P. T.

    2012-09-01

    We study the hydrostatic equilibrium structure of compact stars in the Eddington-inspired Born-Infeld gravity recently proposed by Bañados and Ferreira [Phys. Rev. Lett. 105, 011101 (2010)]. We also develop a framework to study the radial perturbations and stability of compact stars in this theory. We find that the standard results of stellar stability still hold in this theory. The frequency square of the fundamental oscillation mode vanishes for the maximum-mass stellar configuration. The dependence of the oscillation mode frequencies on the coupling parameter κ of the theory is also investigated. We find that the fundamental mode is insensitive to the value of κ, while higher-order modes depend more strongly on κ.

  18. Super-Eddington Accretion in Tidal Disruption Events: the Impact of Realistic Fallback Rates on Accretion Rates

    NASA Astrophysics Data System (ADS)

    Wu, Samantha; Coughlin, Eric R.; Nixon, Chris

    2018-04-01

    After the tidal disruption of a star by a massive black hole, disrupted stellar debris can fall back to the hole at a rate significantly exceeding its Eddington limit. To understand how black hole mass affects the duration of super-Eddington accretion in tidal disruption events, we first run a suite of simulations of the disruption of a Solar-like star by a supermassive black hole of varying mass to directly measure the fallback rate onto the hole, and we compare these fallback rates to the analytic predictions of the "frozen-in" model. Then, adopting a Zero-Bernoulli Accretion flow as an analytic prescription for the accretion flow around the hole, we investigate how the accretion rate onto the black hole evolves with the more accurate fallback rates calculated from the simulations. We find that numerically-simulated fallback rates yield accretion rates onto the hole that can, depending on the black hole mass, be nearly an order of magnitude larger than those predicted by the frozen-in approximation. Our results place new limits on the maximum black hole mass for which super-Eddington accretion occurs in tidal disruption events.

  19. Electromagnetic fields of slowly rotating magnetized compact stars in conformal gravity

    NASA Astrophysics Data System (ADS)

    Turimov, Bobur; Ahmedov, Bobomurat; Abdujabbarov, Ahmadjon; Bambi, Cosimo

    2018-06-01

    In this paper we investigate the exterior vacuum electromagnetic fields of slow-rotating magnetized compact stars in conformal gravity. Assuming the dipolar magnetic field configuration, we obtain an analytical solution of the Maxwell equations for the magnetic and the electric fields outside a slowly rotating magnetized star in conformal gravity. Furthermore, we study the dipolar electromagnetic radiation and energy losses from a rotating magnetized star in conformal gravity. In order to get constraints on the L parameter of conformal gravity, the theoretical results for the magnetic field of a magnetized star in conformal gravity are combined with the precise observational data of radio pulsar period slowdown, and it is found that the maximum value of the parameter of conformal gravity is less than L ≲9.5 ×105 cm (L /M ≲5 ).

  20. Potential cooling of an accretion-heated neutron star crust in the low-mass X-ray binary 1RXS J180408.9-342058

    NASA Astrophysics Data System (ADS)

    Parikh, A. S.; Wijnands, R.; Degenaar, N.; Ootes, L. S.; Page, D.; Altamirano, D.; Cackett, E. M.; Deller, A. T.; Gusinskaia, N.; Hessels, J. W. T.; Homan, J.; Linares, M.; Miller, J. M.; Miller-Jones, J. C. A.

    2017-04-01

    We have monitored the transient neutron star low-mass X-ray binary 1RXS J180408.9-342058 in quiescence after its ˜4.5 month outburst in 2015. The source has been observed using Swift and XMM-Newton. Its X-ray spectra were dominated by a thermal component. The thermal evolution showed a gradual X-ray luminosity decay from ˜18 × 1032 to ˜4 × 1032 (D/5.8 kpc)2 erg s-1 between ˜8 and ˜379 d in quiescence, and the inferred neutron star surface temperature (for an observer at infinity; using a neutron star atmosphere model) decreased from ˜100 to ˜71 eV. This can be interpreted as cooling of an accretion-heated neutron star crust. Modelling the observed temperature curve (using nscool) indicated that the source required ˜1.9 MeV per accreted nucleon of shallow heating in addition to the standard deep crustal heating to explain its thermal evolution. Alternatively, the decay could also be modelled without the presence of deep crustal heating, only having a shallow heat source (again ˜1.9 MeV per accreted nucleon was required). However, the XMM-Newton data statistically required an additional power-law component. This component contributed ˜30 per cent of the total unabsorbed flux in 0.5-10 keV energy range. The physical origin of this component is unknown. One possibility is that it arises from low-level accretion. The presence of this component in the spectrum complicates our cooling crust interpretation because it might indicate that the smooth luminosity and temperature decay curves we observed may not be due to crust cooling but due to some other process.

  1. X-shooter spectroscopy of young stellar objects. IV. Accretion in low-mass stars and substellar objects in Lupus

    NASA Astrophysics Data System (ADS)

    Alcalá, J. M.; Natta, A.; Manara, C. F.; Spezzi, L.; Stelzer, B.; Frasca, A.; Biazzo, K.; Covino, E.; Randich, S.; Rigliaco, E.; Testi, L.; Comerón, F.; Cupani, G.; D'Elia, V.

    2014-01-01

    We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from ~0.03 to ~1.2 M⊙, but mostly with 0.1 M⊙accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (Lacc), and in turn the accretion rate (Ṁacc), was derived by modelling the excess emission from the UV to the near-infrared as the continuum emission of a slab of hydrogen. We computed the flux and luminosity (Lline) of many emission lines of H , He , and Ca ii, observed simultaneously in the range from ~330 nm to 2500 nm. The luminosity of all the lines is well correlated with Lacc. We provide empirical relationships between Lacc and the luminosity of 39 emission lines, which have a lower dispersion than relationships previously reported in the literature. Our measurements extend the Paβ and Brγ relationships to Lacc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies of measuring Lacc and Ṁacc yield significantly different results: Hα line profile modelling may underestimate Ṁacc by 0.6 to 0.8 dex with respect to Ṁacc derived from continuum-excess measures. These differences may explain the probably spurious bi-modal relationships between Ṁacc and other YSOs properties reported in the literature. We derived Ṁacc in the range 2 × 10-12-4 × 10-8 M⊙ yr-1 and conclude that Ṁacc ∝ M⋆1.8(±0.2), with a dispersion lower by a factor of about 2 than in previous studies. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude in Ṁacc, confirming previous suggestions that the geometry of the accretion flow

  2. Swings between rotation and accretion power in a binary millisecond pulsar.

    PubMed

    Papitto, A; Ferrigno, C; Bozzo, E; Rea, N; Pavan, L; Burderi, L; Burgay, M; Campana, S; Di Salvo, T; Falanga, M; Filipović, M D; Freire, P C C; Hessels, J W T; Possenti, A; Ransom, S M; Riggio, A; Romano, P; Sarkissian, J M; Stairs, I H; Stella, L; Torres, D F; Wieringa, M H; Wong, G F

    2013-09-26

    It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star's rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales.

  3. Interplay between Diffusion, Accretion and Nuclear Reactions in the Atmospheres of Sirius and Przybylski's Star

    NASA Astrophysics Data System (ADS)

    Yushchenko, A.; Gopka, V.; Goriely, S.; Lambert, D.; Shavrina, A.; Kang, Y. W.; Rostopchin, S.; Valyavin, G.; Lee, B.-C.; Kim, C.

    2007-06-01

    The abundance anomalies in chemically peculiar B-F stars are usually explained by diffusion of chemical elements in the stable atmospheres of these stars. But it is well known that peculiar stars with similar temperatures and gravities show very different chemical compositions. We show that the abundance patterns of several stars can be influenced by accretion and (or) nuclear reactions in stellar atmospheres. The first case is one of the hottest Am stars - Sirius. We determined the abundances of more than 50 chemical elements in the atmosphere of Sirius A and show that Sirius A was contaminated by s-process enriched matter from Sirius B (now a white dwarf). The second case is the well known Przybylski's star. The abundance pattern of this star is the second most studied one after the Sun with abundances determined for about 60 chemical elements. Spectral lines of radioactive elements with short decay times were found in the spectrum of this star. We report the results of our investigation on the stratification of chemical elements in the atmosphere of Przybylski's star and the new identification of lines corresponding to short-lived actinides in its spectrum. Possible explanations of the abundances pattern of Przybylski's star (as well as HR465 and HD965) can be the natural radioactive decays of thorium and uranium, the explosion of a companion as a supernova or the spallation reactions. These three hypotheses and (or) diffusion can possibly explain the abundance pattern of Przybylski's star and several similar objects such as HR465 and HD965.

  4. Hyper-Eddington accretion in GRB

    NASA Astrophysics Data System (ADS)

    Janiuk, A.; Czerny, B.; Perna, R.; Di Matteo, T.

    2005-05-01

    Popular models of the GRB origin associate this event with a cosmic explosion, birth of a stellar mass black hole and jet ejection. Due to the shock collisions that happen in the jet, the gamma rays are produced and we detect a burst of duration up to several tens of seconds. This burst duration is determined by the lifetime of the central engine, which may be different in various scenarios. Characteristically, the observed bursts have a bimodal distribution and constitute the two classes: short (t < 2s) and long bursts. Theoretical models invoke the mergers of two neutron stars or a neutron star with a black hole, or, on the other hand, a massive star explosion (collapsar). In any of these models we have a phase of disc accretion onto a newly born black hole: the disc is formed from the disrupted neutron star or fed by the material fallback from the ejected collapsar envelope. The disc is extremely hot and dense, and the accretion rate is orders of magnitude higher than the Eddington rate. In such physical conditions the main cooling mechanism is neutrino emission, and one of possible ways of energy extraction from the accretion disc is the neutrino-antineutrino annihilation.

  5. Keck-I MOSFIRE spectroscopy of compact star-forming galaxies at z ≳ 2: high velocity dispersions in progenitors of compact quiescent galaxies

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Barro, Guillermo; Koo, David C.; Faber, Sandra M.

    2014-11-10

    We present Keck-I MOSFIRE near-infrared spectroscopy for a sample of 13 compact star-forming galaxies (SFGs) at redshift 2 ≤ z ≤ 2.5 with star formation rates of SFR ∼ 100 M {sub ☉} yr{sup –1} and masses of log(M/M {sub ☉}) ∼10.8. Their high integrated gas velocity dispersions of σ{sub int} =230{sub −30}{sup +40} km s{sup –1}, as measured from emission lines of Hα and [O III], and the resultant M {sub *}-σ{sub int} relation and M {sub *}-M {sub dyn} all match well to those of compact quiescent galaxies at z ∼ 2, as measured from stellar absorption lines.more » Since log(M {sub *}/M {sub dyn}) =–0.06 ± 0.2 dex, these compact SFGs appear to be dynamically relaxed and evolved, i.e., depleted in gas and dark matter (<13{sub −13}{sup +17}%), and present larger σ{sub int} than their non-compact SFG counterparts at the same epoch. Without infusion of external gas, depletion timescales are short, less than ∼300 Myr. This discovery adds another link to our new dynamical chain of evidence that compact SFGs at z ≳ 2 are already losing gas to become the immediate progenitors of compact quiescent galaxies by z ∼ 2.« less

  6. Hydrodynamical simulations of the tidal stripping of binary stars by massive black holes

    NASA Astrophysics Data System (ADS)

    Mainetti, Deborah; Lupi, Alessandro; Campana, Sergio; Colpi, Monica

    2016-04-01

    In a galactic nucleus, a star on a low angular momentum orbit around the central massive black hole can be fully or partially disrupted by the black hole tidal field, lighting up the compact object via gas accretion. This phenomenon can repeat if the star, not fully disrupted, is on a closed orbit. Because of the multiplicity of stars in binary systems, also binary stars may experience in pairs such a fate, immediately after being tidally separated. The consumption of both the binary components by the black hole is expected to power a double-peaked flare. In this paper, we perform for the first time, with GADGET2, a suite of smoothed particle hydrodynamics simulations of binary stars around a galactic central black hole in the Newtonian regime. We show that accretion luminosity light curves from double tidal disruptions reveal a more prominent knee, rather than a double peak, when decreasing the impact parameter of the encounter and when elevating the difference between the mass of the star which leaves the system after binary separation and the mass of the companion. The detection of a knee can anticipate the onset of periodic accretion luminosity flares if one of the stars, only partially disrupted, remains bound to the black hole after binary separation. Thus knees could be precursors of periodic flares, which can then be predicted, followed up and better modelled. Analytical estimates in the black hole mass range 105-108 M⊙ show that the knee signature is enhanced in the case of black holes of mass 106-107 M⊙.

  7. Bright compact bulges at intermediate redshifts

    NASA Astrophysics Data System (ADS)

    Sachdeva, Sonali; Saha, Kanak

    2018-07-01

    Studying bright (MB < -20), intermediate-redshift (0.4 < z< 1.0), disc-dominated (nB < 2.5) galaxies from Hubble Space Telescope/Advanced Camera for Surveys and Wide Field Camera 3 in Chandra Deep Field-South, in rest-frame B and I band, we found a new class of bulges that is brighter and more compact than ellipticals. We refer to them as `bright, compact bulges' (BCBs) - they resemble neither classical nor pseudo-bulges and constitute ˜12 per cent of the total bulge population at these redshifts. Examining free-bulge + disc decomposition sample and elliptical galaxy sample from Simard et al., we find that only ˜0.2 per cent of the bulges can be classified as BCBs in the local Universe. Bulge to total light ratio of disc galaxies with BCBs is (at ˜0.4) a factor of ˜2 and ˜4 larger than for those with classical and pseudo-bulges. BCBs are ˜2.5 and ˜6 times more massive than classical and pseudo-bulges. Although disc galaxies with BCBs host the most massive and dominant bulge type, their specific star formation rate is 1.5-2 times higher than other disc galaxies. This is contrary to the expectations that a massive compact bulge would lead to lower star formation rates. We speculate that our BCB host disc galaxies are descendant of massive, compact, and passive elliptical galaxies observed at higher redshifts. Those high-redshift ellipticals lack local counterparts and possibly evolved by acquiring a compact disc around them. The overall properties of BCBs support a picture of galaxy assembly in which younger discs are being accreted around massive pre-existing spheroids.

  8. A cooling neutron star crust after recurrent outbursts: modelling the accretion outburst history of Aql X-1

    NASA Astrophysics Data System (ADS)

    Ootes, Laura S.; Wijnands, Rudy; Page, Dany; Degenaar, Nathalie

    2018-07-01

    With our neutron star crust cooling code NSCOOL, we track the thermal evolution of the neutron star in Aql X-1 over the full accretion outburst history from 1996 until 2015. For the first time, we model many outbursts (23 outbursts were detected) collectively and in great detail. This allows us to investigate the influence of previous outbursts on the internal temperature evolution and to test different neutron star crust cooling scenarios. Aql X-1 is an ideal test source for this purpose, because it shows frequent, short outbursts and thermally dominated quiescence spectra. The source goes into outburst roughly once a year for a few months. Assuming that the quiescent Swift/X-Ray Telescope observations of Aql X-1 can be explained within the crust cooling scenario, we find three main conclusions. First, the data are well reproduced by our model if the envelope composition and shallow heating parameters are allowed to change between outbursts. This is not the case if both shallow heating parameters (strength and depth) are tied throughout all accretion episodes, supporting earlier results that the properties of the shallow heating mechanism are not constant between outbursts. Secondly, from our models, shallow heating could not be connected to one specific spectral state during outburst. Thirdly, and most importantly, we find that the neutron star in Aql X-1 does not have enough time between outbursts to cool down to crust-core equilibrium and that heating during one outburst influences the cooling curves of the next.

  9. Monitoring Accreting X-ray Pulsars with the GLAST Burst Monitor

    NASA Technical Reports Server (NTRS)

    Wilson, Colleen A.; Finger, Mark H.; Patel, Sandeep K.; Bhat, P. Narayana; Preece, Robert D.; Meegan, Charles A.

    2007-01-01

    Accreting pulsars are exceptionally good laboratories for probing the detailed physics of accretion onto magnetic stars. While similar accretion flows also occur in other types of astrophysical systems, e.g. magnetic CVs, only neutron stars have a small enough moment of inertia for the accretion of angular momentum to result in measurable changes in spin-frequency in a timescale of days. Long-term monitoring of accreting pulsar spin-frequencies and fluxes was demonstrated with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Here we present sample results from BATSE, discuss measurement techniques appropriate for GBM, and estimate the expected GBM sensitivity.

  10. Simulating the X-ray luminosity of Be X-ray binaries: the case for black holes versus neutron stars

    NASA Astrophysics Data System (ADS)

    Brown, R. O.; Ho, W. C. G.; Coe, M. J.; Okazaki, A. T.

    2018-04-01

    There are over 100 Be stars that are known to have neutron star companions but only one such system with a black hole. Previous theoretical work suggests this is not due to their formation but due to differences in X-ray luminosity. It has also been proposed that the truncation of the Be star's circumstellar disc is dependent on the mass of the compact object. Hence, Be star discs in black hole binaries are smaller. Since accretion onto the compact object from the Be star's disc is what powers the X-ray luminosity, a smaller disc in black hole systems leads to a lower luminosity. In this paper, simulations are performed with a range of eccentricities and compact object mass. The disc's size and density are shown to be dependent on both quantities. Mass capture and, in turn, X-ray luminosity are heavily dependent on the size and density of the disc. Be/black hole binaries are expected to be up to ˜10 times fainter than Be/neutron star binaries when both systems have the same eccentricity and can be 100 times fainter when comparing systems with different eccentricity.

  11. Photometric Determination of the Mass Accretion Rates of Pre-main-sequence Stars. V. Recent Star Formation in the 30 Dor Nebula

    NASA Astrophysics Data System (ADS)

    De Marchi, Guido; Panagia, Nino; Beccari, Giacomo

    2017-09-01

    We report on the properties of the low-mass stars that recently formed in the central ˜ 2\\buildrel{ \\prime}\\over{.} 7× 2\\buildrel{ \\prime}\\over{.} 7 of 30 Dor, including the R136 cluster. Using the photometric catalog of De Marchi et al., based on observations with the Hubble Space Telescope, and the most recent extinction law for this field, we identify 1035 bona fide pre-main-sequence (PMS) stars showing {{H}}α excess emission at the 4σ level with an {{H}}α equivalent width of 20 Å or more. We find a wide spread in age spanning the range ˜ 0.1{--}50 {Myr}. We also find that the older PMS objects are placed in front of the R136 cluster and are separated from it by a conspicuous amount of absorbing material, indicating that star formation has proceeded from the periphery into the interior of the region. We derive physical parameters for all PMS stars, including masses m, ages t, and mass accretion rates {\\dot{M}}{acc}. To identify reliable correlations between these parameters, which are intertwined, we use a multivariate linear regression fit of the type {log}{\\dot{M}}{acc}=a× {log}t+b× {log}m+c. The values of a and b for 30 Dor are compatible with those found in NGC 346 and NGC 602. We extend the fit to a uniform sample of 1307 PMS stars with 0.5< m/{M}⊙ < 1.5 and t< 16 {Myr} in six star-forming regions in the Large and Small Magellanic Clouds and Milky Way with metallicities in the range of 0.1-1.0 {{Z}}⊙ . We find a=-0.59+/- 0.02 and b=0.78+/- 0.08. The residuals are systematically different between the six regions and reveal a strong correlation with metallicity Z, of the type c=(-3.69+/- 0.02)-(0.30+/- 0.04)× {log}Z/{Z}⊙ . A possible interpretation of this trend is that when the metallicity is higher so is the radiation pressure, and this limits the accretion process, in both its rate and duration. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by

  12. Migration of accreting giant planets

    NASA Astrophysics Data System (ADS)

    Robert, C.; Crida, A.; Lega, E.; Méheut, H.

    2017-09-01

    Giant planets forming in protoplanetary disks migrate relative to their host star. By repelling the gas in their vicinity, they form gaps in the disk's structure. If they are effectively locked in their gap, it follows that their migration rate is governed by the accretion of the disk itself onto the star, in a so-called type II fashion. Recent results showed however that a locking mechanism was still lacking, and was required to understand how giant planets may survive their disk. We propose that planetary accretion may play this part, and help reach this slow migration regime.

  13. THE EFFECT OF TRANSIENT ACCRETION ON THE SPIN-UP OF MILLISECOND PULSARS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Bhattacharyya, Sudip; Chakrabarty, Deepto, E-mail: sudip@tifr.res.in

    A millisecond pulsar is a neutron star that has been substantially spun up by accretion from a binary companion. A previously unrecognized factor governing the spin evolution of such pulsars is the crucial effect of nonsteady or transient accretion. We numerically compute the evolution of accreting neutron stars through a series of outburst and quiescent phases, considering the drastic variation of the accretion rate and the standard disk–magnetosphere interaction. We find that, for the same long-term average accretion rate, X-ray transients can spin up pulsars to rates several times higher than can persistent accretors, even when the spin-down due tomore » electromagnetic radiation during quiescence is included. We also compute an analytical expression for the equilibrium spin frequency in transients, by taking spin equilibrium to mean that no net angular momentum is transferred to the neutron star in each outburst cycle. We find that the equilibrium spin rate for transients, which depends on the peak accretion rate during outbursts, can be much higher than that for persistent sources. This explains our numerical finding. This finding implies that any meaningful study of neutron star spin and magnetic field distributions requires the inclusion of the transient accretion effect, since most accreting neutron star sources are transients. Our finding also implies the existence of a submillisecond pulsar population, which is not observed. This may point to the need for a competing spin-down mechanism for the fastest-rotating accreting pulsars, such as gravitational radiation.« less

  14. Some new results on charged compact boson stars

    DOE PAGES

    Kumar, Sanjeev; Kulshreshtha, Usha; Kulshreshtha, Daya Shankar; ...

    2017-07-21

    In this work we present some new results obtained in a study of the phase diagram of charged compact boson stars in a theory involving a complex scalar field with a conical potential coupled to a U(1) gauge field and gravity. We here obtain new bifurcation points in this model. We present a detailed discussion of the various regions of the phase diagram with respect to the bifurcation points. The theory is seen to contain rich physics in a particular domain of the phase diagram.

  15. News on the X-ray emission from hot subdwarf stars

    NASA Astrophysics Data System (ADS)

    Palombara, Nicola La; Mereghetti, Sandro

    2017-12-01

    In latest years, the high sensitivity of the instruments on-board the XMM-Newton and Chandra satellites allowed us to explore the properties of the X-ray emission from hot subdwarf stars. The small but growing sample of X-ray detected hot subdwarfs includes binary systems, in which the X-ray emission is due to wind accretion onto a compact companion (white dwarf or neutron star), as well as isolated sdO stars, in which X-rays are probably due to shock instabilities in the wind. X-ray observations of these low-mass stars provide information which can be useful for our understanding of the weak winds of this type of stars and can lead to the discovery of particularly interesting binary systems. Here we report the most recent results we have recently obtained in this research area.

  16. Mapping out the origins of compact stellar systems

    NASA Astrophysics Data System (ADS)

    Romanowsky, Aaron J.; Brodie, Jean P.; SAGES Collaboration

    2017-03-01

    We present a suite of extragalactic explorations of the origins and nature of globular clusters (GCs) and ultra-compact dwarfs (UCDs), and the connections between them. An example of GC metallicity bimodality is shown to reflect underlying, distinct metal-poor and metal-rich stellar halo populations. Metallicity-matching methods are used to trace the birth sites and epochs of GCs in giant E/S0s, pointing to clumpy disk galaxies at z ~ 3 for the metal-rich GCs, and to a combination of accreted and in-situ formation modes at z ~ 5-6 for the metal-poor GCs. An increasingly diverse zoo of compact stellar systems is being discovered, including objects that bridge the gaps between UCDs and faint fuzzies, and between UCDs and compact ellipticals. Many of these have properties pointing to origins as the stripped nuclei of larger galaxies, and a smoking-gun example is presented of an ω Cen-like star cluster embedded in a tidal stream.

  17. Probing the clumpy winds of giant stars with high mass X-ray binaries

    NASA Astrophysics Data System (ADS)

    Grinberg, Victoria; Hell, Natalie; Hirsch, Maria; Garcia, Javier; Huenemoerder, David; Leutenegger, Maurice A.; Nowak, Michael; Pottschmidt, Katja; Schulz, Norbert S.; Sundqvists, Jon O.; Townsend, Richard D.; Wilms, Joern

    2016-04-01

    Line-driven winds from early type stars are structured, with small, overdense clumps embedded in tenuous hot gas. High mass X-ray binaries (HMXBs), systems where a neutron star or a black hole accretes from the line-driven stellar wind of an O/B-type companion, are ideal for studying such winds: the wind drives the accretion onto the compact object and thus the X-ray production. The radiation from close to the compact object is quasi-pointlike and effectively X-rays the wind.We used RXTE and Chandra-HETG observations of two of the brightest HMXBs, Cyg X-1 and Vela X-1, to decipher their wind structure. In Cyg X-1, we show that the orbital variability of absorption can be only explained by a clumpy wind model and constrain the porosity of the wind as well as the onion-like structure of the clumps. In Vela X-1 we show, using the newest reference energies for low ionization Si-lines obtained with LLNL’s EBIT-I, that the ionized phase of the circumstellar medium and the cold clumps have different velocities.

  18. Accretion disks around neutron and strange stars in R + aR {sup 2} gravity

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Staykov, Kalin V.; Yazadjiev, Stoytcho S.; Doneva, Daniela D., E-mail: kstaykov@phys.uni-sofia.bg, E-mail: daniela.doneva@uni-tuebingen.de, E-mail: yazad@phys.uni-sofia.bg

    2016-08-01

    We study the electromagnetic spectrum of accretion disks around neutron and strange stars in R + aR {sup 2} gravity. Both static and rapidly rotating models are investigated. The results are compared with the General Relativistic results. We found difference between the results in both theories of about 50% for the electromagnetic flux and about 20% in the luminosity for models with equal mass and angular velocity in both theories. The observed differences are much lower for models rotating with Keplerian velocity and with equal masses.

  19. NUSTAR and SUZAKU X-ray spectroscopy of NGC 4151: Evidence for reflection from the inner accretion disk

    DOE PAGES

    Keck, M. L.; Brenneman, L. W.; Ballantyne, D. R.; ...

    2015-06-15

    We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spinmore » $$a\\gt 0.9$$ accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection (IDR) flux variation drives the spectral variability during the observations. As a result, we discuss various physical scenarios for the IDR model and we find that a compact corona is consistent with the observed features.« less

  20. NUSTAR and SUZAKU X-ray spectroscopy of NGC 4151: Evidence for reflection from the inner accretion disk

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Keck, M. L.; Brenneman, L. W.; Ballantyne, D. R.

    We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spinmore » $$a\\gt 0.9$$ accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection (IDR) flux variation drives the spectral variability during the observations. As a result, we discuss various physical scenarios for the IDR model and we find that a compact corona is consistent with the observed features.« less

  1. Impact of accretion on the statistics of neutron star masses

    NASA Astrophysics Data System (ADS)

    Cheng, Z.; Taani, A.; Zhao, Y. H.

    2013-02-01

    We have collected the parameter of 38 neutron stars (NSs) in binary systems with spin periods and measured masses. By adopting the Boot-strap method, we reproduced the procedure of mass calculated for each system separately, to determine the truly mass distribution of the NS that obtained from observation. We also applied the Monte-Carlo simulation and introduce the characteristic spin period 20 ms, in order to distinguish between millisecond pulsars (MSPs) and less recycled pulsars. The mass distributions of MSPs and the less recycled pulsars could be fitted by a Gaussian function as 1.45+/-0.42 M⊙ and 1.31+/-0.17 M⊙ (with 1σ) respectively. As such, the MSP masses are heavier than those in less recycled systems by factor of ~ 0.13M⊙, since the accretion effect during the recycling process.

  2. HIERARCHICAL STRUCTURE FORMATION AND MODES OF STAR FORMATION IN HICKSON COMPACT GROUP 31

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Gallagher, S. C.; Durrell, P. R.; Elmegreen, D. M.

    2010-02-15

    The handful of low-mass, late-type galaxies that comprise Hickson Compact Group 31 (HCG 31) is in the midst of complex, ongoing gravitational interactions, evocative of the process of hierarchical structure formation at higher redshifts. With sensitive, multicolor Hubble Space Telescope imaging, we characterize the large population of < 10 Myr old star clusters (SCs) that suffuse the system. From the colors and luminosities of the young SCs, we find that the galaxies in HCG 31 follow the same universal scaling relations as actively star-forming galaxies in the local universe despite the unusual compact group environment. Furthermore, the specific frequency ofmore » the globular cluster system is consistent with the low end of galaxies of comparable masses locally. This, combined with the large mass of neutral hydrogen and tight constraints on the amount of intragroup light, indicate that the group is undergoing its first epoch of interaction-induced star formation. In both the main galaxies and the tidal-dwarf candidate, F, stellar complexes, which are sensitive to the magnitude of disk turbulence, have both sizes and masses more characteristic of z = 1-2 galaxies. After subtracting the light from compact sources, we find no evidence for an underlying old stellar population in F-it appears to be a truly new structure. The low-velocity dispersion of the system components, available reservoir of H I, and current star formation rate of {approx}10 M {sub sun} yr{sup -1} indicate that HCG 31 is likely to both exhaust its cold gas supply and merge within {approx}1 Gyr. We conclude that the end product will be an isolated, X-ray-faint, low-mass elliptical.« less

  3. Equation of state survey of black hole-neutron star mergers

    NASA Astrophysics Data System (ADS)

    Brege, Wyatt

    2016-03-01

    By varying across several realistic equations of state in the regime in which most neutron star masses are most likely to appear, we can study how important a role these EOS's play in the properties of the post-merger accretion disk in mixed binary systems. In each system considered, the black hole has a mass of MBH = 7M⊙ and a spin of a* = 0 . 9 , and the neutron star has a mass of 1.2 or 1.4 M⊙. The realistic EOS's chosen satisfy experimental and observational constraints, and explore a wide range of neutron star compactnesses. We will address remaining uncertainties in the NS high-density EOS's and, principally, examine differences in the dynamical ejecta and consider implications for nucleosynthesis.

  4. Chromospherically active stars. 11: Giant with compact hot companions and the barium star scenario

    NASA Technical Reports Server (NTRS)

    Fekel, Francis C.; Henry, Gregory W.; Busby, Michael R.; Eitter, Joseph J.

    1993-01-01

    We have determined spectroscopic orbits for three chromsopherically active giants that have hot compact companions. They are HD 160538 (KO III + wd, P = 904 days), HD 165141 (G8 III + wd, P approximately 5200 days), and HD 185510 (KO III + sdB, P = 20.6619 days). By fitting an IUE spectrum with theoretical models, we find the white dwarf companion of HD 165141 has a temperature of about 35,000 K. Spectral types and rotational velocities have been determined for the three giants and distances have been estimated. These three systems and 39 Ceti are compared with the barium star mass-transfer scenario. The long-period mild barium giant HD 165141 as well as HD 185510 and 39 Ceti, which have relatively short periods and normal abundance giants, appear to be consistent with this scenario. The last binary, HD 160538, a system with apparently near solar abundances, a white dwarf companion, and orbital characteristics similar to many barium stars, demonstrates that the existence of a white dwarf companion is insufficient to produce a barium star. The paucity of systems with confirmed white dwarf companions makes abundance analyses of HD 160538 and HD 165141 of great value in examining the role of metallicity in barium star formation.

  5. Chromospherically active stars. 6: Giants with compact hot companions and the barium star scenario

    NASA Technical Reports Server (NTRS)

    Fekel, Francis C.; Henry, Gregory W.; Busby, Michael R.; Eitter, Joseph J.

    1993-01-01

    We have determined spectroscopic orbits for three chromospherically active giants that have hot compact companions. They are HD 160538 (K0 III + wd, P = 904 days), HD 165141 (G8 III + wd, P approximately 5200 days), and HD 185510 (K0 III + sdB, P = 20.6619 days). By fitting an IUE spectrum with theoretical models, we find the white dwarf companion of HD 165141 has a temperature of about 35000 K. Spectral types and rotational velocities have been determined for the three giants and distances have been estimated. These three systems and 39 Ceti are compared with the barium star mass-transfer scenario. The long-period mild barium giant HD 165141 as well as HD 185510 and 39 Ceti, which have relatively short periods and normal abundance giants, appear to be consistent with this scenario. The last binary, HD 160538, a system with apparently near solar abundances, a white dwarf companion, and orbital characteristics similar to many barium stars, demonstrates that the existence of a white-dwarf companion is insufficient to produce a barium star. The paucity of systems with confirmed white-dwarf companions makes abundance analyses of HD 160538 and HD 165141 of great value in examining the role of metallicity in barium star formation.

  6. Accretion geometry in the persistent Be/X-ray binary RXJ0440.9+4431

    NASA Astrophysics Data System (ADS)

    Ferrigno, C.; Farinelli, R.; Bozzo, E.; Pottschmidt, K.; Klochkov, D.; Kretschmar, P.

    2014-01-01

    The persistent Be/X-ray binary RXJ0440.9+4431 flared in 2010 and 2011 and has been followed by various X-ray facilities (Swift, RXTE, XMM-Newton, and INTEGRAL). We studied the source timing and spectral properties as a function of its X-ray luminosity to investigate the transition from normal to flaring activity. The source spectrum can always be described by a bulk-motion Comptonization model of black body seed photons attenuated by a moderate photoelectric absorption. At the highest luminosity, we measured a curvature of the spectrum, which we attribute to a significant contribution of the radiation pressure in the accretion process. This allows us to estimate that the transition from a bulk-motion-dominated flow to a radiatively dominated one happens at a luminosity of ~ 2 × 1036 erg s-1. The luminosity dependency of the size of the black body emission region is found to be rBB ∝ LX0.39±0.02. This suggests that either matter accreting onto the neutron star hosted in RXJ0440.9+4431 penetrates through closed magnetic field lines at the border of the compact object magnetosphere or that the size of the black-body emitting hotspot is larger than the footprint of the accretion column. This phenomenon can be due to illumination of the surface by a growing column or by a a structure of the neutron star magnetic field more complicated than a simple dipole at least close to the surface.

  7. Bright Compact Bulges (BCBs) at intermediate redshifts

    NASA Astrophysics Data System (ADS)

    Sachdeva, Sonali; Saha, Kanak

    2018-04-01

    Studying bright (MB < -20), intermediate-redshift (0.4 < z < 1.0), disc dominated (nB < 2.5) galaxies from HST/ACS and WFC3 in Chandra Deep Field South, in rest-frame B and I-band, we found a new class of bulges which is brighter and more compact than ellipticals. We refer to them as "Bright, Compact Bulges" (BCBs) - they resemble neither classical nor pseudo-bulges and constitute ˜12% of the total bulge population at these redshifts. Examining free-bulge + disc decomposition sample and elliptical galaxy sample from Simard et al. (2011), we find that only ˜0.2% of the bulges can be classified as BCBs in the local Universe. Bulge to total ratio (B/T) of disc galaxies with BCBs is (at ˜0.4) a factor of ˜2 and ˜4 larger than for those with classical and pseudo bulges. BCBs are ˜2.5 and ˜6 times more massive than classical and pseudo bulges. Although disc galaxies with BCBs host the most massive and dominant bulge type, their specific star formation rate is 1.5-2 times higher than other disc galaxies. This is contrary to the expectations that a massive compact bulge would lead to lower star formation rates. We speculate that our BCB host disc galaxies are descendant of massive, compact and passive elliptical galaxies observed at higher redshifts. Those high redshift ellipticals lack local counterparts and possibly evolved by acquiring a compact disc around them. The overall properties of BCBs supports a picture of galaxy assembly in which younger discs are being accreted around massive pre-existing spheroids.

  8. Theory of quasi-spherical accretion in X-ray pulsars

    NASA Astrophysics Data System (ADS)

    Shakura, N.; Postnov, K.; Kochetkova, A.; Hjalmarsdotter, L.

    2012-02-01

    A theoretical model for quasi-spherical subsonic accretion on to slowly rotating magnetized neutron stars is constructed. In this model, the accreting matter subsonically settles down on to the rotating magnetosphere forming an extended quasi-static shell. This shell mediates the angular momentum removal from the rotating neutron star magnetosphere during spin-down episodes by large-scale convective motions. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere. The settling regime of accretion can be realized for moderate accretion rates ? g s-1. At higher accretion rates, a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and accretion becomes highly non-stationary. From observations of the spin-up/spin-down rates (the angular rotation frequency derivative ?, and ? near the torque reversal) of X-ray pulsars with known orbital periods, it is possible to determine the main dimensionless parameters of the model, as well as to estimate the magnetic field of the neutron star. We illustrate the model by determining these parameters for three wind-fed X-ray pulsars GX 301-2, Vela X-1 and GX 1+4. The model explains both the spin-up/spin-down of the pulsar frequency on large time-scales and the irregular short-term frequency fluctuations, which can correlate or anticorrelate with the X-ray flux fluctuations in different systems. It is shown that in real pulsars an almost iso-angular-momentum rotation law with ω˜ 1/R2, due to strongly anisotropic radial turbulent motions sustained by large-scale convection, is preferred.

  9. The Optical Green Valley Versus Mid-infrared Canyon in Compact Groups

    NASA Technical Reports Server (NTRS)

    Walker, Lisa May; Butterfield, Natalie; Johnson, Kelsey; Zucker, Catherine; Gallagher, Sarah; Konstantopoulos, Iraklis; Zabludoff, Ann; Hornschemeier, Ann E.; Tzanavaris, Panayiotis; Charlton, Jane C.

    2013-01-01

    Compact groups of galaxies provide conditions similar to those experienced by galaxies in the earlier universe. Recent work on compact groups has led to the discovery of a dearth of mid-infrared transition galaxies (MIRTGs) in Infrared Array Camera (3.6-8.0 micrometers) color space as well as at intermediate specific star formation rates. However, we find that in compact groups these MIRTGs have already transitioned to the optical ([g-r]) red sequence. We investigate the optical color-magnitude diagram (CMD) of 99 compact groups containing 348 galaxies and compare the optical CMD with mid-infrared (mid-IR) color space for compact group galaxies. Utilizing redshifts available from Sloan Digital Sky Survey, we identified new galaxy members for four groups. By combining optical and mid-IR data, we obtain information on both the dust and the stellar populations in compact group galaxies. We also compare with more isolated galaxies and galaxies in the Coma Cluster, which reveals that, similar to clusters, compact groups are dominated by optically red galaxies. While we find that compact group transition galaxies lie on the optical red sequence, LVL (Local Volume Legacy) + (plus) SINGS (Spitzer Infrared Nearby Galaxies Survey) mid-IR (infrared) transition galaxies span the range of optical colors. The dearth of mid-IR transition galaxies in compact groups may be due to a lack of moderately star-forming low mass galaxies; the relative lack of these galaxies could be due to their relatively small gravitational potential wells. This makes them more susceptible to this dynamic environment, thus causing them to more easily lose gas or be accreted by larger members.

  10. IUE observations of long period eclipsing binaries - A study of accretion onto non-degenerate stars

    NASA Technical Reports Server (NTRS)

    Plavec, M. J.

    1980-01-01

    IUE observations made in 1978-1979 recorded a whole class of interacting long-period binaries similar to beta Lyrae, which includes RX Cas, SX Cas, V 367 Cyg, W Cru, beta Lyr, and W Ser, called the W Serpentis stars. These mass-transferring binaries with relatively high mass transfer rate show two prominent features in the far ultraviolet: a continuum with a color temperature higher than the one observed in the optical region (about 12,000 K), and a strong emission line spectrum with the N V doublet at 1240 A, C IV doublet at 1550 A and lines of Si II, Si III, Si IV, C II, Fe III, AI III, etc. These phenomena are discussed on the assumption that they are due to accretion onto non-degenerate stars.

  11. Physical processes in the strong magnetic fields of accreting neutron stars

    NASA Technical Reports Server (NTRS)

    Meszaros, P.

    1984-01-01

    Analytical formulae are fitted to observational data on physical processes occurring in strong magnetic fields surrounding accreting neutron stars. The propagation of normal modes in the presence of a quantizing magnetic field is discussed in terms of a wave equation in Fourier space, quantum electrodynamic effects, polarization and mode ellipticity. The results are applied to calculating the Thomson scattering, bremsstrahlung and Compton scattering cross-sections, which are a function of the frequency, angle and polarization of the magnetic field. Numerical procedures are explored for solving the radiative transfer equations. When applied to modeling X ray pulsars, a problem arises in the necessity to couple the magnetic angle and frequency dependence of the cross-sections with the hydrodynamic equations. The use of time-dependent averaging and approximation techniques is indicated.

  12. Neutron Stars and Black Holes Seen with the Rossi X-Ray Timing Explorer (RXTE)

    NASA Technical Reports Server (NTRS)

    Swank, Jean

    2008-01-01

    Astrophysical X-rays bring information about location, energy, time, and polarization. X-rays from compact objects were seen in the first explorations to vary in time. Eclipses and pulsations have simple explanations that identified the importance of X-ray binaries and magnetic neutron stars in the first decade of X-ray astronomy. The dynamics of accretion onto stellar and supermassive black holes and onto neutron stars with relatively low magnetic fields shows up as more complex variations, quasi-periodic oscillations, noise with characteristic frequency spectra, broad-band changes in the energy spectra. To study these variations, RXTE instruments needed to have large area and operational flexibility to find transient activity and observe when it was present. Proportional counters and Phoswich scintillators provided it in a modest mission that has made textbook level contributions to understanding of compact objects. The first seen, and the brightest known, X-ray binary, Sco X-1 is one of a class of neutron stars with low mass companions. Before RXTE, none of these had been seen to show pulsations, though they were hypothesized to be the precursors of radio pulsars with millisecond periods and low magnetic fields. RXTE's large area led to identifying coherent millisecond pulsars in a subset which are relatively faint transients. It also led to identifying short episodes of pulsation during thermonuclear bursts, in sources where a steady signal is not seen. The X-ray stage verifies the evolution that produces millisecond radio pulsars.Masses and radii of neutron stars are being determined by various techniques, constraining the equation of state of matter at nuclear densities. Accretion should lead to a range of neutron star masses. An early stage of superstrong magnetic field neutron stars is now known to produce X-ray and gamma-ray bursts in crust quakes and magnetic field reconnection releases of energy. Soft Gamma Repeaters, Anomolous X-ray Pulsars, and high

  13. Mass Accretion Processes in Young Stellar Objects: Role of Intense Flaring Activity

    NASA Astrophysics Data System (ADS)

    Orlando, Salvatore; Reale, Fabio; Peres, Giovanni; Mignone, Andrea

    2014-11-01

    According to the magnetospheric accretion scenario, young low-mass stars are surrounded by circumstellar disks which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn have a signicant role in removing the excess angular momentum from the star-disk system. Although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. On the other hand, strong flaring activity is a common feature of young stellar objects (YSOs). In the Sun, such events give rise to perturbations of the interplanetary medium. Similar but more energetic phenomena occur in YSOs and may influence the circumstellar environment. In fact, a recent study has shown that an intense flaring activity close to the disk may strongly perturb the stability of circumstellar disks, thus inducing mass accretion episodes (Orlando et al. 2011). Here we review the main results obtained in the field and the future perspectives.

  14. General polytropic self-gravitating cylinder free-fall and accreting mass string with a chain of collapsed objects

    NASA Astrophysics Data System (ADS)

    Lou, Yu-Qing; Hu, Xu-Yao

    2016-06-01

    We present a theoretical model framework for general polytropic (GP) hydrodynamic cylinder under self-gravity of infinite length with axial uniformity and axisymmetry. For self-similar dynamic solutions, we derive valuable integrals, analytic asymptotic solutions, sonic critical curves, shock conditions, and global numerical solutions with or without expansion shocks. Among others, we investigate various dynamic solutions featured with central free-fall asymptotic behaviours, corresponding to a collapsed mass string with a sustained dynamic accretion from a surrounding mass reservoir. Depending on the allowed ranges of a scaling index a < -1, such cylindrical dynamic mass accretion rate could be steady, increasing with time and decreasing with time. Physically, such a collapsed mass string or filament would break up into a sequence of sub-clumps and segments as induced by gravitational Jeans instabilities. Depending on the scales involved, such sub-clumps would evolve into collapsed objects or gravitationally bound systems. In diverse astrophysical and cosmological contexts, such a scenario can be adapted on various temporal, spatial and mass scales to form a chain of collapsed clumps and/or compact objects. Examples include the formation of chains of proto-stars, brown dwarfs and gaseous planets along molecular filaments; the formation of luminous massive stars along magnetized spiral arms and circum-nuclear starburst rings in barred spiral galaxies; the formation of chains of compact stellar objects such as white dwarfs, neutron stars, and black holes along a highly condensed mass string. On cosmological scales, one can perceive the formation of chains of galaxies, chains of galaxy clusters or even chains of supermassive and hypermassive black holes in the Universe including the early Universe. All these chains referred to above include possible binaries.

  15. Accretion dynamics in pre-main sequence binaries

    NASA Astrophysics Data System (ADS)

    Tofflemire, B.; Mathieu, R.; Herczeg, G.; Ardila, D.; Akeson, R.; Ciardi, D.; Johns-Krull, C.

    Binary stars are a common outcome of star formation. Orbital resonances, especially in short-period systems, are capable of reshaping the distribution and flows of circumstellar material. Simulations of the binary-disk interaction predict a dynamically cleared gap around the central binary, accompanied by periodic ``pulsed'' accretion events that are driven by orbital motion. To place observational constraints on the binary-disk interaction, we have conducted a long-term monitoring program tracing the time-variable accretion behavior of 9 short-period binaries. In this proceeding we present two results from our campaign: 1) the detection of periodic pulsed accretion events in DQ Tau and TWA 3A, and 2) evidence that the TWA 3A primary is the dominant accretor in the system.

  16. Buchdahl compactness limit for a pure Lovelock static fluid star

    NASA Astrophysics Data System (ADS)

    Dadhich, Naresh; Chakraborty, Sumanta

    2017-03-01

    We obtain the Buchdahl compactness limit for a pure Lovelock static fluid star and verify that the limit following from the uniform-density Schwarzschild's interior solution, which is universal irrespective of the gravitational theory (Einstein or Lovelock), is true in general. In terms of surface potential Φ (r ) , it means at the surface of the star r =r0, Φ (r0)<2 N (d -N -1 )/(d -1 )2, where d and N indicate spacetime dimensions and Lovelock order, respectively. For a given N , Φ (r0) is maximum for d =2 N +2 , while it is always 4 /9 , Buchdahl's limit, for d =3 N +1 . It is also remarkable that for N =1 Einstein gravity, or for pure Lovelock in d =3 N +1 , Buchdahl's limit is equivalent to the criterion that gravitational field energy exterior to the star must be less than half its gravitational mass, having no reference to the interior at all.

  17. Photo-Reverberation Mapping of a Protoplanetary Accretion Disk around a T Tauri star

    NASA Astrophysics Data System (ADS)

    Meng, Huan; Plavchan, Peter; Rieke, George

    2015-12-01

    Theoretical models and spectroscopic observations of newborn stars suggest that protoplantary disks have an inner "wall", where material is depleted by sublimation and/or magnetospheric accretion. Around T Tauri stars, the size of this disk hole is expected to be on a 0.1-AU scale that is unresolved by current adaptive optics imaging, though some model-dependent constraints have been obtained by near-infrared interferometry. Here we report the first measurement of the inner disk wall around a solar-mass young stellar object, YLW 16B in the ρ Ophiuchi star-forming region, by detecting the light travel time of the variable radiation from the stellar surface to the disk. Consistent time lags were detected on two nights, when the time series in H and K bands were synchronized while the 4.5 μm emission lagged by 74.5 ± 3.2 seconds. Considering the nearly edge-on geometry of the disk, the inner rim should be 0.084 ± 0.004 AU from the protostar on average. This size is likely larger than the range of magnetospheric truncations, but consistent with an optically and geometrically thick disk front at the dust sublimation radius of ~1500 K. The detection of a definite time lag places constraints on the geometry of the disk.

  18. Evolving ONe WD+He star systems to intermediate-mass binary pulsars

    NASA Astrophysics Data System (ADS)

    Liu, D.; Wang, B.; Chen, W.; Zuo, Z.; Han, Z.

    2018-06-01

    It has been suggested that accretion-induced collapse (AIC) is a non-negligible path for the formation of the observed neutron stars (NSs). An ONe white dwarf (WD) that accretes material from a He star may experience AIC process and eventually produce intermediate-mass binary pulsars (IMBPs), named as the ONe WD+He star scenario. Note that previous studies can only account for part of the observed IMBPs with short orbital periods. In this work, we investigate the evolution of about 900 ONe WD+He star binaries to explore the distribution of IMBPs. We found that the ONe WD+He star scenario could form IMBPs including pulsars with 5-340 ms spin periods and 0.75-1.38 M_{⊙} WD companions, in which the orbital periods range from 0.04 to 900 d. Compared with the 20 observed IMBPs, this scenario can cover the parameters of 13 sources in the final orbital period-WD mass plane and the Corbet diagram, most of which have short orbital periods. We found that the ONe WD+He star scenario can explain almost all the observed IMBPs with short orbital periods. This work can well match the observed parameters of PSR J1802-2124 (one of the two precisely observed IMBPs), providing a possible evolutional path for its formation. We also speculate that the compact companion of HD 49798 (a hydrogen depleted sdO6 star) may be not a NS based on this work.

  19. A brown dwarf mass donor in an accreting binary.

    PubMed

    Littlefair, S P; Dhillon, V S; Marsh, T R; Gänsicke, Boris T; Southworth, John; Watson, C A

    2006-12-08

    A long-standing and unverified prediction of binary star evolution theory is the existence of a population of white dwarfs accreting from substellar donor stars. Such systems ought to be common, but the difficulty of finding them, combined with the challenge of detecting the donor against the light from accretion, means that no donor star to date has a measured mass below the hydrogen burning limit. We applied a technique that allowed us to reliably measure the mass of the unseen donor star in eclipsing systems. We were able to identify a brown dwarf donor star, with a mass of 0.052 +/- 0.002 solar mass. The relatively high mass of the donor star for its orbital period suggests that current evolutionary models may underestimate the radii of brown dwarfs.

  20. Building the Hot Intra-Group Medium in Spiral-Rich Compact Groups

    NASA Astrophysics Data System (ADS)

    O'Sullivan, Ewan

    2014-11-01

    Galaxy groups provide a natural laboratory for investigating the formation of the hot intergalactic medium (IGM). While galaxy clusters gain most of their hot gas through accretion and gravitational shocks, in groups the processes of galaxy evolution (stripping, collisions, star formation) play an important role in the initial build up of the hot halo. We present Chandra and XMM-Newton observations of groups still in the process of forming their IGM, including the well known compact groups HCG 16 and Stephan's Quintet (HCG 92). We show that starburst winds and shock-heating of stripped HI provide important contributions of gas and metals to the IGM, and discuss the impact of gas stripping, enhanced star formation and nuclear activity in the group member galaxies.

  1. Transitional millisecond pulsars in the low-level accretion state

    NASA Astrophysics Data System (ADS)

    Jaodard, Amruta D.; Hessels, Jason W. T.; Archibald, Anne; Bogdanov, Slavko; Deller, Adam; Hernandez Santisteban, Juan; Patruno, Alessandro; D'Angelo, Caroline; Bassa, Cees; Amruta Jaodand

    2018-01-01

    In the canonical pulsar recycling scenario, a slowly spinning neutron star can be rejuvenated to rapid spin rates by the transfer of angular momentum and mass from a binary companion star. Over the last decade, the discovery of three transitional millisecond pulsars (tMSPs) has allowed us to study recycling in detail. These systems transition between accretion-powered (X-ray) and rotation-powered (radio) pulsar states within just a few days, raising questions such as: what triggers the state transition, when does the recycling process truly end, and what will the radio pulsar’s final spin rate be? Systematic multi-wavelength campaigns over the last decade have provided critical insights: multi-year-long, low-level accretion states showing coherent X-ray pulsations; extremely stable, bi-modal X-ray light curves; outflows probed by radio continuum emission; a surprising gamma-ray brightening during accretion, etc. In my thesis I am trying to bring these clues together to understand the low-level accretion process that recycles a pulsar. For example, recently we timed PSR J1023+0038 in the accretion state and found it to be spinning down ~26% faster compared to the non-accreting radio pulsar state. We are currently conducting simultaneous multi-wavelength campaigns (XMM, HST, Kepler and VLA) to understand the global variability of the accretion flow, as well as high-energy Fermi-LAT observations to probe the gamma-ray emission mechanism. I will highlight these recent developments, while also presenting a broad overview of tMSPs as exciting new laboratories to test low-level accretion onto magnetized neutron stars.

  2. NuSTAR rules out a cyclotron line in the accreting magnetar candidate 4U2206+54.

    NASA Astrophysics Data System (ADS)

    Torrejón, J. M.; Reig, P.; Fürst, F.; Martinez-Chicharro, M.; Postnov, K.; Oskinova, L.

    2018-06-01

    Based on our new NuSTAR X-ray telescope data, we rule out any cyclotron line up to 60 keV in the spectra of the high mass X-ray binary 4U2206+54. In particular, we do not find any evidence of the previously claimed line around 30 keV, independently of the source flux, along the spin pulse. The spin period has increased significantly, since the last observation, up to 5750 ± 10 s, confirming the rapid spin down rate \\dot{ν }=-1.8× 10^{-14} Hz s-1. This behaviour might be explained by the presence of a strongly magnetized neutron star (Bs > several times 1013 G) accreting from the slow wind of its main sequence O9.5 companion.

  3. On Stellar Winds as a Source of Mass: Applying Bondi-Hoyle-Lyttleton Accretion

    NASA Astrophysics Data System (ADS)

    Detweiler, L. G.; Yates, K.; Siem, E.

    2017-12-01

    The interaction between planets orbiting stars and the stellar wind that stars emit is investigated and explored. The main goal of this research is to devise a method of calculating the amount of mass accumulated by an arbitrary planet from the stellar wind of its parent star via accretion processes. To achieve this goal, the Bondi-Hoyle-Lyttleton (BHL) mass accretion rate equation and model is employed. In order to use the BHL equation, various parameters of the stellar wind is required to be known, including the velocity, density, and speed of sound of the wind. In order to create a method that is applicable to arbitrary planets orbiting arbitrary stars, Eugene Parker's isothermal stellar wind model is used to calculate these stellar wind parameters. In an isothermal wind, the speed of sound is simple to compute, however the velocity and density equations are transcendental and so the solutions must be approximated using a numerical approximation method. By combining Eugene Parker's isothermal stellar wind model with the BHL accretion equation, a method for computing planetary accretion rates inside a star's stellar wind is realized. This method is then applied to a variety of scenarios. First, this method is used to calculate the amount of mass that our solar system's planets will accrete from the solar wind throughout our Sun's lifetime. Then, some theoretical situations are considered. We consider the amount of mass various brown dwarfs would accrete from the solar wind of our Sun throughout its lifetime if they were orbiting the Sun at Jupiter's distance. For very high mass brown dwarfs, a significant amount of mass is accreted. In the case of the brown dwarf 15 Sagittae B, it actually accretes enough mass to surpass the mass limit for hydrogen fusion. Since 15 Sagittae B is orbiting a star that is very similar to our Sun, this encouraged making calculations for 15 Sagittae B orbiting our Sun at its true distance from its star, 15 Sagittae. It was found that

  4. The structure and spectrum of the accretion shock in the atmospheres of young stars

    NASA Astrophysics Data System (ADS)

    Dodin, Alexandr

    2018-04-01

    The structure and spectrum of the accretion shock have been self-consistently simulated for a wide range of parameters typical for Classical T Tauri Stars (CTTS). Radiative cooling of the shocked gas was calculated, taking into account the self-absorption and non-equilibrium (time-dependent) effects in the level populations. These effects modify the standard cooling curve for an optically thin plasma in coronal equilibrium, however the shape of high-temperature (T > 3 × 105 K) part of the curve remains unchanged. The applied methods allow us to smoothly describe the transition from the cooling flow to the hydrostatic stellar atmosphere. Thanks to this approach, it has been found that the narrow component of He II lines is formed predominantly in the irradiated stationary atmosphere (hotspot), i.e. at velocities of the settling gas <2 km s-1. The structure of the pre-shock region is calculated simultaneously with the heated atmosphere. The simulation shows that the pre-shock gas produces a noticeable emission component in He II lines and practically does not manifest itself in He I lines (λλ 5876, 10830 Å). The ultraviolet spectrum of the hotspot is distorted by the pre-shock gas, namely numerous red-shifted emission and absorption lines overlap each other forming a pseudo-continuum. The spectrum of the accretion region at high pre-shock densities ˜1014 cm-3 is fully formed in the in-falling gas and can be qualitatively described as a spectrum of a star with an effective temperature derived from the Stefan-Boltzmann law via the full energy flux.

  5. Hydrodynamic modelling of accretion impacts in classical T Tauri stars: radiative heating of the pre-shock plasma

    NASA Astrophysics Data System (ADS)

    Costa, G.; Orlando, S.; Peres, G.; Argiroffi, C.; Bonito, R.

    2017-01-01

    Context. It is generally accepted that, in classical T Tauri stars, the plasma from the circumstellar disc accretes onto the stellar surface with free-fall velocity and the impact generates a shock. The impact region is expected to contribute to emission in different spectral bands; many studies have confirmed that the X-rays arise from the post-shock plasma but, otherwise, there are no studies in the literature investigating the origin of the observed UV emission which is apparently correlated to accretion. Aims: We investigated the effect of radiative heating of the infalling material by the post-shock plasma at the base of the accretion stream, with the aim to identify in which region a significant part of the UV emission originates. Methods: We developed a one-dimensional hydrodynamic model describing the impact of an accretion stream onto the stellar surface; the model takes into account the gravity, the radiative cooling of an optically thin plasma, the thermal conduction, and the heating due to absorption of X-ray radiation. The latter term represents the heating of the infalling plasma due to the absorption of X-rays emitted from the post-shock region. Results: We found that the radiative heating of the pre-shock plasma plays a non-negligible role in the accretion phenomenon. In particular, the dense and cold plasma of the pre-shock accretion column is gradually heated up to a few 105K due to irradiation of X-rays arising from the shocked plasma at the impact region. This heating mechanism does not affect significantly the dynamics of the post-shock plasma. On the other hand, a region of radiatively heated gas (that we consider a precursor) forms in the unshocked accretion column and contributes significantly to UV emission. Our model naturally reproduces the luminosity of UV emission lines correlated to accretion and shows that most of the UV emission originates from the precursor.

  6. The SW Sextantis-type star 2MASS J01074282+4845188: an unusual bright accretion disk with non-steady emission and a hot white dwarf

    NASA Astrophysics Data System (ADS)

    Khruzina, T.; Dimitrov, D.; Kjurkchieva, D.

    2013-03-01

    Context. Cataclysmic variables (CVs) present a short evolutional stage of binary systems. The nova-like stars are rare objects, especially those with eclipses (only several tens). But precisely these allow to determine the global parameters of their configurations and to learn more about the late stage of stellar evolution. Aims: The light curve solution allows one to determine the global parameters of the newly discovered nova-like eclipsing star 2MASS J01074282+4845188 and to estimate the contribution of the different light sources. Methods: We present new photometric and spectral observations of 2MASS J01074282+4845188. To obtain a light curve solution we used a model of a nova-like star whose emission sources are a white dwarf surrounded by an accretion disk, a secondary star filling its Roche lobe, a hot spot and a hot line. The obtained global parameters are compared with those of the eclipsing nova-like UX UMa. Results: 2MASS J01074282+4845188 shows the deepest permanent eclipse among the known nova-like stars. It is reproduced by covering the very bright accretion disk by the secondary component. The luminosity of the disk is much bigger than that of the rest light sources. The determined high temperature of the disk is typical for that observed during the outbursts of CVs. The primary of 2MASS J01074282+4845188 is one of the hottest white dwarfs in CVs. The temperature of 5090 K of its secondary is also quite high and more appropriate for a long-period SW Sex star. It might be explained by the intense heating from the hot white dwarf and the hot accretion disk of the target. Conclusions: The high mass accretion rate Ṁ = 8 × 10-9 M⊙ yr-1, the broad and single-peaked Hα emission profile, and the presence of an S-wave are sure signs for the SW Sex classification of 2MASS J01074282+4845188. The obtained flat temperature distribution along the disk radius as well as the deviation of the energy distribution from the black-body law are evidence of the non

  7. Spontaneous Scalarization of Black Holes and Compact Stars from a Gauss-Bonnet Coupling.

    PubMed

    Silva, Hector O; Sakstein, Jeremy; Gualtieri, Leonardo; Sotiriou, Thomas P; Berti, Emanuele

    2018-03-30

    We identify a class of scalar-tensor theories with coupling between the scalar and the Gauss-Bonnet invariant that exhibit spontaneous scalarization for both black holes and compact stars. In particular, these theories formally admit all of the stationary solutions of general relativity, but these are not dynamically preferred if certain conditions are satisfied. Remarkably, black holes exhibit scalarization if their mass lies within one of many narrow bands. We find evidence that scalarization can occur in neutron stars as well.

  8. Nucleosynthesis inside Supernova-Driven Supercritical Accretion Disks

    NASA Astrophysics Data System (ADS)

    Fujimoto, Shin-Ichirou; Arai, Kenzo; Matsuba, Ryuichi; Hashimoto, Masa-Aki; Koike, Osamu; Mineshige, Shin

    2001-06-01

    We have investigated nucleosynthesis in a supercritical accretion disk around a compact object of 1.4Msolar, using the self-similar solution of an optically thick advection dominated flow. Supercritical accretion is expected to occur in a supernova with fallback material accreting onto a new-born compact object. It has been found that appreciable nuclear reactions take place even for a reasonable value of the viscosity parameter, αvissimeq 0.01, when the accretion rate dot{m}=dot{M}c2/(16LEdd) > 105, where LEdd is the Eddington luminosity. If dot{m} ge 4 × 106, all heavy elements are destroyed to 4He through photodisintegrations at the inner part of the disk. Even 4He is also disintegrated to protons and neutrons near the inner edge when dot{m} ge 2 × 107. If the fallback matter of the supernova explosion has the composition of a helium-rich layer of the progenitor, a considerable amount of 44Ti could be ejected via a jet from the disk.

  9. Type I X-Ray Bursts at Low Accretion Rates

    NASA Astrophysics Data System (ADS)

    Peng, Fang; Brown, E. F.; Truran, J. W.

    2006-06-01

    Neutron stars, with their strong surface gravity, have interestingly short timescales for the sedimentation of heavy elements. Recent observations of unstable thermonuclear burning (observed as X-ray bursts) on the surfaces of slowly accreting neutron stars (< 0.01 of the Eddington rate) motivate us to examine how sedimentation of CNO isotopes affects the ignition of these bursts. For neutron stars accreting at rates less than 0.1 Eddington, there is sufficient time for CNO to settle out of the accreted envelope. We estimate the burst development using a simple one-zone model with a full reaction network. At the lowest accretion rates, 0.1 Eddington, there can still be an effect. We note that the reduced proton-to-seed ratio favors the production of 12C--an important ingredient for subsequent superbursts.This work is supported by the U.S. DOE under grant B523820 to the Center for Astrophysical Thermonuclear Flashes at the University of Chicago, JINA under NSF-PFC grant PHY 02-16783, NSF under grant AST-0507456 and U.S. DOE under contract No. W-31-109-ENG-38.

  10. Filling a SMBH accretion disk atmosphere at small and intermediate radii

    NASA Astrophysics Data System (ADS)

    Karas, Vladimir; Czerny, Bozena; Kunneriath, Devaky

    2017-08-01

    The medium above an accretion disk is highly diluted and hot. An efficient mechanism to deliver particles and dust grains is an open question; apparently, different processes must be in operation. We discuss an interplay of two different scenarios, where the material is elevated from the plane of an equatorial accretion disk into a corona near a supermassive black hole: (i) an electromagnetically induced transport, which can be driven by magnetic field of stars passing across an accretion disk (Karas et al., 2017); and (ii) radiatively driven acceleration by radiation emerging from the disk (Czerny et al 2015), which can launch a dusty wind near above the dust sublimation radius. The former process can operate in the vicinity of a supermassive black hole (SMBH) surrounded by a dense nuclear star-cluster. The latter process involves the effect of radiation pressure from various sources - stars, accretion disc, and the central accreting SMBH; it can help filling the Broad-Line Region against the vertical component of the black hole gravitational attraction and the accretion disk self-gravity at radius about a few $\\times 10^3 R_g$.

  11. Contrasting Behaviour from Two Be/X-ray Binary Pulsars: Insights into Differing Neutron Star Accretion Modes

    NASA Technical Reports Server (NTRS)

    Townsend, L. J.; Drave, S. P.; Hill, A. B.; Coe, M. J.; Corbet, R. H. D.; Bird, A. J.

    2013-01-01

    In this paper we present the identification of two periodic X-ray signals coming from the direction of the Small Magellanic Cloud (SMC). On detection with the Rossi X-ray Timing Explorer (RXTE), the 175.4 s and 85.4 s pulsations were considered to originate from new Be/X-ray binary (BeXRB) pulsars with unknown locations. Using rapid follow-up INTEGRAL and XMM-Newton observations, we show the first pulsar (designated SXP175) to be coincident with a candidate high-mass X-ray binary (HMXB) in the northern bar region of the SMC undergoing a small Type II outburst. The orbital period (87d) and spectral class (B0-B0.5IIIe) of this system are determined and presented here for the first time. The second pulsar is shown not to be new at all, but is consistent with being SXP91.1 - a pulsar discovered at the very beginning of the 13 year long RXTE key monitoring programme of the SMC. Whilst it is theoretically possible for accreting neutron stars to change spin period so dramatically over such a short time, the X-ray and optical data available for this source suggest this spin-up is continuous during long phases of X-ray quiescence, where accretion driven spin-up of the neutron star should be minimal.

  12. On the 'flip-flop' instability of Bondi-Hoyle accretion flows

    NASA Technical Reports Server (NTRS)

    Livio, Mario; Soker, Noam; Matsuda, Takuya; Anzer, Ulrich

    1991-01-01

    A simple physical interpretation is advanced by means of an analysis of the shock cone in the accretion flows past a compact object and with an examination of the accretion-line stability analyses. The stability of the conical shock is examined against small angular deflections with attention given to several simplifying assumptions. A line instability is identified in the Bondi-Hoyle accretion flows that leads to the formation of a large opening-angle shock. When the opening angle becomes large the instability becomes irregular oscillation. The analytical methodology is compared to previous numerical configurations that demonstrate different shock morphologies. The Bondi-Hoyle accretion onto a compact object is concluded to generate a range of nonlinear instabilities in both homogeneous and inhomogeneous cases with a quasiperiodic oscillation in the linear regime.

  13. Constraining the number of compact remnants near Sgr A*

    NASA Astrophysics Data System (ADS)

    Deegan, Patrick; Nayakshin, Sergei

    2007-05-01

    Due to dynamical friction stellar mass black holes and neutron stars are expected to form high-density cusps in the inner parsec of our Galaxy. These compact remnants, expected to number around 20000, may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. Here we build a simple but detailed time-dependent model of such emission. The possibility that these accretion flows are radiatively inefficient is taken into account and brings in some uncertainty in the conclusions. Despite this uncertainty, we find that at least several X-ray sources of this nature should be detectable with Chandra at any one time. Turning this issue around, we also ask a question of what current observational constraints might be telling us about the total number of compact remnants. In our `best guess' model, a cusp of ~40000 remnants overpredicts the number of discrete sources and the total X-ray luminosity of the inner parsec, and is hence ruled out. In the most radiatively inefficient scenario that we consider, the radiative efficiency is set to be as small as ɛ = 10-5. In this rather unlikely scenario, a cusp of ~40000 black holes would be allowed by the data, but several individual sources should still be visible. Future observations of the distribution and orbits of the cold ionized gas in the inner parsec of our Galaxy will put tighter constraints on the cusp of compact remnants.

  14. Searching for X-ray Pulsations from Neutron Stars Using NICER

    NASA Astrophysics Data System (ADS)

    Ray, Paul S.; Arzoumanian, Zaven; Bogdanov, Slavko; Bult, Peter; Chakrabarty, Deepto; Guillot, Sebastien; Kust Harding, Alice; Ho, Wynn C. G.; Lamb, Frederick K.; Mahmoodifar, Simin; Miller, M. Coleman; Strohmayer, Tod E.; Wilson-Hodge, Colleen A.; Wolff, Michael Thomas

    2017-08-01

    The Neutron Star Interior Composition Explorer (NICER) presents an exciting new capability for discovering new modulation properties of X-ray emitting neutron stars, including large area, low background, extremely precise absolute time stamps, superb low-energy response and flexible scheduling. The Pulsation Searches and Multiwavelength Coordination working group has designed a 2.5 Ms observing program to search for pulsations and characterize the modulation properties of about 30 known or suspected neutron star sources across a number of source categories. A key early goal will be to search for pulsations from millisecond pulsars that might exhibit thermal pulsations from the surface suitable for pulse profile modeling to constrain the neutron star equation of state. In addition, we will search for pulsations from transitional millisecond pulsars, isolated neutron stars, LMXBs, accretion-powered millisecond pulsars, central compact objects and other sources. We will present our science plan and initial results from the first months of the NICER mission.

  15. Searching for X-ray Pulsations from Neutron Stars Using NICER

    NASA Astrophysics Data System (ADS)

    Ray, Paul S.; Arzoumanian, Zaven; Gendreau, Keith C.; Bogdanov, Slavko; Bult, Peter; Chakrabarty, Deepto; Chakrabarty, Deepto; Guillot, Sebastien; Harding, Alice; Ho, Wynn C. G.; Lamb, Frederick; Mahmoodifar, Simin; Miller, Cole; Strohmayer, Tod; Wilson-Hodge, Colleen; Wolff, Michael T.; NICER Science Team Working Group on Pulsation Searches and Multiwavelength Coordination

    2018-01-01

    The Neutron Star Interior Composition Explorer (NICER) presents an exciting new capability for discovering new modulation properties of X-ray emitting neutron stars, including large area, low background, extremely precise absolute time stamps, superb low-energy response and flexible scheduling. The Pulsation Searches and Multiwavelength Coordination working group has designed a 2.5 Ms observing program to search for pulsations and characterize the modulation properties of about 30 known or suspected neutron star sources across a number of source categories. A key early goal will be to search for pulsations from millisecond pulsars that might exhibit thermal pulsations from the surface suitable for pulse profile modeling to constrain the neutron star equation of state. In addition, we will search for pulsations from transitional millisecond pulsars, isolated neutron stars, LMXBs, accretion-powered millisecond pulsars, central compact objects and other sources. We present our science plan and initial results from the first months of the NICER mission.

  16. Accretion-driven turbulence in filaments - I. Non-gravitational accretion

    NASA Astrophysics Data System (ADS)

    Heigl, S.; Burkert, A.; Gritschneder, M.

    2018-03-01

    We study accretion-driven turbulence for different inflow velocities in star-forming filaments using the code RAMSES. Filaments are rarely isolated objects and their gravitational potential will lead to radially dominated accretion. In the non-gravitational case, accretion by itself can already provoke non-isotropic, radially dominated turbulent motions responsible for the complex structure and non-thermal line widths observed in filaments. We find that there is a direct linear relation between the absolute value of the total density-weighted velocity dispersion and the infall velocity. The turbulent velocity dispersion in the filaments is independent of sound speed or any net flow along the filament. We show that the density-weighted velocity dispersion acts as an additional pressure term, supporting the filament in hydrostatic equilibrium. Comparing to observations, we find that the projected non-thermal line width variation is generally subsonic independent of inflow velocity.

  17. Accretion Disks and Coronae in the X-Ray Flashlight

    NASA Astrophysics Data System (ADS)

    Degenaar, Nathalie; Ballantyne, David R.; Belloni, Tomaso; Chakraborty, Manoneeta; Chen, Yu-Peng; Ji, Long; Kretschmar, Peter; Kuulkers, Erik; Li, Jian; Maccarone, Thomas J.; Malzac, Julien; Zhang, Shu; Zhang, Shuang-Nan

    2018-02-01

    Plasma accreted onto the surface of a neutron star can ignite due to unstable thermonuclear burning and produce a bright flash of X-ray emission called a Type-I X-ray burst. Such events are very common; thousands have been observed to date from over a hundred accreting neutron stars. The intense, often Eddington-limited, radiation generated in these thermonuclear explosions can have a discernible effect on the surrounding accretion flow that consists of an accretion disk and a hot electron corona. Type-I X-ray bursts can therefore serve as direct, repeating probes of the internal dynamics of the accretion process. In this work we review and interpret the observational evidence for the impact that Type-I X-ray bursts have on accretion disks and coronae. We also provide an outlook of how to make further progress in this research field with prospective experiments and analysis techniques, and by exploiting the technical capabilities of the new and concept X-ray missions ASTROSAT, NICER, Insight-HXMT, eXTP, and STROBE-X.

  18. Theoretical models for stellar X-ray polarization in compact objects

    NASA Technical Reports Server (NTRS)

    Meszaros, P.

    1991-01-01

    Degenerate stellar objects are expected to be strong sources of polarized X-ray emission. This is particularly true for strongly magnetized neutron stars, e.g. accretion or rotation powered pulsars, and gamma ray bursters. In these, linear polarization degrees well in excess of 30 percent are expected. Weaker magnetic field stellar sources, such as old neutron stars in low mass binary systems, white dwarfs and black holes are expected to have polarization degrees in the range 1-3 percent. A great interest attaches to the detection of polarization in these objects, since this would provide invaluable information concerning the geometry, radiation mechanism and magnetic field strength, necessary for testing and proving models of the structure and evolution of stars in their late stages. In this paper we review the theoretical models of the production of polarized radiation in compact stellar X-ray sources, and discuss the possibility of detecting these properties using currently planned detectors to be flown in space.

  19. X-ray observations of the accreting Be/X-ray binary pulsar A 0535+26 in outburst

    NASA Astrophysics Data System (ADS)

    Caballero, I.

    2009-04-01

    Neutron stars are compact objects, characterized by R~10-14 km radius, M~1.4Msun and extremely high central densities ~10e15 g/cm^3. If they are part of a binary system, a flow of matter can take place from the companion star onto the neutron star. The accretion of matter onto neutron stars is one of the most powerful sources of energy in the universe. The accretion of matter takes place under extreme physical conditions, with magnetic fields in the range B~10^(8-15)G, which are impossible to reproduce on terrestrial laboratories. Therefore, accreting neutron stars are unique laboratories to study the matter under extreme conditions. In this thesis, X-ray observations of the accreting Be/X-ray binary A 0535+26 during a normal (type I) outburst are presented. In this system, the neutron star orbits around the optical companion HDE 245770 in an eccentric orbit, and sometimes presents X-ray outbursts (giant or normal) associated with the passage of the neutron star through the periastron. After more than eleven years of quiescence, A 0535+26 showed outbursting activity in 2005. The normal outburst analyzed in this work took place in August/September 2005, and reached a maximum X-ray flux of ~400 mCrab in the 5-100 kev range. The outburst, which lasted for ~30 days, was observed with the RXTE and INTEGRAL observatories. We have measured the spectrum of the source. In particular, two absorption-like features, interpreted as fundamental and first harmonic cyclotron resonant scattering features, have been detected at E~46 kev and E~102 kev with INTEGRAL and RXTE. Cyclotron lines are the only direct way to measure the magnetic field of a neutron star. Our observations have allowed to confirm the magnetic field of A 0535+26 at the site of the X-ray emission to be B~5x10^12 G. We studied the luminosity dependence of the cyclotron line in A 0535+26, and contrary to other sources, we found no significant variation of the cyclotron line energy with the luminosity. Changes of

  20. I-Love-Q anisotropically: Universal relations for compact stars with scalar pressure anisotropy

    NASA Astrophysics Data System (ADS)

    Yagi, Kent; Yunes, Nicolás

    2015-06-01

    Certain physical quantities that characterize neutron stars and quark stars (e.g. their mass, spin angular momentum, and quadrupole moment) have recently been found to be interrelated in a manner that is approximately insensitive to their internal structure. Such approximately universal relations are useful to break degeneracies in data analysis and model selection for future radio, x-ray, and gravitational wave observations. Although the pressure inside compact stars is most likely nearly isotropic, certain scenarios have been put forth that suggest otherwise, for example due to magnetic fields or phase transitions in their interior. We investigate here whether pressure anisotropy affects the approximate universal relations and, if so, whether it prevents their use in future astrophysical observations. We achieve this by numerically constructing slowly rotating and tidally deformed, anisotropic, compact stars in general relativity to third order in stellar rotation relative to the mass shedding limit. We adopt simple models for pressure anisotropy where the matter stress-energy tensor is diagonal for a spherically symmetric spacetime but the tangential pressure differs from the radial one. We find that the equation-of-state variation increases as one increases the amount of anisotropy, but within the anisotropy range studied in this paper (motivated from anisotropy due to crystallization of the core and pion condensation), anisotropy affects the universal relations only weakly. The relations become less universal by a factor of 1.5-3 relative to the isotropic case when anisotropy is maximal, but even then they remain approximately universal to 10%. We find evidence that this increase in variability is strongly correlated to an increase in the eccentricity variation of isodensity contours, which provides further support for the emergent approximate symmetry explanation of universality. Whether one can use universal relations in actual observations ultimately

  1. Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

    NASA Astrophysics Data System (ADS)

    Orlando, S.; Bonito, R.; Argiroffi, C.; Reale, F.; Peres, G.; Miceli, M.; Matsakos, T.; Stehlé, C.; Ibgui, L.; de Sa, L.; Chièze, J. P.; Lanz, T.

    2013-11-01

    Context. According to the magnetospheric accretion model, hot spots form on the surface of classical T Tauri stars (CTTSs) in regions where accreting disk material impacts the stellar surface at supersonic velocity, generating a shock. Aims: We investigate the dynamics and stability of postshock plasma that streams along nonuniform stellar magnetic fields at the impact region of accretion columns. We study how the magnetic field configuration and strength determine the structure, geometry, and location of the shock-heated plasma. Methods: We model the impact of an accretion stream onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our model considers the gravity, the radiative cooling, and the magnetic-field-oriented thermal conduction (including the effects of heat flux saturation). We explore different configurations and strengths of the magnetic field. Results: The structure, stability, and location of the shocked plasma strongly depend on the configuration and strength of the magnetic field. In the case of weak magnetic fields (plasma β ≳ 1 in the postshock region), a large component of B may develop perpendicular to the stream at the base of the accretion column, which limits the sinking of the shocked plasma into the chromosphere and perturbs the overstable shock oscillations induced by radiative cooling. An envelope of dense and cold chromospheric material may also develop around the shocked column. For strong magnetic fields (β < 1 in the postshock region close to the chromosphere), the field configuration determines the position of the shock and its stand-off height. If the field is strongly tapered close to the chromosphere, an oblique shock may form well above the stellar surface at the height where the plasma β ≈ 1. In general, we find that a nonuniform magnetic field makes the distribution of emission measure vs. temperature of the postshock plasma at T > 106 K lower than when there is uniform magnetic field

  2. THE COMPACT STAR-FORMING COMPLEX AT THE HEART OF NGC 253

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Davidge, T. J., E-mail: tim.davidge@nrc.ca

    2016-02-20

    We discuss integral field spectra of the compact star-forming complex that is the brightest near-infrared (NIR) source in the central regions of the starburst galaxy NGC 253. The spectra cover the H and K passbands and were recorded with the Gemini NIR Spectrograph during subarcsecond seeing conditions. Absorption features in the spectrum of the star-forming complex are weaker than in the surroundings. An absorption feature is found near 1.78 μm that coincides with the location of a C{sub 2} bandhead. If this feature is due to C{sub 2} then the star-forming complex has been in place for at least amore » few hundred Myr. Emission lines of Brγ, [Fe ii], and He i 2.06 μm do not track the NIR continuum light. Pockets of star-forming activity that do not have associated concentrations of red supergiants, and so likely have ages <8 Myr, are found along the western edge of the complex, and there is evidence that one such pocket contains a rich population of Wolf–Rayet stars. Unless the star-forming complex is significantly more metal-poor than the surroundings, then a significant fraction of its total mass is in stars with ages <8 Myr. If the present-day star formation rate is maintained then the timescale to double its stellar mass ranges from a few Myr to a few tens of Myr, depending on the contribution made by stars older than ∼8 Myr. If—as suggested by some studies—the star-forming complex is centered on the galaxy’s nucleus, which presumably contains a large population of old and intermediate-age stars, then the nucleus of NGC 253 is currently experiencing a phase of rapid growth in its stellar mass.« less

  3. The (BETA) Pictoris Phenomenon Among Herbig Ae/Be Stars

    NASA Technical Reports Server (NTRS)

    Grady, C. A.; Perez, M. R.; Talavera, A.; Bjorkman, K. S.; deWinter, D.; The, P.-S.; Molster, F. J.; vandenAncker, M. E.; Sitko, M. L.; Morrison, N. D.; hide

    1996-01-01

    We present a survey of high dispersion UV and optical spectra of Herbig Ae/Be (HAeBe) and related stars. We find accreting, circumstellar gas over the velocity range +100 to +400 km/s, and absorption profiles similar to those seen toward Beta Pic, in 36% of the 33 HAeBe stars with IUE data as well as in 3 non-emission B stars. We also find evidence of accretion in 7 HAeBe stars with optical data only. Line profile variability appears ubiquitous. As a group, the stars with accreting gas signatures have higher v sin i than the stars with outflowing material, and tend to exhibit large amplitude (greater than or equal to 1(sup m)) optical light variations. All of the program stars with polarimetric variations that are anti-correlated with the optical light, previously interpreted as the signature of a dust disk viewed close to equator-on, also show spectral signatures of accreting gas. These data imply that accretion activity in HAeBe stars is preferentially observed when the line of sight transits the circumstellar dust disk. Our data imply that the spectroscopic signatures of accreting circumstellar material seen in Beta Pic are not unique to that object, but instead are consistent with interpretation of Beta Pic as a comparatively young A star with its associated circumstellar disk.

  4. An Active Black Hole in a Compact Dwarf

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-05-01

    A new type of galaxy has just been added to the galaxy zoo: a small, compact, and old elliptical galaxy that shows signs of a monster black hole actively accreting material in its center. What can this unusual discovery tell us about how compact elliptical galaxies form?A New Galactic BeastCompact elliptical galaxies are an extremely rare early-type dwarf galaxy. Consistent with their name, compact ellipticals are small, very compact collections of ancient stars; these galaxies exhibit a high surface brightness and arent actively forming stars.Optical view of the ancient compact elliptical galaxy SDSS J085431.18+173730.5 (center of image) in an SDSS color composite image. [Adapted from Paudel et al. 2016]Most compact ellipticals are found in dense environments, particularly around massive galaxies. This has led astronomers to believe that compact ellipticals might form via the tidal stripping of a once-large galaxy in interactions with another, massive galaxy. In this model, once the original galaxys outer layers are stripped away, the compact inner bulge component would be left behind as a compact elliptical galaxy. Recent discoveries of a few isolated compact ellipticals, however, have strained this model.Now a new galaxy has been found to confuse our classification schemes: the first-ever compact elliptical to also display signs of an active galactic nucleus. Led by Sanjaya Paudel (Korea Astronomy and Space Science Institute), a team of scientists discovered SDSS J085431.18+173730.5 serendipitously in Sloan Digital Sky Survey data. The team used SDSS images and spectroscopy in combination with data from the Canada-France-Hawaii Telescope to learn more about this unique galaxy.Puzzling CharacteristicsSDSS J085431.18+173730.5 presents an interesting conundrum. Ancient compact ellipticals are supposed to be devoid of gas, with no fuel left to trigger nuclear activity. Yet SDSS J085431.18+173730.5 clearly shows the emission lines that indicate active accretion onto

  5. Compact Starburst Galaxies with Fast Outflows: Spatially Resolved Stellar Mass Profiles

    NASA Astrophysics Data System (ADS)

    Gottlieb, Sophia; Diamond-Stanic, Aleksandar; Lipscomb, Charles; Ohene, Senyo; Rines, Josh; Moustakas, John; Sell, Paul; Tremonti, Christy; Coil, Alison; Rudnick, Gregory; Hickox, Ryan C.; Geach, James; Kepley, Amanda

    2018-01-01

    Powerful galactic winds driven by stellar feedback and black hole accretion are thought to play an important role in regulating star formation in galaxies. In particular, strong stellar feedback from supernovae, stellar winds, radiation pressure, and cosmic rays is required by simulations of star-forming galaxies to prevent the vast majority of baryons from cooling and collapsing to form stars. However, it remains unclear whether these stellar processes play a significant role in expelling gas and shutting down star formation in massive progenitors of quiescent galaxies. What are the limits of stellar feedback? We present multi-band photometry with HST/WFC3 (F475W, F814W, F160W) for a dozen compact starburst galaxies at z~0.6 with half-light radii that suggest incredibly large central escape velocities. These massive galaxies are driving fast (>1000 km/s) outflows that have been previously attributed to stellar feedback associated with the compact (r~100 pc) starburst. But how compact is the stellar mass? In the context of the stellar feedback hypothesis, it is unclear whether these fast outflows are being driven at velocities comparable to the escape velocity of an incredibly dense stellar system (as predicted by some models of radiation-pressure winds) or at velocities that exceed the central escape velocity by large factor. Our spatially resolved measurements with HST show that the stellar mass is more extended than the light, and this requires that the physical mechanism responsible for driving the winds must be able to launch gas at velocities that are factors of 5-10 beyond the central escape velocity.

  6. Boundary between stable and unstable regimes of accretion. Ordered and chaotic unstable regimes

    NASA Astrophysics Data System (ADS)

    Blinova, A. A.; Romanova, M. M.; Lovelace, R. V. E.

    2016-07-01

    We present a new study of the Rayleigh-Taylor unstable regime of accretion on to rotating magnetized stars in a set of high grid resolution three-dimensional magnetohydrodynamic simulations performed in low-viscosity discs. We find that the boundary between the stable and unstable regimes is determined almost entirely by the fastness parameter ωs = Ω⋆/ΩK(rm), where Ω⋆ is the angular velocity of the star and ΩK(rm) is the angular velocity of the Keplerian disc at the disc-magnetosphere boundary r = rm. We found that accretion is unstable if ωs ≲ 0.6. Accretion through instabilities is present in stars with different magnetospheric sizes. However, only in stars with relatively small magnetospheres, rm/R⋆ ≲ 7, do the unstable tongues produce chaotic hotspots on the stellar surface and irregular light curves. At even smaller values of the fastness parameter, ωs ≲ 0.45, multiple irregular tongues merge, forming one or two ordered unstable tongues that rotate with the angular frequency of the inner disc. This transition occurs in stars with even smaller magnetospheres, rm/R⋆ ≲ 4.2. Most of our simulations were performed at a small tilt of the dipole magnetosphere, Θ = 5°, and a small viscosity parameter α = 0.02. Test simulations at higher α values show that many more cases become unstable, and the light curves become even more irregular. Test simulations at larger tilts of the dipole Θ show that instability is present, however, accretion in two funnel streams dominates if Θ ≳ 15°. The results of these simulations can be applied to accreting magnetized stars with relatively small magnetospheres: Classical T Tauri stars, accreting millisecond X-ray pulsars, and cataclysmic variables.

  7. The INTEGRAL long monitoring of persistent ultra compact X-ray bursters

    NASA Astrophysics Data System (ADS)

    Fiocchi, M.; Bazzano, A.; Ubertini, P.; Bird, A. J.; Natalucci, L.; Sguera, V.

    2008-12-01

    Context: The combination of compact objects, short period variability and peculiar chemical composition of the ultra compact X-ray binaries make up a very interesting laboratory to study accretion processes and thermonuclear burning on the neutron star surface. Improved large optical telescopes and more sensitive X-ray satellites have increased the number of known ultra compact X-ray binaries allowing their study with unprecedented detail. Aims: We analyze the average properties common to all ultra compact bursters observed by INTEGRAL from 0.2 keV to 150 keV. Methods: We have performed a systematic analysis of the INTEGRAL public data and Key-Program proprietary observations of a sample of the ultra compact X-ray binaries. In order to study their average properties in a very broad energy band, we combined INTEGRAL with BeppoSAX and SWIFT data whenever possible. For sources not showing any significant flux variations along the INTEGRAL monitoring, we build the average spectrum by combining all available data; in the case of variable fluxes, we use simultaneous INTEGRAL and SWIFT observations when available. Otherwise we compared IBIS and PDS data to check the variability and combine BeppoSAX with INTEGRAL /IBIS data. Results: All spectra are well represented by a two component model consisting of a disk-blackbody and Comptonised emission. The majority of these compact sources spend most of the time in a canonical low/hard state, with a dominating Comptonised component and accretion rate dot {M} lower than 10-9 {M⊙}/yr, not depending on the model used to fit the data. INTEGRAL is an ESA project with instruments and Science Data Center funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA.

  8. Accretion in Close Pre-Main-Sequence Binaries

    NASA Astrophysics Data System (ADS)

    Ardila, David

    2010-09-01

    We propose to use COS to observe the circumbinary accretion flow in pre-main sequence binaries as a function of orbital phase. These observations will help us understand how the magnetosphere captures circumbinary gas, test model predictions regarding the importance of the mass ratio in directing the accretion flows, and study the kinematics of the gas filling the circumbinary gap. We will observe UZ Tau E {mass ratio q=0.3, e=0.33} and DQ Tau {q=1, e=0.58} in four phases, over three orbital periods, using G160M and G230L. The targets are Classical T Tauri stars for which the circumstellar disks are severely truncated. Our primary observables will be the CIV {1550 A} lines, formed at the footpoints of the accretion flow onto the star. We expect to observe the ebb and flow of the line shape, centroid, and flux as a function of orbital phase. The low-resolution NUV continuum observations will provide an independent measurement of the total accretion rate.

  9. Bondi-Hoyle-Lyttleton Accretion onto Binaries

    NASA Astrophysics Data System (ADS)

    Antoni, Andrea; MacLeod, Morgan; Ramírez-Ruiz, Enrico

    2018-01-01

    Binary stars are not rare. While only close binary stars will eventually interact with one another, even the widest binary systems interact with their gaseous surroundings. The rates of accretion and the gaseous drag forces arising in these interactions are the key to understanding how these systems evolve. This poster examines accretion flows around a binary system moving supersonically through a background gas. We perform three-dimensional hydrodynamic simulations of Bondi-Hoyle-Lyttleton accretion using the adaptive mesh refinement code FLASH. We simulate a range of values of semi-major axis of the orbit relative to the gravitational focusing impact parameter of the pair. On large scales, gas is gravitationally focused by the center-of-mass of the binary, leading to dynamical friction drag and to the accretion of mass and momentum. On smaller scales, the orbital motion imprints itself on the gas. Notably, the magnitude and direction of the forces acting on the binary inherit this orbital dependence. The long-term evolution of the binary is determined by the timescales for accretion, slow down of the center-of-mass, and decay of the orbit. We use our simulations to measure these timescales and to establish a hierarchy between them. In general, our simulations indicate that binaries moving through gaseous media will slow down before the orbit decays.

  10. Accretion Disks and the Formation of Stellar Systems

    NASA Astrophysics Data System (ADS)

    Kratter, Kaitlin Michelle

    2011-02-01

    In this thesis, we examine the role of accretion disks in the formation of stellar systems, focusing on young massive disks which regulate the flow of material from the parent molecular core down to the star. We study the evolution of disks with high infall rates that develop strong gravitational instabilities. We begin in chapter 1 with a review of the observations and theory which underpin models for the earliest phases of star formation and provide a brief review of basic accretion disk physics, and the numerical methods that we employ. In chapter 2 we outline the current models of binary and multiple star formation, and review their successes and shortcomings from a theoretical and observational perspective. In chapter 3 we begin with a relatively simple analytic model for disks around young, high mass stars, showing that instability in these disks may be responsible for the higher multiplicity fraction of massive stars, and perhaps the upper mass to which they grow. We extend these models in chapter 4 to explore the properties of disks and the formation of binary companions across a broad range of stellar masses. In particular, we model the role of global and local mechanisms for angular momentum transport in regulating the relative masses of disks and stars. We follow the evolution of these disks throughout the main accretion phase of the system, and predict the trajectory of disks through parameter space. We follow up on the predictions made in our analytic models with a series of high resolution, global numerical experiments in chapter 5. Here we propose and test a new parameterization for describing rapidly accreting, gravitationally unstable disks. We find that disk properties and system multiplicity can be mapped out well in this parameter space. Finally, in chapter 6, we address whether our studies of unstable disks are relevant to recently detected massive planets on wide orbits around their central stars.

  11. 3D MHD Simulations of Waves Excited in an Accretion Disk by a Rotating Magnetized Star

    NASA Astrophysics Data System (ADS)

    Lovelace, R. V. E.; Romanova, M. M.

    2014-01-01

    We present results of global 3D MHD simulations of warp and density waves in accretion disks excited by a rotating star with a misaligned dipole magnetic field. A wide range of cases are considered. We find for example that if the star's magnetosphere corotates approximately with the inner disk, then a strong one-arm bending wave or warp forms. The warp corotates with the star and has a maximum amplitude (|zω|/r ~ 0.3) between the corotation radius and the radius of the vertical resonance. If the magnetosphere rotates more slowly than the inner disk, then a bending wave is excited at the disk-magnetosphere boundary, but it does not form a large-scale warp. In this case the angular rotation of the disk [Ω(r,z = 0)] has a maximum as a function of r so that there is an inner region where dΩ/dr > 0. In this region we observe radially trapped density waves in approximate agreement with the theoretical prediction of a Rossby wave instability in this region.

  12. New Evidence for a Black Hole in the Compact Binary Cygnus X-3

    NASA Technical Reports Server (NTRS)

    Shrader, Chris R.; Titarchuk, Lev; Shaposhnikov, Nikolai

    2010-01-01

    The bright and highly variable X-ray and radio source known as Cygnus X-3 was among the first X-ray sources discovered, yet it remains in many ways an enigma. Its known to consist of a massive. Wolf-Rayet primary in an extremely tight orbit with a compact object. Yet one of the most basic of pa.ranietern the mass of the compact object - is not known. Nor is it even clear whether its is a neutron star or a black hole. In this Paper we present our analysis of the broad-band high-energy continua covering a substantial range in luminosity and spectral morphology. We apply these results to a recently identified scaling relationship which has been demonstrated to provide reliable estimates of the compact object mass in a number of accretion powered binaries. This analysis leads us to conclude that the compact object in Cygnus X-3 has a mass greater than 4.2 solar mass thus clearly indicative of a black hole and as such resolving a longstanding issue. The full range of uncertainty in our analysis and from using a. range of recently published distance estimates constrains the compact object mass to lie between 4.2 solar mass and 14.4 solar mass. Our favored estimate, based on a 9.0 kpc distance estimate is approx. l0 solar mass, with the. error margin of 3.2 solar masses. This result may thus pose challenges to shared-envelope evolutionary models of compact binaries. as well as establishing Cygnus X-3 as the first confirmed accretion-powered galactic gamma: ray source.

  13. PREFACE: 2nd International Symposium on the Modern Physics of Compact Stars and Relativistic Gravity

    NASA Astrophysics Data System (ADS)

    Edvard Chubaryan, Professor; Aram Saharian, Professor; Armen Sedrakian, Professor

    2014-03-01

    The international conference ''The Modern Physics of Compact Stars and Relativistic Gravity'' took place in Yerevan, Armenia, from 18-21 September 2013. This was the second in a series of conferences which aim to bring together people working in astrophysics of compact stars, physics of dense matter, gravitation and cosmology, observations of pulsars and binary neutron stars and related fields. The conference was held on the occasion of 100th birthday of the founder of the Theoretical Physics Chair at the Department of Physics of Yerevan State University and prominent Armenian scientist Academician Gurgen S Sahakyan. The field of compact stars has seen extraordinary development since the discovery of pulsars in 1967. Even before this discovery, pioneering work of a number of theoretical groups had laid the foundation for this development. A pioneer of this effort was Professor G S Sahakyan who, together with Professor Victor Ambartsumyan and a group of young scientists, started in the early sixties their fundamental work on the properties of superdense matter and on the relativistic structure of compact stellar objects. This conference explored the vast diversity of the manifestations of compact stars, including the modern aspects of the equation of state of superdense matter, its magnetic and thermal properties, rotational dynamics, superfluidity and superconductivity, phase transition from hadronic to quark matter, etc. The articles on these subjects collected in this volume are evidence of liveliness of the field and of the continuous feedback between theory and the experiment. A part of this volume is devoted to the cosmology and the theories of gravity — the subfields of astrophysics that are of fundamental importance to our understanding of the universe. The reader will find here articles touching on the most diverse aspects of these fields such as modern problems in Einstein's classical theory of gravity and its alternatives, string theory motivated

  14. Accretion and Magnetic Reconnection in the Classical T Tauri Binary DQ Tau

    NASA Astrophysics Data System (ADS)

    Tofflemire, Benjamin M.; Mathieu, Robert D.; Ardila, David R.; Akeson, Rachel L.; Ciardi, David R.; Johns-Krull, Christopher; Herczeg, Gregory J.; Quijano-Vodniza, Alberto

    2017-01-01

    The theory of binary star formation predicts that close binaries (a < 100 au) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk (CBD), cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (˜daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominant physical mechanism seen in DQ Tau’s low-cadence observations, we have obtained continuous, moderate-cadence, multiband photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on four separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of five near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit to orbit. We argue that this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the CBD and within the accretion streams themselves. Quasiperiodic apastron accretion events are also observed, which are not predicted by binary accretion theory.

  15. Magnetospheric Accretion in Close Pre-main-sequence Binaries

    NASA Astrophysics Data System (ADS)

    Ardila, David R.; Jonhs-Krull, Christopher; Herczeg, Gregory J.; Mathieu, Robert D.; Quijano-Vodniza, Alberto

    2015-10-01

    The transfer of matter between a circumbinary disk and a young binary system remains poorly understood, obscuring the interpretation of accretion indicators. To explore the behavior of these indicators in multiple systems, we have performed the first systematic time-domain study of young binaries in the ultraviolet. We obtained far- and near-ultraviolet HST/COS spectra of the young spectroscopic binaries DQ Tau and UZ Tau E. Here we focus on the continuum from 2800 to 3200 Å and on the C iv doublet (λλ1548.19, 1550.77 Å) as accretion diagnostics. Each system was observed over three or four consecutive binary orbits, at phases ∼0, 0.2, 0.5, and 0.7. Those observations are complemented by ground-based U-band measurements. Contrary to model predictions, we do not detect any clear correlation between accretion luminosity and phase. Further, we do not detect any correlation between C iv flux and phase. For both stars the appearance of the C iv line is similar to that of single Classical T Tauri Stars (CTTSs), despite the lack of stable long-lived circumstellar disks. However, unlike the case in single CTTSs, the narrow and broad components of the C iv lines are uncorrelated, and we argue that the narrow component is powered by processes other than accretion, such as flares in the stellar magnetospheres and/or enhanced activity in the upper atmosphere. We find that both stars contribute equally to the narrow component C iv flux in DQ Tau, but the primary dominates the narrow component C iv emission in UZ Tau E. The C iv broad component flux is correlated with other accretion indicators, suggesting an accretion origin. However, the line is blueshifted, which is inconsistent with its origin in an infall flow close to the star. It is possible that the complicated geometry of the region, as well as turbulence in the shock region, are responsible for the blueshifted line profiles.

  16. Accretion-induced luminosity spreads in young clusters: evidence from stellar rotation

    NASA Astrophysics Data System (ADS)

    Littlefair, S. P.; Naylor, Tim; Mayne, N. J.; Saunders, Eric; Jeffries, R. D.

    2011-05-01

    We present an analysis of the rotation of young stars in the associations Cepheus OB3b, NGC 2264, 2362 and the Orion Nebula Cluster (ONC). We discover a correlation between rotation rate and position in a colour-magnitude diagram (CMD) such that stars which lie above an empirically determined median pre-main sequence rotate more rapidly than stars which lie below this sequence. The same correlation is seen, with a high degree of statistical significance, in each association studied here. If position within the CMD is interpreted as being due to genuine age spreads within a cluster, then the stars above the median pre-main sequence would be the youngest stars. This would in turn imply that the most rapidly rotating stars in an association are the youngest, and hence those with the largest moments of inertia and highest likelihood of ongoing accretion. Such a result does not fit naturally into the existing picture of angular momentum evolution in young stars, where the stars are braked effectively by their accretion discs until the disc disperses. Instead, we argue that, for a given association of young stars, position within the CMD is not primarily a function of age, but of accretion history. We show that this hypothesis could explain the correlation we observe between rotation rate and position within the CMD.

  17. A comprehensive HST BVI catalogue of star clusters in five Hickson compact groups of galaxies

    NASA Astrophysics Data System (ADS)

    Fedotov, K.; Gallagher, S. C.; Durrell, P. R.; Bastian, N.; Konstantopoulos, I. S.; Charlton, J.; Johnson, K. E.; Chandar, R.

    2015-05-01

    We present a photometric catalogue of star cluster candidates in Hickson compact groups (HCGs) 7, 31, 42, 59, and 92, based on observations with the Advanced Camera for Surveys and the Wide Field Camera 3 on the Hubble Space Telescope. The catalogue contains precise cluster positions (right ascension and declination), magnitudes, and colours in the BVI filters. The number of detected sources ranges from 2200 to 5600 per group, from which we construct the high-confidence sample by applying a number of criteria designed to reduce foreground and background contaminants. Furthermore, the high-confidence cluster candidates for each of the 16 galaxies in our sample are split into two subpopulations: one that may contain young star clusters and one that is dominated by globular older clusters. The ratio of young star cluster to globular cluster candidates varies from group to group, from equal numbers to the extreme of HCG 31 which has a ratio of 8 to 1, due to a recent starburst induced by interactions in the group. We find that the number of blue clusters with MV < -9 correlates well with the current star formation rate in an individual galaxy, while the number of globular cluster candidates with MV < -7.8 correlates well (though with large scatter) with the stellar mass. Analyses of the high-confidence sample presented in this paper show that star clusters can be successfully used to infer the gross star formation history of the host groups and therefore determine their placement in a proposed evolutionary sequence for compact galaxy groups.

  18. ULX spectra revisited: Are accreting, highly magnetized neutron stars the engines of ultraluminous X-ray sources?

    NASA Astrophysics Data System (ADS)

    Koliopanos, F.; Vasilopoulos, G.; Bachetti, M.; Godet, O.; Webb, N.; Barret, D.

    2017-10-01

    In light of recent discoveries of pulsating ULXs and recently introduced models placing neutron stars as the central engines of ULXs, we revisit the spectra of seventeen ULXs, in search of indications that favor this hypothesis. To this end we examined the spectra from XMM-Newton observations of all seventeen sources in our sample. For six sources, these were complimented with spectra from public NuSTAR observations. We demonstrate that the notable ({>}6 keV) spectral curvature observed in most ULXs, is most likely due to thermal emission, with T{>} 1keV. More importantly, we find that a double thermal model (comprised of a 'cool' and 'hot' thermal component) - often associated with emission from neutron star X-ray binaries - describes all ULX spectra in our list. We propose that the dual thermal spectrum is the result of accretion onto highly magnetized NSs, as predicted in recent theoretical models (Mushtukov et al. 2017). We further argue that this finding offers an additional and compelling argument in favor of neutron stars as prime candidates for powering ULXs, as has been recently suggested (King & Lasota 2016; King et al. 2017). In my talk I will discuss the implications of our interpretation along with its merits and shortcomings.

  19. The near-infrared properties of compact binary systems

    NASA Astrophysics Data System (ADS)

    Froning, Cynthia Suzanne

    I present H- and K-band light curves of the dwarf nova cataclysmic variable (CV), IP Peg, and the novalike CV, RW Tri, and an H-band light curve of the novalike CV, SW Sex. All three systems showed contributions from the late-type secondary star and the accretion disk, including a primary eclipse of the accretion disk by the secondary star and a secondary eclipse of the star by the disk. The ellipsoidal variations of the secondary star in IP Peg were modeled and subtracted from the data. The subtracted light curves show a pronounced double-hump variation, resembling those seen in the dwarf novae WZ Sge and AL Com. The primary eclipse was modeled using maximum entropy disk mapping techniques. The accretion disk has a flat intensity distribution and a low brightness temperature (Tbr ~= 3000-4000 K). Superimposed on the face of the disk is the bright spot, where the mass accretion stream impacts the disk; the position of the bright spot is different from the range of positions seen at visible wavelengths. The near-infrared accretion disk flux is dominated by optically thin emission. The eclipse depth is too shallow to be caused by a fully opaque accretion disk. The NIR light curves in RW Tri show a deep primary eclipse of the accretion disk, ellipsoidal variations from the secondary star, a secondary eclipse, and strong flickering in the disk flux. The depth of the secondary eclipse indicates that the accretion disk is opaque. The light curve also has a hump extending from φ = 0.1-0.9 which was successfully modeled as flux from the inner face of the secondary star when heated by a ~0.2 L Lsolar source. The radial brightness temperature profile of the outer disk is consistent with models of a disk in steady-state for a mass transfer rate of M~=5×10- 10 Msolaryr- 1 . At small disk radii, however, the brightness temperature profile is flatter than the steady-state model. The H-band light curve of SW Sex is dominated by emission from the accretion disk. As in RW Tri

  20. Photoionization Models for the Inner Gaseous Disks of Herbig Be Stars: Evidence against Magnetospheric Accretion?

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Patel, P.; Sigut, T. A. A.; Landstreet, J. D., E-mail: ppatel54@uwo.ca

    2017-02-20

    We investigate the physical properties of the inner gaseous disks of three hot Herbig B2e stars, HD 76534, HD 114981, and HD 216629, by modeling CFHT-ESPaDOns spectra using non-LTE radiative transfer codes. We assume that the emission lines are produced in a circumstellar disk heated solely by photospheric radiation from the central star in order to test whether the optical and near-infrared emission lines can be reproduced without invoking magnetospheric accretion. The inner gaseous disk density was assumed to follow a simple power-law in the equatorial plane, and we searched for models that could reproduce observed lines of H imore » (H α and H β ), He i, Ca ii, and Fe ii. For the three stars, good matches were found for all emission line profiles individually; however, no density model based on a single power-law was able to reproduce all of the observed emission lines. Among the single power-law models, the one with the gas density varying as ∼10{sup −10}( R {sub *}/ R ){sup 3} g cm{sup −3} in the equatorial plane of a 25 R {sub *} (0.78 au) disk did the best overall job of representing the optical emission lines of the three stars. This model implies a mass for the H α -emitting portion of the inner gaseous disk of ∼10{sup −9} M {sub *}. We conclude that the optical emission line spectra of these HBe stars can be qualitatively reproduced by a ≈1 au, geometrically thin, circumstellar disk of negligible mass compared to the central star in Keplerian rotation and radiative equilibrium.« less

  1. Massive black hole factories: Supermassive and quasi-star formation in primordial halos

    NASA Astrophysics Data System (ADS)

    Schleicher, Dominik R. G.; Palla, Francesco; Ferrara, Andrea; Galli, Daniele; Latif, Muhammad

    2013-10-01

    Context. Supermassive stars and quasi-stars (massive stars with a central black hole) are both considered as potential progenitors for the formation of supermassive black holes. They are expected to form from rapidly accreting protostars in massive primordial halos. Aims: We explore how long rapidly accreting protostars remain on the Hayashi track, implying large protostellar radii and weak accretion luminosity feedback. We assess the potential role of energy production in the nuclear core, and determine what regulates the evolution of such protostars into quasi-stars or supermassive stars. Methods: We followed the contraction of characteristic mass shells in rapidly accreting protostars, and inferred the timescales for them to reach nuclear densities. We compared the characteristic timescales for nuclear burning with those for which the extended protostellar envelope can be maintained. Results: We find that the extended envelope can be maintained up to protostellar masses of 3.6 × 108 ṁ3 M⊙, where ṁ denotes the accretion rate in solar masses per year. We expect the nuclear core to exhaust its hydrogen content in 7 × 106 yr. If accretion rates ṁ ≫ 0.14 can still be maintained at this point, a black hole may form within the accreting envelope, leading to a quasi-star. Alternatively, the accreting object will gravitationally contract to become a main-sequence supermassive star. Conclusions: Due to the limited gas reservoir in typical 107 M⊙ dark matter halos, the accretion rate onto the central object may drop at late times, implying the formation of supermassive stars as the typical outcome of direct collapse. However, if high accretion rates are maintained, a quasi-star with an interior black hole may form.

  2. The accretion and spreading of matter on white dwarfs

    NASA Astrophysics Data System (ADS)

    Fisker, Jacob Lund; Balsara, Dinshaw S.; Burger, Tom

    2006-10-01

    For a slowly rotating non-magnetized white dwarf the accretion disk extends all the way to the star. At the interface between the accretion disk and the star, the matter moves through a boundary layer (BL) and then spreads toward the poles as new matter continuously piles up behind it. We have solved the 3d compressible Navier-Stokes equations on an axisymmetric grid to determine the structure of this BL for different accretion rates (states). The high states show a spreading BL which sets off a gravity wave in the surface matter. The accretion flow moves supersonically over the cusp making it susceptible to the rapid development of gravity wave and/or Kelvin-Helmholtz instabilities. This BL is optically thick and extends more than 30° to either side of the disk plane after 3/4 of a Keplerian rotation period (tK = 19 s). The low states also show a spreading BL, but here the accretion flow does not set off gravity waves and it is optically thin.

  3. Starquakes, Heating Anomalies, and Nuclear Reactions in the Neutron Star Crust

    NASA Astrophysics Data System (ADS)

    Deibel, Alex Thomas

    When the most massive stars perish, their cores may remain intact in the form of extremely dense and compact stars. These stellar remnants, called neutron stars, are on the cusp of becoming black holes and reach mass densities greater than an atomic nucleus in their centers. Although the interiors of neutron stars were difficult to investigate at the time of their discovery, the advent of modern space-based telescopes (e.g., Chandra X-ray Observatory) has pushed our understanding of the neutron star interior into exciting new realms. It has been shown that the neutron star interior spans an enormous range of densities and contains many phases of matter, and further theoretical progress must rely on numerical calculations of neutron star phenomena built with detailed nuclear physics input. To further investigate the properties of the neutron star interior, this dissertation constructs numerical models of neutron stars, applies models to various observations of neutron star high-energy phenomena, and draws new conclusions about the neutron star interior from these analyses. In particular, we model the neutron star's outermost ? 1 km that encompasses the neutron star's envelope, ocean, and crust. The model must implement detailed nuclear physics to properly simulate the hydrostatic and thermal structure of the neutron star. We then apply our model to phenomena that occur in these layers, such as: thermonuclear bursts in the envelope, g-modes in the ocean, torsional oscillations of the crust, and crust cooling of neutron star transients. A comparison of models to observations provides new insights on the properties of dense matter that are often difficult to probe through terrestrial experiments. For example, models of the quiescent cooling of neutron stars, such as the accreting transient MAXI J0556-332, at late times into quiescence probe the thermal transport properties of the deep neutron star crust. This modeling provides independent data from astronomical

  4. X-ray emitting MHD accretion shocks in classical T Tauri stars. Case for moderate to high plasma-β values

    NASA Astrophysics Data System (ADS)

    Orlando, S.; Sacco, G. G.; Argiroffi, C.; Reale, F.; Peres, G.; Maggio, A.

    2010-02-01

    Context. Plasma accreting onto classical T Tauri stars (CTTS) is believed to impact the stellar surface at free-fall velocities, generating a shock. Current time-dependent models describing accretion shocks in CTTSs are one-dimensional, assuming that the plasma moves and transports energy only along magnetic field lines (β ≪ 1). Aims: We investigate the stability and dynamics of accretion shocks in CTTSs, considering the case of β ⪆ 1 in the post-shock region. In these cases the 1D approximation is not valid and a multi-dimensional MHD approach is necessary. Methods: We model an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere by performing 2D axisymmetric MHD simulations. The model takes into account the stellar magnetic field, the gravity, the radiative cooling, and the thermal conduction (including the effects of heat flux saturation). Results: The dynamics and stability of the accretion shock strongly depend on the plasma β. In the case of shocks with β > 10, violent outflows of shock-heated material (and possibly MHD waves) are generated at the base of the accretion column and intensely perturb the surrounding stellar atmosphere and the accretion column itself (therefore modifying the dynamics of the shock). In shocks with β ≈ 1, the post-shock region is efficiently confined by the magnetic field. The shock oscillations induced by cooling instability are strongly influenced by β: for β > 10, the oscillations may be rapidly dumped by the magnetic field, approaching a quasi-stationary state, or may be chaotic with no obvious periodicity due to perturbation of the stream induced by the post-shock plasma itself; for β≈ 1 the oscillations are quasi-periodic, although their amplitude is smaller and the frequency higher than those predicted by 1D models. Three movies are only available in electronic form at http://www.aanda.org

  5. Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies

    NASA Astrophysics Data System (ADS)

    Izotov, Y. I.; Schaerer, D.; Thuan, T. X.; Worseck, G.; Guseva, N. G.; Orlitová, I.; Verhamme, A.

    2016-10-01

    Following our first detection reported in Izotov et al., we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope. These galaxies, at redshifts of z ˜ 0.3, are characterized by high emission-line flux ratios [O III] λ5007/[O II] λ3727 ≳ 5. The escape fractions of the LyC radiation fesc(LyC) in these galaxies are in the range of ˜6-13 per cent, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Ly α emission lines are detected in the spectra of all four galaxies, compatible with predictions for LyC leakers. We find escape fractions of Ly α, fesc(Ly α) ˜ 20-40 per cent, among the highest known for Ly α emitting galaxies. Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the centre and exponential discs in the outskirts with disc scalelengths α in the range ˜0.6-1.4 kpc. Our galaxies are characterized by low metallicity, ˜1/8-1/5 solar, low stellar mass ˜(0.2-4) × 109 M⊙, high star formation rates, SFR ˜ 14-36 M⊙ yr-1, and high SFR densities, Σ ˜ 2-35 M⊙ yr-1 kpc-2. These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in Izotov et al., reveal that a selection for compact star-forming galaxies showing high [O III] λ5007/[O II] λ3727 ratios appears to pick up very efficiently sources with escaping LyC radiation: all five of our selected galaxies are LyC leakers.

  6. A black hole-white dwarf compact binary model for long gamma-ray bursts without supernova association

    NASA Astrophysics Data System (ADS)

    Dong, Yi-Ze; Gu, Wei-Min; Liu, Tong; Wang, Junfeng

    2018-03-01

    Gamma-ray bursts (GRBs) are luminous and violent phenomena in the Universe. Traditionally, long GRBs are expected to be produced by the collapse of massive stars and associated with supernovae. However, some low-redshift long GRBs have no detection of supernova association, such as GRBs 060505, 060614, and 111005A. It is hard to classify these events convincingly according to usual classifications, and the lack of the supernova implies a non-massive star origin. We propose a new path to produce long GRBs without supernova association, the unstable and extremely violent accretion in a contact binary system consisting of a stellar-mass black hole and a white dwarf, which fills an important gap in compact binary evolution.

  7. Accretion Signatures on Massive Young Stellar Objects

    NASA Astrophysics Data System (ADS)

    Navarete, F.; Damineli, A.; Barbosa, C. L.; Blum, R. D.

    2015-01-01

    We present preliminary results from a survey of molecular H2 (2.12 μm) emission in massive young stellar objects (MYSO) candidates selected from the Red MSX Source survey. We observed 354 MYSO candidates through the H2 S(1) 1-0 transition (2.12 μm) and an adjacent continuum narrow-band filters using the Spartan/SOAR and WIRCam/CFHT cameras. The continuum-subtracted H2 maps were analyzed and extended H2 emission was found in 50% of the sample (178 sources), and 38% of them (66) have polar morphology, suggesting collimated outflows. The polar-like structures are more likely to be driven on radio-quiet sources, indicating that these structures occur during the pre-ultra compact H ii phase. We analyzed the continuum images and found that 54% (191) of the sample displayed extended continuum emission and only ~23% (80) were associated to stellar clusters. The extended continuum emission is correlated to the H2 emission and those sources within stellar clusters does display diffuse H2 emission, which may be due to fluorescent H2 emission. These results support the accretion scenario for massive star formation, since the merging of low-mass stars would not produce jet-like structures. Also, the correlation between jet-like structures and radio-quiet sources indicates that higher inflow rates are required to form massive stars in a typical timescale less than 105 years.

  8. Three-dimensional GRMHD Simulations of Neutrino-cooled Accretion Disks from Neutron Star Mergers

    NASA Astrophysics Data System (ADS)

    Siegel, Daniel M.; Metzger, Brian D.

    2018-05-01

    Merging binaries consisting of two neutron stars (NSs) or an NS and a stellar-mass black hole typically form a massive accretion torus around the remnant black hole or long-lived NS. Outflows from these neutrino-cooled accretion disks represent an important site for r-process nucleosynthesis and the generation of kilonovae. We present the first three-dimensional, general-relativistic magnetohydrodynamic (GRMHD) simulations including weak interactions and a realistic equation of state of such accretion disks over viscous timescales (380 ms). We witness the emergence of steady-state MHD turbulence, a magnetic dynamo with an ∼20 ms cycle, and the generation of a “hot” disk corona that launches powerful thermal outflows aided by the energy released as free nucleons recombine into α-particles. We identify a self-regulation mechanism that keeps the midplane electron fraction low (Y e ∼ 0.1) over viscous timescales. This neutron-rich reservoir, in turn, feeds outflows that retain a sufficiently low value of Y e ≈ 0.2 to robustly synthesize third-peak r-process elements. The quasi-spherical outflows are projected to unbind 40% of the initial disk mass with typical asymptotic escape velocities of 0.1c and may thus represent the dominant mass ejection mechanism in NS–NS mergers. Including neutrino absorption, our findings agree with previous hydrodynamical α-disk simulations that the entire range of r-process nuclei from the first to the third r-process peak can be synthesized in the outflows, in good agreement with observed solar system abundances. The asymptotic escape velocities and quantity of ejecta, when extrapolated to moderately higher disk masses, are consistent with those needed to explain the red kilonova emission following the NS merger GW170817.

  9. Mimetic compact stars

    NASA Astrophysics Data System (ADS)

    Momeni, D.; Moraes, P. H. R. S.; Gholizade, H.; Myrzakulov, R.

    Modified gravity models have been constantly proposed with the purpose of evading some standard gravity shortcomings. Recently proposed by Chamseddine and Mukhanov, the Mimetic Gravity arises as an optimistic alternative. Our purpose in this work is to derive Tolman-Oppenheimer-Volkoff equations and solutions for such a gravity theory. We solve them numerically for quark star and neutron star cases. The results are carefully discussed.

  10. p-Process Nucleosynthesis inside Supernova-driven Supercritical Accretion Disks

    NASA Astrophysics Data System (ADS)

    Fujimoto, Shin-ichirou; Hashimoto, Masa-aki; Koike, Osamu; Arai, Kenzo; Matsuba, Ryuichi

    2003-03-01

    We investigate p-process nucleosynthesis in a supercritical accretion disk around a compact object of 1.4 Msolar, using the self-similar solution of an optically thick advection-dominated flow. Supercritical accretion is expected to occur in a supernova with fallback material accreting onto a newborn compact object. It is found that an appreciable number of p-nuclei are synthesized via the p-process in supernova-driven supercritical accretion disks (SSADs) when the accretion rate m=Mc2/(16LEdd)>105, where LEdd is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs have features similar to those of the oxygen/neon layers in Type II supernovae when the abundance of the fallback gas far from the compact object is that of the oxygen/neon layers in the progenitor. The overall abundance profile is in agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and La, are underproduced in the SSADs as in Type II supernovae. If the fallback gas is mixed with a small fraction of protons through Rayleigh-Taylor instability during the explosion, significant amounts of 92Mo are produced inside the SSADs. Isotopes 96Ru and 138La are also produced when the fallback gas contains abundant protons, although the overall abundance profile of p-nuclei is rather different from that of the solar system. The p-process nucleosynthesis in SSADs contributes to the chemical evolution of p-nuclei, in particular 92Mo, if several percent of the fallback matter are ejected via jets and/or winds.

  11. Variability at the edge: highly accreting objects in Taurus

    NASA Astrophysics Data System (ADS)

    Abraham, Peter; Kospal, Agnes; Szabo, Robert

    2017-04-01

    In Kepler K2, Campaign 13, we will obtain 80-days-long optical light curves of seven highly accreting T Tauri stars in the benchmark Taurus star forming region. Here we propose to monitor our sample simultaneously with Kepler and Spitzer, to be able to separate variability patterns related to different physical processes. Monitoring our targets with Spitzer during the final 11 days of the K2 campaign, we will clean the light curves from non-accretion effects (rotating stellar spots, dips due to passing dust structures), and construct, for the first time, a variability curve which reflects the time-dependent accretion only. We will then study and understand how time-dependent mass accretion affects the density and temperature structure of the protoplanetary disk, which sets the initial conditions for planet formation. The proposed work cannot be done without the unparalleled precision of Kepler and Spitzer. This unique and one-time opportunity motivated our DDT proposal.

  12. Swift/XRT detects a new accretion outburst of the Galactic center neutron star transient GRS 1741-2853

    NASA Astrophysics Data System (ADS)

    Degenaar, N.; Wijnands, R.; Reynolds, M. T.; Miller, J. M.; Kennea, J. A.

    2017-10-01

    Daily Swift/XRT monitoring observations of the Galactic center (Degenaar et al. 2015) have picked up renewed activity of the transient neutron star low-mass X-ray binary and thermonuclear X-ray burster GRS 1741-2853, which is located 10 arcmin NW of Sgr A*. During a 1 ks PC-mode observation performed on 2017 October 11 the source is detected at a net count rate of 0.015 counts/s and it has been steadily brightening since, indicating the onset of a new accretion outburst.

  13. Strange Quark Stars in Binaries: Formation Rates, Mergers, and Explosive Phenomena

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Wiktorowicz, G.; Drago, A.; Pagliara, G.

    2017-09-10

    Recently, the possible coexistence of a first family composed of “normal” neutron stars (NSs) with a second family of strange quark stars (QSs) has been proposed as a solution of problems related to the maximum mass and to the minimal radius of these compact stellar objects. In this paper, we study the mass distribution of compact objects formed in binary systems and the relative fractions of quark and NSs in different subpopulations. We incorporate the strange QS formation model provided by the two-families scenario, and we perform a large-scale population synthesis study in order to obtain the population characteristics. Accordingmore » to our results, the main channel for strange QS formation in binary systems is accretion from a secondary companion on an NS. Therefore, a rather large number of strange QSs form by accretion in low-mass X-ray binaries and this opens the possibility of having explosive GRB-like phenomena not related to supernovae and not due to the merger of two NSs. The number of double strange QS systems is rather small, with only a tiny fraction that merge within a Hubble time. This drastically limits the flux of strangelets produced by the merger, which turns out to be compatible with all limits stemming from Earth and lunar experiments. Moreover, this value of the flux rules out at least one relevant channel for the transformation of all NSs into strange QSs by strangelets’ absorption.« less

  14. Strange Quark Stars in Binaries: Formation Rates, Mergers, and Explosive Phenomena

    NASA Astrophysics Data System (ADS)

    Wiktorowicz, G.; Drago, A.; Pagliara, G.; Popov, S. B.

    2017-09-01

    Recently, the possible coexistence of a first family composed of “normal” neutron stars (NSs) with a second family of strange quark stars (QSs) has been proposed as a solution of problems related to the maximum mass and to the minimal radius of these compact stellar objects. In this paper, we study the mass distribution of compact objects formed in binary systems and the relative fractions of quark and NSs in different subpopulations. We incorporate the strange QS formation model provided by the two-families scenario, and we perform a large-scale population synthesis study in order to obtain the population characteristics. According to our results, the main channel for strange QS formation in binary systems is accretion from a secondary companion on an NS. Therefore, a rather large number of strange QSs form by accretion in low-mass X-ray binaries and this opens the possibility of having explosive GRB-like phenomena not related to supernovae and not due to the merger of two NSs. The number of double strange QS systems is rather small, with only a tiny fraction that merge within a Hubble time. This drastically limits the flux of strangelets produced by the merger, which turns out to be compatible with all limits stemming from Earth and lunar experiments. Moreover, this value of the flux rules out at least one relevant channel for the transformation of all NSs into strange QSs by strangelets’ absorption.

  15. X-Shooter study of accretion in Chamaeleon I

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Fedele, D.; Herczeg, G. J.; Teixeira, P. S.

    2016-01-01

    We present the analysis of 34 new VLT/X-Shooter spectra of young stellar objects in the Chamaeleon I star-forming region, together with four more spectra of stars in Taurus and two in Chamaeleon II. The broad wavelength coverage and accurate flux calibration of our spectra allow us to estimate stellar and accretion parameters for our targets by fitting the photospheric and accretion continuum emission from the Balmer continuum down to ~700 nm. The dependence of accretion on stellar properties for this sample is consistent with previous results from the literature. The accretion rates for transitional disks are consistent with those of full disks in the same region. The spread of mass accretion rates at any given stellar mass is found to be smaller than in many studies, but is larger than that derived in the Lupus clouds using similar data and techniques. Differences in the stellar mass range and in the environmental conditions between our sample and that of Lupus may account for the discrepancy in scatter between Chamaeleon I and Lupus. Complete samples in Chamaeleon I and Lupus are needed to determine whether the difference in scatter of accretion rates and the lack of evolutionary trends are not influenced by sample selection. This work is based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 084.C-1095 and 094.C-0913.

  16. Accretion-induced variability links young stellar objects, white dwarfs, and black holes.

    PubMed

    Scaringi, Simone; Maccarone, Thomas J; Körding, Elmar; Knigge, Christian; Vaughan, Simon; Marsh, Thomas R; Aranzana, Ester; Dhillon, Vikram S; Barros, Susana C C

    2015-10-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies.

  17. Accretion-induced variability links young stellar objects, white dwarfs, and black holes

    PubMed Central

    Scaringi, Simone; Maccarone, Thomas J.; Körding, Elmar; Knigge, Christian; Vaughan, Simon; Marsh, Thomas R.; Aranzana, Ester; Dhillon, Vikram S.; Barros, Susana C. C.

    2015-01-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies. PMID:26601307

  18. Photoionization Models for the Inner Gaseous Disks of Herbig Be Stars: Evidence against Magnetospheric Accretion?

    NASA Astrophysics Data System (ADS)

    Patel, P.; Sigut, T. A. A.; Landstreet, J. D.

    2017-02-01

    We investigate the physical properties of the inner gaseous disks of three hot Herbig B2e stars, HD 76534, HD 114981, and HD 216629, by modeling CFHT-ESPaDOns spectra using non-LTE radiative transfer codes. We assume that the emission lines are produced in a circumstellar disk heated solely by photospheric radiation from the central star in order to test whether the optical and near-infrared emission lines can be reproduced without invoking magnetospheric accretion. The inner gaseous disk density was assumed to follow a simple power-law in the equatorial plane, and we searched for models that could reproduce observed lines of H I (Hα and Hβ), He I, Ca II, and Fe II. For the three stars, good matches were found for all emission line profiles individually; however, no density model based on a single power-law was able to reproduce all of the observed emission lines. Among the single power-law models, the one with the gas density varying as ˜10-10(R */R)3 g cm-3 in the equatorial plane of a 25 R * (0.78 au) disk did the best overall job of representing the optical emission lines of the three stars. This model implies a mass for the Hα-emitting portion of the inner gaseous disk of ˜10-9 M *. We conclude that the optical emission line spectra of these HBe stars can be qualitatively reproduced by a ≈1 au, geometrically thin, circumstellar disk of negligible mass compared to the central star in Keplerian rotation and radiative equilibrium. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l”Univers of the Centre National de la Recherche Scientique of France, and the University of Hawaii.

  19. Quasi-periodic accretion and gravitational waves from oscillating `toroidal neutron stars' around a Schwarzschild black hole

    NASA Astrophysics Data System (ADS)

    Zanotti, Olindo; Rezzolla, Luciano; Font, José A.

    2003-05-01

    We present general relativistic hydrodynamics simulations of constant specific angular momentum tori orbiting a Schwarzschild black hole. These tori are expected to form as a result of stellar gravitational collapse, binary neutron star merger or disruption, can reach very high rest-mass densities and behave effectively as neutron stars but with a toroidal topology (i.e. `toroidal neutron stars'). Here our attention is focused on the dynamical response of these objects to axisymmetric perturbations. We show that upon the introduction of perturbations, these systems either become unstable to the runaway instability or exhibit a regular oscillatory behaviour, resulting in a quasi-periodic variation of the accretion rate as well as of the mass quadrupole. The latter, in particular, is responsible for the emission of intense gravitational radiation for which the signal-to-noise ratio at the detector is comparable to or larger than the typical one expected in stellar-core collapse, making these new sources of gravitational waves potentially detectable. We discuss a systematic investigation of the parameter space in both the linear and non-linear regimes, providing estimates of how the gravitational radiation emitted depends on the mass of the torus and on the strength of the perturbation.

  20. Jets from Merging Neutron Stars

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-06-01

    With the recent discovery of gravitational waves from the merger of two black holes, its especially important to understand the electromagnetic signals resulting from mergers of compact objects. New simulations successfully follow a merger of two neutron stars that produces a short burst of energy via a jet consistent with short gamma-ray burst (sGRB) detections.Still from the authors simulation showing the two neutron stars, and their magnetic fields, before merger. [Adapted from Ruiz et al. 2016]Challenging SystemWe have long suspected that sGRBs are produced by the mergers of compact objects, but this model has been difficult to prove. One major hitch is that modeling the process of merger and sGRB launch is very difficult, due to the fact that these extreme systems involve magnetic fields, fluids and full general relativity.Traditionally, simulations are only able to track such mergers over short periods of time. But in a recent study, Milton Ruiz (University of Illinois at Urbana-Champaign and Industrial University of Santander, Colombia) and coauthors Ryan Lang, Vasileios Paschalidis and Stuart Shapiro have modeled a binary neutron star system all the way through the process of inspiral, merger, and the launch of a jet.A Merger TimelineHow does this happen? Lets walk through one of the teams simulations, in which dipole magnetic field lines thread through the interior of each neutron star and extend beyond its surface(like magnetic fields found in pulsars). In this example, the two neutron stars each have a mass of 1.625 solar masses.Simulation start (0 ms)Loss of energy via gravitational waves cause the neutron stars to inspiral.Merger (3.5 ms)The neutron stars are stretched by tidal effects and make contact. Their merger produces a hypermassive neutron star that is supported against collapse by its differential (nonuniform) rotation.Delayed collapse into a black hole (21.5 ms)Once the differential rotation is redistributed by magnetic fields and partially

  1. Accretion-powered Compact Binaries

    NASA Astrophysics Data System (ADS)

    Mauche, Christopher W.

    2003-12-01

    Preface; The workshop logo; A short history of the CV workshop F. A. Córdova; Part I. Observations: 1. Low mass x-ray binaries A. P. Cowley, P. C. Schmidtke, D. Crampton, J. B. Hutchings, C. A. Haswell, E. L. Robinson, K. D. Horne, H. M. Johnston, S. R. Kulkarni, S. Kitamoto, X. Han, R. M. Hjellming, R. M. Wagner, S. L. Morris, P. Hertz, A. N. Parmar, L. Stella, P. Giommi, P. J. Callanan, T. Naylor, P. A. Charles, C. D. Bailyn, J. N. Imamura, T. Steiman-Cameron, J. Kristian, J. Middleditch, L. Angelini and J. P. Noris; 2. Nonmagnetic cataclysmic variables R. S. Polidan, C. W. Mauche, R. A. Wade, R. H. Kaitchuck, E. M. Schlegel, P. A. Hantzios, R. C. Smith, J. H. Wood, F. Hessman, A. Fiedler, D. H. P. Jones, J. Casares, P. A. Charles, J. van Paradijs, E. Harlaftis, T. Naylor, G. Sonneborn, B. J. M. Hassall, K. Horne, C. A. la Dous, A. W. Shafter, N. A. Hawkins, D. A. H. Buckley, D. J. Sullivan, F. V. Hessman, V. S. Dhillon, T. R. Marsh, J. Singh, S. Seetha, F. Giovannelli, A. Bianchini, E. M. Sion, D. J. Mullan, H. L. Shipman, G. Machin, P. J. Callanan, S. B. Howell, P. Szkody, E. M. Schlegel and R. F. Webbink; 3. Magnetic cataclysmic variables C. Hellier, K. O. Mason, C. W. Mauche, G. S. Miller, J. C. Raymond, F. K. Lamb, J. Patterson, A. J. Norton, M. G. Watson, A. R. King, I. M. McHardy, H. Lehto, J. P. Osborne, E. L. Robinson, A. W. Shafter, S. Balachandran, S. R. Rosen, J. Krautter, W. Buchholz, D. A. H. Buckley, I. R. Tuoly, D. Crampton, B. Warner, R. M. Prestage, B. N. Ashoka, M. Mouchet, J. M. Bonnet-Bidaud, J. M. Hameury, P. Szkody, P. Garnavich, S. Howell, T. Kii, M. Cropper, K. Mason, J. Bailey, D. T. Wickramasinghe, L. Ferrario, K. Beuermann, A. D. Schwope, H.-C. Thomas, S. Jordan, J. Schachter, A. V. Filippenko, S. M. Kahn, F. B. S. Paerels, K. Mukai, M. L. Edgar, S. Larsson, R. F. Jameson, A. R. King, A. Silber, R. Remillard, H. Bradt, M. Ishida, T. Ohashi and G. D. Schmidt; Part II. Accretion Theory: 4. Nonmagnetic W. Kley, F. Geyer, H. Herold, H

  2. Impacts of fragmented accretion streams onto classical T Tauri stars: UV and X-ray emission lines

    NASA Astrophysics Data System (ADS)

    Colombo, S.; Orlando, S.; Peres, G.; Argiroffi, C.; Reale, F.

    2016-10-01

    Context. The accretion process in classical T Tauri stars (CTTSs) can be studied through the analysis of some UV and X-ray emission lines which trace hot gas flows and act as diagnostics of the post-shock downfalling plasma. In the UV-band, where higher spectral resolution is available, these lines are characterized by rather complex profiles whose origin is still not clear. Aims: We investigate the origin of UV and X-ray emission at impact regions of density structured (fragmented) accretion streams. We study if and how the stream fragmentation and the resulting structure of the post-shock region determine the observed profiles of UV and X-ray emission lines. Methods: We modeled the impact of an accretion stream consisting of a series of dense blobs onto the chromosphere of a CTTS through two-dimensional (2D) magnetohydrodynamic (MHD) simulations. We explored different levels of stream fragmentation and accretion rates. From the model results, we synthesize C IV (1550 Å) and O VIII (18.97 Å) line profiles. Results: The impacts of accreting blobs onto the stellar chromosphere produce reverse shocks propagating through the blobs and shocked upflows. These upflows, in turn, hit and shock the subsequent downfalling fragments. As a result, several plasma components differing for the downfalling velocity, density, and temperature are present altoghether. The profiles of C IV doublet are characterized by two main components: one narrow and redshifted to speed ≈ 50 km s-1 and the other broader and consisting of subcomponents with redshift to speed in the range 200-400 km s-1. The profiles of O VIII lines appear more symmetric than C IV and are redshifted to speed ≈ 150 km s-1. Conclusions: Our model predicts profiles of C IV line remarkably similar to those observed and explains their origin in a natural way as due to stream fragmentation. Movies are available at http://www.aanda.org

  3. Foundations of Black Hole Accretion Disk Theory.

    PubMed

    Abramowicz, Marek A; Fragile, P Chris

    2013-01-01

    This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin) disks, slim disks, and advection-dominated accretion flows (ADAFs). After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs).

  4. Evidence for a Broad Relativistic Iron Line from the Neutron Star LMXB Ser X-1

    NASA Technical Reports Server (NTRS)

    Bhattacharyya, Sudip; Strohmayer, Tod E.

    2007-01-01

    We report on an analysis of XMM-Newton data from the neutron star low mass X-ray binary (LMXB) Serpens X-1 (Ser X-1). Spectral analysis of EPIC PN data indicates that the previously known broad iron Ka emission line in this source has a significantly skewed structure with a moderately extended red wing. The asymmetric shape of the line is well described with the laor and diskline models in XSPEC, which strongly supports an inner accretion disk origin of the line. To our knowledge this is the first strong evidence for a relativistic line in a neutron star LMXB. This finding suggests that the broad lines seen in other neutron star LMXBs likely originate from the inner disk as well. Detailed study of such lines opens up a new way to probe neutron star parameters and their strong gravitational fields. The laor model describes the line from Ser X-1 somewhat better than diskline, and suggests that the inner accretion disk radius is less than 6GM/c(exp 2). This is consistent with the weak magnetic fields of LMXBs, and may point towards a high compactness and rapid spin of the neutron star. Finally, the inferred source inclination angle in the approximate range 50-60 deg is consistent with the lack of dipping from Ser X-1.

  5. How young the accretion-powered pulsars could be?

    NASA Astrophysics Data System (ADS)

    Kostina, M. V.; Ikhsanov, N. R.

    2017-12-01

    A question about the age of accretion-powered X-ray pulsars has recently been reopened by a discovery of the X-ray pulsar SXP 1062 in the SMC. This High Mass X-ray Binary (HMXB) contains a neutron star rotating with the period of 1062 s and is associated with a supernova remnant of the age ∼ 104 yr. An attempt to explain the origin of this young long-period X-ray pulsar within the traditional scenario of three basic states (ejector, propeller and accretor) encounters difficulties. Even if this pulsar were born as a magnetar the spin-down time during the propeller stage would exceed 104 yr. Here we explore a more circuitous way of the pulsar spin evolution in HMXBs, in which the propeller stage in the evolutionary track is avoided. We find this way to be possible if the stellar wind of the massive companion to the neutron star is magnetized. The geometry of plasma flow captured by the neutron star in this case differs from spherically symmetrical and the magnetospheric radius of the neutron star is smaller than that evaluated in the convention accretion scenarios. We show that the age of an accretion-powered pulsar in this case can be as small as ∼ 104 years without the need of invoking initial magnetic field in excess of 1013 G.

  6. Imaging accretion sources and circumbinary disks in young brown dwarfs

    NASA Astrophysics Data System (ADS)

    Reiners, Ansgar

    2010-09-01

    We propose to obtain deep WFC3/UVIS imaging observations of two accreting, nearby, young brown dwarf binaries. The first, 2M1207, is a brown dwarf with a planetary mass companion that became a benchmark in low-mass star formation and low-mass evolutionary models. The second, 2M0041, is a nearby young brown dwarf with clear evidence for accretion, but its space motion suggests a slightly higher age than the canonical accretion lifetime of 5-10 Myr. It has recently been discovered to be a binary and is likely to become a second benchmark object in this field. With narrow band images centered on the Halpha line that is indicative of accretion, we aim to determine the accretion ratio between the two components in each system. Halpha was observed in both systems but so far not spatially resolved. In particular, we want to search for accretion in the planetary mass companion of 2M1207. The evidence for accretion in 2M0041 and the possibility that it is in fact older than 10Myr suggests that the accretion lifetime is longer in brown dwarfs than in stars, and in particular that it is longer in brown dwarf binaries. Accretion could be sustained for a longer time if the accreting material is replenished by a circumbinary disk that might exist in both systems. We propose deep WFC/UVIS observations in the optical to search for circumbinary disks, similar to the famous disk around the binary TTauri system GG Tau.

  7. I-Love relations for incompressible stars and realistic stars

    NASA Astrophysics Data System (ADS)

    Chan, T. K.; Chan, AtMa P. O.; Leung, P. T.

    2015-02-01

    In spite of the diversity in the equations of state of nuclear matter, the recently discovered I-Love-Q relations [Yagi and Yunes, Science 341, 365 (2013), 10.1126/science.1236462], which relate the moment of inertia, tidal Love number (deformability), and the spin-induced quadrupole moment of compact stars, hold for various kinds of realistic neutron stars and quark stars. While the physical origin of such universality is still a current issue, the observation that the I-Love-Q relations of incompressible stars can well approximate those of realistic compact stars hints at a new direction to approach the problem. In this paper, by establishing recursive post-Minkowskian expansion for the moment of inertia and the tidal deformability of incompressible stars, we analytically derive the I-Love relation for incompressible stars and show that the so-obtained formula can be used to accurately predict the behavior of realistic compact stars from the Newtonian limit to the maximum mass limit.

  8. X-shooter observations of low-mass stars in the η Chamaeleontis association

    NASA Astrophysics Data System (ADS)

    Rugel, Michael; Fedele, Davide; Herczeg, Gregory

    2018-01-01

    The nearby η Chamaeleontis association is a collection of 4-10 Myr old stars with a disk fraction of 35-45%. In this study, the broad wavelength coverage of VLT/X-shooter is used to measure the stellar and mass accretion properties of 15 low-mass stars in the η Chamaeleontis association. For each star, the observed spectrum is fitted with a non-accreting stellar template and an accretion spectrum obtained from assuming a plane-parallel hydrogen slab. Five of the eight stars with an IR disk excess show excess UV emission, indicating ongoing accretion. The accretion rates measured here are similar to those obtained from previous measurements of excess UV emission, but tend to be higher than past measurements from Hα modeling. The mass accretion rates are consistent with those of other young star forming regions. This work is based on observations made with ESO Telescopes at the Paranal Observatory under program ID 084.C-1095.

  9. ATLASGAL - towards a complete sample of massive star forming clumps

    NASA Astrophysics Data System (ADS)

    Urquhart, J. S.; Moore, T. J. T.; Csengeri, T.; Wyrowski, F.; Schuller, F.; Hoare, M. G.; Lumsden, S. L.; Mottram, J. C.; Thompson, M. A.; Menten, K. M.; Walmsley, C. M.; Bronfman, L.; Pfalzner, S.; König, C.; Wienen, M.

    2014-09-01

    By matching infrared-selected, massive young stellar objects (MYSOs) and compact H II regions in the Red MSX Source survey to massive clumps found in the submillimetre ATLASGAL (APEX Telescope Large Area Survey of the Galaxy) survey, we have identified ˜1000 embedded young massive stars between 280° < ℓ < 350° and 10° < ℓ < 60° with | b | < 1.5°. Combined with an existing sample of radio-selected methanol masers and compact H II regions, the result is a catalogue of ˜1700 massive stars embedded within ˜1300 clumps located across the inner Galaxy, containing three observationally distinct subsamples, methanol-maser, MYSO and H II-region associations, covering the most important tracers of massive star formation, thought to represent key stages of evolution. We find that massive star formation is strongly correlated with the regions of highest column density in spherical, centrally condensed clumps. We find no significant differences between the three samples in clump structure or the relative location of the embedded stars, which suggests that the structure of a clump is set before the onset of star formation, and changes little as the embedded object evolves towards the main sequence. There is a strong linear correlation between clump mass and bolometric luminosity, with the most massive stars forming in the most massive clumps. We find that the MYSO and H II-region subsamples are likely to cover a similar range of evolutionary stages and that the majority are near the end of their main accretion phase. We find few infrared-bright MYSOs associated with the most massive clumps, probably due to very short pre-main-sequence lifetimes in the most luminous sources.

  10. Kinematic Dynamo In Turbulent Circumstellar Disks

    NASA Technical Reports Server (NTRS)

    Stepinski, T.

    1993-01-01

    Many circumstellar disks associated with objects ranging from protoplanetary nebulae, to accretion disks around compact stars allow for the generation of magnetic fields by an (alpha)omega dynamo. We have applied kinematic dynamo formalism to geometrically thin accretion disks. We calculate, in the framework of an adiabatic approximation, the normal mode solutions for dynamos operating in disks around compact stars. We then describe the criteria for a viable dynamo in protoplanetary nebulae, and discuss the particular features that make accretion disk dynamos different from planetary, stellar, and galactic dynamos.

  11. Charged Compact Boson Stars in a Theory of Massless Scalar Field

    NASA Astrophysics Data System (ADS)

    Kumar, Sanjeev

    2018-05-01

    In this work we present some new results obtained in a study of the phase diagram of charged compact boson stars in a theory involving a complex scalar field with a conical potential coupled to a U(1) gauge field and gravity. We obtain new bifurcation points in this model. We present a detailed discussion of the various regions of the phase diagram with respect to the bifurcation points. The theory is seen to contain rich physics in a particular domain of the phase diagram.

  12. Caught in the Act: Gas and Stellar Velocity Dispersions in a Fast Quenching Compact Star-Forming Galaxy at z~1.7

    NASA Astrophysics Data System (ADS)

    Barro, Guillermo; Faber, Sandra M.; Dekel, Avishai; Pacifici, Camilla; Pérez-González, Pablo G.; Toloba, Elisa; Koo, David C.; Trump, Jonathan R.; Inoue, Shigeki; Guo, Yicheng; Liu, Fengshan; Primack, Joel R.; Koekemoer, Anton M.; Brammer, Gabriel; Cava, Antonio; Cardiel, Nicolas; Ceverino, Daniel; Eliche, Carmen; Fang, Jerome J.; Finkelstein, Steven L.; Kocevski, Dale D.; Livermore, Rachael C.; McGrath, Elizabeth

    2016-04-01

    We present Keck I MOSFIRE spectroscopy in the Y and H bands of GDN-8231, a massive, compact, star-forming galaxy at a redshift of z ˜ 1.7. Its spectrum reveals both Hα and [N II] emission lines and strong Balmer absorption lines. The Hα and Spitzer MIPS 24 μm fluxes are both weak, thus indicating a low star-formation rate of SFR ≲ 5{--}10 {M}⊙ yr-1. This, added to a relatively young age of ˜700 Myr measured from the absorption lines, provides the first direct evidence for a distant galaxy being caught in the act of rapidly shutting down its star formation. Such quenching allows GDN-8231 to become a compact, quiescent galaxy, similar to three other galaxies in our sample, by z ˜ 1.5. Moreover, the color profile of GDN-8231 shows a bluer center, consistent with the predictions of recent simulations for an early phase of inside-out quenching. Its line-of-sight velocity dispersion for the gas, {σ }{{{LOS}}}{{gas}} = 127 ± 32 km s-1, is nearly 40% smaller than that of its stars, {σ }{{{LOS}}}\\star = 215 ± 35 km s-1. High-resolution hydro-simulations of galaxies explain such apparently colder gas kinematics of up to a factor of ˜1.5 with rotating disks being viewed at different inclinations and/or centrally concentrated star-forming regions. A clear prediction is that their compact, quiescent descendants preserve some remnant rotation from their star-forming progenitors.

  13. Time-series Photometry of the Pre-Main Sequence Binary V4046 Sgr: Testing the Accretion Stream Theory

    NASA Astrophysics Data System (ADS)

    Tofflemire, Benjamin M.; Mathieu, Robert D.; Ardila, David R.; Ciardi, David R.

    2015-01-01

    Most stars are born in binaries, and the evolution of protostellar disks in pre-main sequence (PMS) binary stars is a current frontier of star formation research. PMS binary stars can have up to three accretion disks: two circumstellar disks and a circumbinary disk separated by a dynamically cleared gap. Theory suggests that mass may periodically flow in an accretion stream from a circumbinary disk across the gap onto circumstellar disks or stellar surfaces. Thus, accretion in PMS binaries is controlled by not only radiation, disk viscosity, and magnetic fields, but also by orbital dynamics.As part of a larger, ongoing effort to characterize mass accretion in young binary systems, we test the predictions of the binary accretion stream theory through continuous, multi-orbit, multi-color optical and near-infrared (NIR) time-series photometry. Observations such as these are capable of detecting and characterizing these modulated accretion streams, if they are generally present. Broad-band blue and ultraviolet photometry trace the accretion luminosity and photospheric temperature while NIR photometry provide a measurement of warm circumstellar material, all as a function of orbital phase. The predicted phase and magnitude of enhanced accretion are highly dependent on the binary orbital parameters and as such, our campaign focuses on 10 PMS binaries of varying periods and eccentricities. Here we present multi-color optical (U, B,V, R), narrowband (Hα), and multi-color NIR (J, H) lightcurves of the PMS binary V4046 Sgr (P=2.42 days) obtained with the SMARTS 1.3m telescope and LCOGT 1m telescope network. These results act to showcase the quality and breadth of data we have, or are currently obtaining, for each of the PMS binaries in our sample. With the full characterization of our sample, these observations will guide an extension of the accretion paradigm from single young stars to multiple systems.

  14. Self-Consistent Models of Accretion Disks

    NASA Technical Reports Server (NTRS)

    Narayan, Ramesh

    2000-01-01

    Research was carried out on several topics in the theory of astrophysical accretion flows around black holes, neutron stars and white dwarfs. The focus of our effort was the advection-dominated accretion flow (ADAF) model which the PI and his collaborators proposed and developed over the last several years. Our group completed a total of 46 papers, of which 36 are in refereed journals and 12 are in conference proceedings. All the papers have either already appeared in print or are in press.

  15. The Origin of X-ray Emission from the Enigmatic Be Star γ Cassiopeiae

    NASA Astrophysics Data System (ADS)

    Hamaguchi, K.; Oskinova, L.; Russell, C. M. P.; Petre, R.; Enoto, T.; Morihana, K.; Ishida, M.

    2017-11-01

    Gamma Cassiopeiae is an enigmatic Be star with unusually hard, strong X-ray emission compared with normal main-sequence B stars. The origin has been debated for decades between two theories: mass accretion onto a hidden compact companion and a magnetic dynamo driven by the star-Be disk differential rotation. There has been no decisive signature found that supports either theory, such as a pulse in X-ray emission or the presence of large-scale magnetic field. In a ~100 ksec duration observation of the star with the Suzaku X-ray observatory in 2011, we detected six rapid X-ray spectral hardening events called ``softness dips''. All the softness dip events show symmetric softness ratio variations, and some of them have flat bottoms apparently due to saturation. The softness dip spectra are best described by either ~40% or ~70% partial covering absorption to kT ~12 keV plasma emission by matter with a neutral hydrogen column density of ~2 - 8 × 1021cm-2, while the spectrum outside of these dips is almost free of absorption. This result suggests that two distinct X-ray emitting spots in the γ Cas system, perhaps on a white dwarf companion with dipole mass accretion, are occulted by blobs in the Be stellar wind, the Be disk, or rotating around the white dwarf companion. The formation of a Be star and white dwarf binary system requires mass transfer between two stars; γ Cas may have experienced such activity in the past.

  16. Ingress observations of the 1980 eclipse of the symbiotic star CI Cyngni

    NASA Technical Reports Server (NTRS)

    Stencel, R. E.; Michalitsianos, A. G.; Kafatos, M.; Boyarchuk, A. A.

    1981-01-01

    One of the major results from the IUE may prove to be the knowledge gained by studies of the ultraviolet spectra of symbiotic stars. Symbiotics combine spectral features of a cool M giant like photosphere with strong high excitation emission lines of nebular origin, superposed. The UV spectra are dominated by intense permitted and semiforbidden emission lines and weak continua indicative of hot compact objects and accretion disks. Two symbiotics, AR Pav and CI Cyg are thought to be eclipsing binaries and IUE observations during the 1980 eclipse of CI Cygni are discussed.

  17. Symbiotic Stars in X-rays

    NASA Technical Reports Server (NTRS)

    Luna, G. J. M.; Sokoloski, J. L.; Mukai, K.; Nelson, T.

    2014-01-01

    Until recently, symbiotic binary systems in which a white dwarf accretes from a red giant were thought to be mainly a soft X-ray population. Here we describe the detection with the X-ray Telescope (XRT) on the Swift satellite of 9 white dwarf symbiotics that were not previously known to be X-ray sources and one that was previously detected as a supersoft X-ray source. The 9 new X-ray detections were the result of a survey of 41 symbiotic stars, and they increase the number of symbiotic stars known to be X-ray sources by approximately 30%. Swift/XRT detected all of the new X-ray sources at energies greater than 2 keV. Their X-ray spectra are consistent with thermal emission and fall naturally into three distinct groups. The first group contains those sources with a single, highly absorbed hard component, which we identify as probably coming from an accretion-disk boundary layer. The second group is composed of those sources with a single, soft X-ray spectral component, which likely arises in a region where low-velocity shocks produce X-ray emission, i.e. a colliding-wind region. The third group consists of those sources with both hard and soft X-ray spectral components. We also find that unlike in the optical, where rapid, stochastic brightness variations from the accretion disk typically are not seen, detectable UV flickering is a common property of symbiotic stars. Supporting our physical interpretation of the two X-ray spectral components, simultaneous Swift UV photometry shows that symbiotic stars with harder X-ray emission tend to have stronger UV flickering, which is usually associated with accretion through a disk. To place these new observations in the context of previous work on X-ray emission from symbiotic stars, we modified and extended the alpha/beta/gamma classification scheme for symbiotic-star X-ray spectra that was introduced by Muerset et al. based upon observations with the ROSAT satellite, to include a new sigma classification for sources with

  18. The rotation of discs around neutron stars: dependence on the Hall diffusion

    NASA Astrophysics Data System (ADS)

    Faghei, Kazem; Salehi, Fatemeh

    2018-01-01

    In this paper, we study the dynamics of a geometrically thin, steady and axisymmetric accretion disc surrounding a rotating and magnetized star. The magnetic field lines of star penetrate inside the accretion disc and are twisted due to the differential rotation between the magnetized star and the disc. We apply the Hall diffusion effect in the accreting plasma, because of the Hall diffusion plays an important role in both fully ionized plasma and weakly ionized medium. In the current research, we show that the Hall diffusion is also an important mechanism in accreting plasma around neutron stars. For the typical system parameter values associated with the accreting X-ray binary pulsar, the angular velocity of the inner regions of disc departs outstandingly from Keplerian angular velocity, due to coupling between the magnetic field of neutron star and the rotating plasma of disc. We found that the Hall diffusion is very important in inner disc and increases the coupling between the magnetic field of neutron star and accreting plasma. On the other word, the rotational velocity of inner disc significantly decreases in the presence of the Hall diffusion. Moreover, the solutions imply that the fastness parameter decreases and the angular velocity transition zone becomes broad for the accreting plasma including the Hall diffusion.

  19. Signs of magnetic accretion in the young Be/X-ray pulsar SXP 1062

    NASA Astrophysics Data System (ADS)

    Ikhsanov, N. R.

    2012-07-01

    The spin behaviour of the neutron star in the newly discovered young Be/X-ray long-period pulsar SXP 1062 is discussed. The star is observed to rotate with the period of 1062 s, and spin down at the rate ˜-2.6 × 10-12 Hz s-1. I show that all of the conventional accretion scenarios encounter major difficulties in explaining the rapid spin-down of the pulsar. These difficulties can be, however, avoided within the magnetic accretion scenario in which the neutron star is assumed to accrete from a magnetized wind. The spin-down rate of the pulsar can be explained within this scenario provided the surface magnetic field of the neutron star is B*˜ 4 × 1013 G. I show that the age of the pulsar in this case lies in the range (2-4) × 104 yr, which is consistent with observations. The spin evolution of the pulsar is briefly discussed.

  20. Embedded class solutions compatible for physical compact stars in general relativity

    NASA Astrophysics Data System (ADS)

    Newton Singh, Ksh.; Pant, Neeraj; Tewari, Neeraj; Aria, Anil K.

    2018-05-01

    We have explored a family of new solutions satisfying Einstein's field equations and Karmarkar condition. We have assumed an anisotropic stress-tensor with no net electric charge. Interestingly, the new solutions yield zero values of all the physical quantities for all even integer n > 0. However, for all n >0 (n ≠ even numbers) they yield physically possible solutions. We have tuned the solution for neutron star Vela X-1 so that the solutions matches the observed mass and radius. For the same star we have extensively discussed the behavior of the solutions. The solutions yield a stiffer equation of state for larger values of n since the adiabatic index increases and speed of sound approaches the speed of light. It is also found that the solution is physically possible for Vela X-1 if 1.8 ≤ n < 7 (with n≠ 2,4,6). All the solutions for n ≥ 7 violates the causality condition and all the solutions with 0 < n < 1.8 lead to complex values of transverse sound speed vt. The range of well-behaved n depends on the mass and radius of compact stars.

  1. Self-Consistent Models of Accretion Disks

    NASA Technical Reports Server (NTRS)

    Narayan, Ramesh

    1997-01-01

    The investigations of advection-dominated accretion flows (ADAFs), with emphasis on applications to X-ray binaries containing black holes and neutron stars is presented. This work is now being recognized as the standard paradigm for understanding the various spectral states of black hole X-ray Binaries (BHXBs). Topics discussed include: (1) Problem in BHXBS, namely that several of these binaries have unusually large concentrations of lithium in their companion stars; (2) A novel test to show that black holes have event horizons; (3) Application of the ADAF model to the puzzling X-ray delay in the recent outburst of the BHXB, GRO J1655-40; (4) Description of the various spectral states in BHXBS; (5) Application of the ADAF model to the famous supermassive black hole at the center of our Galaxy, Sgr A(*); (6) Writing down and solving equations describing steady-state, optically thin, advection-dominated accretion onto a Kerr black hole; (7) The effect of "photon bubble" instability on radiation dominated accretion disks; and (8) Dwarf nova disks in quiescence that have rather low magnetic Reynolds number, of order 10(exp 3).

  2. A Christmas comet falling onto a neutron star

    NASA Astrophysics Data System (ADS)

    Campana, S.

    The Sun and the planets are the main, but not the only, bodies of the Solar System. There are thousands of asteroids and several tens of comets, many of which are still unknown. They are the remnants of the planetesimals that formed at the origin of our Solar System, and they are rocky objects of different dimensions and irregular shape. Sometimes these minor bodies fall onto the Sun or onto planets, like Jupiter. Less dramatic events occur when the infalling bodies do not directly impact onto the target but are tidally disrupted. The tidal disruption of solar mass stars around supermassive black holes has been extensively studied analytically and numerically. In these events the star, as it approaches the black hole, develops into an elongated banana-shaped structure, the most tightly bound debris being at the closer end to the compact object. After completing an (few) eccentric orbit(s), these bound debris fall onto the black hole, emitting energy. Orbital precession may lead to the crossing of the debris orbits producing an accretion disk. Observationally, these events will give rise to luminous events with different temporal decays in different energy bands. Tidal break-up events occur also in planetary systems around normal stars but these events are too faint to be detected. Things change when the star is a compact object. Indeed planets have been discovered around radio pulsars, making likely the existence also of orbiting minor bodies. The direct impact of minor bodies onto neutron stars has been studied in the past and it has been envisaged as a possible (local) explanation for Gamma-Ray Bursts (GRBs), producing short-duration (˜ seconds) events. To explain the peculiarities of GRB 101225A (Christmas burst) we propose that it resulted from the tidal disruption event of a minor body around a neutron star in our Galaxy.

  3. Migration of accreting giant planets

    NASA Astrophysics Data System (ADS)

    Crida, A.; Bitsch, B.; Raibaldi, A.

    2016-12-01

    We present the results of 2D hydro simulations of giant planets in proto-planetary discs, which accrete gas at a more or less high rate. First, starting from a solid core of 20 Earth masses, we show that as soon as the runaway accretion of gas turns on, the planet is saved from type I migration : the gap opening mass is reached before the planet is lost into its host star. Furthermore, gas accretion helps opening the gap in low mass discs. Consequently, if the accretion rate is limited to the disc supply, then the planet is already inside a gap and in type II migration. We further show that the type II migration of a Jupiter mass planet actually depends on its accretion rate. Only when the accretion is high do we retrieve the classical picture where no gas crosses the gap and the planet follows the disc spreading. These results impact our understanding of planet migration and planet population synthesis models. The e-poster presenting these results in French can be found here: L'e-poster présentant ces résultats en français est disponible à cette adresse: http://sf2a.eu/semaine-sf2a/2016/posterpdfs/156_179_49.pdf.

  4. A simple physical model for X-ray burst sources

    NASA Technical Reports Server (NTRS)

    Joss, P. C.; Rappaport, S.

    1977-01-01

    In connection with information considered by Illarianov and Sunyaev (1975) and van den Heuvel (1975), a simple physical model for an X-ray burst source in the galactic disk is proposed. The model includes an unevolved OB star with a relatively weak stellar wind and a compact object in a close binary system. For some reason, the stellar wind from the OB star is unable to accrete steadily on to the compact object. When the stellar wind is sufficiently weak, the compact object accretes irregularly, leading to X-ray bursts.

  5. Theoretical Developments in Understanding Massive Star Formation

    NASA Technical Reports Server (NTRS)

    Yorke, Harold W.; Bodenheimer, Peter

    2007-01-01

    Except under special circumstances massive stars in galactic disks will form through accretion. The gravitational collapse of a molecular cloud core will initially produce one or more low mass quasi-hydrostatic objects of a few Jupiter masses. Through subsequent accretion the masses of these cores grow as they simultaneously evolve toward hydrogen burning central densities and temperatures. We review the evolution of accreting (proto-)stars, including new results calculated with a publicly available stellar evolution code written by the authors.

  6. A Deep NuSTAR Survey of M31: Compact object types in our Nearest Neighbor Galaxy

    NASA Astrophysics Data System (ADS)

    Hornschemeier, Ann E.; Wik, Daniel R.; Yukita, Mihoko; Ptak, Andrew; Venters, Tonia M.; Lehmer, Bret; Maccarone, Thomas J.; Zezas, Andreas; Harrison, Fiona; Stern, Daniel; Williams, Benjamin F.; Vulic, Neven

    2017-08-01

    X-ray binaries (XRBs) trace young and old stellar populations in galaxies, and thus star formation rate and star formation history/stellar mass. X-ray emission from XRBs may be responsible for significant amounts of heating of the early Intergalactic Medium at Cosmic Dawn and may also play a significant role in reionization. Until recently, the E>10 keV (hard X-ray) emission from these populations could only be studied for XRBs in our own galaxy, where it is often difficult to measure accurate distances and thus luminosities. We have observed M31 in 4 NuSTAR fields for a total exposure of 1.4 Ms, covering the young stellar population in a swath of the disk (within the footprint of the Panchromatic Hubble Andromeda Treasury (PHAT) Survey) and older populations in the bulge. We detected more than 100 sources in the 4-25 keV band, where hard band (12-25 keV) emission has allowed us to discriminate between black holes and neutron stars in different accretion states. The luminosity function of the hard band detected sources are compared to Swift/BAT and INTEGRAL-derived luminosity functions of the Milky Way population, which reveals an excess of luminous sources in M31 when correcting for star formation rate and stellar mass.

  7. Ngc7538 Irs1 - A Highly Collimated Ionized Wind Source Powered By Accretion

    NASA Astrophysics Data System (ADS)

    Sandell, Goran H. L.; Wright, M.; Goss, W.; Corder, S.

    2009-01-01

    Recent images show that NGC7538 IRS1 is not a conventional Ultracompact or Hypercompact HII region, but is completely wind-excited (other broad recombination line hypercompact HII regions may be similar to IRS1). NGC 7538 IRS1 is a well studied young high-mass star (L 2 10^5 L_Sun).VLA images at 6 and 2 cm (Cambell 1984; ApJ, 282, L27) showed a compact bipolar core (lobe separation 0.2") with more extended faint lobes. Recombination line observations (Gaume et al. 1995, ApJ, 438, 776) show extremely wide line profiles indicating substantial mass motion of the ionized gas. We re-analyzed high angular resolution VLA archive data from 6 cm to 7 mm, and measured the flux from the compact core and the extended (1.5 - 2") bipolar lobes. We find that the compact core has a spectral index, alpha 0.6, which could be explained by an optically thick hypercompact core with a density gradient. However, the size of the core shrinks with increasing frequency; from 0.24" at 6 cm to 0.1" at 7 mm, consistent with that expected for a collimated jet (Reynolds 1986, ApJ, 304, 713). If we do a crude size correction so that we compare emission from the optically thick inner part of the jet for a set of 2 cm and 7 mm observations we get alpha 1.6, i.e. close to the optically thick value. BIMA and CARMA continuum observations at 3 mm show some dust excess, while. HCO+ J=1-0 observations combined with FCRAO single dish data show a clear inverse P Cygni profile towards IRS1. These observations confirm that IRS1 is heavily accreting with an accretion rate 2 10^-4 M_Sun/year, sufficient to quench the formation of an HII region.

  8. COSMIC probes into compact binary formation and evolution

    NASA Astrophysics Data System (ADS)

    Breivik, Katelyn

    2018-01-01

    The population of compact binaries in the galaxy represents the final state of all binaries that have lived up to the present epoch. Compact binaries present a unique opportunity to probe binary evolution since many of the interactions binaries experience can be imprinted on the compact binary population. By combining binary evolution simulations with catalogs of observable compact binary systems, we can distill the dominant physical processes that govern binary star evolution, as well as predict the abundance and variety of their end products.The next decades herald a previously unseen opportunity to study compact binaries. Multi-messenger observations from telescopes across all wavelengths and gravitational-wave observatories spanning several decades of frequency will give an unprecedented view into the structure of these systems and the composition of their components. Observations will not always be coincident and in some cases may be separated by several years, providing an avenue for simulations to better constrain binary evolution models in preparation for future observations.I will present the results of three population synthesis studies of compact binary populations carried out with the Compact Object Synthesis and Monte Carlo Investigation Code (COSMIC). I will first show how binary-black-hole formation channels can be understood with LISA observations. I will then show how the population of double white dwarfs observed with LISA and Gaia could provide a detailed view of mass transfer and accretion. Finally, I will show that Gaia could discover thousands black holes in the Milky Way through astrometric observations, yielding view into black-hole astrophysics that is complementary to and independent from both X-ray and gravitational-wave astronomy.

  9. A hot compact dust disk around a massive young stellar object.

    PubMed

    Kraus, Stefan; Hofmann, Karl-Heinz; Menten, Karl M; Schertl, Dieter; Weigelt, Gerd; Wyrowski, Friedrich; Meilland, Anthony; Perraut, Karine; Petrov, Romain; Robbe-Dubois, Sylvie; Schilke, Peter; Testi, Leonardo

    2010-07-15

    Circumstellar disks are an essential ingredient of the formation of low-mass stars. It is unclear, however, whether the accretion-disk paradigm can also account for the formation of stars more massive than about 10 solar masses, in which strong radiation pressure might halt mass infall. Massive stars may form by stellar merging, although more recent theoretical investigations suggest that the radiative-pressure limit may be overcome by considering more complex, non-spherical infall geometries. Clear observational evidence, such as the detection of compact dusty disks around massive young stellar objects, is needed to identify unambiguously the formation mode of the most massive stars. Here we report near-infrared interferometric observations that spatially resolve the astronomical-unit-scale distribution of hot material around a high-mass ( approximately 20 solar masses) young stellar object. The image shows an elongated structure with a size of approximately 13 x 19 astronomical units, consistent with a disk seen at an inclination angle of approximately 45 degrees . Using geometric and detailed physical models, we found a radial temperature gradient in the disk, with a dust-free region less than 9.5 astronomical units from the star, qualitatively and quantitatively similar to the disks observed in low-mass star formation. Perpendicular to the disk plane we observed a molecular outflow and two bow shocks, indicating that a bipolar outflow emanates from the inner regions of the system.

  10. A study of accretion discs around rapidly rotating neutron stars in general relativity and its applications to four low mass X-ray binaries

    NASA Astrophysics Data System (ADS)

    Bhattacharyya, Sudip

    2002-02-01

    We calculate the accretion disc temperature profiles, disc luminosities and boundary layer luminosities for rapidly rotating neutron stars considering the full effect of general relativity. We compare the theoretical values of these quantities with their values inferred from EXOSAT data for four low mass X-ray binary sources: XB 1820-30, GX 17+2, GX 9+1 and GX 349+2 and constrain the values of several properties of these sources. According to our calculations, the neutron stars in GX 9+1 and GX 349+2 are rapidly rotating and stiffer equations of state are unfavoured.

  11. Towards a Unified View of Inhomogeneous Stellar Winds in Isolated Supergiant Stars and Supergiant High Mass X-Ray Binaries

    NASA Astrophysics Data System (ADS)

    Martínez-Núñez, Silvia; Kretschmar, Peter; Bozzo, Enrico; Oskinova, Lidia M.; Puls, Joachim; Sidoli, Lara; Sundqvist, Jon Olof; Blay, Pere; Falanga, Maurizio; Fürst, Felix; Gímenez-García, Angel; Kreykenbohm, Ingo; Kühnel, Matthias; Sander, Andreas; Torrejón, José Miguel; Wilms, Jörn

    2017-10-01

    -ray sources in the sky. A large number of them consist of a neutron star accreting from the wind of a massive companion and producing a powerful X-ray source. The characteristics of the stellar wind together with the complex interactions between the compact object and the donor star determine the observed X-ray output from all these systems. Consequently, the use of SgXBs for studies of massive stars is only possible when the physics of the stellar winds, the compact objects, and accretion mechanisms are combined together and confronted with observations. This detailed review summarises the current knowledge on the theory and observations of winds from massive stars, as well as on observations and accretion processes in wind-fed high mass X-ray binaries. The aim is to combine in the near future all available theoretical diagnostics and observational measurements to achieve a unified picture of massive star winds in isolated objects and in binary systems.

  12. Star formation properties of Hickson Compact Groups based on deep Hα imaging

    NASA Astrophysics Data System (ADS)

    Eigenthaler, Paul; Ploeckinger, Sylvia; Verdugo, Miguel; Ziegler, Bodo

    2015-08-01

    We present deep Hα imaging of seven Hickson Compact Groups (HCGs) using the 4.1-m Southern Astrophysics Research (SOAR) Telescope. The high spatial resolution of the observations allows us to study both the integrated star formation properties of the main galaxies as well as the 2D distribution of star-forming knots in the faint tidal arms that form during interactions between the individual galaxies. We derive star formation rates and stellar masses for group members and discuss their position relative to the main sequence of star-forming galaxies. Despite the existence of tidal features within the galaxy groups, we do not find any indication for enhanced star formation in the selected sample of HCGs. We study azimuthally averaged Hα profiles of the galaxy discs and compare them with the g' and r' surface brightness profiles. We do not find any truncated galaxy discs but reveal that more massive galaxies show a higher light concentration in Hα than less massive ones. We also see that galaxies that show a high light concentration in r', show a systematic higher light concentration in Hα. Tidal dwarf galaxy (TDG) candidates have been previously detected in R-band images for two groups in our sample but we find that most of them are likely background objects as they do not show any emission in Hα. We present a new TDG candidate at the tip of the tidal tail in HCG 91.

  13. OGLE-2014-SN-073 as a fallback accretion powered supernova

    NASA Astrophysics Data System (ADS)

    Moriya, Takashi J.; Terreran, Giacomo; Blinnikov, Sergei I.

    2018-03-01

    We investigate the possibility that the energetic Type II supernova OGLE-2014-SN-073 is powered by a fallback accretion following the failed explosion of a massive star. Taking massive hydrogen-rich supernova progenitor models, we estimate the fallback accretion rate and calculate the light-curve evolution of supernovae powered by the fallback accretion. We find that such fallback accretion powered models can reproduce the overall observational properties of OGLE-2014-SN-073. It may imply that some failed explosions could be observed as energetic supernovae like OGLE-2014-SN-073 instead of faint supernovae as previously proposed.

  14. Observations of Accreting Pulsars

    NASA Technical Reports Server (NTRS)

    Bildsten, Lars; Chakrabarty, Deepto; Chiu, John; Finger, Mark H.; Koh, Danny T.; Nelson, Robert W.; Prince, Thomas A.; Rubin, Bradley C.; Scott, D. Matthew; Stollberg, Mark; hide

    1997-01-01

    We summarize 5 years of continuous monitoring of accretion-powered pulsars with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Our 20-70 keV observations have determined or refined the orbital parameters of 13 binaries, discovered five new transient accreting pulsars, measured the pulsed flux history during outbursts of 12 transients (GRO J1744-28, 4U 0115+634, GRO J1750-27, GS 0834-430, 2S 1417-624, GRO J1948+32, EXO 2030+375, GRO J1008-57, A0535+26, GRO J2058+42, 4U 1145-619, and A1118-616), and also measured the accretion torque history during outbursts of six of those transients whose orbital param- eters were also known. We have also continuously measured the pulsed flux and spin frequency for eiaht persistently accreting pulsars (Her X-1, Cen X-3, Vela X-1, OAO 1657-415, GX 301-2, 4U 1626-67, 4U 1538-52, and GX 1+4). Because of their continuity and uniformity over a long baseline, BATSE observations have provided new insights into the long-term behavior of accreting magnetic neutron stars. We have found that all accreting pulsars show stochastic variations in their spin frequencies and luminosities, including those displaying secular spin-up or spin-down on long timescales, which blurs the con- ventional distinction between disk-fed and wind-fed binaries. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars but are uncorrelated, or even anti- correlated, in persistent sources. We describe daily folded pulse profiles, frequency, and flux measurements that are available through the Compton Observatory Science Support Center at NASA/Goddard Space Flight Center.

  15. Accreting binary population synthesis and feedback prescriptions

    NASA Astrophysics Data System (ADS)

    Fragos, Tassos

    2016-04-01

    Studies of extagalactic X-ray binary populations have shown that the characteristics of these populations depend strongly on the characteristics of the host galaxy's parent stellar population (e.g. star-formation history and metallicity). These dependencies not only make X-ray binaries promising for aiding in the measurement of galaxy properties themselves, but they also have important astrophysical and cosmological implications. For example, due to the relatively young stellar ages and primordial metallicities in the early Universe (z > 3), it is predicted that X-ray binaries were more luminous than today. The more energetic X-ray photons, because of their long mean-free paths, can escape the galaxies where they are produced, and interact at long distances with the intergalactic medium. This could result in a smoother spatial distribution of ionized regions, and more importantly in an overall warmer intergalactic medium. The energetic X-ray photons emitted from X-ray binaries dominate the X-ray radiation field over active galactic nuclei at z > 6 - 8, and hence Χ-ray binary feedback can be a non-negligible contributor to the heating and reionization of the inter-galactic medium in the early universe. The spectral energy distribution shape of the XRB emission does not change significantly with redshift, suggesting that the same XRB subpopulation, namely black-hole XRBs in the high-soft state, dominates the cumulative emission at all times. On the contrary, the normalization of the spectral energy distribution does evolve with redshift. To zeroth order, this evolution is driven by the cosmic star-formation rate evolution. However, the metallicity evolution of the universe and the mean stellar population age are two important factors that affect the X-ray emission from high-mass and low-mass XRBs, respectively. In this talk, I will review recent studies on the potential feedback from accreting binary populations in galactic and cosmological scales. Furthermore, I

  16. The MagAO Giant Accreting Protoplanet Survey (GAPlanetS): Recent Results

    NASA Astrophysics Data System (ADS)

    Follette, Katherine; Close, Laird; Males, Jared; Morzinski, Katie; Leonard, Clare; MagAO

    2018-01-01

    I will summarize recent results of the MagAO Giant Accreting Protoplant Survey (GAPlanetS), a search for accreting protoplanets at H-alpha inside of transitional disk gaps. These young, centrally-cleared circumstellar disks are often hosted by stars that are still actively accreting, making it likely that any planets that lie in their central cavities will also be actively accreting. Through differential imaging at Hydrogen-alpha using Magellan's visible light adaptive optics system, we have completed the first systematic search for H-alpha emission from accreting protoplanets in fifteen bright Southern hemisphere transitional disks. I will present results from this survey, including a second epoch on the LkCa 15 system that shows several accreting protoplanet candidates.

  17. Mass-accreting white dwarfs and type Ia supernovae

    NASA Astrophysics Data System (ADS)

    Wang, Bo

    2018-05-01

    Type Ia supernovae (SNe Ia) play a prominent role in understanding the evolution of the Universe. They are thought to be thermonuclear explosions of mass-accreting carbon-oxygen white dwarfs (CO WDs) in binaries, although the mass donors of the accreting WDs are still not well determined. In this article, I review recent studies on mass-accreting WDs, including H- and He-accreting WDs. I also review currently most studied progenitor models of SNe Ia, i.e., the single-degenerate model (including the WD+MS channel, the WD+RG channel and the WD+He star channel), the double-degenerate model (including the violent merger scenario) and the sub-Chandrasekhar mass model. Recent progress on these progenitor models is discussed, including the initial parameter space for producing SNe Ia, the binary evolutionary paths to SNe Ia, the progenitor candidates for SNe Ia, the possible surviving companion stars of SNe Ia, some observational constraints, etc. Some other potential progenitor models of SNe Ia are also summarized, including the hybrid CONe WD model, the core-degenerate model, the double WD collision model, the spin-up/spin-down model and the model of WDs near black holes. To date, it seems that two or more progenitor models are needed to explain the observed diversity among SNe Ia.

  18. Mapping accretion and its variability in the young open cluster NGC 2264: a study based on u-band photometry

    NASA Astrophysics Data System (ADS)

    Venuti, L.; Bouvier, J.; Flaccomio, E.; Alencar, S. H. P.; Irwin, J.; Stauffer, J. R.; Cody, A. M.; Teixeira, P. S.; Sousa, A. P.; Micela, G.; Cuillandre, J.-C.; Peres, G.

    2014-10-01

    Context. The accretion process has a central role in the formation of stars and planets. Aims: We aim at characterizing the accretion properties of several hundred members of the star-forming cluster NGC 2264 (3 Myr). Methods: We performed a deep ugri mapping as well as a simultaneous u-band+r-band monitoring of the star-forming region with CFHT/MegaCam in order to directly probe the accretion process onto the star from UV excess measurements. Photometric properties and stellar parameters are determined homogeneously for about 750 monitored young objects, spanning the mass range ~0.1-2 M⊙. About 40% of the sample are classical (accreting) T Tauri stars, based on various diagnostics (Hα, UV and IR excesses). The remaining non-accreting members define the (photospheric + chromospheric) reference UV emission level over which flux excess is detected and measured. Results: We revise the membership status of cluster members based on UV accretion signatures, and report a new population of 50 classical T Tauri star (CTTS) candidates. A large range of UV excess is measured for the CTTS population, varying from a few times 0.1 to ~3 mag. We convert these values to accretion luminosities and accretion rates, via a phenomenological description of the accretion shock emission. We thus obtain mass accretion rates ranging from a few 10-10 to ~10-7 M⊙/yr. Taking into account a mass-dependent detection threshold for weakly accreting objects, we find a >6σ correlation between mass accretion rate and stellar mass. A power-law fit, properly accounting for censored data (upper limits), yields Ṁacc ∝ M*1.4±0.3. At any given stellar mass, we find a large spread of accretion rates, extending over about 2 orders of magnitude. The monitoring of the UV excess on a timescale of a couple of weeks indicates that its variability typically amounts to 0.5 dex, i.e., much smaller than the observed spread in accretion rates. We suggest that a non-negligible age spread across the star

  19. The formation and assembly of a typical star-forming galaxy at redshift z approximately 3.

    PubMed

    Stark, Daniel P; Swinbank, A Mark; Ellis, Richard S; Dye, Simon; Smail, Ian R; Richard, Johan

    2008-10-09

    Recent studies of galaxies approximately 2-3 Gyr after the Big Bang have revealed large, rotating disks, similar to those of galaxies today. The existence of well-ordered rotation in galaxies during this peak epoch of cosmic star formation indicates that gas accretion is likely to be the dominant mode by which galaxies grow, because major mergers of galaxies would completely disrupt the observed velocity fields. But poor spatial resolution and sensitivity have hampered this interpretation; such studies have been limited to the largest and most luminous galaxies, which may have fundamentally different modes of assembly from those of more typical galaxies (which are thought to grow into the spheroidal components at the centres of galaxies similar to the Milky Way). Here we report observations of a typical star-forming galaxy at z = 3.07, with a linear resolution of approximately 100 parsecs. We find a well-ordered compact source in which molecular gas is being converted efficiently into stars, likely to be assembling a spheroidal bulge similar to those seen in spiral galaxies at the present day. The presence of undisrupted rotation may indicate that galaxies such as the Milky Way gain much of their mass by accretion rather than major mergers.

  20. Star formation: Cosmic feast

    NASA Astrophysics Data System (ADS)

    Scaringi, Simone

    2016-11-01

    Low-mass stars form through a process known as disk accretion, eating up material that orbits in a disk around them. It turns out that the same mechanism also describes the formation of more massive stars.

  1. Star formation: Cosmic feast

    NASA Astrophysics Data System (ADS)

    Scaringi, Simone

    2017-03-01

    Low-mass stars form through a process known as disk accretion, eating up material that orbits in a disk around them. It turns out that the same mechanism also describes the formation of more massive stars.

  2. The X-ray Spectra of Accreting Pulsars: Studies of Three Sources Using Empirical and Phenomenological Models

    NASA Astrophysics Data System (ADS)

    Hemphill, Paul Britton

    Accreting X-ray pulsars are a class of astrophysical objects consisting of a neutron star in a binary system with a stellar companion. Matter expelled by the companion star is captured by the neutron star's gravity; as this matter falls towards the neutron star's surface, is compressed and heated, giving off X-rays. As the matter falls the last few miles above the neutron star surface, a number of physical processes compete for dominance, resulting in a highly complex environment governed by the interplay of magnetic, hydrodynamical, and radiative processes. The resulting spectrum often shows broad absorption-like features called cyclotron lines, which provide the only direct measurement of the magnetic field of a neutron star and act as probes of the properties of the accretion column, and their behavior with respect to changes in the accretion rate onto the neutron star has been of interest in recent years. My work in this dissertation brings together nearly 20 years of data from three X-ray satellites to study the X-ray emission from accreting pulsars, with a focus on the hard X-ray continuum and cyclotron lines. I present results for the accreting pulsars 4U 1538-522 and 4U 1907+09, examining the behavior of their cyclotron lines with respect to their luminosity, finding evidence for a positive correlation between the line energy and luminosity in 4U 1907+09. A combined analysis of most of the available X-ray data for the accreting pulsar 4U 1538-522 shows no such correlation in this source, either positive or negative. However, I do present evidence that the cyclotron line energy in 4U 1538-522 has shifted upwards by ˜ 5% in recent years compared to measurements from 10-20 years ago. I additionally carry out an extensive analysis of the environment around 4U 1538-522 using the soft X-ray detectors aboard the satellite Suzaku. I finally present a set of new results from the transient X-ray pulsar V 0332+53, which I fit with a new physics-based model for the

  3. ALMA reveals starburst-like interstellar medium conditions in a compact star-forming galaxy at z 2 using [CI] and CO

    NASA Astrophysics Data System (ADS)

    Popping, Gergö; Decarli, Roberto; Man, Allison W. S.; Nelson, Erica J.; Béthermin, Matthieu; De Breuck, Carlos; Mainieri, Vincenzo; van Dokkum, Pieter G.; Gullberg, Bitten; van Kampen, Eelco; Spaans, Marco; Trager, Scott C.

    2017-06-01

    We present ALMA detections of the [CI] 1-0, CO J = 3-2, and CO J = 4-3 emission lines, as well as the ALMA band 4 continuum for a compact star-forming galaxy (cSFG) at z = 2.225, 3D-HST GS30274. As is typical for cSFGs, this galaxy has a stellar mass of 1.89 ± 0.47 × 1011M⊙, with a star formation rate (SFR) of 214 ± 44 M⊙ yr-1 putting it on the star-forming "main-sequence", but with an H-band effective radius of 2.5 kpc, making it much smaller than the bulk of "main-sequence" star-forming galaxies. The intensity ratio of the line detections yield an ISM density ( 6 × 104 cm-3) and a UV-radiation field ( 2 × 104G0), similar to the values in local starburst and ultra-luminous infrared galaxy environments. A starburst phase is consistent with the short depletion times (tH2,dep ≤ 140 Myr) we find in 3D-HST GS30274 using three different proxies for the H2 mass ([CI], CO, dust mass). This depletion time is significantly shorter than in more extended SFGs with similar stellar masses and SFRs. Moreover, the gas fraction of 3D-HST GS30274 is smaller than typically found in extended galaxies. We measure the CO and [CI] kinematics and find a FWHM line width of 750 ± 41 km s-1. The CO and [CI] FWHM are consistent with a previously measured Hα FWHM for this source. The line widths are consistent with gravitational motions, suggesting we are seeing a compact molecular gas reservoir. A previous merger event, as suggested by the asymmetric light profile, may be responsible for the compact distribution of gas and has triggered a central starburst event. This event gives rise to the starburst-like ISM properties and short depletion times in 3D-HST GS30274. The centrally located and efficient star formation is quickly building up a dense core of stars, responsible for the compact distribution of stellar light in 3D-HST GS30274.

  4. Black Hole Disk Accretion in Supernovae

    NASA Astrophysics Data System (ADS)

    Mineshige, Shin; Nomura, Hideko; Hirose, Masahito; Nomoto, Ken'ichi; Suzuki, Tomoharu

    1997-11-01

    Massive stars in a certain mass range may form low-mass black holes after supernova explosions. In such massive stars, fallback of ~0.1 M⊙ materials onto a black hole is expected because of a deep gravitational potential or a reverse shock propagating back from the outer composition interface. We study hydrodynamical disk accretion onto a newborn low-mass black hole in a supernova using the smoothed particle hydrodynamics method. If the progenitor was rotating before the explosion, the fallback material should have a certain amount of angular momentum with respect to the black hole, thus forming an accretion disk. The disk material will eventually accrete toward the central object because of viscosity at a supercritical accretion rate, Ṁ/Ṁcrit>106, for the first several tens of days. (Here, Ṁcrit is the Eddington luminosity divided by c2.) We then expect that such an accretion disk is optically thick and advection dominated; that is, the disk is so hot that the produced energy and photons are advected inward rather than being radiated away. Thus, the disk luminosity is much less than the Eddington luminosity. The disk becomes hot and dense; for Ṁ/Ṁcrit~106, for example, T ~ 109(αvis/0.01)-1/4 K and ρ ~ 103(αvis/0.01)-1 g cm-3 (with αvis being the viscosity parameter) in the vicinity of the black hole. Depending on the material mixing, some interesting nucleosynthesis processes via rapid proton and alpha-particle captures are expected even for reasonable viscosity magnitudes (αvis ~ 0.01), and some of them could be ejected in a disk wind or a jet without being swallowed by the black hole.

  5. VARIATIONS OF THE 10 mum SILICATE FEATURES IN THE ACTIVELY ACCRETING T TAURI STARS: DG Tau AND XZ Tau

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Bary, Jeffrey S.; Leisenring, Jarron M.; Skrutskie, Michael F., E-mail: jbary@colgate.ed, E-mail: jml2u@virginia.ed, E-mail: mfs4n@virginia.ed

    2009-11-20

    Using the Infrared Spectrograph aboard the Spitzer Space Telescope, we observed multiple epochs of 11 actively accreting T Tauri stars in the nearby Taurus-Auriga star-forming region. In total, 88 low-resolution mid-infrared spectra were collected over 1.5 years in Cycles 2 and 3. The results of this multi-epoch survey show that the 10 mum silicate complex in the spectra of two sources-DG Tau and XZ Tau-undergoes significant variations with the silicate feature growing both weaker and stronger over month- and year-long timescales. Shorter timescale variations on day- to week-long timescales were not detected within the measured flux errors. The time resolutionmore » coverage of this data set is inadequate for determining if the variations are periodic. Pure emission compositional models of the silicate complex in each epoch of the DG Tau and XZ Tau spectra provide poor fits to the observed silicate features. These results agree with those of previous groups that attempted to fit only single-epoch observations of these sources. Simple two-temperature, two-slab models with similar compositions successfully reproduce the observed variations in the silicate features. These models hint at a self-absorption origin of the diminution of the silicate complex instead of a compositional change in the population of emitting dust grains. We discuss several scenarios for producing such variability including disk shadowing, vertical mixing, variations in disk heating, and disk wind events associated with accretion outbursts.« less

  6. Clumpy wind accretion in Supergiant X-ray Binaries

    NASA Astrophysics Data System (ADS)

    El Mellah, I.; Sundqvist, J. O.; Keppens, R.

    2017-12-01

    Supergiant X-ray binaries (\\sgx) contain a neutron star (NS) orbiting a Supergiant O/B star. The fraction of the dense and fast line-driven wind from the stellar companion which is accreted by the NS is responsible for most of the X-ray emission from those system. Classic \\sgx display photometric variability of their hard X-ray emission, typically from a few 10^{35} to a few 10^{37}erg\\cdots^{-1}. Inhomogeneities (\\aka clumps) in the wind from the star are expected to play a role in this time variability. We run 3D hydrodynamical (HD) finite volume simulations to follow the accretion of the inhomogeneous stellar wind by the NS over almost 3 orders of magnitude. To model the unperturbed wind far upstream the NS, we use recent simulations which managed to resolve its micro-structure. We observe the formation of a Bondi-Hoyle-Lyttleton (BHL) like bow shock around the accretor and follow the clumps as they cross it, down to the NS magnetosphere. Compared to previous estimations discarding the HD effects, we measure lower time variability due to both the damping effect of the shock and the necessity to evacuate angular momentum to enable accretion. We also compute the associated time-variable column density and compare it to recent observations in Vela X-1.

  7. Self-Consistent Thermal Accretion Disk Corona Models for Compact Objects. I: Properties of the Corona and the Spectrum of Escaping Radiation

    NASA Technical Reports Server (NTRS)

    Dove, James B.; Wilms, Jorn; Begelman, Mitchell C.

    1997-01-01

    We present the properties of accretion disk corona (ADC) models in which the radiation field, the temperature, and the total opacity of the corona are determined self-consistently. We use a nonlinear Monte Carlo code to perform the calculations. As an example, we discuss models in which the corona is situated above and below a cold accretion disk with a plane-parallel (slab) geometry, similar to the model of Haardt & Maraschi. By Comptonizing the soft radiation emitted by the accretion disk, the corona is responsible for producing the high-energy component of the escaping radiation. Our models include the reprocessing of radiation in the accretion disk. Here the photons either are Compton-reflected or photoabsorbed, giving rise to fluorescent line emission and thermal emission. The self- consistent coronal temperature is determined by balancing heating (due to viscous energy dissipation) with Compton cooling, determined using the fully relativistic, angle-dependent cross sections. The total opacity is found by balancing pair productions with annihilations. We find that, for a disk temperature kT(sub BB) approx. less than 200 eV, these coronae are unable to have a self-consistent temperature higher than approx. 140 keV if the total optical depth is approx. less than 0.2, regardless of the compactness parameter of the corona and the seed opacity. This limitation corresponds to the angle-averaged spectrum of escaping radiation having a photon index approx. greater than 1.8 within the 5-30 keV band. Finally, all models that have reprocessing features also predict a large thermal excess at lower energies. These constraints make explaining the X-ray spectra of persistent black hole candidates with ADC models very problematic.

  8. Accretion of dark matter by stars.

    PubMed

    Brito, Richard; Cardoso, Vitor; Okawa, Hirotada

    2015-09-11

    Searches for dark matter imprints are one of the most active areas of current research. We focus here on light fields with mass m_{B}, such as axions and axionlike candidates. Using perturbative techniques and full-blown nonlinear numerical relativity methods, we show the following. (i) Dark matter can pile up in the center of stars, leading to configurations and geometries oscillating with a frequency that is a multiple of f=2.5×10^{14}(m_{B}c^{2}/eV)  Hz. These configurations are stable throughout most of the parameter space, and arise out of credible mechanisms for dark-matter capture. Stars with bosonic cores may also develop in other theories with effective mass couplings, such as (massless) scalar-tensor theories. We also show that (ii) collapse of the host star to a black hole is avoided by efficient gravitational cooling mechanisms.

  9. Reconstructing the Accretion History of the Galactic Stellar Halo from Chemical Abundance Ratio Distributions

    NASA Astrophysics Data System (ADS)

    Lee, Duane M.; Johnston, Kathryn V.; Sen, Bodhisattva; Jessop, Will

    2015-03-01

    Observational studies of halo stars during the past two decades have placed some limits on the quantity and nature of accreted dwarf galaxy contributions to the Milky Way (MW) stellar halo by typically utilizing stellar phase-space information to identify the most recent halo accretion events. In this study we tested the prospects of using 2D chemical abundance ratio distributions (CARDs) found in stars of the stellar halo to determine its formation history. First, we used simulated data from 11 “MW-like” halos to generate satellite template sets (STSs) of 2D CARDs of accreted dwarf satellites, which are composed of accreted dwarfs from various mass regimes and epochs of accretion. Next, we randomly drew samples of ˜103-4 mock observations of stellar chemical abundance ratios ([α/Fe], [Fe/H]) from those 11 halos to generate samples of the underlying densities for our CARDs to be compared to our templates in our analysis. Finally, we used the expectation-maximization algorithm to derive accretion histories in relation to the STS used and the sample size. For certain STSs used we typically can identify the relative mass contributions of all accreted satellites to within a factor of two. We also find that this method is particularly sensitive to older accretion events involving low-luminosity dwarfs, e.g., ultra-faint dwarfs—precisely those events that are too ancient to be seen by phase-space studies of stars and too faint to be seen by high-z studies of the early universe. Since our results only exploit two chemical dimensions and near-future surveys promise to provide ˜6-9 dimensions, we conclude that these new high-resolution spectroscopic surveys of the stellar halo will allow us to recover its accretion history—and the luminosity function of infalling dwarf galaxies—across cosmic time.

  10. RECONSTRUCTING THE ACCRETION HISTORY OF THE GALACTIC STELLAR HALO FROM CHEMICAL ABUNDANCE RATIO DISTRIBUTIONS

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Lee, Duane M.; Johnston, Kathryn V.; Sen, Bodhisattva

    Observational studies of halo stars during the past two decades have placed some limits on the quantity and nature of accreted dwarf galaxy contributions to the Milky Way (MW) stellar halo by typically utilizing stellar phase-space information to identify the most recent halo accretion events. In this study we tested the prospects of using 2D chemical abundance ratio distributions (CARDs) found in stars of the stellar halo to determine its formation history. First, we used simulated data from 11 “MW-like” halos to generate satellite template sets (STSs) of 2D CARDs of accreted dwarf satellites, which are composed of accreted dwarfsmore » from various mass regimes and epochs of accretion. Next, we randomly drew samples of ∼10{sup 3–4} mock observations of stellar chemical abundance ratios ([α/Fe], [Fe/H]) from those 11 halos to generate samples of the underlying densities for our CARDs to be compared to our templates in our analysis. Finally, we used the expectation-maximization algorithm to derive accretion histories in relation to the STS used and the sample size. For certain STSs used we typically can identify the relative mass contributions of all accreted satellites to within a factor of two. We also find that this method is particularly sensitive to older accretion events involving low-luminosity dwarfs, e.g., ultra-faint dwarfs—precisely those events that are too ancient to be seen by phase-space studies of stars and too faint to be seen by high-z studies of the early universe. Since our results only exploit two chemical dimensions and near-future surveys promise to provide ∼6–9 dimensions, we conclude that these new high-resolution spectroscopic surveys of the stellar halo will allow us to recover its accretion history—and the luminosity function of infalling dwarf galaxies—across cosmic time.« less

  11. Applying a physical continuum model to describe the broadband X-ray spectra of accreting pulsars at high luminosity

    NASA Astrophysics Data System (ADS)

    Pottschmidt, Katja; Hemphill, Paul B.; Wolff, Michael T.; Cheatham, Diana M.; Iwakiri, Wataru; Gottlieb, Amy M.; Falkner, Sebastian; Ballhausen, Ralf; Fuerst, Felix; Kuehnel, Matthias; Ferrigno, Carlo; Becker, Peter A.; Wood, Kent S.; Wilms, Joern

    2018-01-01

    A new window for better understanding the accretion onto strongly magnetized neutron stars in X-ray binaries is opening. In these systems the accreted material follows the magnetic field lines as it approaches the neutron star, forming accretion columns above the magnetic poles. The plasma falls toward the neutron star surface at near-relativistic speeds, losing energy by emitting X-rays. The X-ray spectral continua are commonly described using phenomenological models, i.e., power laws with different types of curved cut-offs at higher energies. Here we consider high luminosity pulsars. In these systems the mass transfer rate is high enough that the accreting plasma is thought to be decelerated in a radiation-dominated radiative shock in the accretion columns. While the theory of the emission from such shocks had already been developed by 2007, a model for direct comparison with X-ray continuum spectra in xspec or isis has only recently become available. Characteristic parameters of this model are the accretion column radius and the plasma temperature, among others. Here we analyze the broadband X-ray spectra of the accreting pulsars Centaurus X-3 and 4U 1626-67 obtained with NuSTAR. We present results from traditional empirical modeling as well as successfully apply the radiation-dominated radiative shock model. We also take the opportunity to compare to similar recent analyses of both sources using these and other observations.

  12. The neutron star in HESS J1731-347: Central compact objects as laboratories to study the equation of state of superdense matter

    NASA Astrophysics Data System (ADS)

    Klochkov, D.; Suleimanov, V.; Pühlhofer, G.; Yakovlev, D. G.; Santangelo, A.; Werner, K.

    2015-01-01

    Context. Central compact objects (CCOs) in supernova remnants are isolated thermally emitting neutron stars (NSs). They are most probably characterized by a magnetic field strength that is roughly two orders of magnitude lower than that of most of the radio and accreting pulsars. The thermal emission of CCOs can be modeled to obtain constraints on the physical parameters of the star such as its mass, radius, effective temperature, and chemical composition. Aims: The CCO in HESS J1731-347 is one of the brightest objects in this class. Starting from 2007, it was observed several times with different X-ray satellites. Here we present our analysis of two new XMM-Newton observations of the source performed in 2013 which increase the total exposure time of the data available for spectral analysis by a factor of about five compared to the analyses presented before. Methods: We use our numerical spectral models for carbon and hydrogen atmospheres to fit the spectrum of the CCO. From our fits, we derive constraints on the physical parameters of the emitting star such as its mass, radius, distance, and effective temperature. We also use the new data to derive new upper limits on the source pulsations and to confirm the absence of a long-term flux and spectral variability. Results: The analysis shows that atmosphere models are clearly preferred by the fit over the blackbody spectral function. Under the assumption that the X-ray emission is uniformly produced by the entire star surface (supported by the lack of pulsations), hydrogen atmosphere models lead to uncomfortably large distances of the CCO, above 7-8 kpc. On the other hand, the carbon atmosphere model formally excludes distances above 5-6 kpc and is compatible with the source located in the Scutum-Crux (~3 kpc) or Norma-Cygnus (~4.5 kpc) Galactic spiral arm. We provide and discuss the corresponding confidence contours in the NS mass-radius plane. The measured effective temperature indicates that the NS is

  13. 1H 1752 + 081: An eclipsing cataclysmic variable with a small accretion disk

    NASA Technical Reports Server (NTRS)

    Silber, Andrew D.; Remillard, Ronald A.; Horne, Keith; Bradt, Hale V.

    1994-01-01

    We announce the discovery of an eclipsing nova-like cataclysmic variable (CV) as the optical counterpart to the HEAO 1 X-ray source 1H1752 + 081. This CV has an orbital period of 1.882801 hr, a high equivalent width of H-beta, and an average m(sub v) of 16.4 out of the eclipse. A geometric model is constructed from observations of the eclipse ingress and egress in many optical bandpasses. The broad-band emission originates primarily in two regions; the disk/accretion stream 'hot spot' and a compact central component, which may be a spot on the white dwarf surface, the entire white dwarf surface or the boundary layer between the accretion disk and the white dwarf surface. Based on the durations and offsets of the two eclipses we determined the mass ratio q = 2.5 +/- 0.6 and the angle of inclination i = 77 deg +/- 2 deg. If the central component is the entire white dwarf surface the masses of the stars are M(sub 1) = 0.80 +/- 0.06 solar masses and M(sub 2) = 0.32 +/- 0.06 solar masses. The disk is faint and small (R(sub D) = 0.25 +/- 0.05 r(sub L1), where r(sub L1) is the distance from the primary to the L(sub 1) point), compared to other eclipsing CVs. The small disk may result from the removal of angular momentum from the accretion disk by the magnetic field of the white dwarf; this CV may be a DQ Her type with a slowly rotating white dwarf. The emission-line velocities do not show the 'Z-wave' expected from the eclipse of a Keplerian accretion disk, nor do they have the correct phasing to originate near the white dwarf. The most likely origin of the line emission is the hot spot. The secondary star is visible at wavelengths greater than or equal to 6000 A during eclipse. We estimate a spectral type approximately M6 which, together with the observed m(sub 1) = 16.94 during eclipse, results in a distance estimate of 150 +/- 27 pc.

  14. KEY ISSUES REVIEW: Insights from simulations of star formation

    NASA Astrophysics Data System (ADS)

    Larson, Richard B.

    2007-03-01

    Although the basic physics of star formation is classical, numerical simulations have yielded essential insights into how stars form. They show that star formation is a highly nonuniform runaway process characterized by the emergence of nearly singular peaks in density, followed by the accretional growth of embryo stars that form at these density peaks. Circumstellar discs often form from the gas being accreted by the forming stars, and accretion from these discs may be episodic, driven by gravitational instabilities or by protostellar interactions. Star-forming clouds typically develop filamentary structures, which may, along with the thermal physics, play an important role in the origin of stellar masses because of the sensitivity of filament fragmentation to temperature variations. Simulations of the formation of star clusters show that the most massive stars form by continuing accretion in the dense cluster cores, and this again is a runaway process that couples star formation and cluster formation. Star-forming clouds also tend to develop hierarchical structures, and smaller groups of forming objects tend to merge into progressively larger ones, a generic feature of self-gravitating systems that is common to star formation and galaxy formation. Because of the large range of scales and the complex dynamics involved, analytic models cannot adequately describe many aspects of star formation, and detailed numerical simulations are needed to advance our understanding of the subject. 'The purpose of computing is insight, not numbers.' Richard W Hamming, in Numerical Methods for Scientists and Engineers (1962) 'There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.' William Shakespeare, in Hamlet, Prince of Denmark (1604)

  15. Tidal interaction, star formation and chemical evolution in blue compact dwarf galaxy Mrk 22

    NASA Astrophysics Data System (ADS)

    Paswan, A.; Omar, A.; Jaiswal, S.

    2018-02-01

    The optical spectroscopic and radio interferometric H I 21 cm-line observations of the blue compact dwarf galaxy Mrk 22 are presented. The Wolf-Rayet (WR) emission-line features corresponding to high ionization lines of He II λ4686 and C IV λ5808 from young massive stars are detected. The ages of two prominent star-forming regions in the galaxy are estimated as ∼10 and ∼ 4 Myr. The galaxy has non-thermal radio deficiency, which also indicates a young starburst and lack of supernovae events from the current star formation activities, consistent with the detection of WR emission-line features. A significant N/O enrichment is seen in the fainter star-forming region. The gas-phase metallicities [12 + log(O/H)] for the bright and faint regions are estimated as 7.98±0.07 and 7.46±0.09, respectively. The galaxy has a large diffuse H I envelop. The H I images reveal disturbed gas kinematics and H I clouds outside the optical extent of the galaxy, indicating recent tidal interaction or merger in the system. The results strongly indicate that Mrk 22 is undergoing a chemical and morphological evolution due to ongoing star formation, most likely triggered by a merger.

  16. Effects of livestock species and stocking density on accretion rates in grazed salt marshes

    NASA Astrophysics Data System (ADS)

    Nolte, Stefanie; Esselink, Peter; Bakker, Jan P.; Smit, Christian

    2015-01-01

    Coastal ecosystems, such as salt marshes, are threatened by accelerated sea-level rise (SLR). Salt marshes deliver valuable ecosystem services such as coastal protection and the provision of habitat for a unique flora and fauna. Whether salt marshes in the Wadden Sea area are able to survive accelerated SLR depends on sufficient deposition of sediments which add to vertical marsh accretion. Accretion rate is influenced by a number of factors, and livestock grazing was recently included. Livestock grazing is assumed to reduce accretion rates in two ways: (a) directly by increasing soil compaction through trampling, and (b) indirectly by affecting the vegetation structure, which may lower the sediment deposition. For four years, we studied the impact of two livestock species (horse and cattle) at two stocking densities (0.5 and 1.0 animal ha-1) on accretion in a large-scale grazing experiment using sedimentation plates. We found lower cumulative accretion rates in high stocking densities, probably because more animals cause more compaction and create a lower canopy. Furthermore, a trend towards lower accretion rates in horse-compared to cattle-grazed treatments was found, most likely because (1) horses are more active and thus cause more compaction, and (2) herbage intake by horses is higher than by cattle, which causes a higher biomass removal and shorter canopy. During summer periods, negative accretion rates were found. When the grazing and non-grazing seasons were separated, the impact of grazing differed among years. In summer, we only found an effect of different treatments if soil moisture (precipitation) was relatively low. In winter, a sufficiently high inundation frequency was necessary to create differences between grazing treatments. We conclude that stocking densities, and to a certain extent also livestock species, affect accretion rates in salt marshes. Both stocking densities and livestock species should thus be taken into account in management

  17. Mass Measurement of 56Sc Reveals a Small A=56 Odd-Even Mass Staggering, Implying a Cooler Accreted Neutron Star Crust

    DOE PAGES

    Meisel, Z.; George, S.; Ahn, S.; ...

    2015-10-16

    We present the mass excesses of 52-57Sc, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The masses of 56Sc and 57Sc were determined for the first time with atomic mass excesses of -24.85(59)((+0)(-54)) MeV and -21.0(1.3) MeV, respectively, where the asymmetric uncertainty for 56Sc was included due to possible contamination from a long-lived isomer. The 56Sc mass indicates a small odd-even mass staggering in the A = 56 mass chain towards the neutron drip line, significantly deviating from trends predicted by the global FRDM mass model and favoring trends predicted bymore » the UNEDF0 and UNEDF1 density functional calculations. Together with new shell-model calculations of the electron-capture strength function of 56Sc, our results strongly reduce uncertainties in model calculations of the heating and cooling at the 56Ti electron-capture layer in the outer crust of accreting neutron stars. We find that, in contrast to previous studies, neither strong neutrino cooling nor strong heating occurs in this layer. We conclude that Urca cooling in the outer crusts of accreting neutron stars that exhibit superbursts or high temperature steady-state burning, which are predicted to be rich in A approximate to 56 nuclei, is considerably weaker than predicted. Urca cooling must instead be dominated by electron capture on the small amounts of adjacent odd-A nuclei contained in the superburst and high temperature steady-state burning ashes. This may explain the absence of strong crust Urca cooling inferred from the observed cooling light curve of the transiently accreting x-ray source MAXI J0556-332.« less

  18. Mass Measurement of 56Sc Reveals a Small A =56 Odd-Even Mass Staggering, Implying a Cooler Accreted Neutron Star Crust

    NASA Astrophysics Data System (ADS)

    Meisel, Z.; George, S.; Ahn, S.; Bazin, D.; Brown, B. A.; Browne, J.; Carpino, J. F.; Chung, H.; Cole, A. L.; Cyburt, R. H.; Estradé, A.; Famiano, M.; Gade, A.; Langer, C.; Matoš, M.; Mittig, W.; Montes, F.; Morrissey, D. J.; Pereira, J.; Schatz, H.; Schatz, J.; Scott, M.; Shapira, D.; Smith, K.; Stevens, J.; Tan, W.; Tarasov, O.; Towers, S.; Wimmer, K.; Winkelbauer, J. R.; Yurkon, J.; Zegers, R. G. T.

    2015-10-01

    We present the mass excesses of 52-57Sc, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The masses of 56Sc and 57Sc were determined for the first time with atomic mass excesses of -24.85 (59 )(-54+0) MeV and -21.0 (1.3 ) MeV , respectively, where the asymmetric uncertainty for 56Sc was included due to possible contamination from a long-lived isomer. The 56Sc mass indicates a small odd-even mass staggering in the A =56 mass chain towards the neutron drip line, significantly deviating from trends predicted by the global FRDM mass model and favoring trends predicted by the UNEDF0 and UNEDF1 density functional calculations. Together with new shell-model calculations of the electron-capture strength function of 56Sc, our results strongly reduce uncertainties in model calculations of the heating and cooling at the 56Ti electron-capture layer in the outer crust of accreting neutron stars. We find that, in contrast to previous studies, neither strong neutrino cooling nor strong heating occurs in this layer. We conclude that Urca cooling in the outer crusts of accreting neutron stars that exhibit superbursts or high temperature steady-state burning, which are predicted to be rich in A ≈56 nuclei, is considerably weaker than predicted. Urca cooling must instead be dominated by electron capture on the small amounts of adjacent odd-A nuclei contained in the superburst and high temperature steady-state burning ashes. This may explain the absence of strong crust Urca cooling inferred from the observed cooling light curve of the transiently accreting x-ray source MAXI J0556-332.

  19. Gas content of transitional disks: a VLT/X-Shooter study of accretion and winds

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Testi, L.; Natta, A.; Rosotti, G.; Benisty, M.; Ercolano, B.; Ricci, L.

    2014-08-01

    Context. Transitional disks are thought to be a late evolutionary stage of protoplanetary disks whose inner regions have been depleted of dust. The mechanism responsible for this depletion is still under debate. To constrain the various models it is mandatory to have a good understanding of the properties of the gas content in the inner part of the disk. Aims: Using X-Shooter broad band - UV to near-infrared - medium-resolution spectroscopy, we derive the stellar, accretion, and wind properties of a sample of 22 transitional disks. The analysis of these properties allows us to place strong constraints on the gas content in a region very close to the star (≲0.2 AU) that is not accessible with any other observational technique. Methods: We fitted the spectra with a self-consistent procedure to simultaneously derive spectral type, extinction, and accretion properties of the targets. From the continuum excess at near-infrared wavelength we distinguished whether our targets have dust free inner holes. By analyzing forbidden emission lines, we derived the wind properties of the targets. We then compared our findings with results for classical T Tauri stars. Results: The accretion rates and wind properties of 80% of the transitional disks in our sample, which is strongly biased toward stongly accreting objects, are comparable to those of classical T Tauri stars. Thus, there are (at least) some transitional disks with accretion properties compatible with those of classical T Tauri stars, irrespective of the size of the dust inner hole. Only in two cases are the mass accretion rates much lower, while the wind properties remain similar. We detected no strong trend of the mass accretion rates with the size of the dust-depleted cavity or with the presence of a dusty optically thick disk very close to the star. These results suggest that, close to the central star, there is a gas-rich inner disk with a density similar to that of classical T Tauri star disks. Conclusions: The

  20. Mid-IR Spectra Herbig Ae/Be Stars

    NASA Technical Reports Server (NTRS)

    Wooden, Diane; Witteborn, Fred C. (Technical Monitor)

    1997-01-01

    Herbig Ae/Be stars are intermediate mass pre-main sequence stars, the higher mass analogues to the T Tauri stars. Because of their higher mass, they are expected form more rapidly than the T Tauri stars. Whether the Herbig Ae/Be stars accrete only from collapsing infalling envelopes or whether accrete through geometrically flattened viscous accretion disks is of current debate. When the Herbig Ae/Be stars reach the main sequence they form a class called Vega-like stars which are known from their IR excesses to have debris disks, such as the famous beta Pictoris. The evolutionary scenario between the pre-main sequence Herbig Ae/Be stars and the main sequence Vega-like stars is not yet revealed and it bears on the possibility of the presence of Habitable Zone planets around the A stars. Photometric studies of Herbig Ae/Be stars have revealed that most are variable in the optical, and a subset of stars show non-periodic drops of about 2 magnitudes. These drops in visible light are accompanied by changes in their colors: at first the starlight becomes reddened, and then it becomes bluer, the polarization goes from less than 0.1 % to roughly 1% during these minima. The theory postulated by V. Grinnin is that large cometary bodies on highly eccentric orbits occult the star on their way to being sublimed, for systems that are viewed edge-on. This theory is one of several controversial theories about the nature of Herbig Ae/Be stars. A 5 year mid-IR spectrophotometric monitoring campaign was begun by Wooden and Butner in 1992 to look for correlations between the variations in visible photometry and mid-IR dust emission features. Generally the approximately 20 stars that have been observed by the NASA Ames HIFOGS spectrometer have been steady at 10 microns. There are a handful, however, that have shown variable mid-IR spectra, with 2 showing variations in both the continuum and features anti-correlated with visual photometry, and 3 showing variations in the emission

  1. Lense-Thirring Precession of Accretion Disks and Quasi-Periodic Oscillations in X-Ray Binaries

    NASA Astrophysics Data System (ADS)

    Markovic, D.; Lamb, F. K.

    2003-05-01

    It has recently been suggested that gravitomagnetic precession of the inner part of the accretion disk, possibly driven by radiation torques, may be responsible for some of the 20-300 Hz quasi-periodic X-ray brightness oscillations (QPOs) observed in some low-mass binary systems containing accreting neutron stars and black hole candidates. We have explored warping modes of geometrically thin disks in the presence of gravitomagnetic and radiation torques. We have found a family of overdamped, low-frequency gravitomagnetic (LFGM) modes all of which have precession frequencies lower than a certain critical frequency ωcrit, which is 1 Hz for a compact object of solar mass. A radiation warping torque can cause a few of the lowest-frequency LFGM modes to grow with time, but even a strong radiation warping torque has essentially no effect on the LFGM modes with frequencies ≳10-4 Hz. We have also discovered a second family of high-frequency gravitomagnetic (HFGM) modes with precession frequencies that range from ωcrit up to slightly less than the gravitomagnetic precession frequency of a particle at the inner edge of the disk, which is 30 Hz if the disk extends inward to the innermost stable circular orbit around a 2M⊙ compact object with dimensionless angular momentum cJ/GM2 = 0.2. The highest-frequency HFGM modes are very localized spiral corrugations of the inner disk and are weakly damped, with Q values as large as 50. We discuss the implications of our results for the observability of Lense-Thirring precession in X-ray binaries.

  2. TIME-DOMAIN SPECTROSCOPY OF A T TAURI STAR

    NASA Astrophysics Data System (ADS)

    Dupree, Andrea K.; Brickhouse, Nancy S.; Cranmer, Steven R.; Berlind, Perry L.; Strader, Jay; Smith, Graeme H.

    2014-06-01

    High resolution optical and near-infrared spectra of TW Hya, the nearest accreting T Tauri star, cover a decade and reveal the substantial changes in accretion and wind properties. Our spectra suggest that the broad near-IR, optical, and far-uv emission lines, centered on the star, originate in a turbulent post-shock region and can undergo scattering by the overlying stellar wind as well as absorption from infalling material. Stable absorption features appear in H-alpha, apparently caused by an accreting column silhouetted in the stellar wind. The free-fall velocity of material correlates inversely with the strength of the post-shock emission, consistent with a dipole accretion model. Terminal outflow velocities appear to be directly related to the amount of post-shock emission, giving evidence for an accretion-driven stellar wind.

  3. A statistical spectropolarimetric study of Herbig Ae/Be stars

    NASA Astrophysics Data System (ADS)

    Ababakr, K. M.; Oudmaijer, R. D.; Vink, J. S.

    2017-11-01

    We present H α linear spectropolarimetry of a large sample of Herbig Ae/Be stars. Together with newly obtained data for 17 objects, the sample contains 56 objects, the largest such sample to date. A change in linear polarization across the H α line is detected in 42 (75 per cent) objects, which confirms the previous finding that the circumstellar environment around these stars on small spatial scales has an asymmetric structure, which is typically identified with a disc. A second outcome of this research is that we confirm that Herbig Ae stars are similar to T Tauri stars in displaying a line polarization effect, while depolarization is more common among Herbig Be stars. This finding had been suggested previously to indicate that Herbig Ae stars form in the same manner than T Tauri stars through magnetospheric accretion. It appears that the transition between these two differing polarization line effects occurs around the B7-B8 spectral type. This would in turn not only suggest that Herbig Ae stars accrete in a similar fashion as lower mass stars, but also that this accretion mechanism switches to a different type of accretion for Herbig Be stars. We report that the magnitude of the line effect caused by electron scattering close to the stars does not exceed 2 per cent. Only a very weak correlation is found between the magnitude of the line effect and the spectral type or the strength of the H α line. This indicates that the detection of a line effect only relies on the geometry of the line-forming region and the geometry of the scattering electrons.

  4. A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with NuStar

    NASA Astrophysics Data System (ADS)

    Ludlam, R. M.; Miller, J. M.; Bachetti, M.; Barret, D.; Bostrom, A. C.; Cackett, E. M.; Degenaar, N.; Di Salvo, T.; Natalucci, L.; Tomsick, J. A.; Paerels, F.; Parker, M. L.

    2017-02-01

    We present NuSTAR observations of neutron star (NS) low-mass X-ray binaries: 4U 1636-53, GX 17+2, and 4U 1705-44. We observed 4U 1636-53 in the hard state, with an Eddington fraction, {F}{Edd}, of 0.01; GX 17+2 and 4U 1705-44 were in the soft state with fractions of 0.57 and 0.10, respectively. Each spectrum shows evidence for a relativistically broadened Fe K α line. Through accretion disk reflection modeling, we constrain the radius of the inner disk in 4U 1636-53 to be {R}{in}=1.03+/- 0.03 ISCO (innermost stable circular orbit), assuming a dimensionless spin parameter {a}* ={cJ}/{{GM}}2=0.0, and {R}{in}=1.08+/- 0.06 ISCO for {a}* =0.3 (errors quoted at 1σ). This value proves to be model independent. For {a}* =0.3 and M=1.4 {M}⊙ , for example, 1.08 ± 0.06 ISCO translates to a physical radius of R=10.8+/- 0.6 km, and the NS would have to be smaller than this radius (other outcomes are possible for allowed spin parameters and masses). For GX 17+2, {R}{in}=1.00{--}1.04 ISCO for {a}* =0.0 and {R}{in}=1.03{--}1.30 ISCO for {a}* =0.3. For {a}* =0.3 and M=1.4 {M}⊙ , {R}{in}=1.03{--}1.30 ISCO translates to R=10.3{--}13.0 km. The inner accretion disk in 4U 1705-44 may be truncated just above the stellar surface, perhaps by a boundary layer or magnetosphere; reflection models give a radius of 1.46-1.64 ISCO for {a}* =0.0 and 1.69-1.93 ISCO for {a}* =0.3. We discuss the implications our results may have on the equation of state of ultradense, cold matter and our understanding of the innermost accretion flow onto NSs with low surface magnetic fields, and systematic errors related to the reflection models and spacetime metric around less idealized NSs.

  5. Evolution of Pre-Main Sequence Accretion Disks

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee W.

    2004-01-01

    The aim of this project is to develop a comprehensive global picture of the physical conditions in, and evolutionary timescales of, pre-main sequence accretion disks. The results of this work will help constrain the initial conditions for planet formation. To this end we are developing much larger samples of 3-10 Myr-old stars to provide better empirical constraints on protoplanetary disk evolution; measuring disk accretion rates in these systems; and constructing detailed model disk structures consistent with observations to infer physical conditions such as grain growth in protoplanetary disks.

  6. Evolution of Pre-Main Sequence Accretion Disks

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee W.

    2003-01-01

    The aim of this project is to develop a comprehensive global picture of the physical conditions in, and evolutionary timescales of, pre-main sequence accretion disks. The results of this work will help constrain the initial conditions for planet formation. To this end we are developing much larger samples of 3-10 Myr-old stars to provide better empirical constraints on protoplanetary disk evolution; measuring disk accretion rates in these systems; and constructing detailed model disk structures consistent with observations to infer physical conditions such as grain growth in protoplanetary disks.

  7. Evolution of Pre-Main Sequence Accretion Disks

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee W.

    2005-01-01

    The aim of this project was to develop a comprehensive global picture of the physical conditions in, and evolutionary timescales of, premain sequence accretion disks. The results of this work will help constrain the initial conditions for planet formation. To this end we developed much larger samples of 3-10 Myr-old stars to provide better empirical constraints on protoplanetary disk evolution; measured disk accretion rates in these systems; and constructed detailed model disk structures consistent with observations to infer physical conditions such as grain growth in protoplanetary disks.

  8. Accretional Heating by Periodic Dwarf Nova Outburst Events

    NASA Astrophysics Data System (ADS)

    Godon, P.; Sion, E. M.

    2001-12-01

    We carry out simulations of evolutionary models of accreting white dwarfs in dwarf novae to assess the combined effect of boundary layer irradiation and compressional heating on the accreting star. We focus on the behavior of the surface observables of the accreting white dwarf for different value of the mass accretion rate and accretor mass. Outburst of days to weeks are followed by a shut off of the radial infall during quiescences lasting weeks to months. Preliminary results indicate that after a long evolution time of many accretion cycles, the effective surface temperature of the white dwarf will increase substantially. The purpose of this work is to generate a grid of models that will then be used to compared with observations of white dwarf heating and cooling in dwarf nova systems. This work is supported by NASA HST grant GO-8139 and in part by NSF grant AST99-01955 and NASA grant NAG5-8388.

  9. Burst Oscillations: A New Spin on Neutron Stars

    NASA Technical Reports Server (NTRS)

    Strohmayer, Tod

    2007-01-01

    Observations with NASA's Rossi X-ray Timing Explorer (RXTE) have shown that the X-ray flux during thermonuclear X-ray bursts fr-om accreting neutron stars is often strongly pulsed at frequencies as high as 620 Hz. We now know that these oscillations are produced by spin modulation of the thermonuclear flux from the neutron star surface. In addition to revealing the spin frequency, they provide new ways to probe the properties and physics of accreting neutron stars. I will briefly review our current observational and theoretical understanding of these oscillations and discuss what they are telling us about neutron stars.

  10. The evolution of supermassive Population III stars

    NASA Astrophysics Data System (ADS)

    Haemmerlé, Lionel; Woods, T. E.; Klessen, Ralf S.; Heger, Alexander; Whalen, Daniel J.

    2018-02-01

    Supermassive primordial stars forming in atomically cooled haloes at z ˜ 15-20 are currently thought to be the progenitors of the earliest quasars in the Universe. In this picture, the star evolves under accretion rates of 0.1-1 M⊙ yr-1 until the general relativistic instability triggers its collapse to a black hole at masses of ˜105 M⊙. However, the ability of the accretion flow to sustain such high rates depends crucially on the photospheric properties of the accreting star, because its ionizing radiation could reduce or even halt accretion. Here we present new models of supermassive Population III protostars accreting at rates 0.001-10 M⊙ yr-1, computed with the GENEVA stellar evolution code including general relativistic corrections to the internal structure. We compute for the first time evolutionary tracks in the mass range M > 105 M⊙. We use the polytropic stability criterion to estimate the mass at which the collapse occurs, which has been shown to give a lower limit of the actual mass at collapse in recent hydrodynamic simulations. We find that at accretion rates higher than 0.01 M⊙ yr-1, the stars evolve as red, cool supergiants with surface temperatures below 104 K towards masses >105 M⊙. Moreover, even with the lower rates 0.001 M_{⊙} yr{^{-1}}<\\dot{M}< 0.01 M⊙ yr-1, the surface temperature is substantially reduced from 105 to 104 K for M ≳ 600 M⊙. Compared to previous studies, our results extend the range of masses and accretion rates at which the ionizing feedback remains weak, reinforcing the case for direct collapse as the origin of the first quasars. We provide numerical tables for the surface properties of our models.

  11. Probing Radiatively Inefficient Accretion Flow in the Neutron Star X-ray Binary System Aquila X-1

    NASA Astrophysics Data System (ADS)

    Maitra, Dipankar

    2016-09-01

    The nature of radiatively inefficient accretion flows (RIAF) near neutron stars and black holes remains largely enshrouded in mystery, primarily due to their low luminosity. Long term monitoring of Aql X-1 has revealed that during certain outbursts, the system goes into a relatively bright RIAF state for periods lasting several weeks. These low-intensity states offer a unique opportunity to probe radiatively inefficient flows. We request a 75 ksec Chandra/HETG ToO observation of Aql X-1 during a low-intensity state. Emission line diagnostics of the observed spectrum will be used to test different RIAF models and constrain flow properties such as the radial temperature and density profile, existence of an outflowing wind, spatial extent of the RIAF, and gas dynamics within the flow.

  12. Probing Radiatively Inefficient Accretion Flow in the Neutron Star X-ray Binary System Aquila X-1

    NASA Astrophysics Data System (ADS)

    Maitra, Dipankar

    2017-09-01

    The nature of radiatively inefficient accretion flows (RIAF) near neutron stars and black holes remains largely enshrouded in mystery, primarily due to their low luminosity. Long term monitoring of Aql X-1 has revealed that during certain outbursts, the system goes into a relatively bright RIAF state for periods lasting several weeks. These low-intensity states offer a unique opportunity to probe radiatively inefficient flows. We request a 75 ksec Chandra/HETG ToO observation of Aql X-1 during a low-intensity state. Emission line diagnostics of the observed spectrum will be used to test different RIAF models and constrain flow properties such as the radial temperature and density profile, existence of an outflowing wind, spatial extent of the RIAF, and gas dynamics within the flow.

  13. Massive stars, disks, and clustered star formation

    NASA Astrophysics Data System (ADS)

    Moeckel, Nickolas Barry

    The formation of an isolated massive star is inherently more complex than the relatively well-understood collapse of an isolated, low-mass star. The dense, clustered environment where massive stars are predominantly found further complicates the picture, and suggests that interactions with other stars may play an important role in the early life of these objects. In this thesis we present the results of numerical hydrodynamic experiments investigating interactions between a massive protostar and its lower-mass cluster siblings. We explore the impact of these interactions on the orientation of disks and outflows, which are potentially observable indications of encounters during the formation of a star. We show that these encounters efficiently form eccentric binary systems, and in clusters similar to Orion they occur frequently enough to contribute to the high multiplicity of massive stars. We suggest that the massive protostar in Cepheus A is currently undergoing a series of interactions, and present simulations tailored to that system. We also apply the numerical techniques used in the massive star investigations to a much lower-mass regime, the formation of planetary systems around Solar- mass stars. We perform a small number of illustrative planet-planet scattering experiments, which have been used to explain the eccentricity distribution of extrasolar planets. We add the complication of a remnant gas disk, and show that this feature has the potential to stabilize the system against strong encounters between planets. We present preliminary simulations of Bondi-Hoyle accretion onto a protoplanetary disk, and consider the impact of the flow on the disk properties as well as the impact of the disk on the accretion flow.

  14. Simulating X-ray bursts during a transient accretion event

    NASA Astrophysics Data System (ADS)

    Johnston, Zac; Heger, Alexander; Galloway, Duncan K.

    2018-06-01

    Modelling of thermonuclear X-ray bursts on accreting neutron stars has to date focused on stable accretion rates. However, bursts are also observed during episodes of transient accretion. During such events, the accretion rate can evolve significantly between bursts, and this regime provides a unique test for burst models. The accretion-powered millisecond pulsar SAX J1808.4-3658 exhibits accretion outbursts every 2-3 yr. During the well-sampled month-long outburst of 2002 October, four helium-rich X-ray bursts were observed. Using this event as a test case, we present the first multizone simulations of X-ray bursts under a time-dependent accretion rate. We investigate the effect of using a time-dependent accretion rate in comparison to constant, averaged rates. Initial results suggest that using a constant, average accretion rate between bursts may underestimate the recurrence time when the accretion rate is decreasing, and overestimate it when the accretion rate is increasing. Our model, with an accreted hydrogen fraction of X = 0.44 and a CNO metallicity of ZCNO = 0.02, reproduces the observed burst arrival times and fluences with root mean square (rms) errors of 2.8 h, and 0.11× 10^{-6} erg cm^{-2}, respectively. Our results support previous modelling that predicted two unobserved bursts and indicate that additional bursts were also missed by observations.

  15. Panchromatic Observations of SN2011dh Point to a Compact Progenitor Star

    NASA Technical Reports Server (NTRS)

    Soderberg, A. M.; Margutti, R.; Zauerer, B. A.; Krauss, M.; Katz, B.; Chomiuk, L.; Dittmann, J. A.; Nakar, E.; Sakamoto, T.; Kawai, N.; hide

    2011-01-01

    We report the discovery and detailed monitoring of X-ray emission associated with the Type IIb SN2011dh using data from the Swift and Chandra satellites, placing it among the best studied X-ray supernovae to date. We further present millimeter and radio data obtained with the SMA, CARMA, and EVLA during the first three weeks after explosion. Combining these observations with early optical photometry, we show that the panchromatic dataset is well-described by non-thermal synchrotron emission (radio/mm) with inverse Compton scattering (X-ray) of a thermal population of optical photons. We derive the properties of the shockwave and the circumstellar environment and find a time-averaged shock velocity of v approximately equals 0.1c and a progenitor mass loss rate of M-dot approximately equals 6 X 10 (exp 5) Solar M/ yr (wind velocity, v(sub w) = 1000 km/s). We show that these properties are consistent with the sub-class of Type IIb supernovae characterized by compact progenitors (Type cIIb) and dissimilar from those with extended progenitors (Type eIIb). Furthermore, we consider the early optical emission in the context of a cooling envelope model to estimate a progenitor radius of R(sub star) approximately equals 10(exp 11) cm, in line with the expectations for a Type cIIb supernova. Together, these diagnostics suggest that the putative yellow supergiant progenitor star identified in archival HST observations is instead a binary companion or unrelated to the supernova. Finally, we searched for the high energy shock breakout pulse using X-ray and gamma-ray observations obtained during the purported explosion date range. Based on the compact radius of the progenitor, we estimate that the shock breakout pulse was detectable with current instruments but likely missed due to their limited temporal/ spatial coverage. Future all-sky missions will regularly detect shock breakout emission from compact SN progenitors enabling prompt follow-up observations of the shockwave with

  16. Mass Accretion Rate of Very Low Luminosity Objects

    NASA Astrophysics Data System (ADS)

    Sung, Ren-Shiang; Lai, Shih-Ping; Hsieh, Tien-Hao

    2013-08-01

    We propose to measure the mass accretion rate of six Very Low Luminosity Objects (VeLLOs) using Near-infrared Integral Spectrometer (NIFS). The extremely low luminosity of VeLLOs, L_int ≤ 0.1 L_⊙, was previously thought not existing in the nature because the typical accretion rate gives much larger accretion luminosity even for the lowest mass star (``Luminosity Problem''). The commonly accepted solution is that the accretion rate is not constant but episodic. Thus, VeLLOs could be interpreted as protostars being in the quiescent phase of accretion activities. However, there is no observational data directly measuring the mass accretion rate of VeLLOs. The main goal of this proposal is to examine such theory and directly measure the mass accretion rate of VeLLOs for the first time. We propose to measure the blue continuum excess (veiling) of the stellar spectrum, which is the most reliable method for measuring the accretion rate. The measurements have to be made in infrared due to the very high extinction for highly embedded protostars. Our proposal provide a first opportunity to explain the long time ``Luminosity Problem'' through the observational aspects, and Gemini is the only instrument that can provide accurate and high sensitivity infrared spectroscopy measurements within reasonably short time scale.

  17. Compact stars in the braneworld: A new branch of stellar configurations with arbitrarily large mass

    NASA Astrophysics Data System (ADS)

    Lugones, Germán; Arbañil, José D. V.

    2017-03-01

    We study the properties of compact stars in the Randall-Sundrum type-II braneworld (BW) model. To this end, we solve the braneworld generalization of the stellar structure equations for a static fluid distribution with spherical symmetry considering that the spacetime outside the star is described by a Schwarzschild metric. First, the stellar structure equations are integrated employing the so-called causal limit equation of state (EOS), which is constructed using a well-established EOS at densities below a fiducial density, and the causal EOS P =ρ above it. It is a standard procedure in general relativistic stellar structure calculations to use such EOSs for obtaining a limit in the mass radius diagram, known as the causal limit, above which no stellar configurations are possible if the EOS fulfills the condition that the sound velocity is smaller than the speed of light. We find that the equilibrium solutions in the braneworld model can violate the general relativistic causal limit, and for sufficiently large mass they approach asymptotically to the Schwarzschild limit M =2 R . Then, we investigate the properties of hadronic and strange quark stars using two typical EOSs: a nonlinear relativistic mean-field model for hadronic matter and the Massachusetts Institute of Technology (MIT) bag model for quark matter. For masses below ˜1.5 M⊙- 2 M⊙ , the mass versus radius curves show the typical behavior found within the frame of general relativity. However, we also find a new branch of stellar configurations that can violate the general relativistic causal limit and that, in principle, may have an arbitrarily large mass. The stars belonging to this new branch are supported against collapse by the nonlocal effects of the bulk on the brane. We also show that these stars are always stable under small radial perturbations. These results support the idea that traces of extra dimensions might be found in astrophysics, specifically through the analysis of masses and

  18. Accretion rates of protoplanets

    NASA Astrophysics Data System (ADS)

    Greenzweig, Yuval

    The giant planets' solid cores must have formed prior to the dispersal of the primordial solar nebula, to allow the capture of their massive, gaseous envelopes from the nebula. Recent observations of disks of dust surrounding nearby solar-like stars lead to estimates of nebula lifetimes at 106 to 107 years. Thus, theories of solid particle accretion must explain how the solid cores of the giant planets may have formed within comparable timescales. Calculations are presented which support the sole currently hypothesized mechanism of planetary accretion in which the duration of the stage of growth from planetesimals (1 to 10 km size bodies) to moon- or planet-size bodies lies within the widely accepted time constraint mentioned above. It has been shown that under certain conditions a growth advantage is given to the larger bodies of a swarm of Sun-orbiting planetesimals, resulting in runaway growth of the largest body (or bodies) in the swarm. The gravitational cross section of the protoplanet (the largest body in the swarm) increases with its size, eventually requiring the inclusion of the effect of the solar tidal force on the interaction between it and a passing planetesimal. Thus, numerical integrations of the three-body problem (Sun, protoplanet and planetesimal) are needed to determine the accretion rates of protoplanets. Existing analytical formulas are refined for the two-body (no solar tidal force) accretion rates of planetesimals or small protoplanets, and numerically derives the three-body accretion rates of large protoplanets. The three-body accretion rates calculated span a wide range of protoplanetary orbital radii, masses, and densities, and a wide range of planetesimal orbital eccentricities and inclinations. The most useful numerical results are approximated by algebraic expressions, to facilitate their use in accretion calculations, particularly by numerical codes. Since planetary accretion rates depend strongly on planetesimal random velocities

  19. Planet population synthesis driven by pebble accretion in cluster environments

    NASA Astrophysics Data System (ADS)

    Ndugu, N.; Bitsch, B.; Jurua, E.

    2018-02-01

    The evolution of protoplanetary discs embedded in stellar clusters depends on the age and the stellar density in which they are embedded. Stellar clusters of young age and high stellar surface density destroy protoplanetary discs by external photoevaporation and stellar encounters. Here, we consider the effect of background heating from newly formed stellar clusters on the structure of protoplanetary discs and how it affects the formation of planets in these discs. Our planet formation model is built on the core accretion scenario, where we take the reduction of the core growth time-scale due to pebble accretion into account. We synthesize planet populations that we compare to observations obtained by radial velocity measurements. The giant planets in our simulations migrate over large distances due to the fast type-II migration regime induced by a high disc viscosity (α = 5.4 × 10-3). Cold Jupiters (rp > 1 au) originate preferably from the outer disc, due to the large-scale planetary migration, while hot Jupiters (rp < 0.1 au) preferably form in the inner disc. We find that the formation of gas giants via pebble accretion is in agreement with the metallicity correlation, meaning that more gas giants are formed at larger metallicity. However, our synthetic population of isolated stars host a significant amount of giant planets even at low metallicity, in contradiction to observations where giant planets are preferably found around high metallicity stars, indicating that pebble accretion is very efficient in the standard pebble accretion framework. On the other hand, discs around stars embedded in cluster environments hardly form any giant planets at low metallicity in agreement with observations, where these changes originate from the increased temperature in the outer parts of the disc, which prolongs the core accretion time-scale of the planet. We therefore conclude that the outer disc structure and the planet's formation location determines the giant planet

  20. Stable and unstable accretion in the classical T Tauri stars IM Lup and RU Lup as observed by MOST

    NASA Astrophysics Data System (ADS)

    Siwak, Michal; Ogloza, Waldemar; Rucinski, Slavek M.; Moffat, Anthony F. J.; Matthews, Jaymie M.; Cameron, Chris; Guenther, David B.; Kuschnig, Rainer; Rowe, Jason F.; Sasselov, Dimitar; Weiss, Werner W.

    2016-03-01

    Results of the time variability monitoring of the two classical T Tauri stars, RU Lup and IM Lup, are presented. Three photometric data sets were utilized: (1) simultaneous (same field) MOST satellite observations over four weeks in each of the years 2012 and 2013, (2) multicolour observations at the South African Astronomical Observatory in April-May of 2013, (3) archival V-filter All Sky Automated Survey (ASAS) data for nine seasons, 2001-2009. They were augmented by an analysis of high-resolution, public-domain VLT-UT2 Ultraviolet Visual Echelle Spectrograph spectra from the years 2000 to 2012. From the MOST observations, we infer that irregular light variations of RU Lup are caused by stochastic variability of hotspots induced by unstable accretion. In contrast, the MOST light curves of IM Lup are fairly regular and modulated with a period of about 7.19-7.58 d, which is in accord with ASAS observations showing a well-defined 7.247 ± 0.026 d periodicity. We propose that this is the rotational period of IM Lup and is due to the changing visibility of two antipodal hotspots created near the stellar magnetic poles during the stable process of accretion. Re-analysis of RU Lup high-resolution spectra with the broadening function approach reveals signs of a large polar coldspot, which is fairly stable over 13 years. As the star rotates, the spot-induced depression of intensity in the broadening function profiles changes cyclically with period 3.710 58 d, which was previously found by the spectral cross-correlation method.

  1. The dynamic ejecta of compact object mergers and eccentric collisions.

    PubMed

    Rosswog, Stephan

    2013-06-13

    Compact object mergers eject neutron-rich matter in a number of ways: by the dynamical ejection mediated by gravitational torques, as neutrino-driven winds, and probably also a good fraction of the resulting accretion disc finally becomes unbound by a combination of viscous and nuclear processes. If compact binary mergers indeed produce gamma-ray bursts, there should also be an interaction region where an ultra-relativistic outflow interacts with the neutrino-driven wind and produces moderately relativistic ejecta. Each type of ejecta has different physical properties, and therefore plays a different role for nucleosynthesis and for the electromagnetic (EM) transients that go along with compact object encounters. Here, we focus on the dynamic ejecta and present results for over 30 hydrodynamical simulations of both gravitational wave-driven mergers and parabolic encounters as they may occur in globular clusters. We find that mergers eject approximately 1 per cent of a Solar mass of extremely neutron-rich material. The exact amount, as well as the ejection velocity, depends on the involved masses with asymmetric systems ejecting more material at higher velocities. This material undergoes a robust r-process and both ejecta amount and abundance pattern are consistent with neutron star mergers being a major source of the 'heavy' (A>130) r-process isotopes. Parabolic collisions, especially those between neutron stars and black holes, eject substantially larger amounts of mass, and therefore cannot occur frequently without overproducing gala- ctic r-process matter. We also discuss the EM transients that are powered by radioactive decays within the ejecta ('macronovae'), and the radio flares that emerge when the ejecta dissipate their large kinetic energies in the ambient medium.

  2. Runaway gas accretion and gap opening versus type I migration

    NASA Astrophysics Data System (ADS)

    Crida, A.; Bitsch, B.

    2017-03-01

    Growing planets interact with their natal protoplanetary disc, which exerts a torque onto them allowing them to migrate in the disc. Small mass planets do not affect the gas profile and migrate in the fast type-I migration. Although type-I migration can be directed outwards for planets smaller than 20 - 30M⊕ in some regions of the disc, planets above this mass should be lost into the central star long before the disc disperses. Massive planets push away material from their orbit and open a gap. They subsequently migrate in the slower, type II migration, which could save them from migrating all the way to the star. Hence, growing giant planets can be saved if and only if they can reach the gap opening mass, because this extends their migration timescale, allowing them to eventually survive at large orbits until the disc itself disperses. However, most of the previous studies only measured the torques on planets with fixed masses and orbits to determine the migration rate. Additionally, the transition between type-I and type-II migration itself is not well studied, especially when taking the growth mechanism of rapid gas accretion from the surrounding disc into account. Here we use isothermal 2D disc simulations with FARGO-2D1D to study the migration behaviour of gas accreting protoplanets in discs. We find that migrating giant planets always open gaps in the disc. We further show analytically and numerically that in the runaway gas accretion regime, the growth time-scale is comparable to the type-I migration time-scale, indicating that growing planets will reach gap opening masses before migrating all the way to the central star in type-I migration if the disc is not extremely viscous and/or thick. An accretion rate limited to the radial gas flow in the disc, in contrast, is not fast enough. When gas accretion by the planet is taken into account, the gap opening process is accelerated because the planet accretes material originating from its horseshoe region. This

  3. Integral field spectroscopy of nearby quasi-stellar objects - II. Molecular gas content and conditions for star formation

    NASA Astrophysics Data System (ADS)

    Husemann, B.; Davis, T. A.; Jahnke, K.; Dannerbauer, H.; Urrutia, T.; Hodge, J.

    2017-09-01

    We present single-dish 12CO(1-0) and 12CO(2-1) observations for 14 low-redshift quasi-stellar objects (QSOs). In combination with optical integral field spectroscopy, we study how the cold gas content relates to the star formation rate (SFR) and black hole accretion rate. 12CO(1-0) is detected in 8 of 14 targets and 12CO(2-1) is detected in 7 out of 11 cases. The majority of disc-dominated QSOs reveal gas fractions and depletion times matching normal star-forming systems. Two gas-rich major mergers show clear starburst signatures with higher than average gas fractions and shorter depletion times. Bulge-dominated QSO hosts are mainly undetected in 12CO(1-0), which corresponds, on average, to lower gas fractions than in disc-dominated counterparts. Their SFRs, however, imply shorter than average depletion times and higher star formation efficiencies. Negative QSO feedback through removal of cold gas seems to play a negligible role in our sample. We find a trend between black hole accretion rate and total molecular gas content for disc-dominated QSOs when combined with literature samples. We interpret this as an upper envelope for the nuclear activity and it is well represented by a scaling relation between the total and circumnuclear gas reservoir accessible for accretion. Bulge-dominated QSOs significantly differ from that scaling relation and appear uncorrelated with the total molecular gas content. This could be explained either by a more compact gas reservoir, blown out of the gas envelope through outflows, or a different interstellar medium phase composition.

  4. Constraining the dipolar magnetic field of M82 X-2 by the accretion model

    NASA Astrophysics Data System (ADS)

    Chen, Wen-Cong

    2017-02-01

    Recently, ultraluminous X-ray source (ULX) M82 X-2 has been identified to be an accreting neutron star, which has a P = 1.37 s spin period, and is spinning up at a rate dot{P}=-2.0× 10^{-10} s s^{-1}. Interestingly, its isotropic X-ray luminosity Liso = 1.8 × 1040 erg s- 1 during outbursts is 100 times the Eddington limit for a 1.4 M⊙ neutron star. In this Letter, based on the standard accretion model we attempt to constrain the dipolar magnetic field of the pulsar in ULX M82 X-2. Our calculations indicate that the accretion rate at the magnetospheric radius must be super-Eddington during outbursts. To support such a super-Eddington accretion, a relatively high multipole field ( ≳ 1013 G) near the surface of the accretor is invoked to produce an accreting gas column. However, our constraint shows that the surface dipolar magnetic field of the pulsar should be in the range of 1.0-3.5 × 1012 G. Therefore, our model supports that the neutron star in ULX M82 X-2 could be a low-magnetic-field magnetar (proposed by Tong) with a normal dipolar field (˜1012 G) and relatively strong multipole field. For the large luminosity variations of this source, our scenario can also present a self-consistency interpretation.

  5. Accretion onto stellar mass black holes

    NASA Astrophysics Data System (ADS)

    Deegan, Patrick

    2009-12-01

    I present work on the accretion onto stellar mass black holes in several scenarios. Due to dynamical friction stellar mass black holes are expected to form high density cusps in the inner parsec of our Galaxy. These compact remnants may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. I build a simple but detailed time-dependent model of such emission. Future observations of the distribution and orbits of the gas in the inner parsec of Sgr A* will put tighter constraints on the cusp of compact remnants. GRS 1915+105 is an LMXB, whose large orbital period implies a very large accretion disc and explains the extraordinary duration of its current outburst. I present smoothed particle hydrodynamic simulations of the accretion disc. The models includes the thermo-viscous instability, irradiation from the central object and wind loss. I find that the outburst of GRS 1915+105 should last a minimum of 20 years and up to ˜ 100 years if the irradiation is playing a significant role in this system. The predicted recurrence times are of the order of 104 years, making the duty cycle of GRS 1915+105 to be a few 0.1%. I present a simple analytical method to describe the observable behaviour of long period black hole LMXBs, similar to GRS 1915+105. Constructing two simple models for the surface density in the disc, outburst and quiescence times are calculated as a function of orbital period. LMXBs are an important constituent of the X-ray light function (XLF) of giant elliptical galaxies. I find that the duty cycle can vary considerably with orbital period, with implications for modelling the XLF.

  6. Accretion Makes a Splash on TW Hydrae

    NASA Astrophysics Data System (ADS)

    Brickhouse, N. S.

    2011-12-01

    The Chandra Large Program on the Classical T Tauri star TW Hydrae (489 ksec, obtained over the course of one month) brings a wealth of spectral diagnostics to the study of X-ray emission from a young star. The emission measure distribution shows two components separated by a gap (i.e. no emission measure in between). Light curves for the two components can then be constructed from the summed light curves of the appropriate individual lines. The two light curves show uncorrelated variability, with one large flare occurring only in the hot component. We associate the hotter component with the corona, since its peak temperature is ˜10 MK. Ne IX line ratio diagnostics for temperature and density indicate that the source of the cooler component is indeed the accretion shock, as originally reported by Kastner et al. (2002). The temperature and density of the accretion shock are in excellent agreement with models using mass accretion rates derived from the optical. We require a third component, which we call the "post-shock region," from line ratio diagnostics of O VII. The density derived from O VII is lower than the density derived from Ne IX, contrary to standard one-dimensional model expectations and from hydrodynamics simulations to date. The column densities derived from the two ions are also significantly different, with the column density from O VII lower than that from Ne IX. This post-shock region cannot be the settling flow expected from the cooling of the shock column, since its mass is 30 times the mass of material that passes through the shock. Instead this region is the splash of stellar atmosphere that has been hit by the accretion stream and heated by the accretion process (Brickhouse et al. 2010).

  7. Jet-induced star formation in 3C 285 and Minkowski's Object

    NASA Astrophysics Data System (ADS)

    Salomé, Q.; Salomé, P.; Combes, F.

    2015-02-01

    How efficiently star formation proceeds in galaxies is still an open question. Recent studies suggest that active galactic nucleus (AGN) can regulate the gas accretion and thus slow down star formation (negative feedback). However, evidence of AGN positive feedback has also been observed in a few radio galaxies (e.g. Centaurus A, Minkowski's Object, 3C 285, and the higher redshift 4C 41.17). Here we present CO observations of 3C 285 and Minkowski's Object, which are examples of jet-induced star formation. A spot (named 3C 285/09.6 in the present paper) aligned with the 3C 285 radio jet at a projected distance of ~70 kpc from the galaxy centre shows star formation that is detected in optical emission. Minkowski's Object is located along the jet of NGC 541 and also shows star formation. Knowing the distribution of molecular gas along the jets is a way to study the physical processes at play in the AGN interaction with the intergalactic medium. We observed CO lines in 3C 285, NGC 541, 3C 285/09.6, and Minkowski's Object with the IRAM 30 m telescope. In the central galaxies, the spectra present a double-horn profile, typical of a rotation pattern, from which we are able to estimate the molecular gas density profile of the galaxy. The molecular gas appears to be in a compact reservoir, which could be evidence of an early phase of the gas accretion after a recent merger event in 3C 285. No kinematic signature of a molecular outflow is detected by the 30 m telescope. Interestingly, 3C 285/09.6 and Minkowski's Object are not detected in CO. The cold gas mass upper limits are consistent with a star formation induced by the compression of dense ambient material by the jet. The depletion time scales in 3C 285/09.6 and Minkowski's Object are of the order of and even shorter than what is found in 3C 285, NGC 541, and local spiral galaxies (109 yr). The upper limit of the molecular gas surface density in 3C 285/09.6 at least follows a Schmidt-Kennicutt law if the emitting region

  8. PROTOSTELLAR OUTFLOWS AND RADIATIVE FEEDBACK FROM MASSIVE STARS. II. FEEDBACK, STAR-FORMATION EFFICIENCY, AND OUTFLOW BROADENING

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Kuiper, Rolf; Turner, Neal J.; Yorke, Harold W., E-mail: rolf.kuiper@uni-tuebingen.de, E-mail: Neal.J.Turner@jpl.nasa.gov, E-mail: Harold.W.Yorke@jpl.nasa.gov

    2016-11-20

    We perform two-dimensional axially symmetric radiation hydrodynamic simulations to assess the impact of outflows and radiative force feedback from massive protostars by varying when the protostellar outflow starts, and to determine the ratio of ejection to accretion rates and the strength of the wide-angle disk wind component. The star-formation efficiency, i.e., the ratio of final stellar mass to initial core mass, is dominated by radiative forces and the ratio of outflow to accretion rates. Increasing this ratio has three effects. First, the protostar grows slower with a lower luminosity at any given time, lowering radiative feedback. Second, bipolar cavities clearedmore » by the outflow become larger, further diminishing radiative feedback on disk and core scales. Third, the higher momentum outflow sweeps up more material from the collapsing envelope, decreasing the protostar's potential mass reservoir via entrainment. The star-formation efficiency varies with the ratio of ejection to accretion rates from 50% in the case of very weak outflows to as low as 20% for very strong outflows. At latitudes between the low-density bipolar cavity and the high-density accretion disk, wide-angle disk winds remove some of the gas, which otherwise would be part of the accretion flow onto the disk; varying the strength of these wide-angle disk winds, however, alters the final star-formation efficiency by only ±6%. For all cases, the opening angle of the bipolar outflow cavity remains below 20° during early protostellar accretion phases, increasing rapidly up to 65° at the onset of radiation pressure feedback.« less

  9. The Importance of Rotational Time-scales in Accretion Variability

    NASA Astrophysics Data System (ADS)

    Costigan, Gráinne; Vink, Joirck; Scholz, Aleks; Testi, Leonardo; Ray, Tom

    2013-07-01

    For the first few million years, one of the dominant sources of emission from a low mass young stellar object is from accretion. This process regulates the flow of material and angular moments from the surroundings to the central object, and is thought to play an important role in the definition of the long term stellar properties. Variability is a well documented attribute of accretion, and has been observed on time-scales of from days to years. However, where these variations come from is not clear. Th current model for accretion is magnetospheric accretion, where the stellar magnetic field truncates the disc, allowing the matter to flow from the disc onto the surface of the star. This model allows for variations in the accretion rate to come from many different sources, such as the magnetic field, the circumstellar disc and the interaction of the different parts of the system. We have been studying unbiased samples of accretors in order to identify the dominant time-scales and typical magnitudes of variations. In this way different sources of variations can be excluded and any missing physics in these systems identified. Through our previous work with the Long-term Accretion Monitoring Program (LAMP), we found 10 accretors in the ChaI region, whose variability is dominated by short term variations of 2 weeks. This was the shortest time period between spectroscopic observations which spanned 15 months, and rules out large scale processes in the disk as origins of this variability. On the basis of this study we have gone further to study the accretion signature H-alpha, over the time-scales of minutes and days in a set of Herbig Ae and T Tauri stars. Using the same methods as we used in LAMP we found the dominant time-scales of variations to be days. These samples both point towards rotation period of these objects as being an important time-scale for accretion variations. This allows us to indicate which are the most likely sources of these variations.

  10. Accretion and Propeller Torque in the Spin-Down Phase of Neutron Stars: The case of transitional millisecond pulsar PSR J1023+0038

    NASA Astrophysics Data System (ADS)

    Ertan, Ünal

    2018-05-01

    The spin-down rate of PSR J1023+0038, one of the three confirmed transitional millisecond pulsars, was measured in both radio pulsar (RMSP) and X-ray pulsar (LMXB) states. The spin-down rate in the LMXB state is only about 27% greater than in the RMSP state (Jaodand et al. 2016). The inner disk radius, rin, obtained recently by Ertan (2017) for the propeller phase, which is close to the co-rotation radius, rco, and insensitive to the mass-flow rate, can explain the observed torques together with the X-ray luminosities, Lx . The X-ray pulsar and radio pulsar states correspond to accretion with spin-down (weak propeller) and strong propeller situations respectively. Several times increase in the disk mass-flow rate takes the source from the strong propeller with a low Lx to the weak propeller with a higher Lx powered by accretion on to the star. The resultant decrease in rin increases the magnetic torque slightly, explaining the observed small increase in the spin-down rate. We have found that the spin-up torque exerted by accreting material is much smaller than the magnetic spin-down torque exerted by the disk in the LMXB state.

  11. IUE observations of new A star candidate proto-planetary systems

    NASA Technical Reports Server (NTRS)

    Grady, Carol A.

    1994-01-01

    As a result of the detection of accreting gas in the A5e PMS Herbig Ae star, HR 5999, most of the observations for this IUE program were devoted to Herbig Ae stars rather than to main sequence A stars. Mid-UV emission at optical minimum light was detected for UX Ori (A1e), BF Ori (A5e), and CQ Tau (F2e). The presence of accreting gas in HD 45677 and HD 50138 prompted reclassification of these stars as Herbig Be stars rather than as protoplanetary nebulae. Detailed results are discussed.

  12. DIAGNOSING MASS FLOWS AROUND HERBIG Ae/Be STARS USING THE HE I λ10830 LINE

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Cauley, P. Wilson; Johns-Krull, Christopher M., E-mail: pcauley@wesleyan.edu, E-mail: cmj@rice.edu

    2014-12-20

    We examine He I λ10830 profile morphologies for a sample of 56 Herbig Ae/Be stars (HAEBES). We find significant differences between HAEBES and classical T-Tauri stars (CTTS) in the statistics of both blueshifted absorption (i.e., mass outflows) and redshifted absorption features (i.e., mass infall or accretion). Our results suggest that, in general, Herbig Be (HBe) stars do not accrete material from their inner disks in the same manner as CTTS, which are believed to accrete material via magnetospheric accretion, whereas Herbig Ae (HAe) stars generally show evidence for magnetospheric accretion. We find no evidence in our sample of narrow blueshiftedmore » absorption features, which are typical indicators of inner disk winds and are common in He I λ10830 profiles of CTTS. The lack of inner-disk-wind signatures in HAEBES, combined with the paucity of detected magnetic fields on these objects, suggests that accretion through large magnetospheres that truncate the disk several stellar radii above the surface is not as common for HAe and late-type HBe stars as it is for CTTS. Instead, evidence is found for smaller magnetospheres in the maximum redshifted absorption velocities in our HAEBE sample. These velocities are, on average, a smaller fraction of the system escape velocity than is found for CTTS, suggesting accretion is taking place closer to the star. Smaller magnetospheres, and evidence for boundary layer accretion in HBe stars, may explain the less common occurrence of redshifted absorption in HAEBES. Evidence is found that smaller magnetospheres may be less efficient at driving outflows compared to CTTS magnetospheres.« less

  13. Accretion, winds and jets: High-energy emission from young stellar objects

    NASA Astrophysics Data System (ADS)

    Günther, Hans Moritz

    2009-03-01

    Stars form by gravitational collapse from giant molecular clouds. Due to the conservation of angular momentum this collapse does not happen radially, but the matter forms circumstellar disk first and is consequently accreted from the disk onto the star. This thesis deals with the high-energy emission from young stellar objects, which are on the one hand still actively accreting from their disk, and on the other hand are no longer deeply obscured by their natal cloud. Stars of spectral type B and A are called Herbig Ae/Be (HAeBe) stars in this stage, all stars of later spectral type are termed classical T Tauri stars (CTTS); strictly speaking both types are defined by spectroscopic signatures, which are equivalent to the evolutionary stage described above. In this thesis CTTS and HAeBes are studied through high-resolution X-ray and UV spectroscopy and through detailed physical simulations. Spectroscopic X-ray data is reduced and presented for two targets: The CTTS V4046 Sgr was observed with Chandra for 100 ks, using a high-resolution grating spectrometer. The lightcurve contains one flare and the He-like triplets of SiXIII, NeIX and OVII indicate high densities in the X-ray emitting regions. The second target is the HAeBe HD 163296, which was observed with XMM-Newton for 130 ks. The lightcurve shows only moderate variability, the elemental abundance follows a pattern, that is usual for active stars. The He-like triplet of OVII exhibits line ratios similar to coronal sources, indicating that neither a high density nor a strong UV-field is present in the region of the X-ray emission. Using these and similar observations, it can be concluded that at least three mechanisms contribute to the observed high-energy emission from CTTS: First, those stars have active coronae similar to main-sequence stars, second, the accreted material passes through a strong accretion shock at the stellar surface, which heats it to a few MK, and, third, some CTTS drive powerful outflows

  14. Neutron-Star-Black-Hole Binaries Produced by Binary-Driven Hypernovae

    NASA Astrophysics Data System (ADS)

    Fryer, Chris L.; Oliveira, F. G.; Rueda, J. A.; Ruffini, R.

    2015-12-01

    Binary-driven hypernovae (BdHNe) within the induced gravitational collapse paradigm have been introduced to explain energetic (Eiso≳1052 erg ), long gamma-ray bursts (GRBs) associated with type Ic supernovae (SNe). The progenitor is a tight binary composed of a carbon-oxygen (CO) core and a neutron-star (NS) companion, a subclass of the newly proposed "ultrastripped" binaries. The CO-NS short-period orbit causes the NS to accrete appreciable matter from the SN ejecta when the CO core collapses, ultimately causing it to collapse to a black hole (BH) and producing a GRB. These tight binaries evolve through the SN explosion very differently than compact binaries studied in population synthesis calculations. First, the hypercritical accretion onto the NS companion alters both the mass and the momentum of the binary. Second, because the explosion time scale is on par with the orbital period, the mass ejection cannot be assumed to be instantaneous. This dramatically affects the post-SN fate of the binary. Finally, the bow shock created as the accreting NS plows through the SN ejecta transfers angular momentum, braking the orbit. These systems remain bound even if a large fraction of the binary mass is lost in the explosion (well above the canonical 50% limit), and even large kicks are unlikely to unbind the system. Indeed, BdHNe produce a new family of NS-BH binaries unaccounted for in current population synthesis analyses and, although they may be rare, the fact that nearly 100% remain bound implies that they may play an important role in the compact merger rate, important for gravitational waves that, in turn, can produce a new class of ultrashort GRBs.

  15. Black Hole Event Horizons and Advection-Dominated Accretion

    NASA Technical Reports Server (NTRS)

    McClintock, Jeffrey; Mushotzky, Richard F. (Technical Monitor)

    2001-01-01

    The XMM data on black-hole X-ray novae are only now becoming available and they have so far not been included in any publications. This work is part of a larger project that makes use of both XMM and Chandra data. Our first publication on the Chandra results is the following: "New Evidence for Black Hole Event Horizons from Chandra" by M.R. Garcia, J.E. McClintock, R. Narayan, P. Callanan, D. Barret and S. Murray (2001, ApJ, 553, L47). Therein we present the luminosities of the two black-hole X-ray novae, GRO J0422+22 and 4U1 543-47, which were observed by Chandra. These results are combined with the luminosities of four additional black-hole X-ray novae, which were observed as part of a Chandra GTO program (PI: S. Murray). The very low, but nonzero, quiescent X-ray luminosities of these black hole binaries is very difficult to understand in the context of standard viscous accretion disk theory. The principal result of this work is that X-ray novae that contain black hole primaries are about 100 times fainter that X-ray novae that contain neutron star primaries. This result had been suggested in earlier work, but the present work very firmly establishes this large luminosity difference. The result is remarkable because the black-hole and the neutron-star systems are believed to be similar in many respects. Most importantly, the mass transfer rate from the secondary star is believed to be very comparable for the two kinds of systems for similar orbital periods. The advection-dominated accretion flow (ADAF) model provides a natural framework for understanding the extraordinarily low luminosities of the black hole systems and the hundred-fold greater luminosities of the neutron star systems. The chief feature of an ADAF is that the heat energy in the accreting gas is trapped in the gas and travels with it, rather than being radiated promptly. Thus the accreting gas reaches the central object with a huge amount of thermal energy. If the accretor is a black hole, the

  16. Bipolar outflows and Jets From Young Stars

    NASA Astrophysics Data System (ADS)

    Bally, J.

    2000-05-01

    Stars produce powerful jets and winds during their birth. These primary outflows power shock waves (Herbig-Haro objects) and entrain surrounding gas to produce molecular outflows. Many outflows reach parsec-scale dimensions whose dynamical ages can become comparable to the accretion age of the source star. Thus, these giant outflows provide fossil records of the mass loss histories of their parent stars. Jet symmetries provide tantalizing clues about the violent history of stellar accretion and dynamical interactions with nearby companions. These flows inject sufficient energy and momentum into the surrounding medium to alter the physical and chemical state of the gas, generate turbulence, disrupt the parent cloud, and self-regulate the rate of star formation. Recent observations have revealed a new class of externally irradiated jets which are rendered visible by the light of nearby massive stars. Some of these jets appear to be millions of years old, indicating that outflow activity can persist for much longer than previously thought. Stellar jets provide ideal laboratories for the investigation of accretion powered outflows and associated shocks since their time-dependent behavior can be observed with a rich variety of spectral line diagnostics.

  17. Accretion disc origin of the Earth's water.

    PubMed

    Vattuone, Luca; Smerieri, Marco; Savio, Letizia; Asaduzzaman, Abu Md; Muralidharan, Krishna; Drake, Michael J; Rocca, Mario

    2013-07-13

    Earth's water is conventionally believed to be delivered by comets or wet asteroids after the Earth formed. However, their elemental and isotopic properties are inconsistent with those of the Earth. It was thus proposed that water was introduced by adsorption onto grains in the accretion disc prior to planetary growth, with bonding energies so high as to be stable under high-temperature conditions. Here, we show both by laboratory experiments and numerical simulations that water adsorbs dissociatively on the olivine {100} surface at the temperature (approx. 500-1500 K) and water pressure (approx. 10⁻⁸ bar) expected for the accretion disc, leaving an OH adlayer that is stable at least up to 900 K. This may result in the formation of many Earth oceans, provided that a viable mechanism to produce water from hydroxyl exists. This adsorption process must occur in all disc environments around young stars. The inevitable conclusion is that water should be prevalent on terrestrial planets in the habitable zone around other stars.

  18. Rise of the First Super-Massive Stars

    NASA Astrophysics Data System (ADS)

    Regan, John A.; Downes, Turlough P.

    2018-05-01

    We use high resolution adaptive mesh refinement simulations to model the formation of massive metal-free stars in the early Universe. By applying Lyman-Werner (LW) backgrounds of 100 J21 and 1000 J21 respectively we construct environments conducive to the formation of massive stars. We find that only in the case of the higher LW backgrounds are super-critical accretion rates realised that are necessary for super-massive star (SMS) formation. Mild fragmentation is observed for both backgrounds. Violent dynamical interactions between the stars that form in the more massive halo formed (1000 J21 background) results in the eventual expulsion of the two most massive stars from the halo. In the smaller mass halo (100 J21 background) mergers of stars occur before any multibody interactions and a single massive Pop III star is left at the centre of the halo at the end of our simulation. Feedback from the very massive Pop III stars is not effective in generating a large HII region with ionising photons absorbed within a few thousand AU of the star. In all cases a massive black hole seed is the expected final fate of the most massive objects. The seed of the massive Pop III star which remained at the centre of the less massive halo experiences steady accretion rates of almost 10-2M_{⊙}/yr and if these rates continue could potentially experience super-Eddington accretion rates in the immediate aftermath of collapsing into a black hole.

  19. Nonlinear oscillations of compact stars in the vicinity of the maximum mass configuration

    NASA Astrophysics Data System (ADS)

    Brillante, A.; Mishustin, I. N.

    2015-07-01

    We solve the dynamical GR equations for the spherically symmetric evolution of compact stars in the vicinity of the maximum mass, for which instability sets in according to linear perturbation theory. The calculations are done with the analytical Zeldovich-like EOS P=a≤ft(ρ-ρ_0\\right) and with the TM1 parametrisation of the RMF model. The initial configurations for the dynamical calculations are represented by spherical stars with equilibrium density profile, which are perturbed by either i) an artificially added inward velocity field proportional to the radial coordinate, or ii) a rarefaction corresponding to a static and expanded star. These configurations are evolved using a one-dimensional GR hydro code for ideal and barotropic fluids. Depending on the initial conditions we obtain either stable oscillations or the collapse to a black hole. The minimal amplitude of the perturbation, needed to trigger gravitational collapse is evaluated. The approximate independence of this energy on the type of perturbation is pointed out. At the threshold we find type-I critical behaviour for all stellar models considered and discuss the dependence of the time scaling exponent on the baryon mass and EOS.

  20. Strangeon and Strangeon Star

    NASA Astrophysics Data System (ADS)

    Xiaoyu, Lai; Renxin, Xu

    2017-06-01

    The nature of pulsar-like compact stars is essentially a central question of the fundamental strong interaction (explained in quantum chromo-dynamics) at low energy scale, the solution of which still remains a challenge though tremendous efforts have been tried. This kind of compact objects could actually be strange quark stars if strange quark matter in bulk may constitute the true ground state of the strong-interaction matter rather than 56Fe (the so-called Witten’s conjecture). From astrophysical points of view, however, it is proposed that strange cluster matter could be absolutely stable and thus those compact stars could be strange cluster stars in fact. This proposal could be regarded as a general Witten’s conjecture: strange matter in bulk could be absolutely stable, in which quarks are either free (for strange quark matter) or localized (for strange cluster matter). Strange cluster with three-light-flavor symmetry is renamed strangeon, being coined by combining “strange nucleon” for the sake of simplicity. A strangeon star can then be thought as a 3-flavored gigantic nucleus, and strangeons are its constituent as an analogy of nucleons which are the constituent of a normal (micro) nucleus. The observational consequences of strangeon stars show that different manifestations of pulsarlike compact stars could be understood in the regime of strangeon stars, and we are expecting more evidence for strangeon star by advanced facilities (e.g., FAST, SKA, and eXTP).

  1. Star formation towards the Galactic H II region RCW 120. Herschel observations of compact sources

    NASA Astrophysics Data System (ADS)

    Figueira, M.; Zavagno, A.; Deharveng, L.; Russeil, D.; Anderson, L. D.; Men'shchikov, A.; Schneider, N.; Hill, T.; Motte, F.; Mège, P.; LeLeu, G.; Roussel, H.; Bernard, J.-P.; Traficante, A.; Paradis, D.; Tigé, J.; André, P.; Bontemps, S.; Abergel, A.

    2017-04-01

    Context. The expansion of H II regions can trigger the formation of stars. An overdensity of young stellar objects is observed at the edges of H II regions but the mechanisms that give rise to this phenomenon are not clearly identified. Moreover, it is difficult to establish a causal link between H II -region expansion and the star formation observed at the edges of these regions. A clear age gradient observed in the spatial distribution of young sources in the surrounding might be a strong argument in favor of triggering. Aims: We aim to characterize the star formation observed at the edges of H II regions by studying the properties of young stars that form there. We aim to detect young sources, derive their properties and their evolution stage in order to discuss the possible causal link between the first-generation massive stars that form the H II region and the young sources observed at their edges. Methods: We have observed the Galactic H II region RCW 120 with Herschel PACS and SPIRE photometers at 70, 100, 160, 250, 350 and 500 μm. We produced temperature and H2 column density maps and use the getsources algorithm to detect compact sources and measure their fluxes at Herschel wavelengths. We have complemented these fluxes with existing infrared data. Fitting their spectral energy distributions with a modified blackbody model, we derived their envelope dust temperature and envelope mass. We computed their bolometric luminosities and discuss their evolutionary stages. Results: The overall temperatures of the region (without background subtraction) range from 15 K to 24 K. The warmest regions are observed towards the ionized gas. The coldest regions are observed outside the ionized gas and follow the emission of the cold material previously detected at 870 μm and 1.3 mm. The H2 column density map reveals the distribution of the cold medium to be organized in filaments and highly structured. Column densities range from 7 × 1021 cm-2 up to 9 × 1023 cm-2

  2. Computer simulations of planetary accretion dynamics: Sensitivity to initial conditions

    NASA Technical Reports Server (NTRS)

    Isaacman, R.; Sagan, C.

    1976-01-01

    The implications and limitations of program ACRETE were tested. The program is a scheme based on Newtonian physics and accretion with unit sticking efficiency, devised to simulate the origin of the planets. The dependence of the results on a variety of radial and vertical density distribution laws, the ratio of gas to dust in the solar nebula, the total nebular mass, and the orbital eccentricity of the accreting grains was explored. Only for a small subset of conceivable cases are planetary systems closely like our own generated. Many models have tendencies towards one of two preferred configurations: multiple star systems, or planetary systems in which Jovian planets either have substantially smaller masses than in our system or are absent altogether. But for a wide range of cases recognizable planetary systems are generated - ranging from multiple star systems with accompanying planets, to systems with Jovian planets at several hundred AU, to single stars surrounded only by asteroids.

  3. Accretion and Magnetic Reconnection in the Pre-Main Sequence Binary DQ Tau as Revealed through High-Cadence Optical Photometry

    NASA Astrophysics Data System (ADS)

    Tofflemire, Benjamin M.; Mathieu, Robert D.; Ardila, David R.; Akeson, Rachel L.; Ciardi, David R.; Herczeg, Gregory; Johns-Krull, Christopher M.; Vodniza, Alberto

    2016-01-01

    Protostellar disks are integral to the formation and evolution of low-mass stars and planets. A paradigm for the star-disk interaction has been extensively developed through theory and observation in the case of single stars. Most stars, however, form in binaries or higher order systems where the distribution of disk material and mass flows are more complex. Pre-main sequence (PMS) binary stars can have up to three accretion disks: two circumstellar disks and a circumbinary disk separated by a dynamically cleared gap. Theory suggests that mass may periodically flow in an accretion stream from a circumbinary disk across the gap onto circumstellar disks or stellar surfaces.The archetype for this theory is the eccentric, PMS binary DQ Tau. Moderate-cadence broadband photometry (~10 observations per orbital period) has shown pulsed brightening events near most periastron passages, just as numerical simulations would predict for a binary of similar orbital parameters. While this observed behavior supports the accretion stream theory, it is not exclusive to variable accretion rates. Magnetic reconnection events (flares) during the collision of stellar magnetospheres at periastron (when separated by 8 stellar radii) could produce the same periodic, broadband behavior when observed at a one-day cadence. Further evidence for magnetic activity comes from gyrosynchrotron, radio flares (typical of stellar flares) observed near multiple periastron passages. To reveal the physical mechanism seen in DQ Tau's moderate-cadence observations, we have obtained continuous, moderate-cadence, multi-band photometry over 10 orbital periods (LCOGT 1m network), supplemented with 32 nights of minute-cadence photometry centered on 4 separate periastron passages (WIYN 0.9m; APO ARCSAT). With detailed lightcurve morphologies we distinguish between the gradual rise and fall on multi-day time-scales predicted by the accretion stream theory and the hour time-scale, rapid-rise and exponential

  4. Studies of neutron star X-ray binaries

    NASA Astrophysics Data System (ADS)

    Thompson, Thomas W. J.

    Neutron stars represent the endpoint in stellar evolution for stars with initial masses between ~3 and 8 solar masses. They are the densest non- singularities in the universe, cramming more than a solar mass of matter into a sphere with a radius of about 10 km. Such a large mass-to-radius ratio implies deep potential wells, so that when mass transfer is taking place ~10% of the rest-mass is liberated as gravitational binding energy, resulting in prodigious amounts of X-ray emission that contains valuable information on the physical characteristics in accreting binary systems. Much of my research in this dissertation focuses on the spectroscopic and timing properties of the canonical thermonuclear bursting source GS 1826-238. By measuring the relationship between the X-ray flux (which is assumed to trace the accretion rate onto the stellar surface) and the time intervals between subsequent bursts, I find that although the intervals usually decreased proportionately as the persistent flux increased, a few measurements of the flux-recurrence time relationship were significant outliers. Accompanying spectral and timing changes strongly suggest that the accretion disk extends down to smaller radial distances from the source during these atypical episodes. This result is important for understanding the nature of accretion flows around neutron stars because it indicates that accretion disks probably evaporate at some distance from the neutron star surface at lower accretion rates. I also contribute to our understanding of two newly discovered and heavily- absorbed pulsars (neutron stars with strong magnetic fields) by determining the orbital parameters of the systems through pulse timing analysis. Orbital phase- resolved spectroscopy of one source revealed evidence for an "accretion wake" trailing the pulsar through its orbit, showing that X-rays emanating from the surface can ionize the stellar wind in its vicinity. Finally, I develop an innovative application of dust

  5. Cold, clumpy accretion onto an active supermassive black hole

    NASA Astrophysics Data System (ADS)

    Tremblay, Grant R.; Oonk, J. B. Raymond; Combes, Françoise; Salomé, Philippe; O'Dea, Christopher P.; Baum, Stefi A.; Voit, G. Mark; Donahue, Megan; McNamara, Brian R.; Davis, Timothy A.; McDonald, Michael A.; Edge, Alastair C.; Clarke, Tracy E.; Galván-Madrid, Roberto; Bremer, Malcolm N.; Edwards, Louise O. V.; Fabian, Andrew C.; Hamer, Stephen; Li, Yuan; Maury, Anaëlle; Russell, Helen R.; Quillen, Alice C.; Urry, C. Megan; Sanders, Jeremy S.; Wise, Michael W.

    2016-06-01

    Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds—a departure from the ‘hot mode’ accretion model—although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities produce a rain of cold clouds that fall towards the galaxy’s centre, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core. The observations show that these cold clouds also fuel black hole accretion, revealing ‘shadows’ cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.

  6. Cold, clumpy accretion onto an active supermassive black hole.

    PubMed

    Tremblay, Grant R; Oonk, J B Raymond; Combes, Françoise; Salomé, Philippe; O'Dea, Christopher P; Baum, Stefi A; Voit, G Mark; Donahue, Megan; McNamara, Brian R; Davis, Timothy A; McDonald, Michael A; Edge, Alastair C; Clarke, Tracy E; Galván-Madrid, Roberto; Bremer, Malcolm N; Edwards, Louise O V; Fabian, Andrew C; Hamer, Stephen; Li, Yuan; Maury, Anaëlle; Russell, Helen R; Quillen, Alice C; Urry, C Megan; Sanders, Jeremy S; Wise, Michael W

    2016-06-09

    Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds--a departure from the 'hot mode' accretion model--although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities produce a rain of cold clouds that fall towards the galaxy's centre, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core. The observations show that these cold clouds also fuel black hole accretion, revealing 'shadows' cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.

  7. Are Small Radii of Compact Stars Ruled out by GW170817/AT2017gfo?

    NASA Astrophysics Data System (ADS)

    Burgio, G. F.; Drago, A.; Pagliara, G.; Schulze, H.-J.; Wei, J.-B.

    2018-06-01

    The detection of GW170817 and its electromagnetic counterparts allows us to constrain the equation of state of dense matter in new and complementary ways. Very stiff equations of state are ruled out by the upper limit on the average tidal deformability, \\tilde{{{Λ }}}≲ 800, imposed by the detected gravitational wave signal. A lower limit, \\tilde{{{Λ }}}≳ 400, can also be extracted by considering the large amount of ejected matter that powers the kilonova AT2017gfo. By using several microscopic nucleonic equations of state, we first confirm the existence of a monotonic relation between R 1.5 (the radius of the 1.5 M ⊙ configuration) and \\tilde{{{Λ }}}. This translates the limits on \\tilde{{{Λ }}} into limits on the radius: 11.8 km ≲ R 1.5 ≲ 13.1 km. We then show that the monotonic relation is violated if a second branch of compact stars composed of quark matter exists, as in the two-families or twin-stars scenarios. In particular, it is possible to fulfill the limits on \\tilde{{{Λ }}} while having R 1.5 significantly smaller than 12 km. In both of these scenarios, the event GW170817/AT2017gfo originates from the merger of a hadronic star and a star containing quark matter.

  8. Cold-mode Accretion: Driving the Fundamental Mass-Metallicity Relation at z ~ 2

    NASA Astrophysics Data System (ADS)

    Kacprzak, Glenn G.; van de Voort, Freeke; Glazebrook, Karl; Tran, Kim-Vy H.; Yuan, Tiantian; Nanayakkara, Themiya; Allen, Rebecca J.; Alcorn, Leo; Cowley, Michael; Labbé, Ivo; Spitler, Lee; Straatman, Caroline; Tomczak, Adam

    2016-07-01

    We investigate the star formation rate (SFR) dependence on the stellar mass and gas-phase metallicity relation at z = 2 with MOSFIRE/Keck as part of the ZFIRE survey. We have identified 117 galaxies (1.98 ≤ z ≤ 2.56), with 8.9 ≤ log(M/M ⊙) ≤ 11.0, for which we can measure gas-phase metallicities. For the first time, we show a discernible difference between the mass-metallicity relation, using individual galaxies, when dividing the sample by low (<10 M ⊙ yr-1) and high (>10 M ⊙ yr-1) SFRs. At fixed mass, low star-forming galaxies tend to have higher metallicity than high star-forming galaxies. Using a few basic assumptions, we further show that the gas masses and metallicities required to produce the fundamental mass-metallicity relation and its intrinsic scatter are consistent with cold-mode accretion predictions obtained from the OWLS hydrodynamical simulations. Our results from both simulations and observations are suggestive that cold-mode accretion is responsible for the fundamental mass-metallicity relation at z = 2 and it demonstrates the direct relationship between cosmological accretion and the fundamental properties of galaxies.

  9. NuSTAR Observation of the Symbiotic System GX 1+4

    NASA Astrophysics Data System (ADS)

    Wolff, Michael Thomas; Becker, Peter A.; Enoto, Teruaki; Pottschmidt, Katja; Wood, Kent

    2017-08-01

    We report on a NuSTAR observation of the symbiotic binary system GX 1+4. GX 1+4 is one of a small number of systems with a red giant mass donor and a magnetic neutron star in orbit around each other. The accreting pulsar in GX 1+4 has a spin period of ~150 seconds with epochs of both spin-up and spin-down. The orbital period that has not been determined. Magnetic accretion theory in such systems suggests that the neutron star has a magnetic field in the range 1013-1014 Gauss although this is not settled because no cyclotron absorption feature has been observed in the X-ray spectrum. The NuSTAR spectrum shows broad Fe-line emission near ~6.5 keV and also shows a broad power law shape detected up to ~60 keV. We analyze and discuss the NuSTAR X-ray data with particular attention to the question of what can the spectrum tell us about the structure of the accretion flow onto the neutron star and the magnetic field strength.

  10. STELLAR POPULATIONS IN COMPACT GALAXY GROUPS: A MULTI-WAVELENGTH STUDY OF HCGs 16, 22, AND 42, THEIR STAR CLUSTERS, AND DWARF GALAXIES

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Konstantopoulos, I. S.; Maybhate, A.; Charlton, J. C.

    2013-06-20

    We present a multi-wavelength analysis of three compact galaxy groups, Hickson compact groups (HCGs) 16, 22, and 42, which describe a sequence in terms of gas richness, from space- (Swift, Hubble Space Telescope (HST), and Spitzer) and ground-based (Las Campanas Observatory and Cerro Tololo Inter-American Observatory) imaging and spectroscopy. We study various signs of past interactions including a faint, dusty tidal feature about HCG 16A, which we tentatively age-date at <1 Gyr. This represents the possible detection of a tidal feature at the end of its phase of optical observability. Our HST images also resolve what were thought to bemore » double nuclei in HCG 16C and D into multiple, distinct sources, likely to be star clusters. Beyond our phenomenological treatment, we focus primarily on contrasting the stellar populations across these three groups. The star clusters show a remarkable intermediate-age population in HCG 22, and identify the time at which star formation was quenched in HCG 42. We also search for dwarf galaxies at accordant redshifts. The inclusion of 33 members and 27 ''associates'' (possible members) radically changes group dynamical masses, which in turn may affect previous evolutionary classifications. The extended membership paints a picture of relative isolation in HCGs 16 and 22, but shows HCG 42 to be part of a larger structure, following a dichotomy expected from recent studies. We conclude that (1) star cluster populations provide an excellent metric of evolutionary state, as they can age-date the past epochs of star formation; and (2) the extended dwarf galaxy population must be considered in assessing the dynamical state of a compact group.« less

  11. Stellar Populations in Compact Galaxy Groups: a Multi-wavelength Study of HCGs 16, 22, and 42, Their Star Clusters, and Dwarf Galaxies

    NASA Technical Reports Server (NTRS)

    Konstantopoulos, I. S.; Maybhate, A.; Charlton, J. C.; Fedotov, K.; Durrell, P. R.; Mulchaey, J. S.; English, J.; Desjardins, T. D.; Gallagher, S. C.; Walker, L. M.; hide

    2013-01-01

    We present a multi-wavelength analysis of three compact galaxy groups, Hickson compact groups (HCGs) 16, 22, and 42, which describe a sequence in terms of gas richness, from space- (Swift, Hubble Space Telescope (HST), and Spitzer) and ground-based (Las Campanas Observatory and Cerro Tololo Inter-American Observatory) imaging and spectroscopy.We study various signs of past interactions including a faint, dusty tidal feature about HCG 16A, which we tentatively age-date at <1 Gyr. This represents the possible detection of a tidal feature at the end of its phase of optical observability. Our HST images also resolve what were thought to be double nuclei in HCG 16C and D into multiple, distinct sources, likely to be star clusters. Beyond our phenomenological treatment, we focus primarily on contrasting the stellar populations across these three groups. The star clusters show a remarkable intermediate-age population in HCG 22, and identify the time at which star formation was quenched in HCG 42. We also search for dwarf galaxies at accordant redshifts. The inclusion of 33 members and 27 "associates" (possible members) radically changes group dynamical masses, which in turn may affect previous evolutionary classifications. The extended membership paints a picture of relative isolation in HCGs 16 and 22, but shows HCG 42 to be part of a larger structure, following a dichotomy expected from recent studies. We conclude that (1) star cluster populations provide an excellent metric of evolutionary state, as they can age-date the past epochs of star formation; and (2) the extended dwarf galaxy population must be considered in assessing the dynamical state of a compact group.

  12. Gamma-ray bursts from stellar mass accretion disks around black holes

    NASA Technical Reports Server (NTRS)

    Woosley, S. E.

    1993-01-01

    A cosmological model for gamma-ray bursts is explored in which the radiation is produced as a broadly beamed pair fireball along the rotation axis of an accreting black hole. The black hole may be a consequence of neutron star merger or neutron star-black hole merger, but for long complex bursts, it is more likely to come from the collapse of a single Wolf-Rayet star endowed with rotation ('failed' Type Ib supernova). The disk is geometrically thick and typically has a mass inside 100 km of several tenths of a solar mass. In the failed supernova case, the disk is fed for a longer period of time by the collapsing star. At its inner edge the disk is thick to its own neutrino emission and evolves on a viscous time scale of several seconds. In a region roughly 30 km across, interior to the accretion disk and along its axis of rotation, a pair fireball is generated by neutrino annihilation and electron-neutrino scattering which deposit approximately 10 exp 50 ergs/s.

  13. Destruction of a Magnetized Star

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2017-01-01

    What happens when a magnetized star is torn apart by the tidal forces of a supermassive black hole, in a violent process known as a tidal disruption event? Two scientists have broken new ground by simulating the disruption of stars with magnetic fields for the first time.The magnetic field configuration during a simulation of the partial disruption of a star. Top left: pre-disruption star. Bottom left: matter begins to re-accrete onto the surviving core after the partial disruption. Right: vortices form in the core as high-angular-momentum debris continues to accrete, winding up and amplifying the field. [Adapted from Guillochon McCourt 2017]What About Magnetic Fields?Magnetic fields are expected to exist in the majority of stars. Though these fields dont dominate the energy budget of a star the magnetic pressure is a million times weaker than the gas pressure in the Suns interior, for example they are the drivers of interesting activity, like the prominences and flares of our Sun.Given this, we can wonder what role stars magnetic fields might play when the stars are torn apart in tidal disruption events. Do the fields change what we observe? Are they dispersed during the disruption, or can they be amplified? Might they even be responsible for launching jets of matter from the black hole after the disruption?Star vs. Black HoleIn a recent study, James Guillochon (Harvard-Smithsonian Center for Astrophysics) and Michael McCourt (Hubble Fellow at UC Santa Barbara) have tackled these questions by performing the first simulations of tidal disruptions of stars that include magnetic fields.In their simulations, Guillochon and McCourt evolve a solar-mass star that passes close to a million-solar-mass black hole. Their simulations explore different magnetic field configurations for the star, and they consider both what happens when the star barely grazes the black hole and is only partially disrupted, as well as what happens when the black hole tears the star apart

  14. Magnetized Neutron Stars in the Interstellar Medium

    NASA Astrophysics Data System (ADS)

    Toropina, O. D.; Romanova, M. M.; Lovelace, R. V. E.

    2014-09-01

    We investigate the propagation of magnetized, isolated old neutron stars through the interstellar medium. We performed axisymmetric, non-relativistic magnetohydrodynamic simulations of the supersonic motion of neutron star with dipole magnetic field aligned with its velocity through the interstellar medium (ISM). We consider two cases: (1) where the accretion radius is larger than Alfvén radius, i.e. Racc>>RA and gravitational focusing is important; and (2) where Racc<star interacts with the ISM as a “georotator”, without significant gravitational focusing. In the first case we observe Bondi-Hoyle accretion onto an isolated magnetized neutron star. In the second case magnetic field lines are stretched downwind from the star and form a hollow elongated magnetotail. Reconnection of the magnetic field is observed in the tail which may lead to acceleration of particles.

  15. Do Circumnuclear Dense Gas Disks Drive Mass Accretion onto Supermassive Black Holes?

    NASA Astrophysics Data System (ADS)

    Izumi, Takuma; Kawakatu, Nozomu; Kohno, Kotaro

    2016-08-01

    We present a positive correlation between the mass of dense molecular gas ({M}{{dense}}) of ˜100 pc scale circumnuclear disks (CNDs) and the black hole mass accretion rate ({\\dot{M}}{{BH}}) in a total of 10 Seyfert galaxies, based on data compiled from the literature and an archive (median aperture θ med = 220 pc). A typical {M}{{dense}} of CNDs is 107-8 {M}⊙ , estimated from the luminosity of the dense gas tracer, the HCN(1-0) emission line. Because dense molecular gas is the site of star formation, this correlation is virtually equivalent to the one between the nuclear star-formation rate and {\\dot{M}}{{BH}} revealed previously. Moreover, the {M}{{dense}}{--}{\\dot{M}}{{BH}} correlation was tighter for CND-scale gas than for the gas on kiloparsec or larger scales. This indicates that CNDs likely play an important role in fueling black holes, whereas greater than kiloparesec scale gas does not. To demonstrate a possible approach for studying the CND-scale accretion process with the Atacama Large Millimeter/submillimeter Array, we used a mass accretion model where angular momentum loss due to supernova explosions is vital. Based on the model prediction, we suggest that only the partial fraction of the mass accreted from the CND ({\\dot{M}}{{acc}}) is consumed as {\\dot{M}}{{BH}}. However, {\\dot{M}}{{acc}} agrees well with the total nuclear mass flow rate (I.e., {\\dot{M}}{{BH}} + outflow rate). Although these results are still tentative with large uncertainties, they support the view that star formation in CNDs can drive mass accretion onto supermassive black holes in Seyfert galaxies.

  16. Accretion by the Galaxy

    NASA Astrophysics Data System (ADS)

    Binney, J.; Fraternali, F.

    2012-02-01

    Cosmology requires at least half of the baryons in the Universe to be in the intergalactic medium, much of which is believed to form hot coronae around galaxies. Star-forming galaxies must be accreting from their coronae. Hi observations of external galaxies show that they have Hi halos associated with star formation. These halos are naturally modelled as ensembles of clouds driven up by supernova bubbles. These models can fit the data successfully only if clouds exchange mass and momentum with the corona. As a cloud orbits, it is ablated and forms a turbulent wake where cold high-metallicity gas mixes with hot coronal gas causing the prompt cooling of the latter. As a consequence the total mass of Hi increases. This model has recently been used to model the Leiden-Argentina-Bonn survey of Galactic Hi. The values of the model's parameters that are required to model NGC 891, NGC 2403 and our Galaxy show a remarkable degree of consistency, despite the very different natures of the two external galaxies and the dramatic difference in the nature of the data for our Galaxy and the external galaxies. The parameter values are also consistent with hydrodynamical simulations of the ablation of individual clouds. The model predicts that a galaxy that loses its cool-gas disc for instance through a major merger cannot reform it from its corona; it can return to steady star formation only if it can capture a large body of cool gas, for example by accreting a gas-rich dwarf. Thus the model explains how major mergers can make galaxies "red and dead."

  17. A UV-to-MIR monitoring of DR Tau: Exploring how water vapor in the planet formation region is affected by stellar accretion variability

    DOE Office of Scientific and Technical Information (OSTI.GOV)

    Banzatti, A.; Meyer, M. R.; Manara, C. F.

    2014-01-01

    Young stars are known to show variability due to non-steady mass accretion rate from their circumstellar disks. Accretion flares can produce strong energetic irradiation and heating that may affect the disk in the planet formation region, close to the central star. During an extreme accretion outburst in the young star EX Lupi, the prototype of EXor variables, remarkable changes in molecular gas emission from ∼1 AU in the disk have recently been observed. Here, we focus on water vapor and explore how it is affected by variable accretion luminosity in T Tauri stars. We monitored a young highly variable solar-massmore » star, DR Tau, using simultaneously two high/medium-resolution spectrographs at the European Southern Observatory Very Large Telescope: VISIR at 12.4 μm to observe water lines from the disk and X-shooter covering from 0.3 to 2.5 μm to constrain the stellar accretion. Three epochs spanning timescales from several days to several weeks were obtained. The accretion luminosity was estimated to change within a factor of ∼2 and no change in water emission was detected at a significant level. In comparison with EX Lupi and EXor outbursts, DR Tau suggests that the less long-lived and weaker variability phenomena typical of T Tauri stars may leave water at planet-forming radii in the disk mostly unaffected. We propose that these systems may provide evidence for two processes that act over different timescales: ultraviolet photochemistry in the disk atmosphere (faster) and heating of the deeper disk layers (slower).« less

  18. Winds and accretion in delta Sagittae

    NASA Technical Reports Server (NTRS)

    Eaton, Joel A.; Hartkopf, William I.; Mcalister, Harold A.; Mason, Brian D.

    1995-01-01

    The ten-year binary delta Sge (M2 Ib-II+B9.5 V) is a zeta Aur binary containing an abnormally cool component. Combining our analysis of the system as a visual binary with Batten's radial-velocity solution leads to the following properties: i = 40 deg, a = 51 mas = 8.83 A.U. = 1893 solar radius, hence d = 173 pc; M(sub B) = 2.9 solar mass and M(sub M) = 3.8 solar mass; and R(sub B) = 2.6 solar radius and R(sub M) = 152 solar radius. This interpretation of the orbit places the M supergiant on the asymptotic giant branch. We have collected ultraviolet spectra throughout the star's 1980-90 orbit, concentrated around the conjuction of 1990. The wind of the M giant appears in these as narrow shell lines of singly ionized metals, chiefly Fe II, with P-Cyg profiles at many phases, which show the slow variation in strength expected for the orbit but no pronounced atmospheric eclipse. The terminal velocity of the wind is 16-18 km/s, and its excitation temperature is approximately 10,000 K. Most of the broadening of the wind lines is caused by differential expansion of the atmosphere, with (unmeasurably) low turbulent velocities. Nontheless, the mass loss rate (1.1 +/- 0.4 X 10 (exp -8) solar mas/yr) is almost the same as found previously by Reimers and Schroder for very different assumptions about the velocity structure. Also seen in the spectrum throughout the orbit are the effects of a variable, high-speed wind as well as evidence for accretion onto the B9.5 star. This high-speed wind absorbs in species of all ionization stages observed, e. g., C II, Mg II, Al III, SI IV, C IV, and has a terminaal velocity in the range 200-450 km/s. We presume this wind originates at the B dwarf, not the M supergiant, and speculate that it comes from an accretion disk, as suggested by recent models of magnetically moderated accretion. Evidence for accretion is redshifted absorption in the same transitions formed in the high-speed wind, as well as broad emission lines of singly ionized

  19. Magnetohydrodynamic Simulations of Black Hole Accretion

    NASA Astrophysics Data System (ADS)

    Avara, Mark J.

    Black holes embody one of the few, simple, solutions to the Einstein field equations that describe our modern understanding of gravitation. In isolation they are small, dark, and elusive. However, when a gas cloud or star wanders too close, they light up our universe in a way no other cosmic object can. The processes of magnetohydrodynamics which describe the accretion inflow and outflows of plasma around black holes are highly coupled and nonlinear and so require numerical experiments for elucidation. These processes are at the heart of astrophysics since black holes, once they somehow reach super-massive status, influence the evolution of the largest structures in the universe. It has been my goal, with the body of work comprising this thesis, to explore the ways in which the influence of black holes on their surroundings differs from the predictions of standard accretion models. I have especially focused on how magnetization of the greater black hole environment can impact accretion systems.

  20. Hans A. Bethe Prize: Cosmic Collisions Online - Compact Binary Mergers, Gravitational Waves and Gamma-Ray Bursts

    NASA Astrophysics Data System (ADS)

    Shapiro, Stuart

    2017-01-01

    Hans A. Bethe elucidated our understanding of the fundamental forces of Nature by exploring and explaining countless phenomena occurring in nuclear laboratories and in stars. With the dawn of gravitational wave astronomy we now can probe compact binary mergers - Nature's cosmic collision experiments - to deepen our understanding, especially where strong-field gravitation is involved. In addition to gravitational waves, some mergers are likely to generate observable electromagnetic and/or neutrino radiation, heralding a new era of multimessenger astronomy. Robust numerical algorithms now allow us to simulate these events in full general relativity on supercomputers. We will describe some recent magnetohydrodynamic simulations that show how binary black hole-neutron star and neutron star-neutron star mergers can launch jets, lending support to the idea that such mergers could be the engines that power short gamma-ray bursts. We will also show how the magnetorotational collapse of very massive stars to spinning black holes immersed in magnetized accretion disks can launch jets as well, reinforcing the belief that such ``collapsars'' are the progenitors of long gamma-ray bursts. Computer-generated movies highlighting some of these simulations will be shown. We gratefully acknowledge support from NSF Grants 1300903 and 1602536 and NASA Grant NNX13AH44G.