Sample records for accretion shocks application

  1. DIFFUSIVE PARTICLE ACCELERATION IN SHOCKED, VISCOUS ACCRETION DISKS: GREEN'S FUNCTION ENERGY DISTRIBUTION

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

    Becker, Peter A.; Das, Santabrata; Le, Truong, E-mail: pbecker@gmu.edu, E-mail: sbdas@iitg.ernet.in, E-mail: truong.le@nhrec.org

    2011-12-10

    The acceleration of relativistic particles in a viscous accretion disk containing a standing shock is investigated as a possible explanation for the energetic outflows observed around radio-loud black holes. The energy/space distribution of the accelerated particles is computed by solving a transport equation that includes the effects of first-order Fermi acceleration, bulk advection, spatial diffusion, and particle escape. The velocity profile of the accreting gas is described using a model for shocked viscous disks recently developed by the authors, and the corresponding Green's function distribution for the accelerated particles in the disk and the outflow is obtained using a classicalmore » method based on eigenfunction analysis. The accretion-driven, diffusive shock acceleration scenario explored here is conceptually similar to the standard model for the acceleration of cosmic rays at supernova-driven shocks. However, in the disk application, the distribution of the accelerated particles is much harder than would be expected for a plane-parallel shock with the same compression ratio. Hence the disk environment plays a key role in enhancing the efficiency of the shock acceleration process. The presence of the shock helps to stabilize the disk by reducing the Bernoulli parameter, while channeling the excess binding energy into the escaping relativistic particles. In applications to M87 and Sgr A*, we find that the kinetic power in the jet is {approx}0.01 M-dot c{sup 2}, and the outflowing relativistic particles have a mean energy {approx}300 times larger than that of the thermal gas in the disk at the shock radius. Our results suggest that a standing shock may be an essential ingredient in accretion onto underfed black holes, helping to resolve the long-standing problem of the stability of advection-dominated accretion disks.« less

  2. Accretion shock geometries in the magnetic variables

    NASA Technical Reports Server (NTRS)

    Stockman, H. S.

    1988-01-01

    The first self consistent shock models for the AM Herculis-type systems successfully identified the dominant physical processes and their signatures. These homogenous shock models predict unpolarized, Rayleigh-Jeans optical spectra with sharp cutoffs and rising polarizations as the shocks become optically thin in the ultraviolet. However, the observed energy distributions are generally flat with intermediate polarizations over a broad optical band. These and other observational evidence support a non-homogenous accretion profile which may extend over a considerable fraction of the stellar surface. Both the fundamental assumptions underlying the canonical 1-D shock model and the extension of this model to inhomogenous accretion shocks were identified, for both radial and linear structures. The observational evidence was also examined for tall shocks and little evidence was found for relative shock heights in excess of h/R(1) greater than or equal to 0.1. For several systems, upper limits to the shock height can be obtained from either x ray or optical data. These lie in the region h/R(1) is approximately 0.01 and are in general agreement with the current physical picture for these systems. The quasi-periodic optical variations observed in several magnetic variables may eventually prove to be a major aid in further understanding their accretion shock geometries.

  3. Standing shocks in magnetized dissipative accretion flow around black holes

    NASA Astrophysics Data System (ADS)

    Sarkar, Biplob; Das, Santabrata

    2018-02-01

    We explore the global structure of the accretion flow around a Schwarzschild black hole where the accretion disc is threaded by toroidal magnetic fields. The accretion flow is optically thin and advection dominated. The synchrotron radiation is considered to be the active cooling mechanism in the flow. With this, we obtain the global transonic accretion solutions and show that centrifugal barrier in the rotating magnetized accretion flow causes a discontinuous transition of the flow variables in the form of shock waves. The shock properties and the dynamics of the post-shock corona are affected by the flow parameters such as viscosity, cooling rate and strength of the magnetic fields. The shock properties are investigated against these flow parameters. We further show that for a given set of boundary parameters at the outer edge of the disc, accretion flow around a black hole admits shock when the flow parameters are tuned for a considerable range.

  4. Shocks in the relativistic transonic accretion with low angular momentum

    NASA Astrophysics Data System (ADS)

    Suková, P.; Charzyński, S.; Janiuk, A.

    2017-12-01

    We perform 1D/2D/3D relativistic hydrodynamical simulations of accretion flows with low angular momentum, filling the gap between spherically symmetric Bondi accretion and disc-like accretion flows. Scenarios with different directional distributions of angular momentum of falling matter and varying values of key parameters such as spin of central black hole, energy and angular momentum of matter are considered. In some of the scenarios the shock front is formed. We identify ranges of parameters for which the shock after formation moves towards or outwards the central black hole or the long-lasting oscillating shock is observed. The frequencies of oscillations of shock positions which can cause flaring in mass accretion rate are extracted. The results are scalable with mass of central black hole and can be compared to the quasi-periodic oscillations of selected microquasars (such as GRS 1915+105, XTE J1550-564 or IGR J17091-3624), as well as to the supermassive black holes in the centres of weakly active galaxies, such as Sgr A*.

  5. Influence of matter geometry on shocked flows-I: Accretion in the Schwarzschild metric

    NASA Astrophysics Data System (ADS)

    Tarafdar, Pratik; Das, Tapas K.

    2018-07-01

    This work presents a comprehensive and extensive study to illustrate how the geometrical configurations of low angular momentum axially symmetric general relativistic matter flow in the Schwarzschild metric may influence the formation of energy-preserving shocks for adiabatic/polytropic accretion as well as of temperature-preserving dissipative shocks for the isothermal accretion onto non-rotating astrophysical black holes. The dynamical and thermodynamic states of post-shock polytropic and isothermal flow have been studied extensively for three possible matter geometries, and it has been thoroughly discussed about how such states depend on the flow structure, even when the self gravity and the back reaction on the metric are not taken into account. Main purpose of this paper is thus to mathematically demonstrate that for non-self gravitating accretion, various matter geometries, in addition to the corresponding space-time geometry, control the shock induced phenomena as observed within the black hole accretion discs. This work is expected to reveal how the shock generated phenomena (emergence of the outflows/flare in the associated light curves) observed at the close proximity of the horizon depend on the physical environment of the source harbouring a supermassive black hole.

  6. 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

  7. 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

  8. Radiative Reverse Shock Laser Experiments Relevant to Accretion Processes in Cataclysmic Variables

    NASA Astrophysics Data System (ADS)

    Krauland, Christine

    2012-10-01

    We present results from experiments that explore radiative reverse shock waves and their contribution to the evolving dynamics of the cataclysmic variable (CV) system in which they reside. CVs are close binary star systems containing a white dwarf (WD) that accretes matter from its late-type main sequence companion star. In the process of accretion, a reverse shock forms when the supersonic infalling plasma is impeded. It provides the main source of radiation in the binary systems. In the case of a non-magnetic CV, the impact on an accretion disk produces this ``hot spot,'' where the flow obliquely strikes the rotating accretion disk. This collision region has many ambiguities as a radiation hydrodynamic system, but shock development in the infalling flow can be modeled [1]. We discuss the production of radiative reverse shocks in experiments at the Omega-60 laser facility. The ability of this high-intensity laser to create large energy densities in targets having millimeter-scale volumes makes it feasible to create supersonic plasma flows. Obtaining a radiative reverse shock in the laboratory requires a sufficiently fast flow (> 60 km/s) within a material whose opacity is large enough to produce energetically significant emission from experimentally achievable layers. We will show the radiographic and emission data from three campaigns on Omega-60 with accompanying CRASH [2] simulations, and will discuss the implications in the context of the CV system. [4pt] [1] Armitage, P. J. and Livio, M., ApJ, 493, 898 (1998).[0pt] [2] van der Holst, B., Toth, G., Sokolov, I.V., et al., ApJS, 194, 23 (2011).

  9. 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.

  10. 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

  11. Parametric Study of Flow Patterns behind the Standing Accretion Shock Wave for Core-Collapse Supernovae

    NASA Astrophysics Data System (ADS)

    Iwakami, Wakana; Nagakura, Hiroki; Yamada, Shoichi

    2014-05-01

    In this study, we conduct three-dimensional hydrodynamic simulations systematically to investigate the flow patterns behind the accretion shock waves that are commonly formed in the post-bounce phase of core-collapse supernovae. Adding small perturbations to spherically symmetric, steady, shocked accretion flows, we compute the subsequent evolutions to find what flow pattern emerges as a consequence of hydrodynamical instabilities such as convection and standing accretion shock instability for different neutrino luminosities and mass accretion rates. Depending on these two controlling parameters, various flow patterns are indeed realized. We classify them into three basic patterns and two intermediate ones; the former includes sloshing motion (SL), spiral motion (SP), and multiple buoyant bubble formation (BB); the latter consists of spiral motion with buoyant-bubble formation (SPB) and spiral motion with pulsationally changing rotational velocities (SPP). Although the post-shock flow is highly chaotic, there is a clear trend in the pattern realization. The sloshing and spiral motions tend to be dominant for high accretion rates and low neutrino luminosities, and multiple buoyant bubbles prevail for low accretion rates and high neutrino luminosities. It is interesting that the dominant pattern is not always identical between the semi-nonlinear and nonlinear phases near the critical luminosity; the intermediate cases are realized in the latter case. Running several simulations with different random perturbations, we confirm that the realization of flow pattern is robust in most cases.

  12. Parametric study of flow patterns behind the standing accretion shock wave for core-collapse supernovae

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

    Iwakami, Wakana; Nagakura, Hiroki; Yamada, Shoichi, E-mail: wakana@heap.phys.waseda.ac.jp

    2014-05-10

    In this study, we conduct three-dimensional hydrodynamic simulations systematically to investigate the flow patterns behind the accretion shock waves that are commonly formed in the post-bounce phase of core-collapse supernovae. Adding small perturbations to spherically symmetric, steady, shocked accretion flows, we compute the subsequent evolutions to find what flow pattern emerges as a consequence of hydrodynamical instabilities such as convection and standing accretion shock instability for different neutrino luminosities and mass accretion rates. Depending on these two controlling parameters, various flow patterns are indeed realized. We classify them into three basic patterns and two intermediate ones; the former includes sloshingmore » motion (SL), spiral motion (SP), and multiple buoyant bubble formation (BB); the latter consists of spiral motion with buoyant-bubble formation (SPB) and spiral motion with pulsationally changing rotational velocities (SPP). Although the post-shock flow is highly chaotic, there is a clear trend in the pattern realization. The sloshing and spiral motions tend to be dominant for high accretion rates and low neutrino luminosities, and multiple buoyant bubbles prevail for low accretion rates and high neutrino luminosities. It is interesting that the dominant pattern is not always identical between the semi-nonlinear and nonlinear phases near the critical luminosity; the intermediate cases are realized in the latter case. Running several simulations with different random perturbations, we confirm that the realization of flow pattern is robust in most cases.« less

  13. The Planetary Accretion Shock. I. Framework for Radiation-hydrodynamical Simulations and First Results

    NASA Astrophysics Data System (ADS)

    Marleau, Gabriel-Dominique; Klahr, Hubert; Kuiper, Rolf; Mordasini, Christoph

    2017-02-01

    The key aspect determining the postformation luminosity of gas giants has long been considered to be the energetics of the accretion shock at the surface of the planet. We use one-dimensional radiation-hydrodynamical simulations to study the radiative loss efficiency and to obtain postshock temperatures and pressures and thus entropies. The efficiency is defined as the fraction of the total incoming energy flux that escapes the system (roughly the Hill sphere), taking into account the energy recycling that occurs ahead of the shock in a radiative precursor. We focus in this paper on a constant equation of state (EOS) to isolate the shock physics but use constant and tabulated opacities. While robust quantitative results will have to await a self-consistent treatment including hydrogen dissociation and ionization, the results presented here show the correct qualitative behavior and can be understood from semianalytical calculations. The shock is found to be isothermal and supercritical for a range of conditions relevant to the core accretion formation scenario (CA), with Mach numbers { M }≳ 3. Across the shock, the entropy decreases significantly by a few times {k}{{B}}/{{baryon}}. While nearly 100% of the incoming kinetic energy is converted to radiation locally, the efficiencies are found to be as low as roughly 40%, implying that a significant fraction of the total accretion energy is brought into the planet. However, for realistic parameter combinations in the CA scenario, we find that a nonzero fraction of the luminosity always escapes the Hill sphere. This luminosity could explain, at least in part, recent observations in the young LkCa 15 and HD 100546 systems.

  14. Upper Limit of the Viscosity Parameter in Accretion Flows around a Black Hole with Shock Waves

    NASA Astrophysics Data System (ADS)

    Nagarkoti, Shreeram; Chakrabarti, Sandip K.

    2016-01-01

    Black hole accretion is necessarily transonic; thus, flows must become supersonic and, therefore, sub-Keplerian before they enter into the black hole. The viscous timescale is much longer than the infall timescale close to a black hole. Hence, the angular momentum remains almost constant and the centrifugal force ˜ {l}2/{r}3 becomes increasingly dominant over the gravitational force ˜ 1/{r}2. The slowed down matter piles creating an accretion shock. The flow between shock and inner sonic point is puffed up and behaves like a boundary layer. This so-called Comptonizing cloud/corona produces hard X-rays and jets/outflows and, therefore, is an important component of black hole astrophysics. In this paper, we study steady state viscous, axisymmetric, transonic accretion flows around a Schwarzschild black hole. We adopt a viscosity parameter α and compute the highest possible value of α (namely, {α }{cr}) for each pair of two inner boundary parameters (namely, specific angular momentum carried to horizon, lin and specific energy at inner sonic point, E({x}{in})) which is still capable of producing a standing or oscillating shock. We find that while such possibilities exist for α as high as {α }{cr}=0.3 in very small regions of the flow parameter space, typical {α }{cr} appears to be about ˜0.05-0.1. Coincidentally, this also happens to be the typical viscosity parameter achieved by simulations of magnetorotational instabilities in accretion flows. We therefore believe that all realistic accretion flows are likely to have centrifugal pressure supported shocks unless the viscosity parameter everywhere is higher than {α }{cr}.

  15. Radio Observations as a Tool to Investigate Shocks and Asymmetries in Accreting White Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Weston, Jennifer Helen Seng; E-Nova Project

    2017-01-01

    In this dissertation, I use radio observations with the Karl G. Jansky Very Large Array (VLA) to reveal that colliding flows within the ejecta from nova explosions can lead to shocks that accelerate particles and produce radio synchrotron emission. In both novae V1723 Aql and V5589 Sgr, radio emission within the first one to two months deviated strongly from the classic thermal model for radio emission from novae. Three years of radio observations of V1723 Aql show that multiple outflows from the system collided to create non-thermal shocks with a brightness temperature of >106 K. After these shocks faded, the radio light curve became roughly consistent with an expanding thermal shell. However, resolved images of V1723 Aql show elongated material that apparently rotates its major axis over the course of 15 months. In the case of nova V5589 Sgr, I show that the early radio emission is dominated by a shock-powered non-thermal flare that produces strong (kTx > 33 keV) X-rays. These findings have important implications for understanding how normal novae generate GeV gamma-rays.Additionally, I present VLA observations of the symbiotic star CH Cyg and two small surveys of symbiotic binaries. Radio observations of CH Cyg tie the ejection of a collimated jet to a change of state in the accretion disk, strengthening the link between bipolar outflows from accreting white dwarfs and other types of accreting compact objects. Next, I use a survey of eleven accretion-driven symbiotic binaries to determine that the radio brightness of a symbiotic system could potentially be used as an indicator of whether it is powered predominantly by shell burning on the surface of the white dwarf or by accretion. This survey also produces the first radio detections of seven of the target systems. In the second survey of seventeen symbiotic binaries, I spatially resolve extended radio emission in several systems for the first time. The results from these surveys provide some support for the

  16. A numerical study of the stability of radiative shocks. [in accretion flows onto white dwarf stars

    NASA Technical Reports Server (NTRS)

    Imamura, J. N.; Wolff, M. T.; Durisen, R. H.

    1984-01-01

    Attention is given to the oscillatory instability of optically thin radiative shocks in time-dependent numerical calculations of accretion flows onto degenerate dwarfs. The present nonlinear calculations yield good quantitative agreement with the linear results obtained for oscillation frequencies, damping rates, and critical alpha-values. The fundamental mode and the first overtone in the shock radius and luminosity variations can be clearly identified, and evidence is sometimes seen for the second overtone. Time-dependent calculations are also performed which include additional physics relevant to degenerate dwarf accretion, such as electron thermal conduction, unequal electron and ion temperatures, Compton cooling, and relativistic corrections to the bremsstrahlung cooling law. All oscillatory modes are found to be damped, and hence stable, in the case of a 1-solar mass white dwarf accreting in spherical symmetry.

  17. ACCRETION OF SUPERSONIC WINDS ONTO BLACK HOLES IN 3D: STABILITY OF THE SHOCK CONE

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

    Gracia-Linares, M.; Guzmán, F. S.

    Using numerical simulations we present the accretion of supersonic winds onto a rotating black hole in three dimensions. We study five representative directions of the wind with respect to the axis of rotation of the black hole and focus on the evolution and stability of the high-density shock cone that is formed during the process. We explore both the regime in which the shock cone is expected to be stable in order to confirm previous results obtained with two-dimensional simulations, and the regime in which the shock cone is expected to show a flip–flop (FF) type of instability. The methodsmore » used to attempt a triggering of the instability were (i) the accumulation of numerical errors and (ii) the explicit application of a perturbation on the velocity field after the shock cone was formed. The result is negative, that is, we did not find the FF instability within the parameter space we explored, including cases that are expected to be unstable.« less

  18. Reverse Radiative Shock Experiments Relevant to Accreting Stream-Disk Impact in Interacting Binaries

    NASA Astrophysics Data System (ADS)

    Krauland, Christine; Drake, R. P.; Kuranz, C. K.; Huntington, C. M.; Grosskopf, M. J.; Marion, D. C.; Young, R.; Plewa, T.

    2011-05-01

    In many Cataclysmic Binary systems, mass onto an accretion disk produces a `hot spot’ where the infalling supersonic flow obliquely strikes the rotating accretion disk. This collision region has many ambiguities as a radiation hydrodynamic system, but shock development in the infalling flow can be modeled. Depending upon conditions, it has been argued (Armitage & Livio, ApJ 493, 898) that the shocked region may be optically thin, thick, or intermediate, which has the potential to significantly alter the hot spot's structure and emissions. We report the first experimental attempt to produce colliding flows that create a radiative reverse shock at the Omega-60 laser facility. Obtaining a radiative reverse shock in the laboratory requires producing a sufficiently fast flow (> 100 km/s) within a material whose opacity is large enough to produce energetically significant emission from experimentally achievable layers. We will discuss the experimental design, the available data, and our astrophysical context. Funded by the NNSA-DS and SC-OFES Joint Prog. in High-Energy-Density Lab. Plasmas, by the Nat. Laser User Facility Prog. in NNSA-DS and by the Predictive Sci. Acad. Alliances Prog. in NNSA-ASC, under grant numbers are DE-FG52-09NA29548, DE-FG52-09NA29034, and DE-FC52-08NA28616.

  19. Two-fluid dusty shocks: simple benchmarking problems and applications to protoplanetary discs

    NASA Astrophysics Data System (ADS)

    Lehmann, Andrew; Wardle, Mark

    2018-05-01

    The key role that dust plays in the interstellar medium has motivated the development of numerical codes designed to study the coupled evolution of dust and gas in systems such as turbulent molecular clouds and protoplanetary discs. Drift between dust and gas has proven to be important as well as numerically challenging. We provide simple benchmarking problems for dusty gas codes by numerically solving the two-fluid dust-gas equations for steady, plane-parallel shock waves. The two distinct shock solutions to these equations allow a numerical code to test different forms of drag between the two fluids, the strength of that drag and the dust to gas ratio. We also provide an astrophysical application of J-type dust-gas shocks to studying the structure of accretion shocks on to protoplanetary discs. We find that two-fluid effects are most important for grains larger than 1 μm, and that the peak dust temperature within an accretion shock provides a signature of the dust-to-gas ratio of the infalling material.

  20. 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.

  1. STANDING SHOCK INSTABILITY IN ADVECTION-DOMINATED ACCRETION FLOWS

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

    Le, Truong; Wood, Kent S.; Wolff, Michael T.

    2016-03-10

    Depending on the values of the energy and angular momentum per unit mass in the gas supplied at large radii, inviscid advection-dominated accretion flows can display velocity profiles with either preshock deceleration or preshock acceleration. Nakayama has shown that these two types of flow configurations are expected to have different stability properties. By employing the Chevalier and Imamura linearization method and the Nakayama instability boundary conditions, we discover that there are regions of parameter space where disks/shocks with outflows can be stable or unstable. In regions of instability, we find that preshock deceleration is always unstable to the zeroth mode withmore » zero frequency of oscillation, but is always stable to the fundamental mode and overtones. Furthermore, we also find that preshock acceleration is always unstable to the zeroth mode and that the fundamental mode and overtones become increasingly less stable as the shock location moves away from the horizon when the disk half-height expands above ∼12 gravitational radii at the shock radius. In regions of stability, we demonstrate the zeroth mode to be stable for the velocity profiles that exhibit preshock acceleration and deceleration. Moreover, for models that are linearly unstable, our model suggests the possible existence of quasi-periodic oscillations (QPOs) with ratios 2:3 and 3:5. These ratios are believed to occur in stellar and supermassive black hole candidates, for example, in GRS 1915+105 and Sgr A*, respectively. We expect that similar QPO ratios also exist in regions of stable shocks.« less

  2. Hard X-ray emission from accretion shocks around galaxy clusters

    NASA Astrophysics Data System (ADS)

    Kushnir, Doron; Waxman, Eli

    2010-02-01

    We show that the hard X-ray (HXR) emission observed from several galaxy clusters is consistent with a simple model, in which the nonthermal emission is produced by inverse Compton scattering of cosmic microwave background photons by electrons accelerated in cluster accretion shocks: The dependence of HXR surface brightness on cluster temperature is consistent with that predicted by the model, and the observed HXR luminosity is consistent with the fraction of shock thermal energy deposited in relativistic electrons being lesssim0.1. Alternative models, where the HXR emission is predicted to be correlated with the cluster thermal emission, are disfavored by the data. The implications of our predictions to future HXR observations (e.g. by NuStar, Simbol-X) and to (space/ground based) γ-ray observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed.

  3. Stability of stagnation via an expanding accretion shock wave

    NASA Astrophysics Data System (ADS)

    Velikovich, A. L.; Murakami, M.; Taylor, B. D.; Giuliani, J. L.; Zalesak, S. T.; Iwamoto, Y.

    2016-05-01

    Stagnation of a cold plasma streaming to the center or axis of symmetry via an expanding accretion shock wave is ubiquitous in inertial confinement fusion (ICF) and high-energy-density plasma physics, the examples ranging from plasma flows in x-ray-generating Z pinches [Maron et al., Phys. Rev. Lett. 111, 035001 (2013)] to the experiments in support of the recently suggested concept of impact ignition in ICF [Azechi et al., Phys. Rev. Lett. 102, 235002 (2009); Murakami et al., Nucl. Fusion 54, 054007 (2014)]. Some experimental evidence indicates that stagnation via an expanding shock wave is stable, but its stability has never been studied theoretically. We present such analysis for the stagnation that does not involve a rarefaction wave behind the expanding shock front and is described by the classic ideal-gas Noh solution in spherical and cylindrical geometry. In either case, the stagnated flow has been demonstrated to be stable, initial perturbations exhibiting a power-law, oscillatory or monotonic, decay with time for all the eigenmodes. This conclusion has been supported by our simulations done both on a Cartesian grid and on a curvilinear grid in spherical coordinates. Dispersion equation determining the eigenvalues of the problem and explicit formulas for the eigenfunction profiles corresponding to these eigenvalues are presented, making it possible to use the theory for hydrocode verification in two and three dimensions.

  4. Stability of stagnation via an expanding accretion shock wave

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

    Velikovich, A. L.; Giuliani, J. L.; Murakami, M.

    Stagnation of a cold plasma streaming to the center or axis of symmetry via an expanding accretion shock wave is ubiquitous in inertial confinement fusion (ICF) and high-energy-density plasma physics, the examples ranging from plasma flows in x-ray-generating Z pinches [Maron et al., Phys. Rev. Lett. 111, 035001 (2013)] to the experiments in support of the recently suggested concept of impact ignition in ICF [Azechi et al., Phys. Rev. Lett. 102, 235002 (2009); Murakami et al., Nucl. Fusion 54, 054007 (2014)]. Some experimental evidence indicates that stagnation via an expanding shock wave is stable, but its stability has never beenmore » studied theoretically. We present such analysis for the stagnation that does not involve a rarefaction wave behind the expanding shock front and is described by the classic ideal-gas Noh solution in spherical and cylindrical geometry. In either case, the stagnated flow has been demonstrated to be stable, initial perturbations exhibiting a power-law, oscillatory or monotonic, decay with time for all the eigenmodes. This conclusion has been supported by our simulations done both on a Cartesian grid and on a curvilinear grid in spherical coordinates. Dispersion equation determining the eigenvalues of the problem and explicit formulas for the eigenfunction profiles corresponding to these eigenvalues are presented, making it possible to use the theory for hydrocode verification in two and three dimensions.« less

  5. A New Gravitational-wave Signature from Standing Accretion Shock Instability in Supernovae

    NASA Astrophysics Data System (ADS)

    Kuroda, Takami; Kotake, Kei; Takiwaki, Tomoya

    2016-09-01

    We present results from fully relativistic three-dimensional core-collapse supernova simulations of a non-rotating 15{M}⊙ star using three different nuclear equations of state (EoSs). From our simulations covering up to ˜350 ms after bounce, we show that the development of the standing accretion shock instability (SASI) differs significantly depending on the stiffness of nuclear EoS. Generally, the SASI activity occurs more vigorously in models with softer EoS. By evaluating the gravitational-wave (GW) emission, we find a new GW signature on top of the previously identified one, in which the typical GW frequency increases with time due to an accumulating accretion to the proto-neutron star (PNS). The newly observed quasi-periodic signal appears in the frequency range from ˜100 to 200 Hz and persists for ˜150 ms before neutrino-driven convection dominates over the SASI. By analyzing the cycle frequency of the SASI sloshing and spiral modes as well as the mass accretion rate to the emission region, we show that the SASI frequency is correlated with the GW frequency. This is because the SASI-induced temporary perturbed mass accretion strikes the PNS surface, leading to the quasi-periodic GW emission. Our results show that the GW signal, which could be a smoking-gun signature of the SASI, is within the detection limits of LIGO, advanced Virgo, and KAGRA for Galactic events.

  6. DETECTION OF A COOL, ACCRETION-SHOCK-GENERATED X-RAY PLASMA IN EX LUPI DURING THE 2008 OPTICAL ERUPTION

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

    Teets, William K.; Weintraub, David A.; Kastner, Joel H.

    2012-11-20

    EX Lupi is the prototype for a class of young, pre-main-sequence stars which are observed to undergo irregular, presumably accretion-generated, optical outbursts that result in a several magnitude rise of the optical flux. EX Lupi was observed to optically erupt in 2008 January, triggering Chandra ACIS Target of Opportunity observations shortly thereafter. We find very strong evidence that most of the X-ray emission in the first few months after the optical outburst is generated by accretion of circumstellar material onto the stellar photosphere. Specifically, we find a strong correlation between the decreasing optical and X-ray fluxes following the peak ofmore » the outburst in the optical, which suggests that these observed declines in both the optical and X-ray fluxes are the result of declining accretion rate. In addition, in our models of the X-ray spectrum, we find strong evidence for a {approx}0.4 keV plasma component, as expected for accretion shocks on low-mass, pre-main-sequence stars. From 2008 March through October, this cool plasma component appeared to fade as EX Lupi returned to its quiescent level in the optical, consistent with a decrease in the overall emission measure of accretion-shock-generated plasma. The overall small increase of the X-ray flux during the optical outburst of EX Lupi is similar to what was observed in previous X-ray observations of the 2005 optical outburst of the EX Lupi-type star V1118 Ori but contrasts with the large increase of the X-ray flux from the erupting young star V1647 Ori during its 2003 and 2008 optical outbursts.« less

  7. Detection of a Cool, Accretion-Shock-Generated X-Ray Plasma in EX Lupi During the 2008 Optical Eruption

    NASA Technical Reports Server (NTRS)

    Teets, William K.; Weintraub, David A.; Kastner, Joel H.; Grosso, Nicholas; Hamaguchi, Kenji; Richmond, Michael

    2012-01-01

    EX Lupi is the prototype for a class of young, pre-main-sequence stars which are observed to undergo irregular, presumably accretion-generated, optical outbursts that result in a several magnitude rise of the optical flux. EX Lupi was observed to optically erupt in 2008 January, triggering Chandra ACIS Target of Opportunity observations shortly thereafter. We find very strong evidence that most of the X-ray emission in the first few months after the optical outburst is generated by accretion of circumstellar material onto the stellar photosphere. Specifically, we find a strong correlation between the decreasing optical and X-ray fluxes following the peak of the outburst in the optical, which suggests that these observed declines in both the optical and X-ray fluxes are the result of declining accretion rate. In addition, in our models of the X-ray spectrum, we find strong evidence for an approx 0.4 keV plasma component, as expected for accretion shocks on low-mass, pre-main-sequence stars. From 2008 March through October, this cool plasma component appeared to fade as EX Lupi returned to its quiescent level in the optical, consistent with a decrease in the overall emission measure of accretion-shock-generated plasma. The overall small increase of the X-ray flux during the optical outburst of EX Lupi is similar to what was observed in previous X-ray observations of the 2005 optical outburst of the EX Lupi-type star V1118 Ori but contrasts with the large increase of the X-ray flux from the erupting young star V1647 Ori during its 2003 and 2008 optical outbursts.

  8. Are cosmological gas accretion streams multiphase and turbulent?

    NASA Astrophysics Data System (ADS)

    Cornuault, Nicolas; Lehnert, Matthew D.; Boulanger, François; Guillard, Pierre

    2018-03-01

    Simulations of cosmological filamentary accretion reveal flows ("streams") of warm gas, T 104 K, which bring gas into galaxies efficiently. We present a phenomenological scenario in which gas in such flows, if it is shocked as it enters the halo as we assume and depending on the post-shock temperature, stream radius, its relative overdensity, and other factors, becomes biphasic and turbulent. We consider a collimated stream of warm gas that flows into a halo from an overdense filament of the cosmic web. The post-shock streaming gas expands because it has a higher pressure than the ambient halo gas and fragments as it cools. The fragmented stream forms a two phase medium: a warm cloudy phase embedded in hot post-shock gas. We argue that the hot phase sustains the accretion shock. During fragmentation, a fraction of the initial kinetic energy of the infalling gas is converted into turbulence among and within the warm clouds. The thermodynamic evolution of the post-shock gas is largely determined by the relative timescales of several processes. These competing timescales characterize the cooling, expansion of the post-shock gas, amount of turbulence in the clouds, and dynamical time of the halo. We expect the gas to become multiphase when the gas cooling and dynamical times are of the same order of magnitude. In this framework, we show that this mainly occurs in the mass range, Mhalo 1011 to 1013 M⊙, where the bulk of stars have formed in galaxies. Because of the expansion of the stream and turbulence, gas accreting along cosmic web filaments may eventually lose coherence and mix with the ambient halo gas. Through both the phase separation and "disruption" of the stream, the accretion efficiency onto a galaxy in a halo dynamical time is lowered. Decollimating flows make the direct interaction between galaxy feedback and accretion streams more likely, thereby further reducing the overall accretion efficiency. As we discuss in this work, moderating the gas accretion

  9. Delayed Neutrino-Driven Supernova Explosions Aided by the Standing Accretion-Shock Instability

    NASA Astrophysics Data System (ADS)

    Marek, A.; Janka, H.-Th.

    2009-03-01

    We present two-dimensional hydrodynamic simulations of stellar core collapse and develop the framework for a detailed analysis of the energetic aspects of neutrino-powered supernova explosions. Our results confirm that the neutrino-heating mechanism remains a viable explanation of the explosion of a wider mass range of supernova progenitors with iron cores, but the explosion sets in later and develops differently than thought so far. The calculations were performed with an energy-dependent treatment of the neutrino transport based on the "ray-by-ray plus" approximation, in which the neutrino number, energy, and momentum equations are closed with a variable Eddington factor obtained by iteratively solving a model Boltzmann equation. We focus here on the evolution of a 15 M sun progenitor and provide evidence that shock revival and an explosion are initiated at about 600 ms after core bounce, powered by neutrino energy deposition. This is significantly later than previously found for an 11.2 M sun star, for which we also present a continuation of the explosion model published by Buras et al. The onset of the blast is fostered in both cases by the standing accretion-shock instability. This instability exhibits highest growth rates for the dipole and quadrupole modes, which lead to large-amplitude bipolar shock oscillations and push the shock to larger radii, thus increasing the time accreted matter is exposed to neutrino heating in the gain layer. As a consequence, also convective overturn behind the shock is strengthened, which otherwise is suppressed or damped because of the small shock stagnation radius. When the explosion sets in, the shock reveals a pronounced global deformation with a dominant dipolar component. In both the 11.2 M sun and 15 M sun explosions long-lasting equatorial downflows supply the gain layer with fresh gas, of which a sizable fraction is heated by neutrinos and leads to the build-up of the explosion energy of the ejecta over possibly

  10. 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.

  11. 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.

  12. Spherical accretion in giant elliptical galaxies: multi-transonicity, shocks, and implications on AGN feedback

    NASA Astrophysics Data System (ADS)

    Raychaudhuri, Sananda; Ghosh, Shubhrangshu; Joarder, Partha S.

    2018-06-01

    Isolated massive elliptical galaxies, or that are present at the center of cool-core clusters, are believed to be powered by hot gas accretion directly from their surrounding hot X-ray emitting gaseous medium. This leads to a giant Bondi-type spherical/quasi-spherical accretion flow onto their host SMBHs, with the accretion flow region extending well beyond the Bondi radius. In this work, we present a detailed study of Bondi-type spherical flow in the context of these massive ellipticals by incorporating the effect of entire gravitational potential of the host galaxy in the presence of cosmological constant Λ, considering a five-component galactic system (SMBH + stellar + dark matter + hot gas + Λ). The current work is an extension of Ghosh & Banik (2015), who studied only the cosmological aspect of the problem. The galactic contribution to the potential renders the (adiabatic) spherical flow to become multi-transonic in nature, with the flow topology and flow structure significantly deviating from that of classical Bondi solution. More notably, corresponding to moderate to higher values of galactic mass-to-light ratios, we obtain Rankine-Hugoniot shocks in spherical wind flows. Galactic potential enhances the Bondi accretion rate. Our study reveals that there is a strict lower limit of ambient temperature below which no Bondi accretion can be triggered; which is as high as ˜9 × 106 K for flows from hot ISM-phase, indicating that the hot phase tightly regulates the fueling of host nucleus. Our findings may have wider implications, particularly in the context of outflow/jet dynamics, and radio-AGN feedback, associated with these massive galaxies in the contemporary Universe.

  13. 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).

  14. Dissipative advective accretion disc solutions with variable adiabatic index around black holes

    NASA Astrophysics Data System (ADS)

    Kumar, Rajiv; Chattopadhyay, Indranil

    2014-10-01

    We investigated accretion on to black holes in presence of viscosity and cooling, by employing an equation of state with variable adiabatic index and multispecies fluid. We obtained the expression of generalized Bernoulli parameter which is a constant of motion for an accretion flow in presence of viscosity and cooling. We obtained all possible transonic solutions for a variety of boundary conditions, viscosity parameters and accretion rates. We identified the solutions with their positions in the parameter space of generalized Bernoulli parameter and the angular momentum on the horizon. We showed that a shocked solution is more luminous than a shock-free one. For particular energies and viscosity parameters, we obtained accretion disc luminosities in the range of 10- 4 - 1.2 times Eddington luminosity, and the radiative efficiency seemed to increase with the mass accretion rate too. We found steady state shock solutions even for high-viscosity parameters, high accretion rates and for wide range of composition of the flow, starting from purely electron-proton to lepton-dominated accretion flow. However, similar to earlier studies of inviscid flow, accretion shock was not obtained for electron-positron pair plasma.

  15. Redshifted X-rays from the material accreting onto TW Hydrae: Evidence of a low-latitude accretion spot

    NASA Astrophysics Data System (ADS)

    Argiroffi, C.; Drake, J. J.; Bonito, R.; Orlando, S.; Peres, G.; Miceli, M.

    2017-10-01

    Context. High resolution spectroscopy, providing constraints on plasma motions and temperatures, is a powerful means to investigate the structure of accretion streams in classical T Tauri stars (CTTS). In particular, the accretion shock region, where the accreting material is heated to temperatures of a few million degrees as it continues its inward bulk motion, can be probed by X-ray spectroscopy. Aims: In an attempt to detect for the first time the motion of this X-ray-emitting post-shock material, we searched for a Doppler shift in the deep Chandra High Energy Transmission Grating observation of the CTTS TW Hya. This test should unveil the nature of this X-ray emitting plasma component in CTTS and constrain the accretion stream geometry. Methods: We searched for a Doppler shift in the X-ray emission from TW Hya with two different methods: by measuring the position of a selected sample of emission lines and by fitting the whole TW Hya X-ray spectrum, allowing the line-of-sight velocity to vary. Results: We found that the plasma at T 2 - 4 MK has a line-of-sight velocity of 38.3 ± 5.1 km s-1 with respect to the stellar photosphere. This result definitively confirms that this X-ray-emitting material originates in the post-shock region, at the base of the accretion stream, and not in coronal structures. The comparison of the observed velocity along the line of sight, 38.3 ± 5.1 km s-1, with the inferred intrinsic velocity of the post shock of TW Hya, vpost ≈ 110 - 120 km s-1, indicates that the footpoints of the accretion streams on TW Hya are located at low latitudes on the stellar surface. Conclusions: Our results indicate that complex magnetic field geometries, such as those of TW Hya, permit low-latitude accretion spots. Moreover, since on TW Hya the redshift of the soft X-ray emission is very similar to that of the narrow component of the C iv resonance doublet at 1550 Å, then the plasma at 2 - 4 MK and that at 0.1 MK likely originate in the same post-shock

  16. Properties of two-temperature dissipative accretion flow around black holes

    NASA Astrophysics Data System (ADS)

    Dihingia, Indu K.; Das, Santabrata; Mandal, Samir

    2018-04-01

    We study the properties of two-temperature accretion flow around a non-rotating black hole in presence of various dissipative processes where pseudo-Newtonian potential is adopted to mimic the effect of general relativity. The flow encounters energy loss by means of radiative processes acted on the electrons and at the same time, flow heats up as a consequence of viscous heating effective on ions. We assumed that the flow is exposed with the stochastic magnetic fields that leads to Synchrotron emission of electrons and these emissions are further strengthen by Compton scattering. We obtain the two-temperature global accretion solutions in terms of dissipation parameters, namely, viscosity (α) and accretion rate ({\\dot{m}}), and find for the first time in the literature that such solutions may contain standing shock waves. Solutions of this kind are multitransonic in nature, as they simultaneously pass through both inner critical point (xin) and outer critical point (xout) before crossing the black hole horizon. We calculate the properties of shock-induced global accretion solutions in terms of the flow parameters. We further show that two-temperature shocked accretion flow is not a discrete solution, instead such solution exists for wide range of flow parameters. We identify the effective domain of the parameter space for standing shock and observe that parameter space shrinks as the dissipation is increased. Since the post-shock region is hotter due to the effect of shock compression, it naturally emits hard X-rays, and therefore, the two-temperature shocked accretion solution has the potential to explain the spectral properties of the black hole sources.

  17. Dynamical structure of magnetized dissipative accretion flow around black holes

    NASA Astrophysics Data System (ADS)

    Sarkar, Biplob; Das, Santabrata

    2016-09-01

    We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accretion solutions including shocks. We estimate the energy dissipation at the PSC from where a part of the accreting matter can deflect as outflows and jets. We compare the maximum energy that could be extracted from the PSC and the observed radio luminosity values for several supermassive black hole sources and the observational implications of our present analysis are discussed.

  18. 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

  19. 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

  20. Estimation of bipolar jets from accretion discs around Kerr black holes

    NASA Astrophysics Data System (ADS)

    Kumar, Rajiv; Chattopadhyay, Indranil

    2017-08-01

    We analyse flows around a rotating black hole and obtain self-consistent accretion-ejection solutions in full general relativistic prescription. Entire energy-angular momentum parameter space is investigated in the advective regime to obtain shocked and shock-free accretion solutions. Jet equations of motion are solved along the von Zeipel surfaces computed from the post-shock disc, simultaneously with the equations of accretion disc along the equatorial plane. For a given spin parameter, the mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock location. Interestingly, we obtain all types of possible jet solutions, for example, steady shock solution with multiple critical points, bound solution with two critical points and smooth solution with single critical point. Multiple critical points may exist in jet solution for spin parameter as ≥ 0.5. The jet terminal speed generally increases if the accretion shock forms closer to the horizon and is higher for corotating black hole than the counter-rotating and the non-rotating one. Quantitatively speaking, shocks in jet may form for spin parameter as > 0.6 and jet shocks range between 6rg and 130rg above the equatorial plane, while the jet terminal speed vj∞ > 0.35 c if Bernoulli parameter E≥1.01 for as > 0.99.

  1. Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

    NASA Astrophysics Data System (ADS)

    Chattopadhyay, Indranil; Kumar, Rajiv

    2016-07-01

    We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion-ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von Zeipel surfaces and projected the jet equations of motion on to the streamline and solved them simultaneously with the accretion disc equations of motion. We found that steady shock cannot exist beyond α ≳ 0.06 in the general relativistic prescription, but is lower if mass-loss is considered too. We showed that for fixed outer boundary, the shock moves closer to the horizon with increasing viscosity parameter. The mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock location. The jet terminal speed increases with stronger shocks; quantitatively speaking, the terminal speed of jets vj∞ > 0.1 if rsh < 20rg. The maximum of the outflow rate obtained in the general relativistic regime is less than 6 per cent of the mass accretion rate.

  2. Supermassive black hole formation by cold accretion shocks in the first galaxies

    NASA Astrophysics Data System (ADS)

    Inayoshi, Kohei; Omukai, Kazuyuki

    2012-05-01

    We propose a new scenario for supermassive star (SMS: >rsim 105 M⊙) formation in shocked regions of colliding cold accretion flows near the centres of the first galaxies. Recent numerical simulations indicate that assembly of a typical first galaxy with virial temperature Tvir≳104 K proceeds via cold and dense flows penetrating deep to the centre, where supersonic streams collide with each other to develop a hot (˜104 K) and dense (˜103 cm-3) shocked gas. The post-shock layer first cools by efficient Lyα emission and contracts isobarically until ≃8000 K. Whether the layer continues its isobaric contraction depends on the density at this moment: if the density is high enough to excite H2 rovibrational levels collisionally (>rsim 104 cm-3), enhanced H2 collisional dissociation suppresses the gas from cooling further. In this case, the layer fragments into massive (>rsim 105 M⊙) clouds, which collapse isothermally (˜8000 K) by Lyα cooling without subsequent fragmentation. As an outcome, SMSs are expected to form and eventually evolve into the seeds of supermassive black holes (SMBHs). By calculating the thermal evolution of the post-shock gas, we delimit the range of post-shock conditions for SMS formation, which can be expressed as T≳6000 K (nH/104 cm-3)-1 for ? and T>rsim 5000 -6000 K for nH≳104 cm-3, depending somewhat on the initial ionization degree. We found that metal enrichment does not affect the above condition for metallicity below ≃10-3 Z⊙ if metals are in the gas phase, while condensation of several per cent of metals into dust decreases this critical value of metallicity by an order of magnitude. Unlike the previously proposed scenario for SMS formation, which postulates extremely strong ultraviolet radiation to quench H2 cooling, our scenario here naturally explains SMBH seed formation in the assembly process of the first galaxies, even without such strong radiation.

  3. Stellar explosions from accreting white dwarfs

    NASA Astrophysics Data System (ADS)

    Moore, Kevin L.

    Unstable thermonuclear burning on accreting white dwarfs (WDs) can lead to a wide variety of outcomes, and induce shock waves in several contexts. In classical and recurrent novae, a WD accreting hydrogen-rich material from a binary companion can experience thermonuclear runaways, ejecting mass into the interstellar/circumbinary environment at ~1000 km/s. This highly supersonic ejecta drives shock waves into the interstellar gas which may be relevant for sweeping out gas from globular clusters or forming circumstellar absorption regions in interacting supernovae. While runaway nuclear burning in novae releases enough energy for these objects to brighten by a factor of ~10 4 over roughly a weeklong outburst, it does not become dynamically unstable. In contrast, certain helium accretion scenarios may allow for dynamical burning modes, in part due to the higher temperature sensitivity of helium burning reactions and larger accreted envelopes. The majority of this thesis involves such dynamical burning modes, specifically detonations - shock waves sustained by nuclear energy release behind the shock front. We investigate when steady-state detonations are realizable in accreted helium layers on WDs, and model their strength and burning products using both semi-analytic and numerical models. We find the minimum helium layer thickness that will sustain a steady laterally propagating detonation and show that it depends on the density and composition of the helium layer, specifically 12 C and 16O. Though gravitationally unbound, the ashes still have unburned helium (~80% in the thinnest cases) and only reach up to heavy elements such as 40Ca, 44Ti, 48Cr, and 52Fe. It is rare for these thin shells to generate large amounts of radioactive isotopes necessary to power light curves, such as 56Ni. This has important implications on whether the unbound helium burning ashes may create faint and fast peculiar supernovae or events with virtually no radioactivity, as well as on off

  4. High energy spectrum of spherically accreting black holes

    NASA Technical Reports Server (NTRS)

    Meszaros, P.; Ostriker, J. P.

    1983-01-01

    Spherically accreting black holes may sustain strong collisionless shocks, downstream of which the fluid approximation is not valid. The proton-electron Coulomb exchange provides for the downstream matter diffusion into the hole. Energy conversion efficiencies upward of 10-30 percent are obtained, with most of the luminosity in hard X-rays and gamma-rays. The whole spectrum and its application for radio-quiet QSO's and galactic X- and gamma-ray sources are discussed.

  5. The Evolution of Gas Giant Entropy During Formation by Runaway Accretion

    NASA Astrophysics Data System (ADS)

    Berardo, David; Cumming, Andrew; Marleau, Gabriel-Dominique

    2017-01-01

    We calculate the evolution of gas giant planets during the runaway gas accretion phase of formation, to understand how the luminosity of young giant planets depends on the accretion conditions. We construct steady-state envelope models, and run time-dependent simulations of accreting planets with the code Modules for Experiments in Stellar Astrophysics. We show that the evolution of the internal entropy depends on the contrast between the internal adiabat and the entropy of the accreted material, parametrized by the shock temperature T 0 and pressure P 0. At low temperatures ({T}0≲ 300-1000 {{K}}, depending on model parameters), the accreted material has a lower entropy than the interior. The convection zone extends to the surface and can drive a high luminosity, leading to rapid cooling and cold starts. For higher temperatures, the accreted material has a higher entropy than the interior, giving a radiative zone that stalls cooling. For {T}0≳ 2000 {{K}}, the surface-interior entropy contrast cannot be accommodated by the radiative envelope, and the accreted matter accumulates with high entropy, forming a hot start. The final state of the planet depends on the shock temperature, accretion rate, and starting entropy at the onset of runaway accretion. Cold starts with L≲ 5× {10}-6 {L}⊙ require low accretion rates and starting entropy, and the temperature of the accreting material needs to be maintained close to the nebula temperature. If instead the temperature is near the value required to radiate the accretion luminosity, 4π {R}2σ {T}04˜ ({GM}\\dot{M}/R), as suggested by previous work on radiative shocks in the context of star formation, gas giant planets form in a hot start with L˜ {10}-4 {L}⊙ .

  6. The fate or organic matter during planetary accretion - Preliminary studies of the organic chemistry of experimentally shocked Murchison meteorite

    NASA Technical Reports Server (NTRS)

    Tingle, Tracy N.; Tyburczy, James A.; Ahrens, Thomas J.; Becker, Christopher H.

    1992-01-01

    The fate of organic matter in carbonaceous meteorites during hypervelocity (1-2 km/sec) impacts is investigated using results of experiments in which three samples of the Murchison (CM2) carbonaceous chondrite were shocked to 19, 20, and 36 GPa and analyzed by highly sensitive thermal-desorption photoionization mass spectrometry (SALI). The thermal-desorptive SALI mass spectra of unshocked CM2 material revealed presence of indigenous aliphatic, aromatic, sulfur, and organosulfur compounds, and samples shocked to about 20 GPa showed little or no loss of organic matter. On the other hand, samples shocked to 36 GPa exhibited about 70 percent loss of organic material and a lower alkene/alkane ratio than did the starting material. The results suggest that it is unlikely that the indigenous organic matter in carbonaceous chondritelike planetesimals could have survived the impact on the earth in the later stages of earth's accretion.

  7. Simulations of Viscous Accretion Flow around Black Holes in a Two-dimensional Cylindrical Geometry

    NASA Astrophysics Data System (ADS)

    Lee, Seong-Jae; Chattopadhyay, Indranil; Kumar, Rajiv; Hyung, Siek; Ryu, Dongsu

    2016-11-01

    We simulate shock-free and shocked viscous accretion flows onto a black hole in a two-dimensional cylindrical geometry, where initial conditions were chosen from analytical solutions. The simulation code used the Lagrangian total variation diminishing plus remap routine, which enabled us to attain high accuracy in capturing shocks and to handle the angular momentum distribution correctly. The inviscid shock-free accretion disk solution produced a thick disk structure, while the viscous shock-free solution attained a Bondi-like structure, but in either case, no jet activity nor any quasi-periodic oscillation (QPO)-like activity developed. The steady-state shocked solution in the inviscid as well as in the viscous regime matched theoretical predictions well. However, increasing viscosity renders the accretion shock unstable. Large-amplitude shock oscillation is accompanied by intermittent, transient inner multiple shocks. This oscillation of the inner part of the disk is interpreted as the source of QPO in hard X-rays observed in micro-quasars. Strong shock oscillation induces strong episodic jet emission. The jets also show the existence of shocks, which are produced as one shell hits the preceding one. The periodicities of the jets and shock oscillation are similar; the jets for the higher viscosity parameter appear to be stronger and faster.

  8. SIMULATIONS OF VISCOUS ACCRETION FLOW AROUND BLACK HOLES IN A TWO-DIMENSIONAL CYLINDRICAL GEOMETRY

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

    Lee, Seong-Jae; Hyung, Siek; Chattopadhyay, Indranil

    2016-11-01

    We simulate shock-free and shocked viscous accretion flows onto a black hole in a two-dimensional cylindrical geometry, where initial conditions were chosen from analytical solutions. The simulation code used the Lagrangian total variation diminishing plus remap routine, which enabled us to attain high accuracy in capturing shocks and to handle the angular momentum distribution correctly. The inviscid shock-free accretion disk solution produced a thick disk structure, while the viscous shock-free solution attained a Bondi-like structure, but in either case, no jet activity nor any quasi-periodic oscillation (QPO)-like activity developed. The steady-state shocked solution in the inviscid as well as inmore » the viscous regime matched theoretical predictions well. However, increasing viscosity renders the accretion shock unstable. Large-amplitude shock oscillation is accompanied by intermittent, transient inner multiple shocks. This oscillation of the inner part of the disk is interpreted as the source of QPO in hard X-rays observed in micro-quasars. Strong shock oscillation induces strong episodic jet emission. The jets also show the existence of shocks, which are produced as one shell hits the preceding one. The periodicities of the jets and shock oscillation are similar; the jets for the higher viscosity parameter appear to be stronger and faster.« less

  9. Linear analysis on the growth of non-spherical perturbations in supersonic accretion flows

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

    Takahashi, Kazuya; Yamada, Shoichi, E-mail: ktakahashi@heap.phys.waseda.ac.jp

    We analyzed the growth of non-spherical perturbations in supersonic accretion flows. We have in mind an application to the post-bounce phase of core-collapse supernovae (CCSNe). Such non-spherical perturbations have been suggested by a series of papers by Arnett, who has numerically investigated violent convections in the outer layers of pre-collapse stars. Moreover, Couch and Ott demonstrated in their numerical simulations that such perturbations may lead to a successful supernova even for a progenitor that fails to explode without fluctuations. This study investigated the linear growth of perturbations during the infall onto a stalled shock wave. The linearized equations are solvedmore » as an initial and boundary value problem with the use of a Laplace transform. The background is a Bondi accretion flow whose parameters are chosen to mimic the 15 M {sub ☉} progenitor model by Woosley and Heger, which is supposed to be a typical progenitor of CCSNe. We found that the perturbations that are given at a large radius grow as they flow down to the shock radius; the density perturbations can be amplified by a factor of 30, for example. We analytically show that the growth rate is proportional to l, the index of the spherical harmonics. We also found that the perturbations oscillate in time with frequencies that are similar to those of the standing accretion shock instability. This may have an implication for shock revival in CCSNe, which will be investigated in our forthcoming paper in more detail.« less

  10. Minidisks in Binary Black Hole Accretion

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

    Ryan, Geoffrey; MacFadyen, Andrew, E-mail: gsr257@nyu.edu

    Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole (“minidisks”), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using 2D hydrodynamical simulations performed with a new general relativistic version of the moving-mesh code Disco. We introduce a comoving energy variable that enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the innermost stable circular orbit, providing a Reynolds stress thatmore » causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov–Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling. We find that the spiral shock structure is in agreement with the relativistic dispersion relation for tightly wound linear waves. We measure the shock-induced dissipation and find outward angular momentum transport corresponding to an effective alpha parameter of order 0.01. We perform ray-tracing image calculations from the simulations to produce theoretical minidisk spectra and viewing-angle-dependent images for comparison with observations.« less

  11. Radio Observations as a Tool to Investigate Shocks and Asymmetries in Accreting White Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Weston, Jennifer H. S.

    2016-07-01

    This dissertation uses radio observations with the Karl G. Jansky Very Large Array (VLA) to investigate the mechanisms that power and shape accreting white dwarfs (WD) and their ejecta. We test the predictions of both simple spherical and steady-state radio emission models by examining nova V1723 Aql, nova V5589 Sgr, symbiotic CH Cyg, and two small surveys of symbiotic binaries. First, we highlight classical nova V1723 Aql with three years of radio observations alongside optical and X-ray observations. We use these observations to show that multiple outflows from the system collided to create early non-thermal shocks with a brightness temperature of ≥106 K. While the late-time radio light curve is roughly consistent an expanding thermal shell of mass 2x10-4 M⊙ solar masses, resolved images of V1723 Aql show elongated material that apparently rotates its major axis over the course of 15 months, much like what is seen in gamma-ray producing nova V959 Mon, suggesting similar structures in the two systems. Next, we examine nova V5589 Sgr, where we find that the early radio emission is dominated by a shock-powered non-thermal flare that produces strong (kTx > 33 keV) X-rays. We additionally find roughly 10-5 M⊙ solar masses of thermal bremsstrahlung emitting material, all at a distance of ~4 kpc. The similarities in the evolution of both V1723 Aql and V5589 Sgr to that of nova V959 Mon suggest that these systems may all have dense equatorial tori shaping faster flows at their poles. Turning our focus to symbiotic binaries, we first use our radio observations of CH Cyg to link the ejection of a collimated jet to a change of state in the accretion disk. We additionally estimate the amount of mass ejected during this period (10-7 M⊙ masses), and improve measurements of the period of jet precession (P=12013 ± 74 days). We then use our survey of eleven accretion-driven symbiotic systems to determine that the radio brightness of a symbiotic system could potentially

  12. Modelling accretion disc and stellar wind interactions: the case of Sgr A.

    PubMed

    Christie, I M; Petropoulou, M; Mimica, P; Giannios, D

    2016-07-01

    Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ∼10 8  cm s -1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 10 33  erg s -1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of [Formula: see text], n d  = 10 5  cm -3 , and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ∼3000 gravitational radii from the supermassive black hole.

  13. Inclusion of TCAF model in XSPEC to study accretion flow dynamics around black hole candidates

    NASA Astrophysics Data System (ADS)

    Debnath, Dipak; Chakrabarti, Sandip Kumar; Mondal, Santanu

    Spectral and Temporal properties of black hole candidates can be well understood with the Chakrabarti-Titarchuk solution of two component advective flow (TCAF). This model requires two accretion rates, namely, the Keplerian disk accretion rate and the sub-Keplerian halo accretion rate, the latter being composed of a low angular momentum flow which may or may not develop a shock. In this solution, the relevant parameter is the relative importance of the halo (which creates the Compton cloud region) rate with respect to the Keplerian disk rate (soft photon source). Though this model has been used earlier to manually fit data of several black hole candidates quite satisfactorily, for the first time we are able to create a user friendly version by implementing additive Table model FITS file into GSFC/NASA's spectral analysis software package XSPEC. This enables any user to extract physical parameters of accretion flows, such as two accretion rates, shock location, shock strength etc. for any black hole candidate. Most importantly, unlike any other theoretical model, we show that TCAF is capable of predicting timing properties from spectral fits, since in TCAF, a shock is responsible for deciding spectral slopes as well as QPO frequencies.

  14. Numerical simulations of high-energy flows in accreting magnetic white dwarfs

    NASA Astrophysics Data System (ADS)

    Van Box Som, Lucile; Falize, É.; Bonnet-Bidaud, J.-M.; Mouchet, M.; Busschaert, C.; Ciardi, A.

    2018-01-01

    Some polars show quasi-periodic oscillations (QPOs) in their optical light curves that have been interpreted as the result of shock oscillations driven by the cooling instability. Although numerical simulations can recover this physics, they wrongly predict QPOs in the X-ray luminosity and have also failed to reproduce the observed frequencies, at least for the limited range of parameters explored so far. Given the uncertainties on the observed polar parameters, it is still unclear whether simulations can reproduce the observations. The aim of this work is to study QPOs covering all relevant polars showing QPOs. We perform numerical simulations including gravity, cyclotron and bremsstrahlung radiative losses, for a wide range of polar parameters, and compare our results with the astronomical data using synthetic X-ray and optical luminosities. We show that shock oscillations are the result of complex shock dynamics triggered by the interplay of two radiative instabilities. The secondary shock forms at the acoustic horizon in the post-shock region in agreement with our estimates from steady-state solutions. We also demonstrate that the secondary shock is essential to sustain the accretion shock oscillations at the average height predicted by our steady-state accretion model. Finally, in spite of the large explored parameter space, matching the observed QPO parameters requires a combination of parameters inconsistent with the observed ones. This difficulty highlights the limits of one-dimensional simulations, suggesting that multi-dimensional effects are needed to understand the non-linear dynamics of accretion columns in polars and the origins of QPOs.

  15. Properties of quasi-periodic oscillations in accreting magnetic white dwarfs

    NASA Technical Reports Server (NTRS)

    Wu, Kinwah; Chanmugam, G.; Shaviv, G.

    1992-01-01

    Previous studies of time-dependent accretion onto magnetic white dwarfs, in which the cooling was assumed to be due to bremsstrahlung emission, have shown that the accretion shock undergoes oscillations. However, when cyclotron cooling is also included, the oscillations are damped for sufficiently strong magnetic fields. Here we demonstrate that the oscillations can be sustained by accretion-fluctuation-induced excitations. The frequency of the QPOs are shown to increase quadratically with the magnetic field strength. We interpret the oscillations as a two-phase process in which bremsstrahlung cooling dominates in one half-cycle and cyclotron cooling in the other. Such a process may have very different consequences compared to a single-phase process where the functional form of the cooling is essentially the same throughout the cycle. If in the two-phase process damping occurs mainly in the cyclotron cooling half-cycle, there will be a universal effective damping factor which tends to suppress all oscillation modes indiscriminately. The oscillations of the accretion shock also could be a limit cycle process in which the system vacillates between two branches.

  16. Thermodynamics of giant planet formation: shocking hot surfaces on circumplanetary discs

    NASA Astrophysics Data System (ADS)

    Szulágyi, J.; Mordasini, C.

    2017-02-01

    The luminosity of young giant planets can inform about their formation and accretion history. The directly imaged planets detected so far are consistent with the `hot-start' scenario of high entropy and luminosity. If nebular gas passes through a shock front before being accreted into a protoplanet, the entropy can be substantially altered. To investigate this, we present high-resolution, three-dimensional radiative hydrodynamic simulations of accreting giant planets. The accreted gas is found to fall with supersonic speed in the gap from the circumstellar disc's upper layers on to the surface of the circumplanetary disc and polar region of the protoplanet. There it shocks, creating an extended hot supercritical shock surface. This shock front is optically thick; therefore, it can conceal the planet's intrinsic luminosity beneath. The gas in the vertical influx has high entropy which when passing through the shock front decreases significantly while the gas becomes part of the disc and protoplanet. This shows that circumplanetary discs play a key role in regulating a planet's thermodynamic state. Our simulations furthermore indicate that around the shock surface extended regions of atomic - sometimes ionized - hydrogen develop. Therefore, circumplanetary disc shock surfaces could influence significantly the observational appearance of forming gas giants.

  17. Possible Imprints of Cold-mode Accretion on the Present-day Properties of Disk Galaxies

    NASA Astrophysics Data System (ADS)

    Noguchi, Masafumi

    2018-01-01

    Recent theoretical studies suggest that a significant part of the primordial gas accretes onto forming galaxies as narrow filaments of cold gas without building a shock and experiencing heating. Using a simple model of disk galaxy evolution that combines the growth of dark matter halos predicted by cosmological simulations with a hypothetical form of cold-mode accretion, we investigate how this cold-accretion mode affects the formation process of disk galaxies. It is found that the shock-heating and cold-accretion models produce compatible results for low-mass galaxies owing to the short cooling timescale in such galaxies. However, cold accretion significantly alters the evolution of disk galaxies more massive than the Milky Way and puts observable fingerprints on their present properties. For a galaxy with a virial mass {M}{vir}=2.5× {10}12 {M}ȯ , the scale length of the stellar disk is larger by 41% in the cold-accretion model than in the shock-heating model, with the former model reproducing the steep rise in the size–mass relation observed at the high-mass end. Furthermore, the stellar component of massive galaxies becomes significantly redder (0.66 in u ‑ r at {M}{vir}=2.5× {10}12 {M}ȯ ), and the observed color–mass relation in nearby galaxies is qualitatively reproduced. These results suggest that large disk galaxies with red optical colors may be the product of cold-mode accretion. The essential role of cold accretion is to promote disk formation in the intermediate-evolution phase (0.5< z< 1.5) by providing the primordial gas having large angular momentum and to terminate late-epoch accretion, quenching star formation and making massive galaxies red.

  18. Spectral formation in a radiative shock: application to anomalous X-ray pulsars and soft gamma-ray repeaters

    NASA Astrophysics Data System (ADS)

    Kylafis, N. D.; Trümper, J. E.; Ertan, Ü.

    2014-02-01

    Context. In the fallback disk model for the persistent emission of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs), the hard X-ray emission arises from bulk- and thermal Comptonization of bremsstrahlung photons, which are generated in the accretion column. The relatively low X-ray luminosity of these sources implies a moderate transverse optical depth to electron scattering, with photons executing a small number of shock crossings before escaping sideways. Aims: We explore the range of spectral shapes that can be obtained with this model and characterize the most important parameter dependencies. Methods: We use a Monte Carlo code to study the crisscrossing of photons in a radiative shock in an accretion column and compute the resulting spectrum. Results: As expected, high-energy power-law X-ray spectra are produced in radiative shocks with photon-number spectral index Γ ≳ 0.5. We find that the required transverse optical depth is 1 ≲ τ⊥ ≲ 7. Such spectra are observed in low-luminosity X-ray pulsars. Conclusions: We demonstrate here with a simple model that Compton upscattering in the radiative shock in the accretion column can produce hard X-ray spectra similar to those seen in the persistent and transient emission of AXPs and SGRs. In particular, one can obtain a high-energy power-law spectrum, with photon-number spectral-index Γ ~ 1 and a cutoff at 100 - 200 keV, with a transverse Thomson optical depth of ~5, which is shown to be typical in AXPs/SGRs.

  19. Black Hole Disk Accretion in Supernovae

    NASA Astrophysics Data System (ADS)

    Nomura, H.; Mineshige, S.; Hirose, M.; Nomoto, K.; Suzuki, T.

    Hydrodynamical disk accretion flow onto a new-born black hole in a supernova is studied using the SPH (Smoothed Particle Hydrodynamics) method. It has been suggested that a mass of ~0.1Modot falls back to a black hole by a reverse shock. If the progenitor was rotating before the explosion, the accreting material should have a certain amount of angular momentum, thus forming an accretion disk. Disk material will eventually accrete towards the central object via viscosity with a supercritical accretion rate, dotM / dotMc > 106, for first several tens of days. (Here, dotMc 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 produced energy and photons are advected inward rather than being radiated away. Thus, the disk luminosity is much less than the Eddington luminosity (~1038erg s-1). The disk becomes hot and dense; for dotM / dotMc ~106 and the viscosity parameter alphavis ~0.01, for example, T ~109K and rho ~103gcm-3 in the vicinity of the central object. Efficient nucleosynthesis is hence expected even for reasonable viscosity magnitudes, although produced elements may be swallowed by the black hole.

  20. Numerical Treatment of Thin Accretion Disk Dynamics around Rotating Black Holes

    NASA Astrophysics Data System (ADS)

    Yildiran, Deniz; Donmez, Orhan

    In the present study, we perform the numerical simulation of a relativistic thin accretion disk around the nonrotating and rapidly rotating black holes using the general relativistic hydrodynamic code with Kerr in Kerr-Schild coordinate that describes the central rotating black hole. Since the high energy X-rays are produced close to the event horizon resulting the black hole-disk interaction, this interaction should be modeled in the relativistic region. We have set up two different initial conditions depending on the values of thermodynamical variables around the black hole. In the first setup, the computational domain is filled with constant parameters without injecting gas from the outer boundary. In the second, the computational domain is filled with the matter which is then injected from the outer boundary. The matter is assumed to be at rest far from the black hole. Both cases are modeled over a wide range of initial parameters such as the black hole angular momentum, adiabatic index, Mach number and asymptotic velocity of the fluid. It has been found that initial values and setups play an important role in determining the types of the shock cone and in designating the events on the accretion disk. The continuing injection from the outer boundary presents a tail shock to the steady state accretion disk. The opening angle of shock cone grows as long as the rotation parameter becomes larger. A more compressible fluid (bigger adiabatic index) also presents a bigger opening angle, a spherical shock around the rotating black hole, and less accumulated gas in the computational domain. While results from [J. A. Font, J. M. A. Ibanez and P. Papadopoulos, Mon. Not. R. Astron. Soc. 305 (1999) 920] indicate that the tail shock is warped around for the rotating hole, our study shows that it is the case not only for the warped tail shock but also for the spherical and elliptical shocks around the rotating black hole. The warping around the rotating black hole in our case

  1. How do accretion discs break?

    NASA Astrophysics Data System (ADS)

    Dogan, Suzan

    2016-07-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. In this study, we first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. We compare the disc precession torque with the disc viscous torque to determine whether the disc should warp or break. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. To check our analytical findings, we perform 3D hydrodynamical numerical simulations using the PHANTOM smoothed particle hydrodynamics code, and confirm that disc breaking is widespread and enhances accretion on to the central object. For some inclinations, the disc goes through strong Kozai cycles. Disc breaking promotes markedly enhanced and variable accretion and potentially produces high-energy particles or radiation through shocks. This would have significant implications for all binary systems: e.g. accretion outbursts in X-ray binaries and fuelling supermassive black hole (SMBH) binaries. The behaviour we have discussed in this work is relevant to a variety of astrophysical systems, for example X-ray binaries, where the disc plane may be tilted by radiation warping, SMBH binaries, where accretion of misaligned gas can create effectively random inclinations and protostellar binaries, where a disc may be misaligned by a variety of effects such as binary capture/exchange, accretion after binary formation.

  2. General relativistic hydrodynamics with Adaptive-Mesh Refinement (AMR) and modeling of accretion disks

    NASA Astrophysics Data System (ADS)

    Donmez, Orhan

    We present a general procedure to solve the General Relativistic Hydrodynamical (GRH) equations with Adaptive-Mesh Refinement (AMR) and model of an accretion disk around a black hole. To do this, the GRH equations are written in a conservative form to exploit their hyperbolic character. The numerical solutions of the general relativistic hydrodynamic equations is done by High Resolution Shock Capturing schemes (HRSC), specifically designed to solve non-linear hyperbolic systems of conservation laws. These schemes depend on the characteristic information of the system. We use Marquina fluxes with MUSCL left and right states to solve GRH equations. First, we carry out different test problems with uniform and AMR grids on the special relativistic hydrodynamics equations to verify the second order convergence of the code in 1D, 2 D and 3D. Second, we solve the GRH equations and use the general relativistic test problems to compare the numerical solutions with analytic ones. In order to this, we couple the flux part of general relativistic hydrodynamic equation with a source part using Strang splitting. The coupling of the GRH equations is carried out in a treatment which gives second order accurate solutions in space and time. The test problems examined include shock tubes, geodesic flows, and circular motion of particle around the black hole. Finally, we apply this code to the accretion disk problems around the black hole using the Schwarzschild metric at the background of the computational domain. We find spiral shocks on the accretion disk. They are observationally expected results. We also examine the star-disk interaction near a massive black hole. We find that when stars are grounded down or a hole is punched on the accretion disk, they create shock waves which destroy the accretion disk.

  3. Gas Accretion and Angular Momentum

    NASA Astrophysics Data System (ADS)

    Stewart, Kyle R.

    In this chapter, we review the role of gas accretion to the acquisition of angular momentum, both in galaxies and in their gaseous halos. We begin by discussing angular momentum in dark matter halos, with a brief review of tidal torque theory and the importance of mergers, followed by a discussion of the canonical picture of galaxy formation within this framework, where halo gas is presumed to shock-eat to the virial temperature of the halo, following the same spin distribution as the dark matter halo before cooling to the center of the halo to form a galaxy there. In the context of recent observational evidence demonstrating the presence of high angular momentum gas in galaxy halos, we review recent cosmological hydrodynamic simulations that have begun to emphasize the role of "cold flow" accretion—anisotropic gas accretion along cosmic filaments that does not shock-heat before sinking to the central galaxy. We discuss the implications of these simulations, reviewing a number of recent developments in the literature, and suggest a revision to the canonical model as it relates to the expected angular momentum content of gaseous halos around galaxies.

  4. Bimodal gas accretion in the Horizon-MareNostrum galaxy formation simulation

    NASA Astrophysics Data System (ADS)

    Ocvirk, P.; Pichon, C.; Teyssier, R.

    2008-11-01

    The physics of diffuse gas accretion and the properties of the cold and hot modes of accretion on to proto-galaxies between z = 2 and 5.4 is investigated using the large cosmological simulation performed with the RAMSES code on the MareNostrum supercomputing facility. Galactic winds, chemical enrichment, ultraviolet background heating and radiative cooling are taken into account in this very high resolution simulation. Using accretion-weighted temperature histograms, we have performed two different measurements of the thermal state of the gas accreted towards the central galaxy. The first measurement, performed using accretion-weighted histograms on a spherical surface of radius 0.2Rvir centred on the densest gas structure near the halo centre of mass, is a good indicator of the presence of an accretion shock in the vicinity of the galactic disc. We define the hot shock mass, Mshock, as the typical halo mass separating cold dominated from hot dominated accretion in the vicinity of the galaxy. The second measurement is performed by radially averaging histograms between 0.2Rvir and Rvir, in order to detect radially extended structures such as gas filaments: this is a good proxy for detecting cold streams feeding the central galaxy. We define Mstream as the transition mass separating cold dominated from hot dominated accretion in the outer halo, marking the disappearance of these cold streams. We find a hot shock transition mass of Mshock = 1011.6Msolar (dark matter), with no significant evolution with redshift. Conversely, we find that Mstream increases sharply with z. Our measurements are in agreement with the analytical predictions of Birnboim & Dekel and Dekel & Birnboim, if we correct their model by assuming low metallicity (<=10-3Zsolar) for the filaments, correspondingly to our measurements. Metal enrichment of the intergalactic medium is therefore a key ingredient in determining the transition mass from cold to hot dominated diffuse gas accretion. We find that

  5. Diaphragmless shock wave generators for industrial applications of shock waves

    NASA Astrophysics Data System (ADS)

    Hariharan, M. S.; Janardhanraj, S.; Saravanan, S.; Jagadeesh, G.

    2011-06-01

    The prime focus of this study is to design a 50 mm internal diameter diaphragmless shock tube that can be used in an industrial facility for repeated loading of shock waves. The instantaneous rise in pressure and temperature of a medium can be used in a variety of industrial applications. We designed, fabricated and tested three different shock wave generators of which one system employs a highly elastic rubber membrane and the other systems use a fast acting pneumatic valve instead of conventional metal diaphragms. The valve opening speed is obtained with the help of a high speed camera. For shock generation systems with a pneumatic cylinder, it ranges from 0.325 to 1.15 m/s while it is around 8.3 m/s for the rubber membrane. Experiments are conducted using the three diaphragmless systems and the results obtained are analyzed carefully to obtain a relation between the opening speed of the valve and the amount of gas that is actually utilized in the generation of the shock wave for each system. The rubber membrane is not suitable for industrial applications because it needs to be replaced regularly and cannot withstand high driver pressures. The maximum shock Mach number obtained using the new diaphragmless system that uses the pneumatic valve is 2.125 ± 0.2%. This system shows much promise for automation in an industrial environment.

  6. Shock Wave Technology and Application: An Update☆

    PubMed Central

    Rassweiler, Jens J.; Knoll, Thomas; Köhrmann, Kai-Uwe; McAteer, James A.; Lingeman, James E.; Cleveland, Robin O.; Bailey, Michael R.; Chaussy, Christian

    2012-01-01

    Context The introduction of new lithotripters has increased problems associated with shock wave application. Recent studies concerning mechanisms of stone disintegration, shock wave focusing, coupling, and application have appeared that may address some of these problems. Objective To present a consensus with respect to the physics and techniques used by urologists, physicists, and representatives of European lithotripter companies. Evidence acquisition We reviewed recent literature (PubMed, Embase, Medline) that focused on the physics of shock waves, theories of stone disintegration, and studies on optimising shock wave application. In addition, we used relevant information from a consensus meeting of the German Society of Shock Wave Lithotripsy. Evidence synthesis Besides established mechanisms describing initial fragmentation (tear and shear forces, spallation, cavitation, quasi-static squeezing), the model of dynamic squeezing offers new insight in stone comminution. Manufacturers have modified sources to either enlarge the focal zone or offer different focal sizes. The efficacy of extracorporeal shock wave lithotripsy (ESWL) can be increased by lowering the pulse rate to 60–80 shock waves/min and by ramping the shock wave energy. With the water cushion, the quality of coupling has become a critical factor that depends on the amount, viscosity, and temperature of the gel. Fluoroscopy time can be reduced by automated localisation or the use of optical and acoustic tracking systems. There is a trend towards larger focal zones and lower shock wave pressures. Conclusions New theories for stone disintegration favour the use of shock wave sources with larger focal zones. Use of slower pulse rates, ramping strategies, and adequate coupling of the shock wave head can significantly increase the efficacy and safety of ESWL. PMID:21354696

  7. Accretion Flows in Magnetic White Dwarf Systems

    NASA Technical Reports Server (NTRS)

    Imamura, James N.

    2005-01-01

    We received Type A and B funding under the NASA Astrophysics Data Program for the analysis and interpretation of hard x-ray data obtained by the Rossi X-ray Timing Explorer and other NASA sponsored missions for Intermediate Polars (IPS) and Polars. For some targets, optical data was available. We reduced and analyzed the X-ray spectra and the X-ray and optical (obtained at the Cerro Tololo Inter-American Observatory) timing data using detailed shock models (which we constructed) to place constraints on the properties of the accreting white dwarfs, the high energy emission mechanisms of white dwarfs, and the large-scale accretion flows of Polars and IPS. IPS and Polars are white dwarf mass-transfer binaries, members of the larger class of cata,clysmic variables. They differ from the bulk of the cataclysmic variables in that they contain strongly magnetic white dwarfs; the white dwarfs in Polars have B, = 7 to 230 MG and those in IPS have B, less than 10 MG. The IPS and Polars are both examples of funneled accretion flows in strong magnetic field systems. The IPS are similar to x-ray pulsars in that accretion disks form in the systems which are disrupted by the strong stellar magnetic fields of the white dwarfs near the stellar surface from where the plasma is funneled to the surface of the white dwarf. The localized hot spots formed at the footpoints of the funnels coupled with the rotation of the white dwarf leads to coherent pulsed x-ray emission. The Polars offer an example of a different accretion topology; the magnetic field of the white dwarf controls the accretion flow from near the inner Lagrangian point of the system directly to the stellar surface. Accretion disks do not form. The strong magnetic coupling generally leads to synchronous orbital/rotational motion in the Polars. The physical system in this sense resembles the Io/Jupiter system. In both IPS and Polars, pulsed emission from the infrared to x-rays is produced as the funneled flows merge onto the

  8. 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.

  9. 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.

  10. 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.

  11. 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

  12. 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

  13. 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.

  14. 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

  15. 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.

  16. Lessons from accretion disks in cataclysmic variables

    NASA Astrophysics Data System (ADS)

    Horne, Keith

    1998-04-01

    We survey recent progress in the interpretation of observations of cataclysmic variables, whose accretion disks are heated by viscous dissipation rather than irradiation. Many features of standard viscous accretion disk models are confirmed by tomographic imaging studies of dwarf novae. Eclipse maps indicate that steady disk temperature structures are established during outbursts. Doppler maps of double-peaked emission lines suggest disk chromospheres heated by magnetic activity. Gas streams impacting on the disk rim leave expected signatures both in the eclipses and emission lines. Doppler maps of dwarf nova IP Peg at the beginning of an outburst show evidence for tidally-induced spiral shocks. While enjoying these successes, we must still face up to the dreaded ``SW Sex syndrome'' which afflicts most if not all cataclysmic variables in high accretion states. The anomalies include single-peaked emission lines with skewed kinematics, flat temperature-radius profiles, shallow offset line eclipses, and narrow low-ionization absorption lines at phase 0.5. The enigmatic behavior of AE Aqr is now largely understood in terms of a magnetic propeller model in which the rapidly spinning white dwarf magnetosphere expels the gas stream out of the system before an accretion disk can form. A final piece in this puzzle is the realization that an internal shock zone occurs in the exit stream at just the right place to explain the anomalous kinematics and violent flaring of the single-peaked emission lines. Encouraged by this success, we propose that disk-anchored magnetic propellers operate in the high accretion rate systems afflicted by the SW Sex syndrome. Magnetic fields anchored in the Keplerian disk sweep forward and apply a boost that expels gas stream material flowing above the disk plane. This working hypothesis offers a framework on which we can hang all the SW Sex anomalies. The lesson for theorists is that magnetic links appear to be transporting energy and angular

  17. Mass-loss from advective accretion disc around rotating black holes

    NASA Astrophysics Data System (ADS)

    Aktar, Ramiz; Das, Santabrata; Nandi, Anuj

    2015-11-01

    We examine the properties of the outflowing matter from an advective accretion disc around a spinning black hole. During accretion, rotating matter experiences centrifugal pressure-supported shock transition that effectively produces a virtual barrier around the black hole in the form of post-shock corona (hereafter PSC). Due to shock compression, PSC becomes hot and dense that eventually deflects a part of the inflowing matter as bipolar outflows because of the presence of extra thermal gradient force. In our approach, we study the outflow properties in terms of the inflow parameters, namely specific energy (E) and specific angular momentum (λ) considering the realistic outflow geometry around the rotating black holes. We find that spin of the black hole (ak) plays an important role in deciding the outflow rate R_{dot{m}} (ratio of mass flux of outflow to inflow); in particular, R_{dot{m}} is directly correlated with ak for the same set of inflow parameters. It is found that a large range of the inflow parameters allows global accretion-ejection solutions, and the effective area of the parameter space (E, λ) with and without outflow decreases with black hole spin (ak). We compute the maximum outflow rate (R^{max}_{dot{m}}) as a function of black hole spin (ak) and observe that R^{max}_{dot{m}} weakly depends on ak that lies in the range ˜10-18 per cent of the inflow rate for the adiabatic index (γ) with 1.5 ≥ γ ≥ 4/3. We present the observational implication of our approach while studying the steady/persistent jet activities based on the accretion states of black holes. We discuss that our formalism seems to have the potential to explain the observed jet kinetic power for several Galactic black hole sources and active galactic nuclei.

  18. 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

  19. 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.

  20. Shock Waves for Possible Application in Regenerative Medicine

    NASA Astrophysics Data System (ADS)

    Hosseini, S. H. R.; Nejad, S. Moosavi; Akiyama, H.

    The paper reports experimental study of underwater shock waves effects in modification and possible control of embryonic stem cell differentiation and proliferation. The study is motivated by its application in regenerativemedicine. Underwater shock waves have been of interest for various scientific, industrial, and medical applications.

  1. 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).

  2. NEUTRINO-DRIVEN TURBULENT CONVECTION AND STANDING ACCRETION SHOCK INSTABILITY IN THREE-DIMENSIONAL CORE-COLLAPSE SUPERNOVAE

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

    Abdikamalov, Ernazar; Ott, Christian D.; Radice, David

    2015-07-20

    We conduct a series of numerical experiments into the nature of three-dimensional (3D) hydrodynamics in the postbounce stalled-shock phase of core-collapse supernovae using 3D general-relativistic hydrodynamic simulations of a 27 M{sub ⊙} progenitor star with a neutrino leakage/heating scheme. We vary the strength of neutrino heating and find three cases of 3D dynamics: (1) neutrino-driven convection, (2) initially neutrino-driven convection and subsequent development of the standing accretion shock instability (SASI), and (3) SASI-dominated evolution. This confirms previous 3D results of Hanke et al. and Couch and Connor. We carry out simulations with resolutions differing by up to a factor ofmore » ∼4 and demonstrate that low resolution is artificially favorable for explosion in the 3D convection-dominated case since it decreases the efficiency of energy transport to small scales. Low resolution results in higher radial convective fluxes of energy and enthalpy, more fully buoyant mass, and stronger neutrino heating. In the SASI-dominated case, lower resolution damps SASI oscillations. In the convection-dominated case, a quasi-stationary angular kinetic energy spectrum E(ℓ) develops in the heating layer. Like other 3D studies, we find E(ℓ) ∝ℓ{sup −1} in the “inertial range,” while theory and local simulations argue for E(ℓ) ∝ ℓ{sup −5/3}. We argue that current 3D simulations do not resolve the inertial range of turbulence and are affected by numerical viscosity up to the energy-containing scale, creating a “bottleneck” that prevents an efficient turbulent cascade.« less

  3. Active galactic nuclei. II - The acceleration of relativistic particles in a cluster of accreting black holes

    NASA Technical Reports Server (NTRS)

    Pacholczyk, A. G.; Stepinski, T. F.

    1988-01-01

    An accreting cluster of black holes in an active galactic nucleus is a natural site for a system of shock structures with a hierarchy of sizes, corresponding to the distribution of masses in the cluster. Accreted gas containing some magnetic fields and supersonically falling onto the core forms shocks on the outside of each hole and these shocks are capable of accelerating relativistic particles. The energies reached in a single shock are size rather than acceleration time limited and are proportional to the mass of the hole with a proportionality constant being a function of the position of the hole within a cluster and the model of the cluster and the shock formation. These energies are adequate to explain the observed properties of synchrotron and inverse-Compton radiation from these objects. The resulting energy spectrum of particles in the cluster in 'zeroth' approximation has the form of a doubly broken power law with indices of two and three on both extremes of the energy domain respectively, bridged by an index of about 2.5.

  4. Cosmic Rays and Non-thermal Emission Induced by Accretion of Cool Gas onto the Galactic Disk

    NASA Astrophysics Data System (ADS)

    Inoue, Susumu; Uchiyama, Yasunobu; Arakawa, Masanori; Renaud, Matthieu; Wada, Keiichi

    2017-11-01

    On both observational and theoretical grounds, the disk of our Galaxy should be accreting cool gas with temperature ≲ {10}5 K via the halo at a rate ˜1 {{M}⊙ {yr}}-1. At least some of this accretion is mediated by high-velocity clouds (HVCs), observed to be traveling in the halo with velocities of a few 100 km s-1 and occasionally impacting the disk at such velocities, especially in the outer regions of the Galaxy. We address the possibility of particle acceleration in shocks triggered by such HVC accretion events, and the detectability of consequent non-thermal emission in the radio to gamma-ray bands and high-energy neutrinos. For plausible shock velocities ˜ 300 {km} {{{s}}}-1 and magnetic field strengths ˜ 0.3{--}10 μ {{G}}, electrons and protons may be accelerated up to ˜1-10 TeV and ˜ 30{--}{10}3 TeV, respectively, in sufficiently strong adiabatic shocks during their lifetime of ˜ {10}6 {{yr}}. The resultant pion decay and inverse Compton gamma-rays may be the origin of some unidentified Galactic GeV-TeV sources, particularly the “dark” source HESS J1503-582 that is spatially coincident with the anomalous H I structure known as “forbidden-velocity wings.” Correlation of their locations with star-forming regions may be weak, absent, or even opposite. Non-thermal radio and X-ray emission from primary and/or secondary electrons may be detectable with deeper observations. The contribution of HVC accretion to Galactic cosmic rays is subdominant, but could be non-negligible in the outer Galaxy. As the thermal emission induced by HVC accretion is likely difficult to detect, observations of such phenomena may offer a unique perspective on probing gas accretion onto the Milky Way and other galaxies.

  5. Potential applications of low-energy shock waves in functional urology.

    PubMed

    Wang, Hung-Jen; Cheng, Jai-Hong; Chuang, Yao-Chi

    2017-08-01

    A shock wave, which carries energy and can propagate through a medium, is a type of continuous transmitted sonic wave with a frequency of 16 Hz-20 MHz. It is accompanied by processes involving rapid energy transformations. The energy associated with shock waves has been harnessed and used for various applications in medical science. High-energy extracorporeal shock wave therapy is the most successful application of shock waves, and has been used to disintegrate urolithiasis for 30 years. At lower energy levels, however, shock waves have enhanced expression of vascular endothelial growth factor, endothelial nitric oxide synthase, proliferating cell nuclear antigen, chemoattractant factors and recruitment of progenitor cells; shock waves have also improved tissue regeneration. Low-energy shock wave therapy has been used clinically with musculoskeletal disorders, ischemic cardiovascular disorders and erectile dysfunction, through the mechanisms of neovascularization, anti-inflammation and tissue regeneration. Furthermore, low-energy shock waves have been proposed to temporarily increase tissue permeability and facilitate intravesical drug delivery. The present review article provides information on the basics of shock wave physics, mechanisms of action on the biological system and potential applications in functional urology. © 2017 The Japanese Urological Association.

  6. Elliptical Accretion and Low Luminosity from High Accretion Rate Stellar Tidal Disruption Events

    NASA Astrophysics Data System (ADS)

    Svirski, Gilad; Piran, Tsvi; Krolik, Julian

    2017-05-01

    Models for tidal disruption events (TDEs) in which a supermassive black hole disrupts a star commonly assume that the highly eccentric streams of bound stellar debris promptly form a circular accretion disc at the pericentre scale. However, the bolometric peak luminosity of most TDE candidates, ˜ 1044 erg s- 1, implies that we observe only ˜1 per cent of the energy expected from radiatively efficient accretion. Even the energy that must be lost to circularize the returning tidal flow is larger than the observed energy. Recently, Piran et al. suggested that the observed optical TDE emission is powered by shocks at the apocentre between freshly infalling material and earlier arriving matter. This model explains the small radiated energy, the low temperature and the large radius implied by the observations as well as the t-5/3 light curve. However the question of the system's low bolometric efficiency remains unanswered. We suggest that the high orbital energy and low angular momentum of the flow make it possible for magnetic stresses to reduce the matter's already small angular momentum to the point at which it can fall ballistically into the supermassive black hole before circularization. As a result, the efficiency is only ˜1-10 per cent of a standard accretion disc's efficiency. Thus, the intrinsically high eccentricity of the tidal debris naturally explains why most TDE candidates are fainter than expected.

  7. 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.

  8. Accretion and differentiation of carbon in the early Earth.

    PubMed

    Tingle, T N

    1998-05-15

    The abundance of C in carbonaceous and ordinary chondrites decreases exponentially with increasing shock pressure as inferred from the petrologic shock classification of Scott et al. [Scott, E.R.D., Keil, K., Stoffler, D., 1992. Shock metamorphism of carbonaceous chondrites. Geochim. Cosmochim. Acta 56, 4281-4293] and Stoffler et al. [Stoffler, D., Keil, K., Scott, E.R.D., 1991. Shock metamorphism of ordinary chondrites. Geochim. Cosmochim. Acta 55, 3845-3867]. This confirms the experimental results of Tyburczy et al. [Tyburczy, J.A., Frisch, B., Ahrens, T.J., 1986. Shock-induced volatile loss from a carbonaceous chondrite: implications for planetary accretion. Earth Planet. Sci. Lett. 80, 201-207] on shock-induced devolatization of the Murchison meteorite showing that carbonaceous chondrites appear to be completely devolatilized at impact velocities greater than 2 km s-1. Both of these results suggest that C incorporation would have been most efficient in the early stages of accretion, and that the primordial C content of the Earth was between 10(24) and 10(25) g C (1-10% efficiency of incorporation). This estimate agrees well with the value of 3-7 x 10(24) g C based on the atmospheric abundance of 36Ar and the chondritic C/36Ar (Marty and Jambon, 1987). Several observations suggest that C likely was incorporated into the Earth's core during accretion. (1) Graphite and carbides are commonly present in iron meteorites, and those iron meteorites with Widmanstatten patterns reflecting the slowest cooling rates (mostly Group I and IIIb) contain the highest C abundances. The C abundance-cooling rate correlation is consistent with dissolution of C into Fe-Ni liquids that segregated to form the cores of the iron meteorite parent bodies. (2) The carbon isotopic composition of graphite in iron meteorites exhibits a uniform value of -5% [Deines, P., Wickman, F.E. 1973. The isotopic composition of 'graphitic' carbon from iron meteorites and some remarks on the troilitic

  9. On the area of accretion curtains from fast aperiodic time variability of the intermediate polar EX Hya

    NASA Astrophysics Data System (ADS)

    Semena, Andrey N.; Revnivtsev, Mikhail G.; Buckley, David A. H.; Kotze, Marissa M.; Khabibullin, Ildar I.; Breytenbach, Hannes; Gulbis, Amanda A. S.; Coppejans, Rocco; Potter, Stephen B.

    2014-08-01

    We present results of a study of the fast timing variability of the magnetic cataclysmic variable (mCV) EX Hya. It was previously shown that one may expect the rapid flux variability of mCVs to be smeared out at time-scales shorter than the cooling time of hot plasma in the post-shock region of the accretion curtain near the white dwarf (WD) surface. Estimates of the cooling time and the mass accretion rate, thus provide us with a tool to measure the density of the post-shock plasma and the cross-sectional area of the accretion funnel at the WD surface. We have probed the high frequencies in the aperiodic noise of one of the brightest mCV EX Hya with the help of optical telescopes, namely Southern African Large Telescope and the South African Astronomical Observatory 1.9 m telescope. We place upper limits on the plasma cooling time-scale τ < 0.3 s, on the fractional area of the accretion curtain footprint f < 1.6 × 10-4, and a lower limit on the specific mass accretion rate Ṁ/A>3 g s-1 cm-2. We show that measurements of accretion column footprints via eclipse mapping highly overestimate their areas. We deduce a value of Δr/r ≲ 10- 3 as an upper limit to the penetration depth of the accretion disc plasma at the boundary of the magnetosphere.

  10. 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

  11. Radio emission from Sgr A*: pulsar transits through the accretion disc

    NASA Astrophysics Data System (ADS)

    Christie, I. M.; Petropoulou, M.; Mimica, P.; Giannios, D.

    2017-06-01

    Radiatively inefficient accretion flow models have been shown to accurately account for the spectrum and luminosity observed from Sgr A* in the X-ray regime down to mm wavelengths. However, observations at a few GHz cannot be explained by thermal electrons alone but require the presence of an additional non-thermal particle population. Here, we propose a model for the origin of such a population in the accretion flow via means of a pulsar orbiting the supermassive black hole in our Galaxy. Interactions between the relativistic pulsar wind with the disc lead to the formation of a bow shock in the wind. During the pulsar's transit through the accretion disc, relativistic pairs, accelerated at the shock front, are injected into the disc. The radio-emitting particles are long lived and remain within the disc long after the pulsar's transit. Periodic pulsar transits through the disc result in regular injection episodes of non-thermal particles. We show that for a pulsar with spin-down luminosity Lsd ˜ 3 × 1035 erg s-1 and a wind Lorentz factor of γw ˜ 104 a quasi-steady synchrotron emission is established with luminosities in the 1-10 GHz range comparable to the observed one.

  12. 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) .

  13. Numerical Simulations of Wind Accretion in Symbiotic Binaries

    NASA Astrophysics Data System (ADS)

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

    2009-08-01

    About half of the binary systems are close enough to each other for mass to be exchanged between them at some point in their evolution, yet the accretion mechanism in wind accreting binaries is not well understood. We study the dynamical effects of gravitational focusing by a binary companion on winds from late-type stars. In particular, we investigate the mass transfer and formation of accretion disks around the secondary in detached systems consisting of an asymptotic giant branch (AGB) mass-losing star and an accreting companion. The presence of mass outflows is studied as a function of mass-loss rate, wind temperature, and binary orbital parameters. A two-dimensional hydrodynamical model is used to study the stability of mass transfer in wind accreting symbiotic binary systems. In our simulations we use an adiabatic equation of state and a modified version of the isothermal approximation, where the temperature depends on the distance from the mass losing star and its companion. The code uses a block-structured adaptive mesh refinement method that allows us to have high resolution at the position of the secondary and resolve the formation of bow shocks and accretion disks. We explore the accretion flow between the components and formation of accretion disks for a range of orbital separations and wind parameters. Our results show the formation of stream flow between the stars and accretion disks of various sizes for certain orbital configurations. For a typical slow and massive wind from an AGB star the flow pattern is similar to a Roche lobe overflow with accretion rates of 10% of the mass loss from the primary. Stable disks with exponentially decreasing density profiles and masses of the order 10-4 solar masses are formed when wind acceleration occurs at several stellar radii. The disks are geometrically thin with eccentric streamlines and close to Keplerian velocity profiles. The formation of tidal streams and accretion disks is found to be weakly dependent on

  14. Structure and Dynamics of the Accretion Process and Wind in TW Hya

    NASA Astrophysics Data System (ADS)

    Dupree, A. K.; Brickhouse, N. S.; Cranmer, S. R.; Berlind, P.; Strader, Jay; Smith, Graeme H.

    2014-07-01

    Time-domain spectroscopy of the classical accreting T Tauri star, TW Hya, covering a decade and spanning the far UV to the near-infrared spectral regions can identify the radiation sources, the atmospheric structure produced by accretion, and properties of the stellar wind. On timescales from days to years, substantial changes occur in emission line profiles and line strengths. Our extensive time-domain spectroscopy suggests 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 some absorption from infalling material. Stable absorption features appear in Hα, apparently caused by an accreting column silhouetted in the stellar wind. Inflow of material onto the star is revealed by the near-IR He I 10830 Å line, and its free-fall velocity correlates inversely with the strength of the post-shock emission, consistent with a dipole accretion model. However, the predictions of hydrogen line profiles based on accretion stream models are not well-matched by these observations. Evidence of an accelerating warm to hot stellar wind is shown by the near-IR He I line, and emission profiles of C II, C III, C IV, N V, and O VI. The outflow of material changes substantially in both speed and opacity in the yearly sampling of the near-IR He I line over a decade. Terminal outflow velocities that range from 200 km s-1 to almost 400 km s-1 in He I appear to be directly related to the amount of post-shock emission, giving evidence for an accretion-driven stellar wind. Calculations of the emission from realistic post-shock regions are needed. Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support

  15. 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.

  16. Shock vaporization and the accretion of the icy satellites of Jupiter and Saturn

    NASA Technical Reports Server (NTRS)

    Ahrens, T. J.; Okeefe, J. D.

    1984-01-01

    The role of impact vaporization acting during the formation of the Jovian and Saturnian satellites is examined in an attempt to explain the observed density in terms of composition of these rock and ice objects. A hypothesis is examined which states that the smaller satellites of Saturn having mean densities in the 1.1 to 1.4 Mg/cu m range represent primordial accreted planetesimal condensates formed in the proto-Jovian and Saturnian accretionary planetary discs. These densities are in the range expected for water-ice/silicate mixtures constrained in the solar values of O/Si and O/Mg atomic ratios. It is demonstrated that if the large satellites accreted from the same group of planetesimals which formed the small Saturnian satellites impact vaporization of water upon accretion in a porous regolith, at low H2O partial pressure, can account for the increase in mean planetesimal density from 1.6 Mg/cu m (43% H2O + 57% silicate) to a mean planetary density of 1.9 Mg/cu m for Ganymedean-sized water silicate objects. If impact volatilization of initially porous planetesimals is assumed, it can be demonstrated starting with planetesimals composed of 54% H2O and 40% silicate partial devolatilization upon accretion will yield a Ganymede-sized planet, having a radius of 2600 km and a density of 1.85 kg/cu m, similar to that of Ganymede, Callisto, and Titan.

  17. Influence of the geometric configuration of accretion flow on the black hole spin dependence of relativistic acoustic geometry

    NASA Astrophysics Data System (ADS)

    Tarafdar, Pratik; Das, Tapas K.

    Linear perturbation of general relativistic accretion of low angular momentum hydrodynamic fluid onto a Kerr black hole leads to the formation of curved acoustic geometry embedded within the background flow. Characteristic features of such sonic geometry depend on the black hole spin. Such dependence can be probed by studying the correlation of the acoustic surface gravity κ with the Kerr parameter a. The κ-a relationship further gets influenced by the geometric configuration of the accretion flow structure. In this work, such influence has been studied for multitransonic shocked accretion where linear perturbation of general relativistic flow profile leads to the formation of two analogue black hole-type horizons formed at the sonic points and one analogue white hole-type horizon which is formed at the shock location producing divergent acoustic surface gravity. Dependence of the κ-a relationship on the geometric configuration has also been studied for monotransonic accretion, over the entire span of the Kerr parameter including retrograde flow. For accreting astrophysical black holes, the present work thus investigates how the salient features of the embedded relativistic sonic geometry may be determined not only by the background spacetime, but also by the flow configuration of the embedding matter.

  18. Relativistic sonic geometry for isothermal accretion in the Kerr metric

    NASA Astrophysics Data System (ADS)

    Arif Shaikh, Md

    2018-03-01

    We linearly perturb advective isothermal transonic accretion onto rotating astrophysical black holes to study the emergence of the relativistic acoustic spacetime and to investigate how the salient features of this spacetime is influenced by the spin angular momentum of the black hole. We have perturbed three different quantities—the velocity potential, the mass accretion rate and the relativistic Bernoulli’s constant to show that the acoustic metric obtained for these three cases are the same up to a conformal factor. By constructing the required causal structures, it has been demonstrated that the acoustic black holes are formed at the transonic points of the flow and the acoustic white holes are formed at the shock location. The corresponding acoustic surface gravity has been computed in terms of the relevant accretion variables and the background metric elements. We have performed a linear stability analysis of the background stationary flow.

  19. Stationary radiation hydrodynamics of accreting magnetic white dwarfs.

    NASA Astrophysics Data System (ADS)

    Woelk, U.; Beuermann, K.

    1996-02-01

    Using an artificial viscosity, we solved the one-dimensional time-independent two-fluid hydrodynamic equations simultaneously to the fully frequency and angle dependent radiation transport in an accretion flow directed towards the surface of a magnetic white dwarf. We consider energy transfer from ions to electrons by Coulomb encounters and cooling by bremsstrahlung and by cyclotron radiation in fields between B=5 and 70MG. Electron and ion temperatures relax in the post-shock regime and the cooling flow settles onto the white dwarf surface. For high mass flow rates ˙(m) (in g/cm^2^/s), cooling takes place mainly by bremsstrahlung and the solutions approach the non-magnetic case. For low ˙(m) and high B, cooling is dominated by cyclotron radiation which causes the thickness of the cooling region to collapse by 1-2 orders of magnitude compared to the non-magnetic case. The electron temperature behind the shock drops from a few 10^8^ to a few 10^7^K and the ratio of cyclotron vs. total radiative flux approaches unity. For high ˙(m) and low B values, bremsstrahlung dominates, but cyclotron losses can never be neglected. We find a smooth transition from particle-heated to shock-heated atmospheres in the maximum electron temperature and also in the thickness of the heated layer. With these results, the stationary radiation-hydrodynamics of accreting magnetic white dwarfs with cyclotron and bremsstrahlung cooling has been solved for the whole range of observed mass flow rates and field strengths.

  20. 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.

  1. 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

  2. Automated Detection and Analysis of Interplanetary Shocks with Real-Time Application

    NASA Astrophysics Data System (ADS)

    Vorotnikov, V.; Smith, C. W.; Hu, Q.; Szabo, A.; Skoug, R. M.; Cohen, C. M.

    2006-12-01

    The ACE real-time data stream provides web-based now-casting capabilities for solar wind conditions upstream of Earth. Our goal is to provide an automated code that finds and analyzes interplanetary shocks as they occur for possible real-time application to space weather nowcasting. Shock analysis algorithms based on the Rankine-Hugoniot jump conditions exist and are in wide-spread use today for the interactive analysis of interplanetary shocks yielding parameters such as shock speed and propagation direction and shock strength in the form of compression ratios. Although these codes can be automated in a reasonable manner to yield solutions not far from those obtained by user-directed interactive analysis, event detection presents an added obstacle and the first step in a fully automated analysis. We present a fully automated Rankine-Hugoniot analysis code that can scan the ACE science data, find shock candidates, analyze the events, obtain solutions in good agreement with those derived from interactive applications, and dismiss false positive shock candidates on the basis of the conservation equations. The intent is to make this code available to NOAA for use in real-time space weather applications. The code has the added advantage of being able to scan spacecraft data sets to provide shock solutions for use outside real-time applications and can easily be applied to science-quality data sets from other missions. Use of the code for this purpose will also be explored.

  3. 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.

  4. Dehydration kinetics of shocked serpentine

    NASA Technical Reports Server (NTRS)

    Tyburczy, James A.; Ahrens, Thomas J.

    1988-01-01

    Experimental rates of dehydration of shocked and unshocked serpentine were determined using a differential scanning calorimetric technique. Dehydration rates in shocked serpentine are enhanced by orders of magnitude over corresponding rates in unshocked material, even though the impact experiments were carried out under conditions that inhibited direct impact-induced devolatilization. Extrapolation to temperatures of the Martian surface indicates that dehydration of shocked material would occur 20 to 30 orders of magnitude more rapidly than for unshocked serpentine. The results indicate that impacted planetary surfaces and associated atmospheres would reach chemical equilibrium much more quickly than calculations based on unshocked material would indicate, even during the earliest, coldest stages of accretion. Furthermore, it is suggested that chemical weathering of shocked planetary surfaces by solid-gas reactions would be sufficiently rapid that true equilibrium mineral assemblages should form.

  5. 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.

  6. X-ray and gamma-ray emission of Sagittarius A* as a wind-accreting black hole

    NASA Technical Reports Server (NTRS)

    Mastichiadis, A.; Ozernoy, L. M.

    1994-01-01

    If, as many believe, Sgr A* is a massive black hole at the Galactic center, one should expect it to be a source of X-ray and gamma-ray activity, behaving basically as a scaled-down active galactic nucleus. An unavoidable source of accretion is the wind from IRS 16, a nearby group of hot, massive stars. Since the density and velocity of the accreting matter are known from observations, the accretion rate is basically a function of the putative black hole mass, M(sub h), only; this value represents a reliable lower limit to a real rate, given the other possible sources of accreting matter. Based on this and on the theories about shock acceleration in active galactic nuclei, we have estimated the expected production of relativistic particles and their hard radiation. These values turn out to be a function of M(sub h) as well. Comparing our results with available X-ray and gamma-ray observations which show Sgr A* to have a relatively low activity level, we conclude tentatively that the putative black hole in the Galactic center cannot have a mass greater than approximately 6 x 10(exp 3) solar mass. This conclusion is consistent with the upper limits to the black hole mass found by different methods earlier, although much more work is needed to make calculations of shock acceleration around black holes more reliable.

  7. 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.

  8. Evolution of a steam atmosphere during earth's accretion

    NASA Astrophysics Data System (ADS)

    Zahnle, K. J.; Kasting, J. F.; Pollack, J. B.

    1988-04-01

    The evolution of an impact-generated steam atmosphere around an accreting earth is presently modeled under the assumption of Safronov (1978) accretion, in a scheme that encompasses the degassing of planetesimals on impact, thermal blanketing by the steam atmosphere, surface-to-interior water exchange, the shock heating and convective cooling of the earth's interior, and hydrogen escape due both to solar EUV-powered planetary wind and impact erosion. The model yields four distinct classes of impact-generated atmospheres: the first, on which emphasis is placed, has as its salient feature a molten surface that is maintained by the opacity of a massive water vapor atmosphere; the second occurs when the EUV-limited escape exceeds the impact degassing rate, while the third is dominated by impact erosion and the fourth is characterized by an atmosphere more massive than any thus far encountered.

  9. Evolution of a steam atmosphere during earth's accretion

    NASA Technical Reports Server (NTRS)

    Zahnle, Kevin J.; Kasting, James F.; Pollack, James B.

    1988-01-01

    The evolution of an impact-generated steam atmosphere around an accreting earth is presently modeled under the assumption of Safronov (1978) accretion, in a scheme that encompasses the degassing of planetesimals on impact, thermal blanketing by the steam atmosphere, surface-to-interior water exchange, the shock heating and convective cooling of the earth's interior, and hydrogen escape due both to solar EUV-powered planetary wind and impact erosion. The model yields four distinct classes of impact-generated atmospheres: the first, on which emphasis is placed, has as its salient feature a molten surface that is maintained by the opacity of a massive water vapor atmosphere; the second occurs when the EUV-limited escape exceeds the impact degassing rate, while the third is dominated by impact erosion and the fourth is characterized by an atmosphere more massive than any thus far encountered.

  10. Effects of Shocks on Emission from Central Engines of Active Galactic Nuclei. I

    NASA Technical Reports Server (NTRS)

    Sivron, R.; Caditz, D.; Tsuruta, S.

    1996-01-01

    In this paper we show that perturbations of the accretion flow within the central engines of some active galactic nuclei (AGNS) are likely to form shock waves in the accreting plasma. Such shocks, which may be either collisional or collisionless, can contribute to the observed high-energy temporal and spectral variability. Our rationale is the following: Observations show that the continuum emission probably originates in an optically thin, hot plasma in the AGN central engine. The flux and spectrum from this hot plasma varies significantly over light crossing timescales. Several authors have suggested that macroscopic perturbations contained within this plasma are the sources of this variability. In order to produce the observed emission the perturbations must be radiatively coupled with the optically thin hot matter and must also move with high velocities. We suggest that shocks, which can be very effective in randomizing the bulk motion of the perturbations, are responsible for this coupling. Shocks should form in the central engine, because the temperatures and magnetic fields are probably reduced below their virial values by radiative dissipation. Perturbations moving at Keplerian speeds, or strong non-linear excitations, result in supersonic and super-Alfvenic velocities leading to shock waves within the hot plasma. We show that even a perturbation smaller than the emitting region can form a shock that significantly modifies the continuum emission in an AGN, and that the spectral and temporal variability from such a shock generally resembles those of radio-quiet AGNS. As an example, the shock inducing perturbation in our model is a small main-sequence star, the capturing and eventual accretion of which are known to be a plausible process. We argue that shocks in the central engine may also provide a natural triggering mechanism for the "cold" component of Guilbert & Rees two-phase medium and an efficient mecha- nism for angular momentum transfer. Current and

  11. FeO and H-2O and the homogeneous accretion of the earth

    NASA Technical Reports Server (NTRS)

    Lange, M. A.; Ahrens, T. J.

    1983-01-01

    Shock devolatilization recovery data for brunite (Mg(OH)2) shocked to 13 and 23 GPa are presented. These data combined with previous data for serpentine (Mg3Si2O5(OH)4) are used to constrain the minimum size terrestrial planet for which planetesimal infall will result in an impact generated water atmosphere. Assuming, in hydrous phyllosilicates, model calculations simulating the interaction of metallic iron with impact released free water on the surface of the accreting Earth were carried out. It is assumed that the reaction of water with iron in the presence of enstatite is the prime source of the terrestrial FeO component of silicates and oxides. Lower and upper bounds on the terrestrial FeO budget are based on mantle FeO content and possible incorporation of FeO in the outer core. We demonstrate that the iron water reaction would result in the absence of atmospheric/hydrospheric water, if homogeneous accretion is assumed.

  12. FeO and H2O and the homogeneous accretion of the earth

    NASA Technical Reports Server (NTRS)

    Lange, M. A.; Ahrens, T. J.

    1984-01-01

    Shock devolatilization recovery data for brunite (Mg(OH)2) shocked to 13 and 23 GPa are presented. These data combined with previous data for serpentine (Mg3Si2O5(OH)4) are used to constrain the minimum size terrestrial planet for which planetesimal infall will result in an impact generated water atmosphere. Assuming, in hydrous phyllosilicates, model calculations simulating the interaction of metallic iron with impact released free water on the surface of the accreting earth were carried out. It is assumed that the reaction of water with iron in the presence of enstatite is the prime source of the terrestrial FeO component of silicates and oxides. Lower and upper bounds on the terrestrial FeO budget are based on mantle FeO content and possible incorporation of FeO in the outer core. We demonstrate that the iron water reaction would resuit in the absence of atmospheric/hydrospheric water, if homogeneous accretion is assumed.

  13. Low-radiative efficiency accretion: Microphysics and applications to low-luminosity AGN

    NASA Astrophysics Data System (ADS)

    Quataert, Eliot James Leo

    There is growing dynamical evidence that most nearby galaxies contain central ``massive dark objects,'' most likely supermassive black holes. Accretion onto a supermassive black hole may therefore be commonplace, and not just restricted to quasars and active galactic nuclei (AGN). This hypothesis is supported by observational surveys which show that the majority of nearby galaxies have nuclear emission properties reminiscent of AGN. Their emission-line and bolometric luminosities are, however, ~102 - 105 times smaller than typical AGN. In this thesis I explore several issues related to the physics of these low luminosity active galactic nuclei (LLAGN). In particular, it has been proposed that LLAGN are supermassive black holes accreting mass via a radiatively inefficient advection-dominated accretion flow, in which most of the energy dissipated by turbulence is carried with the gas through the event horizon rather than being radiated. This requires that turbulence dissipate most of its energy into the protons, rather than the electrons. I calculate the heating of electrons and protons by the collisionless dissipation of magneto-hydrodynamic turbulence and argue that preferential proton heating can only be achieved for relatively subthermal magnetic fields (roughly β >~ 10, where β is the average ratio of the gas pressure to the magnetic pressure in the accretion flow). For stronger, near equipartition, magnetic fields (β ~ 1), the electrons receive most of the turbulent energy. I give an independent argument, based on a fluid model for the radial evolution of the magnetic energy density in the accretion flow, that magnetic fields in advection- dominated accretion flows may be somewhat subthermal. An alternative explanation for LLAGN is that they accrete mass at very low rates. This is, however, inconsistent with accretion rate estimates (based on Bondi's method) in nearby massive elliptical galaxies and the center of our Galaxy. I give a detailed discussion of

  14. 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.

  15. An impact-induced terrestrial atmosphere and iron-water reactions during accretion of the Earth

    NASA Technical Reports Server (NTRS)

    Lange, M. A.; Ahrens, T. J.

    1985-01-01

    Shock wave data and theoretical calculations were used to derive models of an impact-generated terrestrial atmosphere during accretion of the Earth. The models showed that impacts of infalling planetesimals not only provided the entire budget of terrestrial water but also led to a continuous depletion of near-surface layers of water-bearing minerals of their structural water. This resulted in a final atmospheric water reservoir comparable to the present day total water budget of the Earth. The interaction of metallic iron with free water at the surface of the accreting Earth is considered. We carried out model calcualtions simulating these processes during accretion. It is assumed that these processes are the prime source of the terrestrial FeO component of silicates and oxides. It is demonstrated that the iron-water reaction would result in the absence of atmospheric/hydrospheric water, if homogeneous accretion is assumed. In order to obtain the necessary amount of terrestrial water, slightly heterogeneous accretion with initially 36 wt% iron planetesimals, as compared with a homogeneous value of 34 wt% is required.

  16. Mixed ice accretion on aircraft wings

    NASA Astrophysics Data System (ADS)

    Janjua, Zaid A.; Turnbull, Barbara; Hibberd, Stephen; Choi, Kwing-So

    2018-02-01

    Ice accretion is a problematic natural phenomenon that affects a wide range of engineering applications including power cables, radio masts, and wind turbines. Accretion on aircraft wings occurs when supercooled water droplets freeze instantaneously on impact to form rime ice or runback as water along the wing to form glaze ice. Most models to date have ignored the accretion of mixed ice, which is a combination of rime and glaze. A parameter we term the "freezing fraction" is defined as the fraction of a supercooled droplet that freezes on impact with the top surface of the accretion ice to explore the concept of mixed ice accretion. Additionally we consider different "packing densities" of rime ice, mimicking the different bulk rime densities observed in nature. Ice accretion is considered in four stages: rime, primary mixed, secondary mixed, and glaze ice. Predictions match with existing models and experimental data in the limiting rime and glaze cases. The mixed ice formulation however provides additional insight into the composition of the overall ice structure, which ultimately influences adhesion and ice thickness, and shows that for similar atmospheric parameter ranges, this simple mixed ice description leads to very different accretion rates. A simple one-dimensional energy balance was solved to show how this freezing fraction parameter increases with decrease in atmospheric temperature, with lower freezing fraction promoting glaze ice accretion.

  17. Shock-induced nanobubble collapse and its applications

    NASA Astrophysics Data System (ADS)

    Vedadi, Mohammad Hossein

    The shock-induced collapse of nanobubbles in water is investigated using molecular dynamics simulations based on a reactive force field. Monitoring the collapse of a cavitation nanobubble, we observe a focused nanojet at the onset of bubble shrinkage and a water hammer shock wave upon bubble collapse. The nanojet length scales linearly with the nanobubble radius, as observed in experiments on micron-to-millimeter size bubbles. The shock induces dramatic structural changes, including an ice-VII-like structural motif at a particle velocity of approximately 1 km/s. The incipient ice VII formation and the calculated Hugoniot curve are in good agreement with experimental results. Moreover, a substantial number of positive and negative ions appear when the nanojet hits the distal side of the nanobubble and the water hammer shock forms. Furthermore, two promising applications of shock-induced nanobubble collapse have been explored. Our simulations of poration in lipid bilayers due to shock-induced collapse of nanobubbles reveal penetration of nanojets into lipid bilayers. The nanojet impact generates shear flow of water on bilayer leaflets and pressure gradients across them, which transiently enhance the bilayer permeability by creating nanopores through which water molecules translocate across the bilayer. The effects of nanobubble size and temperature on the porosity of lipid bilayers are examined. Finally, the shock-induced collapse of CO2-filled nanobubbles in water is investigated. The energetic nanojet and high-pressure water hammer shock formed during and after collapse of the nanobubble trigger mechano-chemical H2O-CO2 reactions, some of which lead to splitting of water molecules. The dominant pathways through which splitting of water molecules occur are identified.

  18. Design of Experiments Relevant to Accreting Stream-Disk Impact in Interacting Binaries

    NASA Astrophysics Data System (ADS)

    Krauland, Christine; Drake, R. P.; Kuranz, C. C.; Grosskopf, M. J.; Young, R.; Plewa, T.

    2010-05-01

    In many Cataclysmic Binary systems, mass transfer via Roche lobe overflow onto an accretion disk occurs. This produces a hot spot from the heating created by the supersonic impact of the infalling flow with the rotating accretion disk, which can produce a radiative reverse shock in the infalling flow. This collision region has many ambiguities as a radiation hydrodynamic system. Depending upon conditions, it has been argued (Armitgae & Livio, ApJ 493, 898) that the shocked region may be optically thin, thick, or intermediate, which has the potential to significantly alter its structure and emissions. Laboratory experiments have yet to produce colliding flows that create a radiative reverse shock or to produce obliquely incident colliding flows, both of which are aspects of these Binary systems. We have undertaken the design of such an experiment, aimed at the Omega-60 laser facility. The design elements include the production of postshock flows within a dense material layer or ejecta flows by release of material from a shocked layer. Obtaining a radiative reverse shock in the laboratory requires producing a sufficiently fast flow (> 100 km/s) within a material whose opacity is large enough to produce energetically significant emission from experimentally achievable layers. In this poster we will discuss the astrophysical context, the experimental design work we have done, and the challenges of implementing and diagnosing an actual experiment. This work is funded by the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, by the National Laser User Facility Program in NNSA-DS and by the Predictive Sciences Academic Alliances Program in NNSA-ASC. The corresponding grant numbers are DE-FG52-09NA29548, DE-FG52-09NA29034, and DE-FC52-08NA28616.

  19. 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

  20. Shock Structure: Application to the heliospheric termination shock and an interstellar shock

    NASA Astrophysics Data System (ADS)

    Mostafavi, P.; Zank, G. P.

    2017-12-01

    The structure of parallel and perpendicular shocks is often mediated by energetic particles. Here we describe shock structure when mediated by energetic particle heat flux and viscosity. We present a general theoretical model of shock mediation, which is then applied to Voyager 2 observations of the heliospheric termination shock (HTS) and Voyage 1 observations of a shock in very local interstellar medium (VLISM). Voyager 2 observations showed that the downstream HTS flow remained supersonic with respect to the thermal gas [Richardson et al., 2008]. Thus the thermal gas remains cold through the HTS and does not provide the dissipation to account for the deceleration of the supersonic solar wind. We show that PUIs are the primary dissipation mechanism and gain most of the solar wind kinetic energy in crossing the HTS. The interstellar shock observed by Voyager 1 [Burlaga et al., 2013] was extremely broad and so far there no theoretical explanation has been provided that describes the VLISM shock structure. Using the Chandrasekhar function, we show that the VLISM is collisional with respect to the thermal plasma and that electron and proton collisional mean free paths are very small. Thus, thermal collisionality should determine the structure of VLISM shocks. PUIs outside the heliosphere are generated by secondary charge exchange and contribute a very small pressure. Since PUIs and the dissipation associated with them cannot mediate the shock observed in the VLISM, we suggest that the thickness of the shock observed in the VLISM is due to collisional thermal gas dissipation.

  1. Photon Bubbles and the Vertical Structure of Accretion Disks

    NASA Astrophysics Data System (ADS)

    Begelman, Mitchell C.

    2006-06-01

    We consider the effects of ``photon bubble'' shock trains on the vertical structure of radiation pressure-dominated accretion disks. These density inhomogeneities are expected to develop spontaneously in radiation-dominated accretion disks where magnetic pressure exceeds gas pressure, even in the presence of magnetorotational instability (MRI). They increase the rate at which radiation escapes from the disk and may allow disks to exceed the Eddington limit by a substantial factor without blowing themselves apart. To refine our earlier analysis of photon bubble transport in accretion disks, we generalize the theory of photon bubbles to include the effects of finite optical depths and radiation damping. Modifications to the diffusion law at low τ tend to ``fill in'' the low-density regions of photon bubbles, while radiation damping inhibits the formation of photon bubbles at large radii, small accretion rates, and small heights above the equatorial plane. Accretion disks dominated by photon bubble transport may reach luminosities from 10 to >100 times the Eddington limit (LEdd), depending on the mass of the central object, while remaining geometrically thin. However, photon bubble-dominated disks with α-viscosity are subject to the same thermal and viscous instabilities that plague standard radiation pressure-dominated disks, suggesting that they may be intrinsically unsteady. Photon bubbles can lead to a ``core-halo'' vertical disk structure. In super-Eddington disks the halo forms the base of a wind, which carries away substantial energy and mass, but not enough to prevent the luminosity from exceeding LEdd. Photon bubble-dominated disks may have smaller color corrections than standard accretion disks of the same luminosity. They remain viable contenders for some ultraluminous X-ray sources and may play a role in the rapid growth of supermassive black holes at high redshift.

  2. Self-similar dynamic converging shocks - I. An isothermal gas sphere with self-gravity

    NASA Astrophysics Data System (ADS)

    Lou, Yu-Qing; Shi, Chun-Hui

    2014-07-01

    We explore novel self-similar dynamic evolution of converging spherical shocks in a self-gravitating isothermal gas under conceivable astrophysical situations. The construction of such converging shocks involves a time-reversal operation on feasible flow profiles in self-similar expansion with a proper care for the increasing direction of the specific entropy. Pioneered by Guderley since 1942 but without self-gravity so far, self-similar converging shocks are important for implosion processes in aerodynamics, combustion, and inertial fusion. Self-gravity necessarily plays a key role for grossly spherical structures in very broad contexts of astrophysics and cosmology, such as planets, stars, molecular clouds (cores), compact objects, planetary nebulae, supernovae, gamma-ray bursts, supernova remnants, globular clusters, galactic bulges, elliptical galaxies, clusters of galaxies as well as relatively hollow cavity or bubble structures on diverse spatial and temporal scales. Large-scale dynamic flows associated with such quasi-spherical systems (including collapses, accretions, fall-backs, winds and outflows, explosions, etc.) in their initiation, formation, and evolution are likely encounter converging spherical shocks at times. Our formalism lays an important theoretical basis for pertinent astrophysical and cosmological applications of various converging shock solutions and for developing and calibrating numerical codes. As examples, we describe converging shock triggered star formation, supernova explosions, and void collapses.

  3. RADIO TRANSIENTS FROM ACCRETION-INDUCED COLLAPSE OF WHITE DWARFS

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

    Moriya, Takashi J., E-mail: takashi.moriya@nao.ac.jp

    2016-10-20

    We investigate observational properties of accretion-induced collapse (AIC) of white dwarfs (WDs) in radio frequencies. If AIC is triggered by accretion from a companion star, a dense circumstellar medium can be formed around the progenitor system. Then, the ejecta from AIC collide with the dense circumstellar medium, creating a strong shock. The strong shock can produce synchrotron emission that can be observed in radio frequencies. Even if AIC occurs as a result of WD mergers, we argue that AIC may cause fast radio bursts (FRBs) if a certain condition is satisfied. If AIC forms neutron stars (NSs) that are somore » massive that rotation is required to support themselves (i.e., supramassive NSs), the supramassive NSs may immediately lose their rotational energy by the r-mode instability and collapse to black holes. If the collapsing supramassive NSs are strongly magnetized, they may emit FRBs, as previously proposed. The AIC radio transients from single-degenerate systems may be detected in future radio transient surveys like the Very Large Array Sky Survey or the Square Kilometer Array transient survey. Because AIC has been proposed as a source of gravitational waves (GWs), GWs from AIC may be accompanied by radio-bright transients that can be used to confirm the AIC origin of observed GWs.« less

  4. Relativistic low angular momentum accretion: long time evolution of hydrodynamical inviscid flows

    NASA Astrophysics Data System (ADS)

    Mach, Patryk; Piróg, Michał; Font, José A.

    2018-05-01

    We investigate relativistic low angular momentum accretion of inviscid perfect fluid onto a Schwarzschild black hole. The simulations are performed with a general-relativistic, high-resolution (second-order), shock-capturing, hydrodynamical numerical code. We use horizon-penetrating Eddington–Finkelstein coordinates to remove inaccuracies in regions of strong gravity near the black hole horizon and show the expected convergence of the code with the Michel solution and stationary Fishbone–Moncrief toroids. We recover, in the framework of relativistic hydrodynamics, the qualitative behavior known from previous Newtonian studies that used a Bondi background flow in a pseudo-relativistic gravitational potential with a latitude-dependent angular momentum at the outer boundary. Our models exhibit characteristic ‘turbulent’ behavior and the attained accretion rates are lower than those of the Bondi–Michel radial flow. For sufficiently low values of the asymptotic sound speed, geometrically thick tori form in the equatorial plane surrounding the black hole horizon while accretion takes place mainly through the poles.

  5. 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.

  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. 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).

  8. The Role of the Outer Boundary Condition in Accretion Disk Models: Theory and Application

    NASA Astrophysics Data System (ADS)

    Yuan, Feng; Peng, Qiuhe; Lu, Ju-fu; Wang, Jianmin

    2000-07-01

    In a previous paper, we find that the outer boundary conditions (OBCs) of an optically thin accretion flow play an important role in determining the structure of the flow. Here in this paper, we further investigate the influence of OBCs on the dynamics and radiation of the accretion flow on a more detailed level. Bremsstrahlung and synchrotron radiations amplified by Comptonization are taken into account, and two-temperature plasma assumption is adopted. The three OBCs we adopted are the temperatures of the electrons and ions and the specific angular momentum of the accretion flow at a certain outer boundary. We investigate the individual role of each of the three OBCs on the dynamical structure and the emergent spectrum. We find that when the general parameters such as the mass accretion rate M and the viscous parameter α are fixed the peak flux at various bands such as radio, IR, and X-ray can differ by as much as several orders of magnitude under different OBCs in our example. Our results indicate that the OBC is both dynamically and radiatively important and therefore should be regarded as a new ``parameter'' in accretion disk models. As an illustrative example, we further apply the above results to the compact radio source Sgr A* located at the center of our Galaxy. The advection-dominated accretion flow (ADAF) model has turned out to be a great success in explaining its luminosity and spectrum. However, there exists a discrepancy between the mass accretion rate favored by ADAF models in the literature and that favored by the three-dimensional hydrodynamical simulation, with the former being 10-20 times smaller than the latter. By seriously considering the outer boundary condition of the accretion flow, we find that because of the low specific angular momentum of the accretion gas the accretion in Sgr A* should belong to a new accretion pattern, which is characterized by the possession of a very large sonic radius. This accretion pattern can significantly

  9. The Physics of Molecular Shocks in Star-Forming Regions

    NASA Technical Reports Server (NTRS)

    Hollenbach, David; Cuzzi, Jeffrey (Technical Monitor)

    1996-01-01

    Molecular shocks are produced by the impact of the supersonic infall of gas and dust onto protostars and by the interaction of the supersonic outflow from the protostar with the circumstellar material. Infalling gas creates an accretion shock around the circumstellar disk which emits a unique infrared spectrum and which processes the interstellar dust as it enters the disk. The winds and jets from protostars also impact the disk, the infalling material, and the ambient molecular cloud core creating shocks whose spectrum and morphology diagnose the mass loss processes of the protostar and the orientation and structure of the star forming system. We discuss the physics of these shocks, the model spectra derived from theoretical models, and comparisons with observations of H2O masers, H2 emission, as well as other shocks tracers. We show the strong effect of magnetic fields on molecular shock structure, and elucidate the chemical changes induced by the shock heating and compression.

  10. Shock Wave Propagation in Layered Planetary Interiors: Revisited

    NASA Astrophysics Data System (ADS)

    Arkani-Hamed, J.; Monteux, J.

    2017-12-01

    The end of the terrestrial planet accretion is characterized by numerous large impacts. About 90% of the mass of a large planet is accreted while the core mantle separation is occurring, because of the accretionary and the short-lived radio-isotope heating. The characteristics of the shockwave propagation, hence the existing scaling laws are poorly known within the layered planets. Here, we use iSALE-2D hydrocode simulations to calculate shock pressure in a differentiated Mars type body for impact velocities of 5-20 km/s, and impactor sizes of 100-400 km. We use two different rheologies for the target interior, an inviscid model ("no-stress model") and a pressure and damage-dependent strength model ("elaborated model"). To better characterize the shock pressure within the whole mantle as a function of distance from the impact site, we propose the following distribution: (1) a near field zone larger than the isobaric core that extends to 7-15 times the projectile radius into the target, where the peak shock pressure decays exponentially with increasing distance, (2) a far field zone where the pressure decays with distance following a power law. The shock pressure decreases more rapidly with distance in the near field for the elaborated model than for the no-stress model because of the influence of acoustic fluidization and damage. However to better illustrate the influence of the rheology on the shock propagation, we use the same expressions to fit the shock pressure with distance for both models. At the core-mantle boundary, CMB, the peak shock pressure jumps as the shock wave enters the core. We derived the boundary condition at CMB for the peak shock pressure. It is less sensitive to the impact velocity or the impactor size, but strongly depends on the rheology of the planet's mantle. Because of the lower shock wave velocity in the core compared to that in the mantle, the refracted shockwave propagates toward the symmetry axis of the planet, and the shock

  11. Hydrodynamic simulations of viscous accretion flows around black holes

    NASA Astrophysics Data System (ADS)

    Giri, Kinsuk; Chakrabarti, Sandip K.

    2012-03-01

    We study the time evolution of a rotating, axisymmetric, viscous accretion flow around black holes using a grid-based finite difference method. We use the Shakura-Sunyaev viscosity prescription. However, we compare with the results obtained when all the three independent components of the viscous stress are kept. We show that the centrifugal pressure supported shocks became weaker with the inclusion of viscosity. The shock is formed farther out when the viscosity is increased. When the viscosity is above a critical value, the shock disappears altogether and the flow becomes subsonic and Keplerian everywhere except in a region close to the horizon, where it remains supersonic. We also find that as the viscosity is increased, the amount of outflowing matter in the wind is decreased to less than a percentage of the inflow matter. Since the post-shock region could act as a reservoir of hot electrons or the so-called 'Compton cloud', the size of which changes with viscosity, the spectral properties are expected to depend on viscosity strongly: the harder states are dominated by low angular momentum and the low-viscosity flow with significant outflows while the softer states are dominated by the high-viscosity Keplerian flow having very few outflows.

  12. Fundamentals of collisionless shocks for astrophysical application, 2. Relativistic shocks

    NASA Astrophysics Data System (ADS)

    Bykov, A. M.; Treumann, R. A.

    2011-08-01

    In this concise review of the recent developments in relativistic shock theory in the Universe we restrict ourselves to shocks that do not exhibit quantum effects. On the other hand, emphasis is given to the formation of shocks under both non-magnetised and magnetised conditions. We only briefly discuss particle acceleration in relativistic shocks where much of the results are still preliminary. Analytical theory is rather limited in predicting the real shock structure. Kinetic instability theory is briefed including its predictions and limitations. A recent self-similar relativistic shock theory is described which predicts the average long-term shock behaviour to be magnetised and to cause reasonable power-law distributions for energetic particles. The main focus in this review is on numerical experiments on highly relativistic shocks in (i) pair and (ii) electron-nucleon plasmas and their limitations. These simulations do not validate all predictions of analytic and self-similar theory and so far they do not solve the injection problem and the self-modification by self-generated cosmic rays. The main results of the numerical experiments discussed in this review are: (i) a confirmation of shock evolution in non-magnetised relativistic plasma in 3D due to either the lepton-Weibel instability (in pair plasmas) or to the ion-Weibel instability; (ii) the sensitive dependence of shock formation on upstream magnetisation which causes suppression of Weibel modes for large upstream magnetisation ratios σ>10-3; (iii) the sensitive dependence of particle dynamics on the upstream magnetic inclination angle θ Bn , where particles of θ Bn >34° cannot escape upstream, leading to the distinction between `subluminal' and `superluminal' shocks; (iv) particles in ultra-relativistic shocks can hardly overturn the shock and escape to upstream; they may oscillate around the shock ramp for a long time, so to speak `surfing it' and thereby becoming accelerated by a kind of SDA; (v

  13. 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.

  14. Interactive navigation system for shock wave applications.

    PubMed

    Hagelauer, U; Russo, S; Gigliotti, S; de Durante, C; Corrado, E M

    2001-01-01

    The latest generation of shock wave lithotripters, with therapy heads mounted on articulated arms, have found widespread application in the treatment of orthopedic diseases. Currently, integration of an ultrasound probe in the therapy head is the dominant modality for positioning the shock wave focus on the treatment area. For orthopedic applications, however, X-ray imaging is often preferred. This article describes a new method to locate the therapy head of a lithotripter. In the first step, the surgeon positions the tissue to be treated at the isocenter of a C-arc. This is achieved using AP and 30-degree lateral projections, with corresponding horizontal and vertical movements of the patient under fluoroscopic guidance. These movements register the anatomic location in the coordinate system of the C-arc. In the second step, the therapy head is navigated to align the shock wave focus with the isocenter. Position data are reported from an optical tracker mounted on the X-ray system, which tracks an array of infrared LEDs on the therapy head. The accuracy of the tracking system was determined on a test bench, and was calculated to be 1.55 mm (RMS) for an angular movement of +/-15 degrees around a calibrated position. Free-hand navigation and precise alignment are performed with a single virtual reality display. The display is calculated by a computer system in real time, and uses graphical symbols to represent the shock wave path and isocenter. In an interactive process, the physician observes the display while navigating the therapy head towards the isocenter. Precise alignment is achieved by displaying an enlarged view of the intersecting graphical symbols. Results from the first tests on 100 patients demonstrate the feasibility of this approach in a clinical environment. Copyright 2001 Wiley-Liss, Inc.

  15. 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.

  16. Ion streaming instabilities with application to collisionless shock wave structure

    NASA Technical Reports Server (NTRS)

    Golden, K. I.; Linson, L. M.; Mani, S. A.

    1973-01-01

    The electromagnetic dispersion relation for two counterstreaming ion beams of arbitrary relative strength flowing parallel to a dc magnetic field is derived. The beams flow through a stationary electron background and the dispersion relation in the fluid approximation is unaffected by the electron thermal pressure. The dispersion relation is solved with a zero net current condition applied and the regions of instability in the k-U space (U is the relative velocity between the two ion beams) are presented. The parameters are then chosen to be applicable for parallel shocks. It was found that unstable waves with zero group velocity in the shock frame can exist near the leading edge of the shock for upstream Alfven Mach numbers greater than 5.5. It is suggested that this mechanism could generate sufficient turbulence within the shock layer to scatter the incoming ions and create the required dissipation for intermediate strength shocks.

  17. EVAPORATION OF ICY PLANETESIMALS DUE TO BOW SHOCKS

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

    Tanaka, Kyoko K.; Yamamoto, Tetsuo; Tanaka, Hidekazu

    2013-02-20

    We present the novel concept of evaporation of planetesimals as a result of bow shocks associated with planetesimals orbiting with supersonic velocities relative to the gas in a protoplanetary disk. We evaluate the evaporation rates of the planetesimals based on a simple model describing planetesimal heating and evaporation by the bow shock. We find that icy planetesimals with radius {approx}>100 km evaporate efficiently even outside the snow line in the stage of planetary oligarchic growth, where strong bow shocks are produced by gravitational perturbations from protoplanets. The obtained results suggest that the formation of gas giant planets is suppressed owingmore » to insufficient accretion of icy planetesimals onto the protoplanet within the {approx}<5 AU disk region.« less

  18. Scaling impact and shock-compression response for porous materials: Application to planetary formation

    NASA Astrophysics Data System (ADS)

    Jeanloz, R.

    2016-12-01

    A thermodynamic model based on the Mie-Grüneisen equation of state does a good job of describing the response of porous materials to impact, so can provide insights into the accretion and cohesion of planetesimals too small to be significantly held together by gravity (e.g., tens of km or less in average diameter). The model identifies an offset in Hugoniot pressure (ΔPH) due to porosity that is found to be in agreement with experimental shock-compression measurements for samples having a wide range of initial porosities. Assuming the Grüneisen parameter (γ) is proportional to volume (γ/V = constant), the relative offset in Hugoniot pressure as a function of initial porosity (φ = 1 - V0/V0por) and compression (η = 1 - V/V0) is ΔPH/PH = γ0 φ/[2(1 - φ) - γ0 (φ + η(1 - φ))] where subscripts 0 and por represent zero-pressure (non-porous) conditions and a porous sample, respectively. This additional thermal pressure at a given volume is due to the extra internal energy and corresponding temperature increase associated with collapsing pores (Fig. 1: near-identical curves for φ = 0.001 and 0.01). This result can be interpreted as indicating that upon collapse individual pores create hot spots with temperatures of order 103-104K above the background, suggesting that impact into an initially porous target can result in cohesion due to partial melting and vaporization. Moreover, the waste heat associated with pore closure far exceeds the dissipation in shock loading of non-porous material, reflecting the ability of a porous target to absorb and dissipate impact energy. The Mie-Grüneisen model along with analysis of waste heat thus provides a scaling for planetesimal impact that might explain how rock and regolith accrete into a gravitationally bound planet. Fig. 1. Porosity-induced anomaly in Hugoniot temperature per unit of porosity, shown as a function of compression for samples with initial porosity φ = 0.001 (green), 0.01 (blue) and 0.1 (gold) assuming

  19. THE NuSTAR X-RAY SPECTRUM OF HERCULES X-1: A RADIATION-DOMINATED RADIATIVE SHOCK

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

    Wolff, Michael T.; Wood, Kent S.; Becker, Peter A.

    2016-11-10

    We report on new spectral modeling of the accreting X-ray pulsar Hercules X-1. Our radiation-dominated radiative shock model is an implementation of the analytic work of Becker and Wolff on Comptonized accretion flows onto magnetic neutron stars. We obtain a good fit to the spin-phase-averaged 4–78 keV X-ray spectrum observed by the Nuclear Spectroscopic Telescope Array during a main-on phase of the Her X-1 35 day accretion disk precession period. This model allows us to estimate the accretion rate, the Comptonizing temperature of the radiating plasma, the radius of the magnetic polar cap, and the average scattering opacity parameters inmore » the accretion column. This is in contrast to previous phenomenological models that characterized the shape of the X-ray spectrum, but could not determine the physical parameters of the accretion flow. We describe the spectral fitting details and discuss the interpretation of the accretion flow physical parameters.« less

  20. Multi-scale simulations of black hole accretion in barred galaxies. Self-gravitating disk models

    NASA Astrophysics Data System (ADS)

    Jung, M.; Illenseer, T. F.; Duschl, W. J.

    2018-06-01

    Due to the non-axisymmetric potential of the central bar, in addition to their characteristic arms and bar, barred spiral galaxies form a variety of structures within the thin gas disk, such as nuclear rings, inner spirals, and dust lanes. These structures in the inner kiloparsec are extremely important in order to explain and understand the rate of black hole feeding. The aim of this work is to investigate the influence of stellar bars in spiral galaxies on the thin self-gravitating gas disk. We focus on the accretion of gas onto the central supermassive black hole and its time-dependent evolution. We conducted multi-scale simulations simultaneously resolving the galactic disk and the accretion disk around the central black hole. In all the simulations we varied the initial gas disk mass. As an additional parameter we chose either the gas temperature for isothermal simulations or the cooling timescale for non-isothermal simulations. Accretion was either driven by a gravitationally unstable or clumpy accretion disk or by energy dissipation in strong shocks. Most of the simulations show a strong dependence of the accretion rate at the outer boundary of the central accretion disk (r < 300 pc) on the gas flow at kiloparsec scales. The final black hole masses reach up to 109 M⊙ after 1.6 Gyr. Our models show the expected influence of the Eddington limit and a decline in growth rate at the corresponding sub-Eddington limit.

  1. The effect of accretion environment at large radius on hot accretion flows

    NASA Astrophysics Data System (ADS)

    Yang, Xiao-Hong; Bu, De-Fu

    2018-05-01

    We study the effects of accretion environment (gas density, temperature, and angular momentum) at large radii (˜10 pc) on luminosity of hot accretion flows. The radiative feedback effects from the accretion flow on the accretion environment are also self-consistently taken into account. We find that the slowly rotating flows at large radii can significantly deviate from Bondi accretion when radiation heating and cooling are considered. We further find that when the temperature of environment gas is low (e.g. T = 2 × 107 K), the luminosity of hot accretion flows is high. When the temperature of gas is high (e.g. T ≥ 4 × 107 K), the luminosity of hot accretion flow significantly deceases. The environment gas density can also significantly influence the luminosity of accretion flows. When density is higher than ˜4 × 10-22 g cm-3 and temperature is lower than 2 × 107 K, hot accretion flow with luminosity lower than 2 per cent LEdd is not present. Therefore, the parsec-scale environment density and temperature are two important parameters to determine the luminosity. The results are also useful for the subgrid models adopted by the cosmological simulations.

  2. To accrete or not accrete, that is the question

    USGS Publications Warehouse

    von Huene, Roland E.

    1986-01-01

    Along modern convergent margins tectonic processes span a spectrum from accretion to erosion. The process of accretion is generally recognized because it leaves a geologic record, whereas the process of erosion is generally hypothetical because it produces a geologic hiatus. Major conditions that determine the dominance of accretion or erosion at modern convergent margins are: 1) rate and direction of plate convergence, 2) sediment supply and type in the trench, and 3) topography of the subducting ocean floor. Most change in structure has been ascribed to plate motion, but both erosion and accretion are observed along the same convergence margin. Thus sediment supply and topography are probably of equivalent importance to plate motion because both erosion and accretion are observed under constant conditions of plate convergence. The dominance of accretion or erosion at a margin varies with the thickness of trench sediment. In a sediment flooded trench, the proportions of subducted and accreted sediment are commonly established by the position of a decollement along a weak horizon in the sediment section. Thus, the vertical variation of sediment strength and the distribution of horizontal stress are important factors. Once deformation begins, the original sediment strength is decreased by sediment remolding and where sediment thickens rapidly, increases in pore fluid pressure can be pronounced. In sediment-starved trenches, where the relief of the subducting ocean floor is not smoothed over, the front of the margin must respond to the topography subducted as well as that accreted. The hypothesized erosion by the drag of positive features against the underside of the upper plate (a high stress environment) may alternate with erosion due to the collapse of a margin front into voids such as graben (a low stress environment). ?? 1986 Ferdinand Enke Verlag Stuttgart.

  3. Principles and application of shock-tubes and shock tunnels

    NASA Technical Reports Server (NTRS)

    Ried, R. C.; Clauss, H. G., Jr.

    1963-01-01

    The principles, theoretical flow equations, calculation techniques, limitations and practical performance characteristics of basic and high performance shock tubes and shock tunnels are presented. Selected operating curves are included.

  4. 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.

  5. [Biochemical indicators of anaphylactic shock and the application in forensic medicine].

    PubMed

    Mi, Li; Chen, Jie; Gao, Wei-Min; Du, Zhong-Bo; Cao, Zhi-Peng; Zhang, Yuan; Zhu, Bao-Li

    2014-04-01

    Fatal anaphylactic shock is common in forensic practice. However, it is difficult to diagnose for lacking specific pathological and morphologic changes in forensic autopsy. The application of some biochemical indicators is of great significance. This paper reviews the biological characteristics of some biochemical indicators and detection methods. The forensic application, problems and prospects of these indicators are also introduced in details. The stable biochemical indicators, IgE, tryptase and chymase, show great potential and advantages in the identification of fatal anaphylactic shock in forensic medicine.

  6. Accreting CO material onto ONe white dwarfs towards accretion-induced collapse

    NASA Astrophysics Data System (ADS)

    Wu, Cheng-Yuan; Wang, Bo

    2018-03-01

    The final outcomes of accreting ONe white dwarfs (ONe WDs) have been studied for several decades, but there are still some issues that are not resolved. Recently, some studies suggested that the deflagration of oxygen would occur for accreting ONe WDs with Chandrasekhar masses. In this paper, we aim to investigate whether ONe WDs can experience accretion-induced collapse (AIC) or explosions when their masses approach the Chandrasekhar limit. Employing the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), we simulate the long-term evolution of ONe WDs with accreting CO material. The ONe WDs undergo weak multicycle carbon flashes during the mass-accretion process, leading to mass increase of the WDs. We found that different initial WD masses and mass-accretion rates influence the evolution of central density and temperature. However, the central temperature cannot reach the explosive oxygen ignition temperature due to neutrino cooling. This work implies that the final outcome of accreting ONe WDs is electron-capture induced collapse rather than thermonuclear explosion.

  7. Application of shock wave data to earth and planetary science

    NASA Technical Reports Server (NTRS)

    Ahrens, T. J.

    1985-01-01

    It is pointed out that shock wave data for: (1) low temperature condensable gases H2 and He, (2) H2O, CH4, NH3, CO, CO2, and N2 ices, and (3) silicates, metals, oxides and sulfides have many applications in geophysics and planetary science. The present paper is concerned with such applications. The composition of planetary interiors is discussed, taking into account the division of the major constituent of the planets in three groups on the basis of 'cosmic abundance' arguments, the H-He mixtures in the case of Jupiter and Saturn, shock wave data for hydrogen, and constraints on the internal structure of Uranus and Neptune. Attention is also given to the earth's mantle, shock wave data for mantle materials, the earth's core, impacts on planetary surfaces, elastic wave velocities as a function of pressure along the Hugoniot of iron, and reactions which yield the CO2 bearing atmospheres for Venus, earth, and Mars.

  8. Repetitive extracorporeal shock wave applications are superior in inducing angiogenesis after full thickness burn compared to single application.

    PubMed

    Goertz, O; von der Lohe, L; Lauer, H; Khosrawipour, T; Ring, A; Daigeler, A; Lehnhardt, M; Kolbenschlag, J

    2014-11-01

    Burn wounds remain a challenge due to subsequent wound infection and septicemia, which can be prevented by acceleration of wound healing. The aim of the study was to analyze microcirculation and leukocyte endothelium interaction with particular focus on angiogenesis after full-thickness burn using three different repetitions of low energy shock waves. Full-thickness burns were inflicted to the ears of hairless mice (n=44; area: 1.6±0.05 mm2 (mean±SEM)). Mice were randomized into four groups: the control group received a burn injury but no shock waves; group A received ESWA (0.03 mJ/mm2) on day one after burn injury; group B received shock waves on day one and day three after burn injury; group C ESWA on day one, three and seven after burn injury. Intravital fluorescent microscopy was used to assess microcirculatory parameters, angiogenesis and leukocyte interaction. Values were obtained before burn (baseline value) immediately after and on days 1, 3, 7 and 12 after burn. Shock-wave treated groups showed significantly accelerated angiogenesis compared to the control group. The non-perfused area (NPA) is regarded as a parameter for angiogenesis and showed the following data on day 12 2.7±0.4% (group A, p=0.001), 1.4±0.5% (group B, p<0.001), 1.0±0.3% (group C, p<0.001), 6.1±0.9% (control group). Edema formation is positively correlated with the number of shock wave applications: day 12: group A: 173.2±9.8%, group B: 184.2±6.6%, group C: 201.1±6.9%, p=0.009 vs. control: 162.3±8.7% (all data: mean±SEM). According to our data shock waves positively impact the wound healing process following burn injury. Angiogenesis showed significantly improved activity after shock wave application. In all three treatment groups angiogenesis was higher compared to the control group. Within the ESWA groups, double applications showed better results than single application and three applications showed better results than single or double applications. Copyright © 2014

  9. 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.

  10. Ion Viscosity Mediated by Tangled Magnetic Fields: An Application to Black Hole Accretion Disks

    NASA Technical Reports Server (NTRS)

    Subramanian, Prasad; Becker, Peter A.; Kafatos, Menas

    1996-01-01

    We examine the viscosity associated with the shear stress exerted by ions in the presence of a tangled magnetic field. As an application, we consider the effect of this mechanism on the structure of black hole accretion disks. We do not attempt to include a self-consistent description of the magnetic field. Instead, we assume the existence of a tangled field with coherence length lambda(sub coh), which is the average distance between the magnetic 'kinks' that scatter the particles. For simplicity, we assume that the field is self-similar, and take lambda(sub coh) to be a fixed fraction zeta of the local disk height H. Ion viscosity in the presence of magnetic fields is generally taken to be the cross-field viscosity, wherein the effective mean free path is the ion Larmor radius lambda(sub L), which is much less than the ion-ion Coulomb mean free path A(sub ii) in hot accretion disks. However, we arrive at a formulation for a 'hybrid' viscosity in which the tangled magnetic field acts as an intermediary in the transfer of momentum between different layers in the shear flow. The hybrid viscosity greatly exceeds the standard cross-field viscosity when (lambda/lambda(sub L)) much greater than (lambda(sub L)/lambda(sub ii)), where lambda = ((lambda(sub ii)(sup -1) + lambda(sub (coh)(sup -1))(sup -1) is the effective mean free path for the ions. This inequality is well satisfied in hot accretion disks, which suggests that the ions may play a much larger role in the momentum transfer process in the presence of magnetic fields than was previously thought. The effect of the hybrid viscosity on the structure of a steady-state, two-temperature, quasi-Keplerian accretion disk is analyzed. The hybrid viscosity is influenced by the degree to which the magnetic field is tangled (represented by zeta = lambda(sub coh)), and also by the relative accretion rate M/M(sub E), where M(sub E) = L(sub E)/c(sup 2) and L(sub E) is the Eddington luminosity. We find that ion viscosity in the

  11. Cooling of the magma ocean due to accretional disruption of the surface insulating layer

    NASA Technical Reports Server (NTRS)

    Sasaki, Sho

    1992-01-01

    Planetary accretion has been considered as a process to heat planets. Some fraction of the kinetic energy of incoming planetesimals is trapped to heat the planetary interior (Kaula, 1979; Davies, 1984). Moreover, blanketing effect of a primary atmosphere (Hayashi et al., 1979; Sasaki, 1990) or a degassed atmosphere (Abe and Matsui, 1986; Zahnle et al., 1988) would raise the surface temperature of the Earth-size planets to be higher than the melting temperature. The primordial magma ocean was likely to be formed during accretion of terrestrial planets. In the magma ocean, if crystallized fractions were heavier than melt, they would sink. But if solidified materials were lighter than the melt (like anorthosite of the lunar early crust) they would float to form a solid shell surrounding the planet. (In an icy satellite, solidified water ice should easily float on liquid water because of its small density.) The surface solid lid would prevent efficient convective heat transfer and slow the interior cooling. Consider that the accretion of planetesimals still continues in this cooling stage. Shock disruption at planetesimal impact events may destroy the solid insulating layer. Even if the layer survives impacts, the surface layer is finally overturned by Rayleigh-Taylor instability, since accreting materials containing metals are heavier than the surface solidified lid of silicates.

  12. 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.

  13. Hard X-Ray-emitting Black Hole Fed by Accretion of Low Angular Momentum Matter

    NASA Astrophysics Data System (ADS)

    Igumenshchev, Igor V.; Illarionov, Andrei F.; Abramowicz, Marek A.

    1999-05-01

    Observed spectra of active galactic nuclei and luminous X-ray binaries in our Galaxy suggest that both hot (~109 K) and cold (~106 K) plasma components exist close to the central accreting black hole. The hard X-ray component of the spectra is usually explained by Compton upscattering of optical/UV photons from optically thick cold plasma by hot electrons. Observations also indicate that some of these objects are quite efficient in converting gravitational energy of accretion matter into radiation. Existing theoretical models have difficulties in explaining the two plasma components and high intensity of hard X-rays. Most of the models assume that the hot component emerges from the cold one because of some kind of instability, but no one offers a satisfactory physical explanation for this. Here we propose a solution to these difficulties that reverses what was imagined previously: in our model, the hot component forms first and afterward it cools down to form the cold component. In our model, the accretion flow initially has a small angular momentum, and thus it has a quasi-spherical geometry at large radii. Close to the black hole, the accreting matter is heated up in shocks that form because of the action of the centrifugal force. The hot postshock matter is very efficiently cooled down by Comptonization of low-energy photons and condensates into a thin and cool accretion disk. The thin disk emits the low-energy photons which cool the hot component. All the properties of our model, in particular the existence of hot and cold components, follow from an exact numerical solution of standard hydrodynamical equations--we postulate no unknown processes operating in the flow. In contrast to the recently discussed advection-dominated accretion flow, the particular type of accretion flow considered in this Letter is both very hot and quite radiatively efficient.

  14. Scaling Law of Impact Induced Shock Pressure in Planetary Mantle

    NASA Astrophysics Data System (ADS)

    Monteux, Julien; Arkani-Hamed, Jafar

    2015-04-01

    While hydrocode simulation of impact induced shock pressure inside planetary mantle is more accurate, it is not suitable for studying several hundreds of impacts occurring during the accretion of a planet. Not only simulation of each impact takes over two orders of magnitude longer computer time than that of a scaling law simulation [1], but also it is cumbersome to apply for growing proto-planets where size of a proto-planet and impact velocities of the accreting bodies increase significantly. This is compounded by the formation of the iron core during the accretion with increasing size. Major impacting bodies during accretion of a Mars type planet have very low velocities. We use iSale hydrocode simulations and adopt physical properties of dunite for the mantle to calculate shock pressure and particle velocity in a Mars type body for 11 impact velocities ranging from 4 to 60 km/s. Large impactors of 100 to 1000 km in diameter, comparable to those impacted on Mars and created giant impact basins, are examined. The results are in good agreement with those of Pierazzo et al. [2] which were calculated for impact velocities higher than 10 km/s and impactor of 0.2 to 10 km in diameter. The internal consistency of our models indicates that our scaling laws are also accurate for lower impact velocities. We found no distinct isobaric region, rather the peak shock pressure changes relatively slowly versus distance from the impact site in the near field zone, within ~ 3 times the impactor radius, compare to that in the far field zone as also suggested by Ahrens and O'Keefe [3]. Hence we propose two distinct scaling laws, the power law distribution of shock pressure P as a function of distance R from the impact site at the surface, one for the near field zone and the other for the far field zone: Log P = a + n Log (R/Rimp) With n = 1.72 - 2.44 Log(Vimp) for R < ~3 Rimp, and n = -0.84 -0.51 Log(Vimp) for R > ~3 Rimp where a is a constant, Rimp is the impactor radius, and Vimp

  15. Proceedings of the 15th International Symposium on Shock Waves and Shock Tubes

    NASA Astrophysics Data System (ADS)

    Bershader, Daniel; Hanson, Ronald

    1986-09-01

    One hundred ten papers were presented in 32 sessions. Topics included: The application of Hook-method spectroscopy to the diagnosis of shock-heated gases. The nonintrusive destruction of kidney stones by underwater focused shock waves. Several of the papers reflect the recent and continuing interest in shock wave phenomena in dusty gases and other multiphase and heterogeneous systems, including chemically reactive configurations. The major subject areas were: shock propagation and interactions; shock-general chemical kinetics; shock computation, modeling, and stability problems; shock wave aerodynamics; experimental methods; shocks in multiphase and heterogeneous media; high energy gas excitation and wave phenomena; and technical applications and shocks in condensed matter.

  16. Automated shock detection and analysis algorithm for space weather application

    NASA Astrophysics Data System (ADS)

    Vorotnikov, Vasiliy S.; Smith, Charles W.; Hu, Qiang; Szabo, Adam; Skoug, Ruth M.; Cohen, Christina M. S.

    2008-03-01

    Space weather applications have grown steadily as real-time data have become increasingly available. Numerous industrial applications have arisen with safeguarding of the power distribution grids being a particular interest. NASA uses short-term and long-term space weather predictions in its launch facilities. Researchers studying ionospheric, auroral, and magnetospheric disturbances use real-time space weather services to determine launch times. Commercial airlines, communication companies, and the military use space weather measurements to manage their resources and activities. As the effects of solar transients upon the Earth's environment and society grow with the increasing complexity of technology, better tools are needed to monitor and evaluate the characteristics of the incoming disturbances. A need is for automated shock detection and analysis methods that are applicable to in situ measurements upstream of the Earth. Such tools can provide advance warning of approaching disturbances that have significant space weather impacts. Knowledge of the shock strength and speed can also provide insight into the nature of the approaching solar transient prior to arrival at the magnetopause. We report on efforts to develop a tool that can find and analyze shocks in interplanetary plasma data without operator intervention. This method will run with sufficient speed to be a practical space weather tool providing useful shock information within 1 min of having the necessary data to ground. The ability to run without human intervention frees space weather operators to perform other vital services. We describe ways of handling upstream data that minimize the frequency of false positive alerts while providing the most complete description of approaching disturbances that is reasonably possible.

  17. Shock-induced CO2 loss from CaCO3: Implications for early planetary atmospheres

    NASA Technical Reports Server (NTRS)

    Lange, M. A.; Ahrens, T. J.

    1984-01-01

    Recovered samples from shock recovery experiments on single crystal calcite were subjected to thermogravimetric analysis to determine the amount of post-shock CO2, the decarbonization interval and the activation energy, for the removal of remaining CO2 in shock-loaded calcite. Comparison of post-shock CO2 with that initially present determines shock-induced CO2 loss as a function of shock pressure. Incipient to complete CO2 loss occurs over a pressure range of approximately 10 to approximately 70 GPa. Optical and scanning electron microscopy reveal structural changes, which are related to the shock-loading. The occurrence of dark, diffuse areas, which can be resolved as highly vesicular areas as observed with a scanning electron microscope are interpreted as representing quenched partial melts, into which shock-released CO2 was injected. The experimental results are used to constrain models of shock-produced, primary CO2 atmospheres on the accreting terrestrial planets.

  18. Shock loading predictions from application of indicial theory to shock-turbulence interactions

    NASA Technical Reports Server (NTRS)

    Keefe, Laurence R.; Nixon, David

    1991-01-01

    A sequence of steps that permits prediction of some of the characteristics of the pressure field beneath a fluctuating shock wave from knowledge of the oncoming turbulent boundary layer is presented. The theory first predicts the power spectrum and pdf of the position and velocity of the shock wave, which are then used to obtain the shock frequency distribution, and the pdf of the pressure field, as a function of position within the interaction region. To test the validity of the crucial assumption of linearity, the indicial response of a normal shock is calculated from numerical simulation. This indicial response, after being fit by a simple relaxation model, is used to predict the shock position and velocity spectra, along with the shock passage frequency distribution. The low frequency portion of the shock spectra, where most of the energy is concentrated, is satisfactorily predicted by this method.

  19. Shock Driven Multiphase Instabilities in Scramjet Applications

    NASA Astrophysics Data System (ADS)

    McFarland, Jacob

    2016-11-01

    Shock driven multiphase instabilities (SDMI) arise in many applications from dust production in supernovae to ejecta distribution in explosions. At the limit of small, fast reacting particles the instability evolves similar to the Richtmyer-Meshkov (RM) instability. However, as additional particle effects such as lag, phase change, and collisions become significant the required parameter space becomes much larger and the instability deviates significantly from the RM instability. In scramjet engines the SDMI arises during a cold start where liquid fuel droplets are injected and processed by shock and expansion waves. In this case the particle evaporation and mixing is important to starting and sustaining combustion, but the particles are large and slow to react, creating significant multiphase effects. This talk will examine multiphase mixing in scramjet relevant conditions in 3D multiphase hydrodynamic simulations using the FLASH code from the University of Chicago FLASH center.

  20. On the wind production from hot accretion flows with different accretion rates

    NASA Astrophysics Data System (ADS)

    Bu, De-Fu; Gan, Zhao-Ming

    2018-02-01

    We perform two-dimensional simulations to study how the wind strength changes with accretion rate. We take into account bremsstrahlung, synchrotron radiation and the Comptonization. We find that when the accretion rate is low, radiative cooling is not important, and the accretion flow is hot. For the hot accretion flow, wind is very strong. The mass flux of wind can be ˜ 50 per cent of the mass inflow rate. When the accretion rate increases to a value at which radiative cooling rate is roughly equal to or slightly larger than viscous heating rate, cold clumps can form around the equatorial plane. In this case, the gas pressure gradient force is small and wind is very weak. Our results may be useful for the sub-grid model of active galactic nuclear feedback study.

  1. Laser shock wave and its applications

    NASA Astrophysics Data System (ADS)

    Yang, Chaojun; Zhang, Yongkang; Zhou, Jianzhong; Zhang, Fang; Feng, Aixin

    2007-12-01

    The technology of laser shock wave is used to not only surface modification but also metal forming. It can be divided into three parts: laser shock processing, laser shock forming (LSF) and laser peenforming(LPF). Laser shock processing as a surface treatment to metals can make engineering components have a residual compressive stress so that it obviously improves their fatigue strength and stress corrosion performances, while laser shock forming (LSF) is a novel technique that is used in plastic deformation of sheet metal recently and Laser peen forming (LPF) is another new sheet metal forming process presented in recent years. They all can be carried out by a high-power and repetition pulse Nd:Glass laser device made by Jiangsu University. Laser shock technology has characterized of ultrahigh pressure and high strain rate (10 6 - 10 7s -1). Now, for different materials, we are able to form different metals to contours and shapes and simultaneity leave their surfaces in crack-resistant compressive stress state. The results show that the technology of laser shock wave can strengthen surface property and prolong fatigue life and especially can deform metals to shapes that could not be adequately made using conventional methods. With the development of the technology of laser shock wave, the applied fields of laser will become greater and greater.

  2. Two examples of industrial applications of shock physics research

    NASA Astrophysics Data System (ADS)

    Sanai, Mohsen

    1996-05-01

    An in-depth understanding of shock physics phenomena has led to many industrial applications. Two recent applications discussed in this paper are a method for assessing explosion safety in industrial plants and a bomb-resistant luggage container for widebody aircraft. Our explosion safety assessment is based on frequent use of computer simulation of postulated accidents to model in detail the detonation of energetic materials, the formation and propagation of the resulting airblast, and the projection of fragments of known material and mass. Using a general load-damage analysis technique referred to as the pressure-impulse (PI) method, we have developed a PC-based computer algorithm that includes a continually expanding library of PI load and damage curves, which can predict and graphically display common structural damage modes and the response of humans to postulated explosion accidents. A second commercial application of shock physics discussed here is a bomb-resistant luggage container for widebody aircraft that can protect the aircraft from a terrorist bomb hidden inside the luggage. This hardened luggage container (HLC) relies on blast management and debris containment provided by a flexible flow-through blanket woven from threads made with a strong lightweight material, such as Spectra or Kevlar. This mitigation blanket forms a continuous and seamless shell around the sides of the luggage container that are parallel to the aircraft axis, leaving the two ends of the container unprotected. When an explosion occurs, the mitigation blanket expands into a nearly circular shell that contains the flying debris while directing the flow into the adjacent containers. The HLC concept has been demonstrated through full-scale experiments conducted at SRI. We believe that these two examples represent a broad class of potential industrial hazard applications of the experimental, analytical, and computational tools possessed by the shock physics community.

  3. 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.

  4. New Occurrence of Shocked Graphite Aggregates at Barringer Crater

    NASA Astrophysics Data System (ADS)

    Miura, Y.; Noma, Y.; Iancu, O. G.

    1993-07-01

    High-pressure carbon minera]s are considered to be formed by solid-solid transformation under static or impact high-pressure condition, but shocked quartz aggregates of impact craters are considered to be formed by quenched accretion of various aggregates by dynamic impact process [1-3]. The main purpose of this study is to elucidate new findings and occurrences of shocked graphite (SG) aggregates [2,3] at the Barringer meteorite crater. The graphite nodule block of Barringer Crater used in this study is collected near the rim. The sample is compared with standard graphite samples of Korea, Madagascar, and artificial impact graphites. There are four different mineral aggregates of the Barringer graphite nodule sample: (1) shocked graphite-1, (2) shocked graphite-2 and hexagonal diamond in the vein, (3) shocked quartz-1 (with kamacite) in the rim, and (4) calcite in the rim (Table 1). X-ray diffraction peaks of shocked graphite reveal low X-ray intensity, high Bragg-angle shift of X-ray diffraction peak, and multiple splitting of X-ray diffraction peaks. X-ray calculated density (rho) has been determined by X-ray diffractometer by the equation of density deviation Delta rho (%) = 100 x {(rho-rho(sub)0)/rho(sub)0}, where standard density rho(sub)0 is 2.255 g/cm^3 in Korean graphite [2,3]. The high-density value of shocked graphite grain obtained in Barringer is Delta rho = +0.6 +/- 0.1%. Shocked hexagonal diamonds (chaoite) show a high value of Delta rho = +0.6 +/- 0.9%. Analytical electron microscopy data reveal three different aggregates in the graphite nodule samples (Table 1): (1) shocked graphite-1 in the matrix, which contains uniformly Fe and Ca elements formed under gas state; (2) shocked graphite-2 in the vein, where crystallized shocked graphites and hexagonal diamonds are surrounded by kamacite-rich metals formed under gas-melt states of mixed compositions from iron meteorite and target rocks; and (3) shocked quartz-1 and kamacite in the rim, where

  5. Bondi or not Bondi: the impact of resolution on accretion and drag force modelling for supermassive black holes

    NASA Astrophysics Data System (ADS)

    Beckmann, R. S.; Slyz, A.; Devriendt, J.

    2018-07-01

    Whilst in galaxy-size simulations, supermassive black holes (SMBHs) are entirely handled by sub-grid algorithms, computational power now allows the accretion radius of such objects to be resolved in smaller scale simulations. In this paper, we investigate the impact of resolution on two commonly used SMBH sub-grid algorithms; the Bondi-Hoyle-Lyttleton (BHL) formula for accretion on to a point mass, and the related estimate of the drag force exerted on to a point mass by a gaseous medium. We find that when the accretion region around the black hole scales with resolution, and the BHL formula is evaluated using local mass-averaged quantities, the accretion algorithm smoothly transitions from the analytic BHL formula (at low resolution) to a supply-limited accretion scheme (at high resolution). However, when a similar procedure is employed to estimate the drag force, it can lead to significant errors in its magnitude, and/or apply this force in the wrong direction in highly resolved simulations. At high Mach numbers and for small accretors, we also find evidence of the advective-acoustic instability operating in the adiabatic case, and of an instability developing around the wake's stagnation point in the quasi-isothermal case. Moreover, at very high resolution, and Mach numbers above M_∞ ≥ 3, the flow behind the accretion bow shock becomes entirely dominated by these instabilities. As a result, accretion rates on to the black hole drop by about an order of magnitude in the adiabatic case, compared to the analytic BHL formula.

  6. Bondi or not Bondi: the impact of resolution on accretion and drag force modelling for Supermassive Black Holes

    NASA Astrophysics Data System (ADS)

    Beckmann, R. S.; Slyz, A.; Devriendt, J.

    2018-04-01

    Whilst in galaxy-size simulations, supermassive black holes (SMBH) are entirely handled by sub-grid algorithms, computational power now allows the accretion radius of such objects to be resolved in smaller scale simulations. In this paper, we investigate the impact of resolution on two commonly used SMBH sub-grid algorithms; the Bondi-Hoyle-Lyttleton (BHL) formula for accretion onto a point mass, and the related estimate of the drag force exerted onto a point mass by a gaseous medium. We find that when the accretion region around the black hole scales with resolution, and the BHL formula is evaluated using local mass-averaged quantities, the accretion algorithm smoothly transitions from the analytic BHL formula (at low resolution) to a supply limited accretion (SLA) scheme (at high resolution). However, when a similar procedure is employed to estimate the drag force it can lead to significant errors in its magnitude, and/or apply this force in the wrong direction in highly resolved simulations. At high Mach numbers and for small accretors, we also find evidence of the advective-acoustic instability operating in the adiabatic case, and of an instability developing around the wake's stagnation point in the quasi-isothermal case. Moreover, at very high resolution, and Mach numbers above M_∞ ≥ 3, the flow behind the accretion bow shock becomes entirely dominated by these instabilities. As a result, accretion rates onto the black hole drop by about an order of magnitude in the adiabatic case, compared to the analytic BHL formula.

  7. Conservative GRMHD simulations of moderately thin, tilted accretion disks

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

    Teixeira, Danilo Morales; Fragile, P. Chris; Zhuravlev, Viacheslav V.

    2014-12-01

    This paper presents our latest numerical simulations of accretion disks that are misaligned with respect to the rotation axis of a Kerr black hole. In this work, we use a new, fully conservative version of the Cosmos++ general relativistic magnetohydrodynamics (GRMHD) code, coupled with an ad hoc cooling function designed to control the thickness of the disk. Together these allow us to simulate the thinnest tilted accretion disks ever using a GRMHD code. In this way, we are able to probe the regime where the dimensionless stress and scale height of the disk become comparable. We present results for bothmore » prograde and retrograde cases. The simulated prograde tilted disk shows no sign of Bardeen-Petterson alignment even in the innermost parts of the disk. The simulated retrograde tilted disk, however, does show modest alignment. The implication of these results is that the parameter space associated with Bardeen-Petterson alignment for prograde disks may be rather small, only including very thin disks. Unlike our previous work, we find no evidence for standing shocks in our simulated tilted disks. We ascribe this to the black hole spin, tilt angle, and disk scale height all being small in these simulations. We also add to the growing body of literature pointing out that the turbulence driven by the magnetorotational instability in global simulations of accretion disks is not isotropic. Finally, we provide a comparison between our moderately thin, untilted reference simulation and other numerical simulations of thin disks in the literature.« less

  8. Simulating Shock Triggered Star Formation with AstroBEAR2.0

    NASA Astrophysics Data System (ADS)

    Li, Shule; Frank, Adam; Blackman, Eric

    2013-07-01

    Star formation can be triggered by the compression from shocks running over stable clouds. Triggered star formation is a favored explanation for the traces of SLRI's in our solar system. Previous research has shown that when parameters such as shock speed are within a certain range, the gravitational collapse of otherwise stable, dense cloud cores is possible. However, these studies usually focus on the precursors of star formation, and the conditions for the triggering. We use AstroBEAR2.0 code to simulate the collapse and subsequent evolution of a stable Bonnor-Ebert cloud by an incoming shock. Through our simulations, we show that interesting physics happens when the newly formed star interacts with the cloud residue and the post-shock flow. We identify these interactions as controlled by the initial conditions of the triggering and study the flow pattern as well as the evolution of important physics quantities such as accretion rate and angular momentum.

  9. Incompressible Wind Accretion

    NASA Astrophysics Data System (ADS)

    Tejeda, E.

    2018-04-01

    We present a simple, analytic model of an incompressible fluid accreting onto a moving gravitating object. This solution allows us to probe the highly subsonic regime of wind accretion. Moreover, it corresponds to the Newtonian limit of a previously known relativistic model of a stiff fluid accreting onto a black hole. Besides filling this blank in the literature, the new solution should be useful as a benchmark test for numerical hydrodynamics codes. Given its simplicity, it can also be used as an illustrative example in a gas dynamics course.

  10. General relativistic study of astrophysical jets with internal shocks

    NASA Astrophysics Data System (ADS)

    Vyas, Mukesh K.; Chattopadhyay, Indranil

    2017-08-01

    We explore the possibility of the formation of steady internal shocks in jets around black holes. We consider a fluid described by a relativistic equation of state, flowing about the axis of symmetry (θ = 0) in a Schwarzschild metric. We use two models for the jet geometry: (I) a conical geometry and (II) a geometry with non-conical cross-section. A jet with conical geometry has a smooth flow, while the jet with non-conical cross-section undergoes multiple sonic points and even standing shock. The jet shock becomes stronger, as the shock location is situated farther from the central black hole. Jets with very high energy and very low energy do not harbour shocks, but jets with intermediate energies do harbour shocks. One advantage of these shocks, as opposed to shocks mediated by external medium, is that these shocks have no effect on the jet terminal speed, but may act as possible sites for particle acceleration. Typically, a jet with specific energy 1.8c2 will achieve a terminal speed of v∞ = 0.813c for jet with any geometry, where, c is the speed of light in vacuum. But for a jet of non-conical cross-section for which the length scale of the inner torus of the accretion disc is 40rg, then, in addition, a steady shock will form at rsh ˜ 7.5rg and compression ratio of R ˜ 2.7. Moreover, electron-proton jet seems to harbour the strongest shock. We will discuss possible consequences of such a scenario.

  11. Non-thermal plasma jet without electrical shock for biomedical applications

    NASA Astrophysics Data System (ADS)

    Baik, Ku Youn; Kang, Han Lim; Kim, Junseong; Park, Shin Young; Bang, Ji Yun; Uhm, Han S.; Choi, Eun Ha; Cho, Guangsup

    2013-10-01

    A plasma jet without an electrical shock was generated through a Y-shaped tube in which voltages with opposite phases were applied to a pair of tubes. The plasma plume generated at the intersection had a plasma potential of a 60-90 V and high concentrations of reactive species sufficient to induce a high level of lethality on gram-negative bacteria on a tissue mimic. The selective lethality of bacteria on an epithelial-cell-containing tissue mimic could be modulated using oxidant and antioxidant chemicals, thereby leading to the possibility of a shock-reduced plasma jet for biomedical applications.

  12. Observations of accreting pulsars

    NASA Technical Reports Server (NTRS)

    Prince, Thomas A.; Bildsten, Lars; Chakrabarty, Deepto; Wilson, Robert B.; Finger, Mark H.

    1994-01-01

    We discuss recent observations of accreting binary pulsars with the all-sky BATSE instrument on the Compton Gamma Ray Observatory. BATSE has detected and studied nearly half of the known accreting pulsar systems. Continuous timing studies over a two-year period have yielded accurate orbital parameters for 9 of these systems, as well as new insights into long-term accretion torque histories.

  13. 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

  14. SIMULATIONS OF THE SYMBIOTIC RECURRENT NOVA V407 CYG. I. ACCRETION AND SHOCK EVOLUTIONS

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

    Pan, Kuo-Chuan; Ricker, Paul M.; Taam, Ronald E., E-mail: kuo-chuan.pan@unibas.ch, E-mail: pmricker@illinois.edu, E-mail: r-taam@northwestern.edu, E-mail: taam@asiaa.sinica.edu.tw

    2015-06-10

    The shock interaction and evolution of nova ejecta with wind from a red giant (RG) star in a symbiotic binary system are investigated via three-dimensional hydrodynamics simulations. We specifically model the 2010 March outburst of the symbiotic recurrent nova V407 Cygni from its quiescent phase to its eruption phase. The circumstellar density enhancement due to wind–white-dwarf interaction is studied in detail. It is found that the density-enhancement efficiency depends on the ratio of the orbital speed to the RG wind speed. Unlike another recurrent nova, RS Ophiuchi, we do not observe a strong disk-like density enhancement, but instead observe anmore » aspherical density distribution with ∼20% higher density in the equatorial plane than at the poles. To model the 2010 outburst, we consider several physical parameters, including the RG mass-loss rate, nova eruption energy, and ejecta mass. A detailed study of the shock interaction and evolution reveals that the interaction of shocks with the RG wind generates strong Rayleigh–Taylor instabilities. In addition, the presence of the companion and circumstellar density enhancement greatly alter the shock evolution during the nova phase. Depending on the model, the ejecta speed after sweeping out most of the circumstellar medium decreases to ∼100–300 km s{sup −1}, which is consistent with the observed extended redward emission in [N ii] lines in 2011 April.« less

  15. Corona accretion in active galactic nuclei and the observational test

    NASA Astrophysics Data System (ADS)

    Qiao, E.; Liu, B.; Taam, R.; Yuan, W.

    2017-10-01

    In this talk, we propose a new accretion model, in which the matter is accreted initially in the form of a vertically extended, hot gas (corona) to the central supermassive black hole by capturing the interstellar medium or the stellar wind in active galactic nuclei (AGNs). In this scenario, when the initial mass accretion rate is greater than about 0.01 \\dot M_{Edd}, at a critical radius r_{d}, part of the hot gas begins to condense on to the equatorial disc plane of the black hole, forming an inner cold accretion disc. Then, the matter is accreted in the form of a disc-corona structure extending down to the ISCO of the black hole. We calculate the theoretical structure and the corresponding emergent spectra of the model. It is shown that the model can naturally explain the origin of the X-ray emission in AGNs. Meanwhile the model predicts a new geometry of the accretion flow, which can very well explain some observations, such as the correlation between the hard X-ray slope Γ and the reflection scaling factor R found in AGNs. Finally, we discuss the potential applications of the model to high mass X-ray binaries.

  16. Gas-grain energy transfer in solar nebula shock waves: Implications for the origin of chondrules

    NASA Technical Reports Server (NTRS)

    Hood, L. L.; Horanyi, M.

    1993-01-01

    Meteoritic chondrules provide evidence for the occurrence of rapid transient heating events in the protoplanetary nebula. Astronomical evidence suggests that gas dynamic shock waves are likely to be excited in protostellar accretion disks by processes such as protosolar mass ejections, nonaxisymmetric structures in an evolving disk, and impact on the nebula surface of infalling 'clumps' of circumstellar gas. Previous detailed calculations of gas-grain energy and momentum transfer have supported the possibility that such shock waves could have melted pre-existing chondrule-sized grains. The main requirement for grains to reach melting temperatures in shock waves with plausibly low Mach numbers is that grains existed in dust-rich zones (optical depth greater than 1) where radiative cooling of a given grain can be nearly balanced by radiation from surrounding grains. Localized dust-rich zones also provide a means of explaining the apparent small spatial scale of heating events. For example, the scale size of at least some optically thick dust-rich zones must have been relatively small (less than 10 kilometers) to be consistent with petrologic evidence for accretion of hot material onto cold chondrules. The implied number density of mm-sized grains for these zones would be greater than 30 m(exp -3). In this paper, we make several improvements of our earlier calculations to include radiation self-consistently in the shock jump conditions, and we include heating of grains due to radiation from the shocked gas. In addition, we estimate the importance of momentum feedback of dust concentrations onto the shocked gas which would tend to reduce the efficiency of gas dynamic heating of grains in the center of the dust cloud.

  17. New trends in laser shock wave physics and applications

    NASA Astrophysics Data System (ADS)

    Peyre, Patrice; Carboni, Christelle; Sollier, Arnault; Berthe, Laurent; Richard, Caroline; de Los Rios, E.; Fabbro, Remy

    2002-09-01

    Recent applications for laser-induced shock waves have been demonstrated in the aeronautical and nuclear industries, due to the development of new generations of lasers that enable high cadency rates with rather small designs. In this paper, we first aim at making an overview on basic physical processes involved in Laser Shock Processing, and a presentation of pressure loadings generated by different laser conditions. In a second part, a specific focus is given to new ranges of applications like wear resistance, uniform and localized corrosion or modeling of fatigue behaviour after LSP. For instance it is demonstrated that the pitting corrosion behaviour of 316L steel in saline medium can be improved by laser-induced pure mechanical effects surrounding inclusions. It is also shown that wear rates of a 100Cr6 tool steel can be reduced after LSP provided applied pressures are kept below a material deposit threshold. Last but not least, the fatigue cracking behaviour of 2024-T351 aluminum alloy after LSP was improved and calculated through a computed program taking into account work hardening together with residual stress effects.

  18. The Emerging Paradigm of Pebble Accretion

    NASA Astrophysics Data System (ADS)

    Ormel, Chris W.

    Pebble accretion is the mechanism in which small particles ("pebbles") accrete onto big bodies big (planetesimals or planetary embryos) in gas-rich environments. In pebble accretion accretion , accretion occurs by settling and depends only on the mass of the gravitating body gravitating , not its radius. I give the conditions under which pebble accretion operates and show that the collisional cross section can become much larger than in the gas-free, ballistic, limit. In particular, pebble accretion requires the pre-existence of a massive planetesimal seed. When pebbles experience strong orbital decay by drift motions or are stirred by turbulence, the accretion efficiency is low and a great number of pebbles are needed to form Earth-mass cores. Pebble accretion is in many ways a more natural and versatile process than the classical, planetesimal-driven paradigm, opening up avenues to understand planet formation in solar and exoplanetary systems.

  19. A second-order shock-expansion method applicable to bodies of revolution near zero lift

    NASA Technical Reports Server (NTRS)

    1957-01-01

    A second-order shock-expansion method applicable to bodies of revolution is developed by the use of the predictions of the generalized shock-expansion method in combination with characteristics theory. Equations defining the zero-lift pressure distributions and the normal-force and pitching-moment derivatives are derived. Comparisons with experimental results show that the method is applicable at values of the similarity parameter, the ratio of free-stream Mach number to nose fineness ratio, from about 0.4 to 2.

  20. Three-dimensional hydrodynamic Bondi-Hoyle accretion. 2: Homogeneous medium at Mach 3 with gamma = 5/3

    NASA Technical Reports Server (NTRS)

    Ruffert, Maximilian; Arnett, David

    1994-01-01

    We investigate the hydrodynamics of three-dimensional classical Bondi-Hoyle accretion. Totally absorbing spheres of varying sizes (from 10 down to 0.01 accretion radii) move at Mach 3 relative to a homogeneous and slightly perturbed medium, which is taken to be an ideal gas (gamma = 5/3). To accommodate the long-range gravitational forces, the extent of the computational volume is 32(exp 3) accretion radii. We examine the influence of numerical procedure on physical behavior. The hydrodynamics is modeled by the 'piecewise parabolic method.' No energy sources (nuclear burning) or sinks (radiation, conduction) are included. The resolution in the vicinity of the accretor is increased by multiply nesting several (5-10) grids around the sphere, each finer grid being a factor of 2 smaller in zone dimension that the next coarser grid. The largest dynamic range (ratio of size of the largest grid to size of the finest zone) is 16,384. This allows us to include a coarse model for the surface of the accretor (vacuum sphere) on the finest grid, while at the same time evolving the gas on the coarser grids. Initially (at time t = 0-10), a shock front is set up, a Mach cone develops, and the accretion column is observable. Eventually the flow becomes unstable, destroying axisymmetry. This happens approximately when the mass accretion rate reaches the values (+/- 10%) predicted by the Bondi-Hoyle accretion formula (factor of 2 included). However, our three-dimensional models do not show the highly dynamic flip-flop flow so prominent in two-dimensional calculations performed by other authors. The flow, and thus the accretion rate of all quantities, shows quasi-periodic (P approximately equals 5) cycles between quiescent and active states. The interpolation formula proposed in an accompanying paper is found to follow the collected numerical data to within approximately 30%. The specific angular momentum accreted is of the same order of magnitude as the values previously found for

  1. 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.

  2. Global Evolution of an Accretion Disk with a Net Vertical Field: Coronal Accretion, Flux Transport, and Disk Winds

    NASA Astrophysics Data System (ADS)

    Zhu, Zhaohuan; Stone, James M.

    2018-04-01

    We report results from global ideal MHD simulations that study thin accretion disks (with thermal scale height H/R = 0.1 and 0.05) threaded by net vertical magnetic fields. Our computations span three orders of magnitude in radius, extend all the way to the pole, and are evolved for more than 1000 innermost orbits. We find that (1) inward accretion occurs mostly in the upper magnetically dominated regions of the disk at z ∼ R, similar to predictions from some previous analytical work and the “coronal accretion” flows found in GRMHD simulations. (2) A quasi-static global field geometry is established in which flux transport by inflows at the surface is balanced by turbulent diffusion. The resulting field is strongly pinched inwards at the surface. A steady-state advection–diffusion model, with a turbulent magnetic Prandtl number of order unity, reproduces this geometry well. (3) Weak unsteady disk winds are launched beyond the disk corona with the Alfvén radius R A /R 0 ∼ 3. Although the surface inflow is filamentary and the wind is episodic, we show that the time-averaged properties are well-described by steady-wind theory. Even with strong fields, β 0 = 103 at the midplane initially, only 5% of the angular momentum transport is driven by the wind, and the wind mass flux from the inner decade of the radius is only ∼0.4% of the mass accretion rate. (4) Within the disk, most of the accretion is driven by the Rϕ stress from the MRI and global magnetic fields. Our simulations have many applications to astrophysical accretion systems.

  3. 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

  4. Equation-of-state dependent features in shock-oscillation modulated neutrino and gravitational-wave signals from supernovae

    NASA Astrophysics Data System (ADS)

    Marek, A.; Janka, H.-T.; Müller, E.

    2009-03-01

    We present two-dimensional (axisymmetric) neutrino-hydrodynamic simulations of the long-time accretion phase of a 15 M_⊙ progenitor star after core bounce and before the launch of a supernova explosion, when non-radial hydrodynamic instabilities like convection occur in different regions of the collapsing stellar core and the standing accretion shock instability (SASI) leads to large-amplitude oscillations of the stalled shock with a period of tens of milliseconds. Our simulations were performed with the Prometheus-Vertex code, which includes a multi-flavor, energy-dependent neutrino transport scheme and employs an effective relativistic gravitational potential. Testing the influence of a stiff and a soft equation of state for hot neutron star matter, we find that the non-radial mass motions in the supernova core impose a time variability on the neutrino and gravitational-wave signals with larger amplitudes, as well as higher frequencies in the case of a more compact nascent neutron star. After the prompt shock-breakout burst of electron neutrinos, a more compact accreting remnant produces higher neutrino luminosities and higher mean neutrino energies. The observable neutrino emission in the SASI sloshing direction exhibits a modulation of several ten percent in the luminosities and around 1 MeV in the mean energies with most power at typical SASI frequencies between roughly 20 and 100 Hz. The modulation is caused by quasi-periodic variations in the mass accretion rate of the neutron star in each hemisphere. At times later than ~50-100 ms after bounce, the gravitational-wave amplitude is dominated by the growing low-frequency (⪉200 Hz) signal associated with anisotropic neutrino emission. A high-frequency wave signal results from nonradial gas flows in the outer layers of the anisotropically accreting neutron star. Right after bounce such nonradial mass motions occur due to prompt post-shock convection in both considered cases and contribute mostly to the early

  5. (BARS) -- Bibliographic Retrieval System Sandia Shock Compression (SSC) database Shock Physics Index (SPHINX) database. Volume 1: UNIX version query guide customized application for INGRES

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

    Herrmann, W.; von Laven, G.M.; Parker, T.

    1993-09-01

    The Bibliographic Retrieval System (BARS) is a data base management system specially designed to retrieve bibliographic references. Two databases are available, (i) the Sandia Shock Compression (SSC) database which contains over 5700 references to the literature related to stress waves in solids and their applications, and (ii) the Shock Physics Index (SPHINX) which includes over 8000 further references to stress waves in solids, material properties at intermediate and low rates, ballistic and hypervelocity impact, and explosive or shock fabrication methods. There is some overlap in the information in the two data bases.

  6. 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.

  7. 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.

  8. Rethinking Black Hole Accretion Discs

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg

    Accretion discs are staples of astrophysics. Tapping into the gravitational potential energy of the accreting material, these discs are highly efficient machines that produce copious radiation and extreme outflows. While interesting in their own right, accretion discs also act as tools to study black holes and directly influence the properties of the Universe. Black hole X-ray binaries are fantastic natural laboratories for studying accretion disc physics and black hole phenomena. Among many of the curious behaviors exhibited by these systems are black hole state transitions -- complicated cycles of dramatic brightening and dimming. Using X-ray observations with high temporal cadence, we show that the evolution of the accretion disc spectrum during black hole state transitions can be described by a variable disc atmospheric structure without invoking a radially truncated disc geometry. The accretion disc spectrum can be a powerful diagnostic for measuring black hole spin if the effects of the disc atmosphere on the emergent spectrum are well-understood; however, properties of the disc atmosphere are largely unconstrained. Using statistical methods, we decompose this black hole spin measurement technique and show that modest uncertainties regarding the disc atmosphere can lead to erroneous spin measurements. The vertical structure of the disc is difficult to constrain due to our ignorance of the contribution to hydrostatic balance by magnetic fields, which are fundamental to the accretion process. Observations of black hole X-ray binaries and the accretion environments near supermassive black holes provide mounting evidence for strong magnetization. Performing numerical simulations of accretion discs in the shearing box approximation, we impose a net vertical magnetic flux that allows us to effectively control the level of disc magnetization. We study how dynamo activity and the properties of turbulence driven by the magnetorotational instability depend on the

  9. Theory and High-Energy-Density Laser Experiments Relevant to Accretion Processes in Cataclysmic Variables

    NASA Astrophysics Data System (ADS)

    Krauland, Christine; Drake, R.; Loupias, B.; Falize, E.; Busschaert, C.; Ravasio, A.; Yurchak, R.; Pelka, A.; Koenig, M.; Kuranz, C. C.; Plewa, T.; Huntington, C. M.; Kaczala, D. N.; Klein, S.; Sweeney, R.; Villete, B.; Young, R.; Keiter, P. A.

    2012-05-01

    We present results from high-energy-density (HED) laboratory experiments that explore the contribution of radiative shock waves to the evolving dynamics of the cataclysmic variable (CV) systems in which they reside. CVs can be classified under two main categories, non-magnetic and magnetic. In the process of accretion, both types involve strongly radiating shocks that provide the main source of radiation in the binary systems. This radiation can cause varying structure to develop depending on the optical properties of the material on either side of the shock. The ability of high-intensity lasers to create large energy densities in targets of millimeter-scale volume makes it feasible to create similar radiative shocks in the laboratory. We provide an overview of both CV systems and their connection to the designed and executed laboratory experiments preformed on two laser facilities. Available data and accompanying simulations will likewise be shown. Funded by the NNSA-DS and SC-OFES Joint Prog. in High-Energy-Density Lab. Plasmas, by the Nat. Laser User Facility Prog. in NNSA-DS and by the Predictive Sci. Acad. Alliances Prog. in NNSA-ASC, under grant numbers are DE-FG52-09NA29548, DE-FG52-09NA29034, and DE-FC52-08NA28616.

  10. 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

  11. Shock wave treatment in medicine.

    PubMed

    Shrivastava, S K; Kailash

    2005-03-01

    Extracorporeal shock wave therapy in orthopedics and traumatology is still a young therapy method. Since the last few years the development of shock wave therapy has progressed rapidly. Shock waves have changed the treatment of urolithiasis substantially. Today shock waves are the first choice to treat kidney and urethral stones. Urology has long been the only medical field for shock waves in medicine. Meanwhile shock waves have been used in orthopedics and traumatology to treat insertion tendinitis, avascular necrosis of the head of femur and other necrotic bone alterations. Another field of shock wave application is the treatment of tendons, ligaments and bones on horses in veterinary medicine. In the present paper we discuss the basic theory and application of shock waves and its history in medicine. The idea behind using shock wave therapy for orthopedic diseases is the stimulation of healing in tendons, surrounding tissue and bones.

  12. 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.

  13. Cosmic-ray shock acceleration in oblique MHD shocks

    NASA Technical Reports Server (NTRS)

    Webb, G. M.; Drury, L. OC.; Volk, H. J.

    1986-01-01

    A one-dimensional, steady-state hydrodynamical model of cosmic-ray acceleration at oblique MHD shocks is presented. Upstream of the shock the incoming thermal plasma is subject to the adverse pressure gradient of the accelerated particles, the J x B force, as well as the thermal gas pressure gradient. The efficiency of the acceleration of cosmic-rays at the shock as a function of the upstream magnetic field obliquity and upstream plasma beta is investigated. Astrophysical applications of the results are briefly discussed.

  14. Flares, Magnetic Reconnections and Accretion Disk Viscosity

    NASA Astrophysics Data System (ADS)

    Welsh, William

    2001-07-01

    Accretion disks are invoked to explain a host of astrophysical phenomena, from protostellar objects to AGN. And yet the mechanism allowing accretion disks to operate are completely unknown. This proposal seeks to observe the ``smoking gun'' signature of magnetically-driven viscosity in accretion disks. Magnetically-induced viscosity is a plausible and generally accepted hypothesis {for esthetic reasons}, but it is completely untested. Determining the cause of accretion disk viscosity is of major significance to all accretion-disk powered systems {e.g. CVs, X-ray binaries, AGN and protostellar disks}. These data will also firmly establish the importance of magnetic fields in accretion disks. Because of its known flaring properites, we will observe the accretion disk in EM Cyg simulataneously with STIS/FUV and CHANDRA. The simultaneous X-rays are absolutely necessary for the unambiguous detection of accretion disk magnetic reconnection flares.

  15. Miniature shock tube for laser driven shocks.

    PubMed

    Busquet, Michel; Barroso, Patrice; Melse, Thierry; Bauduin, Daniel

    2010-02-01

    We describe in this paper the design of a miniature shock tube (smaller than 1 cm(3)) that can be placed in a vacuum vessel and allows transverse optical probing and longitudinal backside extreme ultraviolet emission spectroscopy in the 100-500 A range. Typical application is the study of laser launched radiative shocks, in the framework of what is called "laboratory astrophysics."

  16. 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.

  17. Formation of the Giant Planets by Concurrent Accretion of Solids and Gas

    NASA Technical Reports Server (NTRS)

    Hubickyj, Olenka

    1997-01-01

    Models were developed to simulate planet formation. Three major phases are characterized in the simulations: (1) planetesimal accretion rate, which dominates that of gas, rapidly increases owing to runaway accretion, then decreases as the planet's feeding zone is depleted; (2) occurs when both solid and gas accretion rates are small and nearly independent of time; and (3) starts when the solid and gas masses are about equal and is marked by runaway gas accretion. The models applicability to planets in our Solar System are judged using two basic "yardsticks". The results suggest that the solar nebula dissipated while Uranus and Neptune were in the second phase, during which, for a relatively long time, the masses of their gaseous envelopes were small but not negligible compared to the total masses. Background information, results and a published article are included in the report.

  18. MASS ACCRETION AND ITS EFFECTS ON THE SELF-SIMILARITY OF GAS PROFILES IN THE OUTSKIRTS OF GALAXY CLUSTERS

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

    Lau, Erwin T.; Nagai, Daisuke; Avestruz, Camille

    2015-06-10

    Galaxy clusters exhibit remarkable self-similar behavior which allows us to establish simple scaling relationships between observable quantities and cluster masses, making galaxy clusters useful cosmological probes. Recent X-ray observations suggested that self-similarity may be broken in the outskirts of galaxy clusters. In this work, we analyze a mass-limited sample of massive galaxy clusters from the Omega500 cosmological hydrodynamic simulation to investigate the self-similarity of the diffuse X-ray emitting intracluster medium (ICM) in the outskirts of galaxy clusters. We find that the self-similarity of the outer ICM profiles is better preserved if they are normalized with respect to the mean densitymore » of the universe, while the inner profiles are more self-similar when normalized using the critical density. However, the outer ICM profiles as well as the location of accretion shock around clusters are sensitive to their mass accretion rate, which causes the apparent breaking of self-similarity in cluster outskirts. We also find that the collisional gas does not follow the distribution of collisionless dark matter (DM) perfectly in the infall regions of galaxy clusters, leading to 10% departures in the gas-to-DM density ratio from the cosmic mean value. Our results have a number implications for interpreting observations of galaxy clusters in X-ray and through the Sunyaev–Zel’dovich effect, and their applications to cosmology.« less

  19. Application of cosmic-ray shock theories to the Cygnus Loop - An alternative model

    NASA Technical Reports Server (NTRS)

    Boulares, Ahmed; Cox, Donald P.

    1988-01-01

    Steady state cosmic-ray shock models are investigated here in the light of observations of the Cygnus Loop supernova remnant. The predicted downstream temperature is derived for each model. The Cygnus Loop data and the application of the models to them, including wave dissipation, are presented. Heating rate and ionization fraction structures are provided along with an estimate of the cosmic-ray diffusion coefficient. It is found that the model of Voelk, Drury, and McKenzie (1984), in which the plasma waves are generated by the streaming instability of the cosmic rays and are dissipated into the gas, can be made consistent with some observed characteristics of the Cygnus Loop shocks. The model is used to deduce upstream densities and shock velocities and, compared to the usual pure gas shock interpretation, it is found that lower densities and approximately three times higher velocities are required.

  20. Gravity signatures of terrane accretion

    NASA Astrophysics Data System (ADS)

    Franco, Heather; Abbott, Dallas

    1999-01-01

    In modern collisional environments, accreted terranes are bracketed by forearc gravity lows, a gravitational feature which results from the abandonment of the original trench and the initiation of a new trench seaward of the accreted terrane. The size and shape of the gravity low depends on the type of accreted feature and the strength of the formerly subducting plate. Along the Central American trench, the accretion of Gorgona Island caused a seaward trench jump of 48 to 66 km. The relict trench axes show up as gravity lows behind the trench with minimum values of -78 mgal (N of Gorgona) and -49 mgal (S of Gorgona) respectively. These forearc gravity lows have little or no topographic expression. The active trench immediately seaward of these forearc gravity lows has minimum gravity values of -59 mgal (N of Gorgona) and -58 mgal (S of Gorgona), respectively. In the north, the active trench has a less pronounced gravity low than the sediment covered forearc. In the Mariana arc, two Cretaceous seamounts have been accreted to the Eocene arc. The northern seamount is most likely a large block, the southern seamount may be a thrust slice. These more recent accretion events have produced modest forearc topographic and gravity lows in comparison with the topographic and gravity lows within the active trench. However, the minimum values of the Mariana forearc gravity lows are modest only by comparison to the Mariana Trench (-216 mgal); their absolute values are more negative than at Gorgona Island (-145 to -146 mgal). We speculate that the forearc gravity lows and seaward trench jumps near Gorgona Island were produced by the accretion of a hotspot island from a strong plate. The Mariana gravity lows and seaward trench jumps (or thrust slices) were the result of breaking a relatively weak plate close to the seamount edifice. These gravity lows resulting from accretion events should be preserved in older accreted terranes.

  1. OXIDIZING PROTO-ATMOSPHERE ON TITAN: CONSTRAINT FROM N{sub 2} FORMATION BY IMPACT SHOCK

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

    Ishimaru, Ryo; Matsui, Takafumi; Sekine, Yasuhito

    2011-11-01

    Titan is the only satellite that possesses a thick atmosphere, composed mainly of N{sub 2} and CH{sub 4}. However, its origin and evolution remain largely unknown. Knowledge of the acquirement of a N{sub 2} atmosphere on Titan would provide insights into nitrogen evolution in planetary atmospheres as well as the formation of satellite systems around gas giants. Previous studies have proposed that the atmospheric N{sub 2} would have been converted from NH{sub 3} via shock heating by accreting satellitesimals in the highly reducing proto-atmosphere composed of NH{sub 3} and CH{sub 4}. Nevertheless, the validity of this mechanism strongly depends onmore » both the composition of the proto-atmosphere and kinetics of shock chemistry. Here, we show that a CO{sub 2}-rich oxidizing proto-atmosphere is necessary to form N{sub 2} from NH{sub 3} efficiently by atmospheric shock heating. Efficient shock production of N{sub 2} is inhibited in a reducing proto-atmosphere composed of NH{sub 3} and CH{sub 4}, because CH{sub 4} plays as the coolant gas owing to its large heat capacity. Our calculations show that the amount of N{sub 2} produced in a CO{sub 2}-rich proto-atmosphere could have reached {approx}20 times that on the present Titan. Although further quantitative analysis are required (especially, the occurrence of catalytic reactions), our results imply that the chemical composition of satellitesimals that formed the Saturnian system is required to be oxidizing if the current atmospheric N{sub 2} is derived from the shock heating in the proto-atmosphere during accretion. This supports the formation of regular satellites in an actively supplied circumplanetary disk using CO{sub 2}-rich materials originated from the solar nebula at the final stage of gas giant formation.« less

  2. NGC 4051: Black hole mass and photon index-mass accretion rate correlation

    NASA Astrophysics Data System (ADS)

    Seifina, Elena; Chekhtman, Alexandre; Titarchuk, Lev

    2018-05-01

    We present a discovery of the correlation between the X-ray spectral (photon) index and mass accretion rate observed in an active galactic nucleus, NGC 4051. We analyzed spectral transition episodes observed in NGC 4051 using XMM-Newton, Suzaku and RXTE. We applied a scaling technique for a black hole (BH) mass evaluation which uses a correlation between the photon index and normalization of the seed (disk) component, which is proportional to a mass accretion rate. We developed an analytical model that shows the spectral (photon) index of the BH emergent spectrum undergoes an evolution from lower to higher values depending on a mass accretion rate in the accretion disk. We considered Cygnus X-1 and GRO J1550-564 as reference sources for which distances, inclination angles and the BH masses are evaluated by dynamical measurements. Application of the scaling technique for the photon index-mass accretion rate correlation provides an estimate of the black hole mass in NGC 4051 to be more than 6 × 105 solar masses.

  3. Relationships among physical properties as indicators of high temperature deformation or post-shock thermal annealing in ordinary chondrites

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

    Friedrich, Jon M.; Ruzicka, Alex; Macke, Robert J.

    Collisions and attendant shock compaction must have been important for the accretion and lithification of planetesimals, including the parent bodies of chondrites, but the conditions under which these occurred are not well constrained. A simple model for the compaction of chondrites predicts that shock intensity as recorded by shock stage should be related to porosity and grain fabric. To test this model, we studied sixteen ordinary chondrites of different groups (H, L, LL) using X-ray computed microtomography (μCT) to measure porosity and metal fabric, ideal gas pycnometry and 3D laser scanning to determine porosity, and optical microscopy (OM) to determinemore » shock stage. These included a subsample of six chondrites previously studied using transmission electron microscopy (TEM) to characterize microstructures in olivine. Combining with previous data, results support the simple model in general, but not for chondrites with low shock-porosity-foliation (low-SPF chondrites). These include Kernouvé (H6), Portales Valley (H6/7), Butsura (H6), Park (L6), GRO 85209 (L6), Estacado (H6), MIL 99301 (LL6), Spade (H6), and Queen’s Mercy (H6), among others. The data for these meteorites are best explained by high ambient heat during or after shock. Low-SPF chondrites tend to have older 40Ar/39Ar ages (~4435–4526 Ma) than other, non-low-SPF type 6 chondrites in this study. We conclude that the H, L, and LL asteroids all were shock-compacted at an early stage while warm, with collisions occurring during metamorphic heating of the parent bodies. Results ultimately bear on whether chondrite parent bodies have internal structures more akin to a metamorphosed onion shell or metamorphosed rubble pile, and on the nature of accretion and lithification processes for planetesimals.« less

  4. Relationships among physical properties as indicators of high temperature deformation or post-shock thermal annealing in ordinary chondrites

    NASA Astrophysics Data System (ADS)

    Friedrich, Jon M.; Ruzicka, Alex; Macke, Robert J.; Thostenson, James O.; Rudolph, Rebecca A.; Rivers, Mark L.; Ebel, Denton S.

    2017-04-01

    Collisions and attendant shock compaction must have been important for the accretion and lithification of planetesimals, including the parent bodies of chondrites, but the conditions under which these occurred are not well constrained. A simple model for the compaction of chondrites predicts that shock intensity as recorded by shock stage should be related to porosity and grain fabric. To test this model, we studied sixteen ordinary chondrites of different groups (H, L, LL) using X-ray computed microtomography (μCT) to measure porosity and metal fabric, ideal gas pycnometry and 3D laser scanning to determine porosity, and optical microscopy (OM) to determine shock stage. These included a subsample of six chondrites previously studied using transmission electron microscopy (TEM) to characterize microstructures in olivine. Combining with previous data, results support the simple model in general, but not for chondrites with low shock-porosity-foliation (low-SPF chondrites). These include Kernouvé (H6), Portales Valley (H6/7), Butsura (H6), Park (L6), GRO 85209 (L6), Estacado (H6), MIL 99301 (LL6), Spade (H6), and Queen's Mercy (H6), among others. The data for these meteorites are best explained by high ambient heat during or after shock. Low-SPF chondrites tend to have older 40Ar/39Ar ages (∼4435-4526 Ma) than other, non-low-SPF type 6 chondrites in this study. We conclude that the H, L, and LL asteroids all were shock-compacted at an early stage while warm, with collisions occurring during metamorphic heating of the parent bodies. Results ultimately bear on whether chondrite parent bodies have internal structures more akin to a metamorphosed onion shell or metamorphosed rubble pile, and on the nature of accretion and lithification processes for planetesimals.

  5. 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.

  6. Lagrangian technique to calculate window interface velocity from shock velocity measurements: Application for quartz windows

    DOE PAGES

    McCoy, Chad A.; Knudson, Marcus D.

    2017-08-24

    Measurement of the window interface velocity is a common technique for investigating the dynamic response materials at high strain rates. However, these measurements are limited in pressure to the range where the window remains transparent. The most common window material for this application is lithium fluoride, which under single shock compression becomes opaque at ~200 GPa. To date, no other window material has been identified for use at higher pressures. Here, we present a Lagrangian technique to calculate the interface velocity from a continuously measured shock velocity, with application to quartz. The quartz shock front becomes reflective upon melt, atmore » ~100 GPa, enabling the use of velocity interferometry to continuously measure the shock velocity. This technique overlaps with the range of pressures accessible with LiF windows and extends the region where wave profile measurements are possible to pressures in excess of 2000 GPa. Lastly, we show through simulated data that the technique accurately reproduces the interface velocity within 20% of the initial state, and that the Lagrangian technique represents a significant improvement over a simple linear approximation.« less

  7. Lagrangian technique to calculate window interface velocity from shock velocity measurements: Application for quartz windows

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

    McCoy, Chad A.; Knudson, Marcus D.

    Measurement of the window interface velocity is a common technique for investigating the dynamic response materials at high strain rates. However, these measurements are limited in pressure to the range where the window remains transparent. The most common window material for this application is lithium fluoride, which under single shock compression becomes opaque at ~200 GPa. To date, no other window material has been identified for use at higher pressures. Here, we present a Lagrangian technique to calculate the interface velocity from a continuously measured shock velocity, with application to quartz. The quartz shock front becomes reflective upon melt, atmore » ~100 GPa, enabling the use of velocity interferometry to continuously measure the shock velocity. This technique overlaps with the range of pressures accessible with LiF windows and extends the region where wave profile measurements are possible to pressures in excess of 2000 GPa. Lastly, we show through simulated data that the technique accurately reproduces the interface velocity within 20% of the initial state, and that the Lagrangian technique represents a significant improvement over a simple linear approximation.« less

  8. Nuclear Spiral Shocks and Induced Gas Inflows in Weak Oval Potentials

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

    Kim, Woong-Tae; Elmegreen, Bruce G., E-mail: wkim@astro.snu.ac.kr, E-mail: bge@us.ibm.com

    Nuclear spirals are ubiquitous in galaxy centers. They exist not only in strong barred galaxies but also in galaxies without noticeable bars. We use high-resolution hydrodynamic simulations to study the properties of nuclear gas spirals driven by weak bar-like and oval potentials. The amplitude of the spirals increases toward the center by a geometric effect, readily developing into shocks at small radii even for very weak potentials. The shape of the spirals and shocks depends rather sensitively on the background shear. When shear is low, the nuclear spirals are loosely wound and the shocks are almost straight, resulting in largemore » mass inflows toward the center. When shear is high, on the other hand, the spirals are tightly wound and the shocks are oblique, forming a circumnuclear disk through which gas flows inward at a relatively lower rate. The induced mass inflow rates are enough to power black hole accretion in various types of Seyfert galaxies as well as to drive supersonic turbulence at small radii.« less

  9. 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.

  10. Numerical Modeling of Tidal Effects in Polytropic Accretion Disks

    NASA Technical Reports Server (NTRS)

    Godon, Patrick

    1997-01-01

    A two-dimensional time-dependent hybrid Fourier-Chebyshev method of collocation is developed and used for the study of tidal effects in accretion disks, under the assumptions of a polytropic equation of state and a standard alpha viscosity prescription. Under the influence of the m = 1 azimuthal component of the tidal potential, viscous oscillations in the outer disk excite an m = 1 eccentric instability in the disk. While the m = 2 azimuthal component of the tidal potential excites a Papaloizou-Pringle instability in the inner disk (a saturated m = 2 azimuthal mode), with an elliptic pattern rotating at about a fraction (approx. = 1/3) of the local Keplerian velocity in the inner disk. The period of the elliptic mode corresponds well to the periods of the short-period oscillations observed in cataclysmic variables. In cold disks (r(Omega)/c(sub s) = M approx. = 40) we also find a critical value of the viscosity parameter (alpha approx. = 0.01), below which shock dissipation dominates and is balanced by the wave amplification due to the wave action conservation. In this case the double spiral shock propagates all the way to the inner boundary with a Mach number M(sub s) approx. = 1.3.

  11. 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

  12. Accretion flows onto supermassive black holes

    NASA Technical Reports Server (NTRS)

    Begelman, Mitchell C.

    1988-01-01

    The radiative and hydrodynamic properties of an angular momentum-dominated accretion flow onto a supermassive black hole depend largely on the ratio of the accretion rate to the Eddington accretion rate. High values of this ratio favor optically thick flows which produce largely thermal radiation, while optically thin 'two-temperature' flows may be present in systems with small values of this ratio. Observations of some AGN suggest that thermal and nonthermal sources of radiation may be of comparable importance in the 'central engine'. Consideration is given to the possibilities for coexistence of different modes of accretion in a single flow. One intriguing possibility is that runaway pair production may cause an optically thick 'accretion annulus' to form at the center of a two-temperature inflow.

  13. Pebble Accretion in Turbulent Protoplanetary Disks

    NASA Astrophysics Data System (ADS)

    Xu, Ziyan; Bai, Xue-Ning; Murray-Clay, Ruth A.

    2017-09-01

    It has been realized in recent years that the accretion of pebble-sized dust particles onto planetary cores is an important mode of core growth, which enables the formation of giant planets at large distances and assists planet formation in general. The pebble accretion theory is built upon the orbit theory of dust particles in a laminar protoplanetary disk (PPD). For sufficiently large core mass (in the “Hill regime”), essentially all particles of appropriate sizes entering the Hill sphere can be captured. However, the outer regions of PPDs are expected to be weakly turbulent due to the magnetorotational instability (MRI), where turbulent stirring of particle orbits may affect the efficiency of pebble accretion. We conduct shearing-box simulations of pebble accretion with different levels of MRI turbulence (strongly turbulent assuming ideal magnetohydrodynamics, weakly turbulent in the presence of ambipolar diffusion, and laminar) and different core masses to test the efficiency of pebble accretion at a microphysical level. We find that accretion remains efficient for marginally coupled particles (dimensionless stopping time {τ }s˜ 0.1{--}1) even in the presence of strong MRI turbulence. Though more dust particles are brought toward the core by the turbulence, this effect is largely canceled by a reduction in accretion probability. As a result, the overall effect of turbulence on the accretion rate is mainly reflected in the changes in the thickness of the dust layer. On the other hand, we find that the efficiency of pebble accretion for strongly coupled particles (down to {τ }s˜ 0.01) can be modestly reduced by strong turbulence for low-mass cores.

  14. TRIGGERING COLLAPSE OF THE PRESOLAR DENSE CLOUD CORE AND INJECTING SHORT-LIVED RADIOISOTOPES WITH A SHOCK WAVE. II. VARIED SHOCK WAVE AND CLOUD CORE PARAMETERS

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

    Boss, Alan P.; Keiser, Sandra A., E-mail: boss@dtm.ciw.edu, E-mail: keiser@dtm.ciw.edu

    2013-06-10

    A variety of stellar sources have been proposed for the origin of the short-lived radioisotopes that existed at the time of the formation of the earliest solar system solids, including Type II supernovae (SNe), asymptotic giant branch (AGB) and super-AGB stars, and Wolf-Rayet star winds. Our previous adaptive mesh hydrodynamics models with the FLASH2.5 code have shown which combinations of shock wave parameters are able to simultaneously trigger the gravitational collapse of a target dense cloud core and inject significant amounts of shock wave gas and dust, showing that thin SN shocks may be uniquely suited for the task. However,more » recent meteoritical studies have weakened the case for a direct SN injection to the presolar cloud, motivating us to re-examine a wider range of shock wave and cloud core parameters, including rotation, in order to better estimate the injection efficiencies for a variety of stellar sources. We find that SN shocks remain as the most promising stellar source, though planetary nebulae resulting from AGB star evolution cannot be conclusively ruled out. Wolf-Rayet (WR) star winds, however, are likely to lead to cloud core shredding, rather than to collapse. Injection efficiencies can be increased when the cloud is rotating about an axis aligned with the direction of the shock wave, by as much as a factor of {approx}10. The amount of gas and dust accreted from the post-shock wind can exceed that injected from the shock wave, with implications for the isotopic abundances expected for a SN source.« less

  15. Sources of Shock Waves in the Protoplanetary Disk

    NASA Astrophysics Data System (ADS)

    Boss, A. P.; Durisen, R. H.

    2005-12-01

    Finding an appropriate heat source for melting the chondrules that constitute the bulk of many primitive meteorites is perhaps the most important outstanding problem in all of meteoritics. Shock waves within the Solar Nebula are one possible means for accomplishing this provided that they move with respect to the precursor aggregates at speeds of ~ 6 to 9 km s-1 in environments with appropriate nebular pressures and densities. Here we briefly review the status of four different mechanisms which have been proposed as sources of such shock fronts. We argue that two of them, the accretion shock at the nebular surface and shocks propagating inside the nebula launched by the impact of gas clumps falling onto the disk, are unlikely to work. Bow shocks driven by 1000-km-size planetesimals show more promise, but require the presence of Jupiter to raise the eccentricities of the planetesimals. We then focus this chapter on the fourth mechanism, which may be the dominant source of shocks in the early nebula. Wood (1996) proposed that the chondrule-producing shocks were due to nebular spiral arms. This hypothesis is now strongly supported by recent calculations of the evolution of gravitationally unstable disks. In a gaseous disk capable of forming Jupiter, the disk gas must have been close to marginal gravitational instability near or beyond Jupiter's orbit. Massive clumps and spirals due to such instability can drive spiral shock fronts inward with shock speeds as large as ~ 10 km s-1 at asteroidal orbits, sufficient to account for chondrule formation. Once Jupiter forms, it may either continue to drive strong shock fronts at asteroidal distances, or it may pump up the eccentricity of planetesimals, leading to chondrule processing for as long as the inner disk gas survives, a few Myr or so. Mixing and transport of solids in an unstable disk results in a scenario that unifies chondrite formation from chondrules, refractory inclusions, and matrix grains with disk processes

  16. Impact-induced devolatilization and hydrogen isotopic fractionation of serpentine: Implications for planetary accretion

    NASA Technical Reports Server (NTRS)

    Tyburczy, James A.; Krishnamurthy, R. V.; Epstein, Samuel; Ahrens, Thomas J.

    1988-01-01

    Impact-induced devolatilization of porous serpentine was investigated using two independent experimental methods, the gas recovery and the solid recovery method, each yielding nearly identical results. For shock pressures near incipient devolatilization, the hydrogen isotopic composition of the evolved H2O is very close to that of the starting material. For shock pressures at which up to 12 percent impact-induced devolatilization occurs, the bulk evolved gas is significantly lower in deuterium than the starting material. There is also significant reduction of H2O to H2 in gases recovered at these higher shock pressures, probably caused by reaction of evolved H2O with the metal gas recovery fixture. Gaseous H2O-H2 isotopic fractionation suggests high temperature isotopic equilibrium between the gaseous species, indicating initiation of devolatilization at sites of greater than average energy deposition. Bulk gas-residual solid isotopic fractionations indicate nonequilibrium, kinetic control of gas-solid isotopic ratios. Impact-induced hydrogen isotopic fractionation of hydrous silicates during accretion can strongly affect the long-term planetary isotopic ratios of planetary bodies, leaving the interiors enriched in deuterium. Depending on the model used for extrapolation of the isotopic fractionation to devolatilization fractions greater than those investigated experimentally can result from this process.

  17. Modeling Soft Excess with GRMHD Accretion for XMM-Newton Spectra of Bright AGNs

    NASA Astrophysics Data System (ADS)

    Fukumura, K.; Haba, Y.; Takahashi, M.; Tombesi, F.

    2017-10-01

    Despite a number of well-studied X-ray observations of the so called soft excess (SE) from a certain class of AGNs in the past decades, its physical identification has remained to be elusive to date. With the absence of a single leading model, a few competing scenarios have been proposed. In this presentation, we show that the innermost plasma accretion under strong gravity can develop into an MHD shock front at r < 5 r_{g} where incoming thermal disk photons (of ˜ 10 eV) are efficiently Compton up-scattered by shock-accelerated electrons in its downstream region to produce the observed SE feature. Considering all the relativistic effects in our treatment, our GRMHD Comptonization model, consisting of (1) disk photon temperature (kT_{bb}), electron energy (Θ_{e}) and inclination (θ_{obs}) for a given black hole spin (a/m), can naturally provide the SE spectra for a fiducial parameter set by solving GRMHD flows. Our calculations indicate that the Comptonizing region is very compact just outside the black hole event horizon resembling a putative 'coronae' with a characteristic electron energy on the order of ˜ 100 keV determined by shock strength. We also show preliminary spectral analysis results for some stereotypical PG and NLS1 AGNs.

  18. Application of a relativistic accretion disc model to X-ray spectra of LMC X-1 and GRO J1655-40

    NASA Astrophysics Data System (ADS)

    Gierliński, Marek; Maciołek-Niedźwiecki, Andrzej; Ebisawa, Ken

    2001-08-01

    We present a general relativistic accretion disc model and its application to the soft-state X-ray spectra of black hole binaries. The model assumes a flat, optically thick disc around a rotating Kerr black hole. The disc locally radiates away the dissipated energy as a blackbody. Special and general relativistic effects influencing photons emitted by the disc are taken into account. The emerging spectrum, as seen by a distant observer, is parametrized by the black hole mass and spin, the accretion rate, the disc inclination angle and the inner disc radius. We fit the ASCA soft-state X-ray spectra of LMC X-1 and GRO J1655-40 by this model. We find that, having additional limits on the black hole mass and inclination angle from optical/UV observations, we can constrain the black hole spin from X-ray data. In LMC X-1 the constraint is weak, and we can only rule out the maximally rotating black hole. In GRO J1655-40 we can limit the spin much better, and we find 0.68<=a<=0.88. Accretion discs in both sources are radiation-pressure dominated. We do not find Compton reflection features in the spectra of any of these objects.

  19. Accretion Disks Around Binary Black Holes of Unequal Mass: GRMHD Simulations Near Decoupling

    NASA Technical Reports Server (NTRS)

    Gold, Roman; Paschalidis, Vasileios; Etienne, Zachariah B.; Shapiro, Stuart L.; Pfeiffer, Harald, P.

    2013-01-01

    We report on simulations in general relativity of magnetized disks onto black hole binaries. We vary the binary mass ratio from 1:1 to 1:10 and evolve the systems when they orbit near the binary disk decoupling radius. We compare (surface) density profiles, accretion rates (relative to a single, non-spinning black hole), variability, effective alpha-stress levels and luminosities as functions of the mass ratio. We treat the disks in two limiting regimes: rapid radiative cooling and no radiative cooling. The magnetic field lines clearly reveal jets emerging from both black hole horizons and merging into one common jet at large distances. The magnetic fields give rise to much stronger shock heating than the pure hydrodynamic flows, completely alter the disk structure, and boost accretion rates and luminosities. Accretion streams near the horizons are among the densest structures; in fact, the 1:10 no-cooling evolution results in a refilling of the cavity. The typical effective temperature in the bulk of the disk is approx. 10(exp5) (M / 10(exp 8)M solar mass (exp -1/4(L/L(sub edd) (exp 1/4K) yielding characteristic thermal frequencies approx. 10 (exp 15) (M /10(exp 8)M solar mass) (exp -1/4(L/L (sub edd) (1+z) (exp -1)Hz. These systems are thus promising targets for many extragalactic optical surveys, such as LSST, WFIRST, and PanSTARRS.

  20. Interpreting MAD within multiple accretion regimes

    NASA Astrophysics Data System (ADS)

    Mocz, Philip; Guo, Xinyi

    2015-02-01

    General relativistic magnetohydrodynamic (GRMHD) simulations of accreting black holes in the radiatively inefficient regime show that systems with sufficient magnetic poloidal flux become magnetically arrested disc (MAD) systems, with a well-defined relationship between the magnetic flux and the mass accretion rate. Recently, Zamaninasab et al. report that the jet magnetic flux and accretion disc luminosity are tightly correlated over 7 orders of magnitude for a sample of 76 radio-loud active galaxies, concluding that the data are explained by the MAD mode of accretion. Their analysis assumes radiatively efficient accretion, and their sample consists primarily of radiatively efficient sources, while GRMHD simulations of MAD thus far have been carried out in the radiatively inefficient regime. We propose a model to interpret MAD systems in the context of multiple accretion regimes, and apply it to the sample in Zamaninasab et al., along with additional radiatively inefficient sources from archival data. We show that most of the radiatively inefficient radio-loud galaxies are consistent with being MAD systems. Assuming the MAD relationship found in radiatively inefficient simulations holds at other accretion regimes, a significant fraction of our sample can be candidates for MAD systems. Future GRMHD simulations have yet to verify the validity of this assumption.

  1. 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.

  2. Protostellar Collapse with a Shock

    NASA Technical Reports Server (NTRS)

    Tsai, John C.; Hsu, Juliana J.

    1995-01-01

    We reexamine both numerically and analytically the collapse of the singular isothermal sphere in the context of low-mass star formation. We consider the case where the onset of collapse is initiated by some arbitrary process which is accompanied by a central output of either heat or kinetic energy. We find two classes of numerical solutions describing this manner of collapse. The first approaches in time the expansion wave solution of Shu, while the second class is characterized by an ever-decreasing central accretion rate and the presence of an outwardly propagating weak shock. The collapse solution which represents the dividing case between these two classes is determined analytically by a similarity analysis. This solution shares with the expansion wave solution the properties that the gas remains stationary with an r(exp -2) density profile at large radius and that, at small radius, the gas free-falls onto a nascent core at a constant rate which depends only on the isothermal sound speed. This accretion rate is a factor of approx. 0.1 that predicted by the expansion wave solution. This reduction is due in part to the presence of a weak shock which propagates outward at 1.26 times the sound speed. Gas in the postshock region first moves out subsonically but is then decelerated and begins to collapse. The existence of two classes of numerical collapse solutions is explained in terms of the instability to radial perturbations of the analytic solution. Collapse occurring in the manner described by some of our solutions would eventually unbind a finite-sized core. However, this does not constitute a violation of the instability properties of the singular isothermal sphere which is unstable both to collapse and to expansion. To emphasize this, we consider a purely expanding solution for isothermal spheres. This solution is found to be self-similar and results in a uniform density core in the central regions of the gas. Our solutions may be relevant to the 'luminosity

  3. Protostellar Collapse with a Shock

    NASA Technical Reports Server (NTRS)

    Tsai, John C.; Hsu, Juliana J. L.

    1995-01-01

    We reexamine both numerically and analytically the collapse of the singular isothermal sphere in the context of low-mass star formation. We consider the case where the onset of collapse is initiated by some arbitrary process which is accompanied by a central output of either heat or kinetic energy. We find two classes of numerical solutions describing this manner of collapse. The first approaches in time the expansion wave solution of Shu, while the second class is characterized by an ever-decreasing central accretion rate and the presence of an outwardly propagating weak shock. The collapse solution which represents the dividing case between these two classes is determined analytically by a similarity analysis. This solution shares with the expansion wave solution the properties that the gas remains stationary with an r(sup -2) density profile at large radius and that, at small radius, the gas free-falls onto a nascent core at a constant rate which depends only on the isothermal sound speed. This accretion rate is a factor of approx. 0.1 that predicted by the expansion wave solution. This reduction is due in part to the presence of a weak shock which propagates outward at 1.26 times the sound speed. Gas in the postshock region first moves out subsonically but is then decelerated and begins to collapse. The existence of two classes of numerical collapse solutions is explained in terms of the instability to radial perturbations of the analytic solution. Collapse occurring in the manner described by some of our solutions would eventually unbind a finite-sized core. However, this does not constitute a violation of the instability properties of the singular isothermal sphere which is unstable both to collapse and to expansion. To emphasize this, we consider a purely expanding solution for isothermal spheres. This solution is found to be self-similar and results in a uniform density core in the central regions of the gas. Our solutions may be relevant to the 'luminosity

  4. 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.

  5. The End of Accretion: The X-Ray Binary/Millisecond Pulsar Transition Object PSR J1023+0038

    NASA Astrophysics Data System (ADS)

    Archibald, Anne

    2015-04-01

    Millisecond radio pulsars (MSRPs), those spinning hundreds of times per second, have long been understood to be old pulsars that have been spun up by the accretion of matter from a companion in a low-mass X-ray binary (LMXB) phase. Yet the details of this transformation, particularly the end of the accretion process and the birth of a radio pulsar, remain mysterious. I will describe the discovery and detailed study of the first object known to transition between MSRP and LMXB states, PSR J1023+0038. By dint of a multiwavelength campaign of observations in the RMSP state, we are able to measure all the key system parameters and show the existence of an X-ray shock close to the pulsar-facing side of the companion. Since the discovery of PSR J1023+0038, two more objects (XSS J12270-4859 and M28I) have been found to make the same transition, and the study of these transitioning objects has become an active field of research. Most interestingly, PSR J1023+0038 has transitioned back into an LMXB state, with an active accretion disk and a puzzling increase in gamma-ray flux. Our detailed picture of the system allows us to test models of accretion against the phenomena we observe in PSR J1023+0038, and in fact these observations challenge current models: in spite of the low luminosity of the system (and low inferred accretion rate) some material is penetrating the centrifugal barrier and falling on the neutron-star surface. Key evidence for explaining this puzzling behaviour will come when PSR J1023+0038 returns to an MSRP state and we are able to compare pulsar timing models from after the LMXB state with those we obtained in this work.

  6. A model for accretion of the terrestrial planets

    NASA Technical Reports Server (NTRS)

    Weidenschilling, S. J.

    1974-01-01

    One possible origin of the terrestrial planets involves their formation by gravitational accretion of particles originally in Keplerian orbits about the sun. Some implications of this theory are considered. A formal expression for the rate of mass accretion by a planet is developed. The formal singularity of the gravitational collision cross section for low relative velocities is shown to be without physical significance when the accreting bodies are in heliocentric orbits. The distribution of particle velocities relative to an accreting planet is considered; the mean velocity increases with time. The internal temperature of an accreting planet is shown to depend simply on the accretion rate. A simple and physically reasonable approximate expression for a planetary accretion rate is proposed.

  7. Current topics in shock waves; Proceedings of the International Symposium on Shock Waves and Shock Tubes, 17th, Lehigh University, Bethlehem, PA, July 17-21, 1989

    NASA Astrophysics Data System (ADS)

    Kim, Yong W.

    Various papers on shock waves are presented. The general topics addressed include: shock formation, focusing, and implosion; shock reflection and diffraction; turbulence; laser-produced plasmas and waves; ionization and shock-plasma interaction; chemical kinetics, pyrolysis, and soot formation; experimental facilities, techniques, and applications; ignition of detonation and combustion; particle entrainment and shock propagation through particle suspension; boundary layers and blast simulation; computational methods and numerical simulation.

  8. Ion Ramp Structure of Bow shocks and Interplanetary Shocks: Differences and Similarities

    NASA Astrophysics Data System (ADS)

    Goncharov, O.; Safrankova, J.; Nemecek, Z.; Koval, A.; Szabo, A.; Prech, L.; Zastenker, G. N.; Riazantseva, M.

    2017-12-01

    Collisionless shocks play a significant role in the solar wind interaction with the Earth. Fast forward shocks driven by coronal mass ejections or by interaction of fast and slow solar wind streams can be encountered in the interplanetary space, whereas the bow shock is a standing fast reverse shock formed by an interaction of the supersonic solar wind with the Earth magnetic field. Both types of shocks are responsible for a transformation of a part of the energy of the directed solar wind motion to plasma heating and to acceleration of reflected particles to high energies. These processes are closely related to the shock front structure. In present paper, we compares the analysis of low-Mach number fast forward interplanetary shocks registered in the solar wind by the DSCOVR, WIND, and ACE with observations of bow shock crossings observed by the Cluster, THEMIS, MMS, and Spektr-R spacecraft. An application of the high-time resolution data facilitates further discussion on formation mechanisms of both types of shocks.

  9. 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

  10. HEROIC: 3D general relativistic radiative post-processor with comptonization for black hole accretion discs

    NASA Astrophysics Data System (ADS)

    Narayan, Ramesh; Zhu, Yucong; Psaltis, Dimitrios; Saḑowski, Aleksander

    2016-03-01

    We describe Hybrid Evaluator for Radiative Objects Including Comptonization (HEROIC), an upgraded version of the relativistic radiative post-processor code HERO described in a previous paper, but which now Includes Comptonization. HEROIC models Comptonization via the Kompaneets equation, using a quadratic approximation for the source function in a short characteristics radiation solver. It employs a simple form of accelerated lambda iteration to handle regions of high scattering opacity. In addition to solving for the radiation field, HEROIC also solves for the gas temperature by applying the condition of radiative equilibrium. We present benchmarks and tests of the Comptonization module in HEROIC with simple 1D and 3D scattering problems. We also test the ability of the code to handle various relativistic effects using model atmospheres and accretion flows in a black hole space-time. We present two applications of HEROIC to general relativistic magnetohydrodynamics simulations of accretion discs. One application is to a thin accretion disc around a black hole. We find that the gas below the photosphere in the multidimensional HEROIC solution is nearly isothermal, quite different from previous solutions based on 1D plane parallel atmospheres. The second application is to a geometrically thick radiation-dominated accretion disc accreting at 11 times the Eddington rate. Here, the multidimensional HEROIC solution shows that, for observers who are on axis and look down the polar funnel, the isotropic equivalent luminosity could be more than 10 times the Eddington limit, even though the spectrum might still look thermal and show no signs of relativistic beaming.

  11. 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

  12. Conditions for shock revival by neutrino heating in core-collapse supernovae

    NASA Astrophysics Data System (ADS)

    Janka, H.-Th.

    2001-03-01

    Energy deposition by neutrinos can rejuvenate the stalled bounce shock and can provide the energy for the supernova explosion of a massive star. This neutrino-heating mechanism, though investigated by numerical simulations and analytic studies, is not finally accepted or proven as the trigger of the explosion. Part of the problem is that different groups have obtained seemingly discrepant results, and the complexity of the hydrodynamic models often hampers a clear and simple interpretation of the results. This demands a deeper theoretical understanding of the requirements of a successful shock revival. A toy model is developed here for discussing the neutrino heating phase analytically. The neutron star atmosphere between the neutrinosphere and the supernova shock can well be considered to be in hydrostatic equilibrium, with a layer of net neutrino cooling below the gain radius and a layer of net neutrino heating above. Since the mass infall rate to the shock is in general different from the rate at which gas is advected into the neutron star, the mass in the gain layer varies with time. Moreover, the gain layer receives additional energy input by neutrinos emitted from the neutrinosphere and the cooling layer. Therefore the determination of the shock evolution requires a time-dependent treatment. To this end the hydrodynamical equations of continuity and energy are integrated over the volume of the gain layer to obtain conservation laws for the total mass and energy in this layer. The radius and velocity of the supernova shock can then be calculated from global properties of the gain layer as solutions of an initial value problem, which expresses the fact that the behavior of the shock is controlled by the cumulative effects of neutrino heating and mass accumulation in the gain layer. The described toy model produces steady-state accretion and mass outflow from the nascent neutron star as special cases. The approach is useful to illuminate the conditions that can

  13. A scaling law for accretion zone sizes

    NASA Technical Reports Server (NTRS)

    Greenzweig, Yuval; Lissauer, Jack J.

    1987-01-01

    Current theories of runaway planetary accretion require small random velocities of the accreted particles. Two body gravitational accretion cross sections which ignore tidal perturbations of the Sun are not valid for the slow encounters which occur at low relative velocities. Wetherill and Cox have studied accretion cross sections for rocky protoplanets orbiting at 1 AU. Using analytic methods based on Hill's lunar theory, one can scale these results for protoplanets that occupy the same fraction of their Hill sphere as does a rocky body at 1 AU. Generalization to bodies of different sizes is achieved here by numerical integrations of the three-body problem. Starting at initial positions far from the accreting body, test particles are allowed to encounter the body once, and the cross section is computed. A power law is found relating the cross section to the radius of the accreting body (of fixed mass).

  14. Shock and vibration technology with applications to electrical systems

    NASA Technical Reports Server (NTRS)

    Eshleman, R. L.

    1972-01-01

    A survey is presented of shock and vibration technology for electrical systems developed by the aerospace programs. The shock environment is surveyed along with new techniques for modeling, computer simulation, damping, and response analysis. Design techniques based on the use of analog computers, shock spectra, optimization, and nonlinear isolation are discussed. Shock mounting of rotors for performance and survival, and vibration isolation techniques are reviewed.

  15. Accretion Rate: An Axis Of Agn Unification

    NASA Astrophysics Data System (ADS)

    Trump, Jonathan R.; Impey, C. D.; Kelly, B. C.

    2011-01-01

    We show how accretion rate governs the physical properties of broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rate by using accurate accretion luminosities from well-sampled multiwavelength SEDs from the Cosmic Evolution Survey (COSMOS), and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L/L_Edd>0.01), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L/L_Edd<0.01) are unobscured and yet lack a broad line region. The disappearance of the broad emission lines is caused by an expanding radiatively inefficient accretion flow (RIAF) at the inner radius of the accretion disk. The presence of the RIAF also drives L/L_Edd<0.01 narrow-line and lineless AGNs to be 10-100 times more radio-luminous than broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L/L_Edd<0.01 AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together these results suggest that specific accretion rate is an important physical "axis" of AGN unification, described by a simple model.

  16. X-Ray Spectra from MHD Simulations of Accreting Black Holes

    NASA Technical Reports Server (NTRS)

    Schnittman, Jeremy D.; Noble, Scott C.; Krolik, Julian H.

    2011-01-01

    We present new global calculations of X-ray spectra from fully relativistic magneto-hydrodynamic (MHO) simulations of black hole (BH) accretion disks. With a self consistent radiative transfer code including Compton scattering and returning radiation, we can reproduce the predominant spectral features seen in decades of X-ray observations of stellar-mass BHs: a broad thermal peak around 1 keV, power-law continuum up to >100 keV, and a relativistically broadened iron fluorescent line. By varying the mass accretion rate, different spectral states naturally emerge: thermal-dominant, steep power-law, and low/hard. In addition to the spectral features, we briefly discuss applications to X-ray timing and polarization.

  17. Accretion Discs Around Black Holes: Developement of Theory

    NASA Astrophysics Data System (ADS)

    Bisnovatyi-Kogan, G. S.

    Standard accretion disk theory is formulated which is based on the local heat balance. The energy produced by a turbulent viscous heating is supposed to be emitted to the sides of the disc. Sources of turbulence in the accretion disc are connected with nonlinear hydrodynamic instability, convection, and magnetic field. In standard theory there are two branches of solution, optically thick, and optically thin. Advection in accretion disks is described by the differential equations what makes the theory nonlocal. Low-luminous optically thin accretion disc model with advection at some suggestions may become advectively dominated, carrying almost all the energy inside the black hole. The proper account of magnetic filed in the process of accretion limits the energy advected into a black hole, efficiency of accretion should exceed ˜ 1/4 of the standard accretion disk model efficiency.

  18. Shock waves and shock tubes; Proceedings of the Fifteenth International Symposium, Berkeley, CA, July 28-August 2, 1985

    NASA Technical Reports Server (NTRS)

    Bershader, D. (Editor); Hanson, R. (Editor)

    1986-01-01

    A detailed survey is presented of shock tube experiments, theoretical developments, and applications being carried out worldwide. The discussions explore shock tube physics and the related chemical, physical and biological science and technology. Extensive attention is devoted to shock wave phenomena in dusty gases and other multiphase and heterogeneous systems, including chemically reactive mixtures. Consideration is given to techniques for measuring, visualizing and theoretically modeling flowfield, shock wave and rarefaction wave characteristics. Numerical modeling is explored in terms of the application of computational fluid dynamics techniques to describing flowfields in shock tubes. Shock interactions and propagation, in both solids, fluids, gases and mixed media are investigated, along with the behavior of shocks in condensed matter. Finally, chemical reactions that are initiated as the result of passage of a shock wave are discussed, together with methods of controlling the evolution of laminar separated flows at concave corners on advanced reentry vehicles.

  19. Shock waves and shock tubes; Proceedings of the Fifteenth International Symposium, Berkeley, CA, July 28-August 2, 1985

    NASA Astrophysics Data System (ADS)

    Bershader, D.; Hanson, R.

    A detailed survey is presented of shock tube experiments, theoretical developments, and applications being carried out worldwide. The discussions explore shock tube physics and the related chemical, physical and biological science and technology. Extensive attention is devoted to shock wave phenomena in dusty gases and other multiphase and heterogeneous systems, including chemically reactive mixtures. Consideration is given to techniques for measuring, visualizing and theoretically modeling flowfield, shock wave and rarefaction wave characteristics. Numerical modeling is explored in terms of the application of computational fluid dynamics techniques to describing flowfields in shock tubes. Shock interactions and propagation, in both solids, fluids, gases and mixed media are investigated, along with the behavior of shocks in condensed matter. Finally, chemical reactions that are initiated as the result of passage of a shock wave are discussed, together with methods of controlling the evolution of laminar separated flows at concave corners on advanced reentry vehicles.

  20. Accretion onto a higher dimensional black hole

    NASA Astrophysics Data System (ADS)

    John, Anslyn J.; Ghosh, Sushant G.; Maharaj, Sunil D.

    2013-11-01

    We examine the steady-state spherically symmetric accretion of relativistic fluids, with a polytropic equation of state, onto a higher-dimensional Schwarzschild black hole. The mass accretion rate, critical radius, and flow parameters are determined and compared with results obtained in standard four dimensions. The accretion rate, M˙, is an explicit function of the black hole mass, M, as well as the gas boundary conditions and the dimensionality, D, of the spacetime. We also find the asymptotic compression ratios and temperature profiles below the accretion radius and at the event horizon. This analysis is a generalization of Michel’s solution to higher dimensions and of the Newtonian expressions of Giddings and Mangano, which consider the accretion of TeV black holes.

  1. 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.

  2. 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

  3. 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.

  4. Increases to Inferred Rates of Planetesimal Accretion due to Thermohaline Mixing in Metal-accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    Bauer, Evan B.; Bildsten, Lars

    2018-06-01

    Many isolated, old white dwarfs (WDs) show surprising evidence of metals in their photospheres. Given that the timescale for gravitational sedimentation is astronomically short, this is taken as evidence for ongoing accretion, likely of tidally disrupted planetesimals. The rate of such accretion, {\\dot{M}}acc}, is important to constrain, and most modeling of this process relies on assuming an equilibrium between diffusive sedimentation and metal accretion supplied to the WD’s surface convective envelope. Building on the earlier work of Deal and collaborators, we show that high {\\dot{M}}acc} models with only diffusive sedimentation are unstable to thermohaline mixing and that models that account for the enhanced mixing from the active thermohaline instability require larger accretion rates, sometimes reaching {\\dot{M}}acc}≈ {10}13 {{g}} {{{s}}}-1 to explain observed calcium abundances. We present results from a grid of MESA models that include both diffusion and thermohaline mixing. These results demonstrate that both mechanisms are essential for understanding metal pollution across the range of polluted WDs with hydrogen atmospheres. Another consequence of active thermohaline mixing is that the observed metal abundance ratios are identical to accreted material.

  5. Chondrule Formation in Bow Shocks around Eccentric Planetary Embryos

    NASA Astrophysics Data System (ADS)

    Morris, Melissa A.; Boley, Aaron C.; Desch, Steven J.; Athanassiadou, Themis

    2012-06-01

    Recent isotopic studies of Martian meteorites by Dauphas & Pourmand have established that large (~3000 km radius) planetary embryos existed in the solar nebula at the same time that chondrules—millimeter-sized igneous inclusions found in meteorites—were forming. We model the formation of chondrules by passage through bow shocks around such a planetary embryo on an eccentric orbit. We numerically model the hydrodynamics of the flow and find that such large bodies retain an atmosphere with Kelvin-Helmholtz instabilities allowing mixing of this atmosphere with the gas and particles flowing past the embryo. We calculate the trajectories of chondrules flowing past the body and find that they are not accreted by the protoplanet, but may instead flow through volatiles outgassed from the planet's magma ocean. In contrast, chondrules are accreted onto smaller planetesimals. We calculate the thermal histories of chondrules passing through the bow shock. We find that peak temperatures and cooling rates are consistent with the formation of the dominant, porphyritic texture of most chondrules, assuming a modest enhancement above the likely solar nebula average value of chondrule densities (by a factor of 10), attributable to settling of chondrule precursors to the midplane of the disk or turbulent concentration. We calculate the rate at which a planetary embryo's eccentricity is damped and conclude that a single planetary embryo scattered into an eccentric orbit can, over ~105 years, produce ~1024 g of chondrules. In principle, a small number (1-10) of eccentric planetary embryos can melt the observed mass of chondrules in a manner consistent with all known constraints.

  6. Accretion flow dynamics during 1999 outburst of XTE J1859+226—modeling of broadband spectra and constraining the source mass

    NASA Astrophysics Data System (ADS)

    Nandi, Anuj; Mandal, S.; Sreehari, H.; Radhika, D.; Das, Santabrata; Chattopadhyay, I.; Iyer, N.; Agrawal, V. K.; Aktar, R.

    2018-05-01

    We examine the dynamical behavior of accretion flow around XTE J1859+226 during the 1999 outburst by analyzing the entire outburst data (˜166 days) from RXTE Satellite. Towards this, we study the hysteresis behavior in the hardness intensity diagram (HID) based on the broadband (3-150 keV) spectral modeling, spectral signature of jet ejection and the evolution of Quasi-periodic Oscillation (QPO) frequencies using the two-component advective flow model around a black hole. We compute the flow parameters, namely Keplerian accretion rate (\\dot{m}d), sub-Keplerian accretion rate (\\dot{m}h), shock location (rs) and black hole mass (M_{bh}) from the spectral modeling and study their evolution along the q-diagram. Subsequently, the kinetic jet power is computed as L^{obs}_{jet} ˜3-6 ×10^{37} erg s^{-1} during one of the observed radio flares which indicates that jet power corresponds to 8-16% mass outflow rate from the disc. This estimate of mass outflow rate is in close agreement with the change in total accretion rate (˜14%) required for spectral modeling before and during the flare. Finally, we provide a mass estimate of the source XTE J1859+226 based on the spectral modeling that lies in the range of 5.2-7.9 M_{⊙} with 90% confidence.

  7. 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

  8. 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.

  9. Accretion of magnetized matter into a black hole.

    NASA Astrophysics Data System (ADS)

    Bisnovatyj-Kogan, G. S.

    1999-12-01

    Accretion is the main source of energy in binary X-ray sources inside the Galaxy, and most probably in active galactic nuclei, where numerous observational data for the existence of supermassive black holes have been obtained. Standard accretion disk theory is formulated which is based on local heat balance. The whole energy produced by turbulent viscous heating is supposed to be emitted to the sides of the disk. Sources of turbulence in the accretion disk are discussed, including nonlinear hydrodynamic turbulence, convection and magnetic field. In standard theory there are two branches of solution, optically thick, anti-optically thin, which are individually self-consistent. The choice between these solutions should be done on the basis of a stability analysis. Advection in the accretion disks is described by differential equations, which makes the theory nonlocal. The low-luminosity optically thin accretion disk model with advection under some conditions may become advectively dominated, carrying almost all the energy inside the black hole. A proper account for magnetic field in the process of accretion limits the energy advected into a black hole, and does not allow the radiative efficiency of accretion to become lower than about 1/4 of the standard accretion disk model efficiency.

  10. Laser-launched flyer plate and confined laser ablation for shock wave loading: validation and applications.

    PubMed

    Paisley, Dennis L; Luo, Sheng-Nian; Greenfield, Scott R; Koskelo, Aaron C

    2008-02-01

    We present validation and some applications of two laser-driven shock wave loading techniques: laser-launched flyer plate and confined laser ablation. We characterize the flyer plate during flight and the dynamically loaded target with temporally and spatially resolved diagnostics. With transient imaging displacement interferometry, we demonstrate that the planarity (bow and tilt) of the loading induced by a spatially shaped laser pulse is within 2-7 mrad (with an average of 4+/-1 mrad), similar to that in conventional techniques including gas gun loading. Plasma heating of target is negligible, in particular, when a plasma shield is adopted. For flyer plate loading, supported shock waves can be achieved. Temporal shaping of the drive pulse in confined laser ablation allows for flexible loading, e.g., quasi-isentropic, Taylor-wave, and off-Hugoniot loading. These techniques can be utilized to investigate such dynamic responses of materials as Hugoniot elastic limit, plasticity, spall, shock roughness, equation of state, phase transition, and metallurgical characteristics of shock-recovered samples.

  11. Accretion of the terrestrial planets. II

    NASA Technical Reports Server (NTRS)

    Weidenschilling, S. J.

    1976-01-01

    The theory of gravitational accretion of the terrestrial planets is examined. The concept of a 'closed feeding zone' is somewhat unrealistic, but provides a lower bound on the accretion time. A velocity relation for planetesimals which includes an initial velocity component is suggested. The orbital parameters of the planetesimals and the dimensions of the feeding zone are related to their relative velocities. The assumption of an initial velocity does not seriously change the accretion time. Mercury, Venus, and the earth have accretion times on the order of 100 million years. Mars requires well over one billion years to accrete by the same assumptions. The lunar cratering history makes a late formation of Mars unlikely. If Mars is as old as the earth, nongravitational forces or a violation of the feeding zone concept is required. One such possibility is the removal of matter from the zone of Mars by Jupiter's influence. The final sweeping up by Mars would result in the scattering of a considerable mass among the other terrestrial planets. The late postaccretional bombardments inferred for the moon and Mercury may have had this source.

  12. Classical Accreting Pulsars with NICER

    NASA Technical Reports Server (NTRS)

    Wilson-Hodge, Colleen A.

    2014-01-01

    Soft excesses are very common center dot Lx > 1038 erg/s - reprocessing by optically thick material at the inner edge of the accretion disk center dot Lx < 1036 erg/s - photoionized or collisionally heated diffuse gas or thermal emission from the NS surface center dot Lx 1037 erg/s - either or both types of emission center dot NICER observations of soft excesses in bright X-ray pulsars combined with reflection modeling will constrain the ionization state, metalicity and dynamics of the inner edge of the magnetically truncated accretion disk Reflection models of an accretion disk for a hard power law - Strong soft excess below 3 keV from hot X-ray heated disk - For weakly ionized case: strong recombination lines - Are we seeing changes in the disk ionization in 4U1626-26? 13 years of weekly monitoring with RXTE PCA center dot Revealed an unexpectedly large population of Be/X-ray binaries compared to the Milky Way center dot Plotted luminosities are typical of "normal" outbursts (once per orbit) center dot The SMC provides an excellent opportunity to study a homogenous population of HMXBs with low interstellar absorption for accretion disk studies. Monitoring with NICER will enable studies of accretion disk physics in X-ray pulsars center dot The SMC provides a potential homogeneous low-absorption population for this study center dot NICER monitoring and TOO observations will also provide measurements of spinfrequencies, QPOs, pulsed fluxes, and energy spectra.

  13. THE MAGELLANIC STREAM: BREAK-UP AND ACCRETION ONTO THE HOT GALACTIC CORONA

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

    Tepper-García, Thor; Bland-Hawthorn, Joss; Sutherland, Ralph S.

    The Magellanic H i Stream (≈2 × 10{sup 9} M{sub ⊙} [d/55 kpc]{sup 2}) encircling the Galaxy at a distance d is arguably the most important tracer of what happens to gas accreting onto a disk galaxy. Recent observations reveal that the Stream’s mass is in fact dominated (3:1) by its ionized component. Here we revisit the origin of the mysterious Hα recombination emission observed along much of its length that is overly bright (∼150–200 mR) for the known Galactic ultraviolet (UV) background (≈20–40 mR [d/55 kpc]{sup −2}). In an earlier model, we proposed that a slow shock cascade wasmore » operating along the Stream due to its interaction with the extended Galactic hot corona. We find that for a smooth coronal density profile, this model can explain the bright Hα emission if the coronal density satisfies 2 × 10{sup −4} < (n/cm{sup −3}) < 4 × 10{sup −4} at d = 55 kpc. But in view of updated parameters for the Galactic halo and mounting evidence that most of the Stream must lie far beyond the Magellanic Clouds (d > 55 kpc), we revisit the shock cascade model in detail. At lower densities, the H i gas is broken down by the shock cascade but mostly mixes with the hot corona without significant recombination. At higher densities, the hot coronal mass (including the other baryonic components) exceeds the baryon budget of the Galaxy. If the Hα emission arises from the shock cascade, the upper limit on the smooth coronal density constrains the Stream’s mean distance to ≲75 kpc. If, as some models indicate, the Stream is even further out, either the shock cascade is operating in a regime where the corona is substantially mass-loaded with recent gas debris, or an entirely different ionization mechanism is responsible.« less

  14. Fractionation and Accretion of Meteorite Parent Bodies

    NASA Technical Reports Server (NTRS)

    Weidenschilling, Stuart J.

    2005-01-01

    Senior Scientist Stuart J. Weidenschilling presents his final administrative report for the research program on which he was the Principal Investigator. The research program resulted in the following publications: 1) Particle-gas dynamics and primary accretion. J. N. Cuzzi and S. J . Weidenschilling. To appear in Meteorites and the Early Solar System 11 (D. Lauretta et a]., Eds.), Univ. Arizona Press. 2005; 2) Timescales of the solar protoplanetary disk. S. Russell, L. Hartmann, J . N. Cuzzi, A. Krot, M. Gounelle and S. J. Weidenschilling. To appear in Meteorites and the Early Solar System II (D. Lauretta et al., Eds.), Univ. Arizona Press, 2005; 3) Nebula evolution of thermally processed solids: Reconciling astrophysical models and chondritic meteorites. J. N. Cuzzi, F. J. Ciesla, M. I. Petaev, A. N. Krot, E. R. D. Scott and S . J. Weidenschilling. To appear in Chondrites and the Protoplanetary Disk (A. Krot et a]., Eds.), ASP Conference Series, 2005; 4) Possible chondrule formation in planetesimal bow shocks: Physical processes in the near vicinity of the planetesimal. L. L. Hood, F. J. Ciesla and S. J. Weidenschilling. To appear in Chondrites and the Protoplanetary Disk (A. Krot et al., Eds.), ASP Conference Series, 2005; 5) From icy grains to comets. In Comets II (M. Festou et al., Eds.), Univ. Arizona Press, pp. 97- 104, 2005; 6) Evaluating planetesimal bow shocks as sites for chondrule formation. F. J . Ciesla, L. L. Hood and S. J. Weidenschilling. Meteoritics & Planetary Science 39, 1809-1 821, 2004; and 7) Radial drift of particles in the solar nebula: Implications for planetesimal formation. Icarus 165, 438-442, 2003.

  15. Spectroscopic Detection of a Stellar-like Photosphere in an Accreting Protostar

    NASA Technical Reports Server (NTRS)

    Greene, Thomas P.; Lada, Charles J.; DeVincenzi, Donald L. (Technical Monitor)

    2002-01-01

    We present high-resolution (R is approximately equal to 18,000), high signal-to-noise 2 micron spectra of two luminous, X-ray flaring Class I protostars in the rho Ophiuchi cloud acquired with the NIRSPEC (near infrared spectrograph) of the Keck II telescope. We present the first spectrum of a highly veiled, strongly accreting protostar which shows photospheric absorption features and demonstrates the stellar nature of its central core. We find the spectrum of the luminous (L (sub bol) = 10 solar luminosity) protostellar source, YLW 15, to be stellar-like with numerous atomic and molecular absorption features, indicative of a K5 IV/V spectral type and a continuum veiling r(sub k) = 3.0. Its derived stellar luminosity (3 stellar luminosity) and stellar radius (3.1 solar radius) are consistent with those of a 0.5 solar mass pre-main-sequence star. However, 70% of its bolometric luminosity is due to mass accretion, whose rate we estimate to be 1.7 x 10(exp -6) solar masses yr(exp -1). We determine that excess infrared emission produced by the circumstellar accretion disk, the inner infalling envelope, and accretion shocks at the surface of the stellar core of YLW 15 all contribute significantly to its near-IR (infrared) continuum veiling. Its rotational velocity v sin i = 50 km s(exp -1) is comparable to those of flat-spectrum protostars but considerably higher than those of classical T Tauri stars in the rho Oph cloud. The protostar may be magnetically coupled to its circumstellar disk at a radius of 2 - 3 R(sub *). It is also plausible that this protostar can shed over half its angular momentum and evolve into a more slowly rotating classical T Tauri star by remaining coupled to its circumstellar disk (at increasing radius) as its accretion rate drops by an order of magnitude during the rapid transition between the Class I and Class II phases of evolution. The spectrum of WL 6 does not show any photospheric absorption features, and we estimate that its continuum

  16. 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.

  17. 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

  18. The case for electron re-acceleration at galaxy cluster shocks

    NASA Astrophysics Data System (ADS)

    van Weeren, Reinout J.; Andrade-Santos, Felipe; Dawson, William A.; Golovich, Nathan; Lal, Dharam V.; Kang, Hyesung; Ryu, Dongsu; Brìggen, Marcus; Ogrean, Georgiana A.; Forman, William R.; Jones, Christine; Placco, Vinicius M.; Santucci, Rafael M.; Wittman, David; Jee, M. James; Kraft, Ralph P.; Sobral, David; Stroe, Andra; Fogarty, Kevin

    2017-01-01

    On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-sized radio sources have been found1,2 . Also known as radio relics, these regions of diffuse radio emission are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach-number shocks generated by cluster-cluster merger events 3 . A long-standing problem is how low-Mach-number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here, we report the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411-3412 by combining radio, X-ray and optical observations. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.

  19. The case for electron re-acceleration at galaxy cluster shocks

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

    van Weeren, Reinout J.; Andrade-Santos, Felipe; Dawson, William A.

    On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-sized radio sources have been found. Also known as radio relics, these regions of diffuse radio emission are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach-number shocks generated by cluster–cluster merger events. A long-standing problem is how low-Mach-number shocks can acceleratemore » electrons so efficiently to explain the observed radio relics. Here, we report the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411–3412 by combining radio, X-ray and optical observations. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. Lastly, it also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.« less

  20. The case for electron re-acceleration at galaxy cluster shocks

    DOE PAGES

    van Weeren, Reinout J.; Andrade-Santos, Felipe; Dawson, William A.; ...

    2017-01-04

    On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-sized radio sources have been found. Also known as radio relics, these regions of diffuse radio emission are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach-number shocks generated by cluster–cluster merger events. A long-standing problem is how low-Mach-number shocks can acceleratemore » electrons so efficiently to explain the observed radio relics. Here, we report the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411–3412 by combining radio, X-ray and optical observations. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. Lastly, it also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.« less

  1. Stochastic events lead to accretion in Saturn's rings

    NASA Astrophysics Data System (ADS)

    Esposito, Larry W.

    2010-05-01

    UVIS occultations indicate accretion is triggered at the B ring edge, in strong density waves in ring A and in the F ring. Moons may trigger accretion by streamline crowding (Lewis & Stewart); which enhances collisions, leading to accretion; increasing random velocities; leading to more collisions and more accretion. Cassini occultations of these strongly perturbed locations show not only accretion but also disaggregation, with time scales of hours to weeks. The collisions may lead to temporary aggregations via stochastic events: collisions can compress unconsolidated objects, trigger adhesion or bring small pieces into contact with larger or higher-density seeds. Disaggregation then can follow from disruptive collisions or tidal shedding. In the accretion/disruption balance, increased random motions could eventually give the upper hand to disruption… just as ‘irrational exuberance' can lead to financial panic in the economy; or the overpopulation of hares can lead to boom-and-bust in the population of foxes. This unstable equilibrium can similarly give rise to episodic cycles in accretion: explaining why the observable ring features that indicate embedded objects have been increasing since the beginning of Cassini's observations of Saturn in 2004.

  2. The formation of massive molecular filaments and massive stars triggered by a magnetohydrodynamic shock wave

    NASA Astrophysics Data System (ADS)

    Inoue, Tsuyoshi; Hennebelle, Patrick; Fukui, Yasuo; Matsumoto, Tomoaki; Iwasaki, Kazunari; Inutsuka, Shu-ichiro

    2018-05-01

    Recent observations suggest an that intensive molecular cloud collision can trigger massive star/cluster formation. The most important physical process caused by the collision is a shock compression. In this paper, the influence of a shock wave on the evolution of a molecular cloud is studied numerically by using isothermal magnetohydrodynamics simulations with the effect of self-gravity. Adaptive mesh refinement and sink particle techniques are used to follow the long-time evolution of the shocked cloud. We find that the shock compression of a turbulent inhomogeneous molecular cloud creates massive filaments, which lie perpendicularly to the background magnetic field, as we have pointed out in a previous paper. The massive filament shows global collapse along the filament, which feeds a sink particle located at the collapse center. We observe a high accretion rate \\dot{M}_acc> 10^{-4} M_{⊙}yr-1 that is high enough to allow the formation of even O-type stars. The most massive sink particle achieves M > 50 M_{⊙} in a few times 105 yr after the onset of the filament collapse.

  3. Magnetized, mass-loaded, rotating accretion flows

    NASA Astrophysics Data System (ADS)

    Toniazzo, T.; Hartquist, T. W.; Durisen, R. H.

    2001-03-01

    We present a semi-analytical investigation of a simple one-dimensional, steady-state model for a mass-loaded, rotating, magnetized, hydrodynamical flow. Our approach is analogous to one used in early studies of magnetized winds. The model represents the infall towards a central point mass of the gas generated in a cluster of stars surrounding it, as is likely to occur in some active nuclei and starburst galaxies. We describe the properties of the different classes of infall solutions. We find that the flow becomes faster than the fast-mode speed, and hence decoupled from the centre, only for a limited range of parameter values, and when magnetic stresses are ineffective. Such flow is slowed as it approaches a centrifugal barrier, implying the existence of an accretion disc. When the flow does not become super-fast and the magnetic torque is insufficient, no steady solution extending inward to the centre exists. Finally, with a larger magnetic torque, solutions representing steady sub-Alfvénic flows are found, which can resemble spherical hydrodynamical infall. Such solutions, if applicable, would imply that rotation is not important and that any accretion disc formed would be of very limited size.

  4. Preliminary Evidence for a Virial Shock around the Coma Galaxy Cluster

    NASA Astrophysics Data System (ADS)

    Keshet, Uri; Kushnir, Doron; Loeb, Abraham; Waxman, Eli

    2017-08-01

    Galaxy clusters, the largest gravitationally bound objects in the universe, are thought to grow by accreting mass from their surroundings through large-scale virial shocks. Due to electron acceleration in such a shock, it should appear as a γ-ray, hard X-ray, and radio ring, elongated toward the large-scale filaments feeding the cluster, coincident with a cutoff in the thermal Sunyaev-Zel’dovich (SZ) signal. However, no such signature was found until now, and the very existence of cluster virial shocks has remained a theory. We find preliminary evidence for a large γ-ray ring of ˜ 5 {Mpc} minor axis around the Coma cluster, elongated toward the large-scale filament connecting Coma and Abell 1367, detected at the nominal 2.7σ confidence level (5.1σ using control signal simulations). The γ-ray ring correlates both with a synchrotron signal and with the SZ cutoff, but not with Galactic tracers. The γ-ray and radio signatures agree with analytic and numerical predictions if the shock deposits ˜ 1 % of the thermal energy in relativistic electrons over a Hubble time and ˜ 1 % in magnetic fields. The implied inverse Compton and synchrotron cumulative emission from similar shocks can contribute significantly to the diffuse extragalactic γ-ray and low-frequency radio backgrounds. Our results, if confirmed, reveal the prolate structure of the hot gas in Coma, the feeding pattern of the cluster, and properties of the surrounding large-scale voids and filaments. The anticipated detection of such shocks around other clusters would provide a powerful new cosmological probe.

  5. Collisionless Shocks and Particle Acceleration.

    NASA Astrophysics Data System (ADS)

    Malkov, M.

    2016-12-01

    Collisionless shocks emerged in the 50s and 60s of the last century as an important branch of plasma physics and have remained ever since. New applications pose new challenges to our understanding of collisionless shock mechanisms. Particle acceleration in astrophysical settings, primarily studied concerning the putative origin of cosmic rays (CR) in supernova remnant (SNR) shocks, stands out with the collisionless shock mechanism being the key. Among recent laboratory applications, a laser-based tabletop proton accelerator is an affordable compact alternative to big synchrotron accelerators. The much-anticipated proof of cosmic ray (CR) acceleration in supernova remnants is hindered by our limited understanding of collisionless shock mechanisms. Over the last decade, dramatically improved observations were puzzling the theorists with unexpected discoveries. The difference between the helium/carbon and proton CR rigidity (momentum to charge ratio) spectra, seemingly inconsistent with the acceleration and propagation theories, and the perplexing positron excess in the 10-300 GeV range are just two recent examples. The latter is now also actively discussed in the particle physics and CR communities as a possible signature of decay or annihilation of hypothetical dark matter particles. By considering an initial (injection) phase of a diffusive shock acceleration mechanism, including particle reflection off the shock front - where an elemental similarity of particle dynamics does not apply - I will discuss recent suggestions of how to address the new data from the collisionless shock perspective. The backreaction of accelerated particles on the shock structure, its environment, and visibility across the electromagnetic spectrum from radio to gamma rays is another key aspect of collisionless shock that will be discussed.

  6. Gravitomagnetic acceleration from black hole accretion disks

    NASA Astrophysics Data System (ADS)

    Poirier, J.; Mathews, G. J.

    2016-05-01

    We demonstrate how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near an accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism contributing to the production of jets, it presents a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  7. Shock waves data for minerals

    NASA Technical Reports Server (NTRS)

    Ahrens, Thomas J.; Johnson, Mary L.

    1994-01-01

    Shock compression of the materials of planetary interiors yields data which upon comparison with density-pressure and density-sound velocity profiles constrain internal composition and temperature. Other important applications of shock wave data and related properties are found in the impact mechanics of terrestrial planets and solid satellites. Shock wave equation of state, shock-induced dynamic yielding and phase transitions, and shock temperature are discussed. In regions where a substantial phase change in the material does not occur, the relationship between the particle velocity, U(sub p), and the shock velocity, U(sub s), is given by U(sub s) = C(sub 0) + S U(sub p), where C(sub 0) is the shock velocity at infinitesimally small particle velocity, or the ambient pressure bulk sound velocity. Numerical values for the shock wave equation of state for minerals and related materials of the solar system are provided.

  8. 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.

  9. SPREADING LAYERS IN ACCRETING OBJECTS: ROLE OF ACOUSTIC WAVES FOR ANGULAR MOMENTUM TRANSPORT, MIXING, AND THERMODYNAMICS

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

    Philippov, Alexander A.; Rafikov, Roman R.; Stone, James M., E-mail: sashaph@princeton.edu

    Disk accretion at a high rate onto a white dwarf (WD) or a neutron star has been suggested to result in the formation of a spreading layer (SL)—a belt-like structure on the object's surface, in which the accreted matter steadily spreads in the poleward (meridional) direction while spinning down. To assess its basic characteristics, we perform two-dimensional hydrodynamic simulations of supersonic SLs in the relevant morphology with a simple prescription for cooling. We demonstrate that supersonic shear naturally present at the base of the SL inevitably drives sonic instability that gives rise to large-scale acoustic modes governing the evolution ofmore » the SL. These modes dominate the transport of momentum and energy, which is intrinsically global and cannot be characterized via some form of local effective viscosity (e.g., α-viscosity). The global nature of the wave-driven transport should have important implications for triggering Type I X-ray bursts in low-mass X-ray binaries. The nonlinear evolution of waves into a system of shocks drives effective rearrangement (sensitively depending on thermodynamical properties of the flow) and deceleration of the SL, which ultimately becomes transonic and susceptible to regular Kelvin–Helmholtz instability. We interpret this evolution in terms of the global structure of the SL and suggest that mixing of the SL material with the underlying stellar fluid should become effective only at intermediate latitudes on the accreting object's surface, where the flow has decelerated appreciably. In the near-equatorial regions the transport is dominated by acoustic waves and mixing is less efficient. We speculate that this latitudinal nonuniformity of mixing in accreting WDs may be linked to the observed bipolar morphology of classical nova ejecta.« less

  10. Counter-rotating accretion discs

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

    Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud on to the surface of an existing corotating disc or from the counter-rotating gas moving radially inwards to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc centre. We discuss high-resolution axisymmetric hydrodynamic simulations of viscous counter-rotating discs for the cases where the two components are vertically separated and radially separated. The viscosity is described by an isotropic α-viscosity including all terms in the viscous stress tensor. For the vertically separated components, a shear layer forms between them and the middle part of this layer free-falls to the disc centre. The accretion rates are increased by factors of ˜102-104 over that for a conventional disc rotating in one direction with the same viscosity. The vertical width of the shear layer and the accretion rate are strongly dependent on the viscosity and the mass fraction of the counter-rotating gas. In the case of radially separated components where the inner disc corotates and the outer disc rotates in the opposite direction, a gap between the two components opens and closes quasi-periodically. The accretion rates are ≳25 times larger than those for a disc rotating in one direction with the same viscosity.

  11. The Effects of the Critical Ice Accretion on Airfoil and Wing Performance

    NASA Technical Reports Server (NTRS)

    Selig, Michael S.; Bragg, Michael B.; Saeed, Farooq

    1998-01-01

    In support of the NASA Lewis Modern Airfoils Ice Accretion Test Program, the University of Illinois at Urbana-Champaign provided expertise in airfoil design and aerodynamic analysis to determine the aerodynamic effect of ice accretion on modern airfoil sections. The effort has concentrated on establishing a design/testing methodology for "hybrid airfoils" or "sub-scale airfoils," that is, airfoils having a full-scale leading edge together with a specially designed and foreshortened aft section. The basic approach of using a full-scale leading edge with a foreshortened aft section was considered to a limited extent over 40 years ago. However, it was believed that the range of application of the method had not been fully exploited. Thus a systematic study was being undertaken to investigate and explore the range of application of the method so as to determine its overall potential.

  12. Reynolds-averaged Navier-Stokes based ice accretion for aircraft wings

    NASA Astrophysics Data System (ADS)

    Lashkajani, Kazem Hasanzadeh

    algorithm. A PDE based multi-block structured grid generation code, NSGRID, is developed for this purpose. The study includes the developments of novel mesh generation algorithms over complex glaze ice shapes containing multi-curvature ice accretion geometries, such as single/double ice horns. The twofold approaches tackle surface geometry discretization as well as field mesh generation. An adaptive curvilinear curvature control algorithm is constructed solving a 1D elliptic PDE equation with periodic source terms. This method controls the arclength grid spacing so that high convex and concave curvature regions around ice horns are appropriately captured and is shown to effectively treat the grid shock problem. Then, a novel blended method is developed by defining combinations of source terms with 2D elliptic equations. The source terms include two common control functions, Sorenson and Spekreijse, and an additional third source term to improve orthogonality. This blended method is shown to be very effective for improving grid quality metrics for complex glaze ice meshes with RANS resolution. The performance in terms of residual reduction per non-linear iteration of several solution algorithms (Point-Jacobi, Gauss-Seidel, ADI, Point and Line SOR) are discussed within the context of a full Multi-grid operator. Details are given on the various formulations used in the linearization process. It is shown that the performance of the solution algorithm depends on the type of control function used. Finally, the algorithms are validated on standard complex experimental ice shapes, demonstrating the applicability of the methods. Finally, the automated framework of RANS based two-dimensional multi-step ice accretion, CANICE2D-NS is developed, coupled with a Multi-Block Navier-Stokes CFD code, NSCODE2D, a Multi-Block elliptic grid generation code, NSGRID2D, and a Multi-Block Eulerian droplet solver, NSDROP2D (developed at Polytechnique Montreal). The framework allows Lagrangian and

  13. Review of gravitomagnetic acceleration from accretion disks

    NASA Astrophysics Data System (ADS)

    Poirier, J.; Mathews, G. J.

    2015-11-01

    We review the development of the equations of gravitoelectromagnetism and summarize how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near the accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism to produce collimated jets, it is a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  14. Accretion onto a noncommutative geometry inspired black hole

    NASA Astrophysics Data System (ADS)

    Kumar, Rahul; Ghosh, Sushant G.

    2017-09-01

    The spherically symmetric accretion onto a noncommutative (NC) inspired Schwarzschild black hole is treated for a polytropic fluid. The critical accretion rate \\dot{M}, sonic speed a_s and other flow parameters are generalized for the NC inspired static black hole and compared with the results obtained for the standard Schwarzschild black holes. Also explicit expressions for gas compression ratios and temperature profiles below the accretion radius and at the event horizon are derived. This analysis is a generalization of Michel's solution to the NC geometry. Owing to the NC corrected black hole, the accretion flow parameters also have been modified. It turns out that \\dot{M} ≈ {M^2} is still achievable but r_s seems to be substantially decreased due to the NC effects. They in turn do affect the accretion process.

  15. Helium shell flashes and evolution of accreting white dwarfs

    NASA Astrophysics Data System (ADS)

    Fujimoto, M. Y.; Sugimoto, D.

    1982-06-01

    The evolution of accreting white dwarfs is investigated from the onset of accretion through the helium shell flash. Properties of the helium shell flashes are studied by means of a generalized theory of shell flash and by numerical computations, and it is found that the shell flash grows up to the strength of a supernova explosion when the mass of the helium zone is large enough on a massive white dwarf. Although accretion onto a hot white dwarf causes a weaker shell flash than those onto cool ones, a strong tendency exists for the strength to be determined mainly by the accretion rate. For fast accretion, the shell flashes are weak and triggered recurrently, while for slow accretion the helium shell flash, once triggered, develops into a detonation supernova.

  16. Pulsed Accretion in the T Tauri Binary TWA 3A

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

    Tofflemire, Benjamin M.; Mathieu, Robert D.; Herczeg, Gregory J.

    TWA 3A is the most recent addition to a small group of young binary systems that both actively accrete from a circumbinary disk and have spectroscopic orbital solutions. As such, it provides a unique opportunity to test binary accretion theory in a well-constrained setting. To examine TWA 3A’s time-variable accretion behavior, we have conducted a two-year, optical photometric monitoring campaign, obtaining dense orbital phase coverage (∼20 observations per orbit) for ∼15 orbital periods. From U -band measurements we derive the time-dependent binary mass accretion rate, finding bursts of accretion near each periastron passage. On average, these enhanced accretion events evolvemore » over orbital phases 0.85 to 1.05, reaching their peak at periastron. The specific accretion rate increases above the quiescent value by a factor of ∼4 on average but the peak can be as high as an order of magnitude in a given orbit. The phase dependence and amplitude of TWA 3A accretion is in good agreement with numerical simulations of binary accretion with similar orbital parameters. In these simulations, periastron accretion bursts are fueled by periodic streams of material from the circumbinary disk that are driven by the binary orbit. We find that TWA 3A’s average accretion behavior is remarkably similar to DQ Tau, another T Tauri binary with similar orbital parameters, but with significantly less variability from orbit to orbit. This is only the second clear case of orbital-phase-dependent accretion in a T Tauri binary.« less

  17. Shock-induced bubble jetting into a viscous fluid with application to tissue injury in shock-wave lithotripsy.

    PubMed

    Freund, J B; Shukla, R K; Evan, A P

    2009-11-01

    Shock waves in liquids are known to cause spherical gas bubbles to rapidly collapse and form strong re-entrant jets in the direction of the propagating shock. The interaction of these jets with an adjacent viscous liquid is investigated using finite-volume simulation methods. This configuration serves as a model for tissue injury during shock-wave lithotripsy, a medical procedure to remove kidney stones. In this case, the viscous fluid provides a crude model for the tissue. It is found that for viscosities comparable to what might be expected in tissue, the jet that forms upon collapse of a small bubble fails to penetrate deeply into the viscous fluid "tissue." A simple model reproduces the penetration distance versus viscosity observed in the simulations and leads to a phenomenological model for the spreading of injury with multiple shocks. For a reasonable selection of a single efficiency parameter, this model is able to reproduce in vivo observations of an apparent 1000-shock threshold before wide-spread tissue injury occurs in targeted kidneys and the approximate extent of this injury after a typical clinical dose of 2000 shock waves.

  18. Shock-induced bubble jetting into a viscous fluid with application to tissue injury in shock-wave lithotripsy

    PubMed Central

    Freund, J. B.; Shukla, R. K.; Evan, A. P.

    2009-01-01

    Shock waves in liquids are known to cause spherical gas bubbles to rapidly collapse and form strong re-entrant jets in the direction of the propagating shock. The interaction of these jets with an adjacent viscous liquid is investigated using finite-volume simulation methods. This configuration serves as a model for tissue injury during shock-wave lithotripsy, a medical procedure to remove kidney stones. In this case, the viscous fluid provides a crude model for the tissue. It is found that for viscosities comparable to what might be expected in tissue, the jet that forms upon collapse of a small bubble fails to penetrate deeply into the viscous fluid “tissue.” A simple model reproduces the penetration distance versus viscosity observed in the simulations and leads to a phenomenological model for the spreading of injury with multiple shocks. For a reasonable selection of a single efficiency parameter, this model is able to reproduce in vivo observations of an apparent 1000-shock threshold before wide-spread tissue injury occurs in targeted kidneys and the approximate extent of this injury after a typical clinical dose of 2000 shock waves. PMID:19894850

  19. General-relativistic Simulations of Four States of Accretion onto Millisecond Pulsars

    NASA Astrophysics Data System (ADS)

    Parfrey, Kyle; Tchekhovskoy, Alexander

    2017-12-01

    Accreting neutron stars can power a wide range of astrophysical phenomena including short- and long-duration gamma-ray bursts, ultra-luminous X-ray sources, and X-ray binaries. Numerical simulations are a valuable tool for studying the accretion-disk–magnetosphere interaction that is central to these problems, most clearly for the recently discovered transitional millisecond pulsars. However, magnetohydrodynamic (MHD) methods, widely used for simulating accretion, have difficulty in highly magnetized stellar magnetospheres, while force-free methods, suitable for such regions, cannot include the accreting gas. We present an MHD method that can stably evolve essentially force-free, highly magnetized regions, and describe the first time-dependent relativistic simulations of magnetized accretion onto millisecond pulsars. Our axisymmetric general-relativistic MHD simulations for the first time demonstrate how the interaction of a turbulent accretion flow with a pulsar’s electromagnetic wind can lead to the transition of an isolated pulsar to the accreting state. This transition naturally leads to the formation of relativistic jets, whose power can greatly exceed the power of the isolated pulsar’s wind. If the accretion rate is below a critical value, the pulsar instead expels the accretion stream. More generally, our simulations produce for the first time the four possible accretion regimes, in order of decreasing mass accretion rate: (a) crushed 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 wind.

  20. 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.

  1. Development of 3D Ice Accretion Measurement Method

    NASA Technical Reports Server (NTRS)

    Lee, Sam; Broeren, Andy P.; Addy, Harold E., Jr.; Sills, Robert; Pifer, Ellen M.

    2012-01-01

    Icing wind tunnels are designed to simulate in-flight icing environments. The chief product of such facilities is the ice accretion that forms on various test articles. Documentation of the resulting ice accretion key piece of data in icing-wind-tunnel tests. Number of currently used options for documenting ice accretion in icing-wind-tunnel testing.

  2. The Advanced Composition Explorer Shock Database and Application to Particle Acceleration Theory

    NASA Technical Reports Server (NTRS)

    Parker, L. Neergaard; Zank, G. P.

    2015-01-01

    The theory of particle acceleration via diffusive shock acceleration (DSA) has been studied in depth by Gosling et al. (1981), van Nes et al. (1984), Mason (2000), Desai et al. (2003), Zank et al. (2006), among many others. Recently, Parker and Zank (2012, 2014) and Parker et al. (2014) using the Advanced Composition Explorer (ACE) shock database at 1 AU explored two questions: does the upstream distribution alone have enough particles to account for the accelerated downstream distribution and can the slope of the downstream accelerated spectrum be explained using DSA? As was shown in this research, diffusive shock acceleration can account for a large population of the shocks. However, Parker and Zank (2012, 2014) and Parker et al. (2014) used a subset of the larger ACE database. Recently, work has successfully been completed that allows for the entire ACE database to be considered in a larger statistical analysis. We explain DSA as it applies to single and multiple shocks and the shock criteria used in this statistical analysis. We calculate the expected injection energy via diffusive shock acceleration given upstream parameters defined from the ACE Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM) data to construct the theoretical upstream distribution. We show the comparison of shock strength derived from diffusive shock acceleration theory to observations in the 50 keV to 5 MeV range from an instrument on ACE. Parameters such as shock velocity, shock obliquity, particle number, and time between shocks are considered. This study is further divided into single and multiple shock categories, with an additional emphasis on forward-forward multiple shock pairs. Finally with regard to forward-forward shock pairs, results comparing injection energies of the first shock, second shock, and second shock with previous energetic population will be given.

  3. The Advanced Composition Explorer Shock Database and Application to Particle Acceleration Theory

    NASA Technical Reports Server (NTRS)

    Parker, L. Neergaard; Zank, G. P.

    2015-01-01

    The theory of particle acceleration via diffusive shock acceleration (DSA) has been studied in depth by Gosling et al. (1981), van Nes et al. (1984), Mason (2000), Desai et al. (2003), Zank et al. (2006), among many others. Recently, Parker and Zank (2012, 2014) and Parker et al. (2014) using the Advanced Composition Explorer (ACE) shock database at 1 AU explored two questions: does the upstream distribution alone have enough particles to account for the accelerated downstream distribution and can the slope of the downstream accelerated spectrum be explained using DSA? As was shown in this research, diffusive shock acceleration can account for a large population of the shocks. However, Parker and Zank (2012, 2014) and Parker et al. (2014) used a subset of the larger ACE database. Recently, work has successfully been completed that allows for the entire ACE database to be considered in a larger statistical analysis. We explain DSA as it applies to single and multiple shocks and the shock criteria used in this statistical analysis. We calculate the expected injection energy via diffusive shock acceleration given upstream parameters defined from the ACE Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM) data to construct the theoretical upstream distribution. We show the comparison of shock strength derived from diffusive shock acceleration theory to observations in the 50 keV to 5 MeV range from an instrument on ACE. Parameters such as shock velocity, shock obliquity, particle number, and time between shocks are considered. This study is further divided into single and multiple shock categories, with an additional emphasis on forward-forward multiple shock pairs. Finally with regard to forwardforward shock pairs, results comparing injection energies of the first shock, second shock, and second shock with previous energetic population will be given.

  4. The multiplicity and anisotropy of galactic satellite accretion

    NASA Astrophysics Data System (ADS)

    Shao, Shi; Cautun, Marius; Frenk, Carlos S.; Grand, Robert J. J.; Gómez, Facundo A.; Marinacci, Federico; Simpson, Christine M.

    2018-05-01

    We study the incidence of group and filamentary dwarf galaxy accretion into Milky Way (MW) mass haloes using two types of hydrodynamical simulations: EAGLE, which resolves a large cosmological volume, and the AURIGA suite, which are very high resolution zoom-in simulations of individual MW-sized haloes. The present-day 11 most massive satellites are predominantly (75 per cent) accreted in single events, 14 per cent in pairs, and 6 per cent in triplets, with higher group multiplicities being unlikely. Group accretion becomes more common for fainter satellites, with 60 per cent of the top 50 satellites accreted singly, 12 per cent in pairs, and 28 per cent in richer groups. A group similar in stellar mass to the Large Magellanic Cloud would bring on average 15 members with stellar mass larger than 104 M⊙. Half of the top 11 satellites are accreted along the two richest filaments. The accretion of dwarf galaxies is highly anisotropic, taking place preferentially perpendicular to the halo minor axis, and, within this plane, preferentially along the halo major axis. The satellite entry points tend to be aligned with the present-day central galaxy disc and satellite plane, but to a lesser extent than with the halo shape. Dwarfs accreted in groups or along the richest filament have entry points that show an even larger degree of alignment with the host halo than the full satellite population. We also find that having most satellites accreted as a single group or along a single filament is unlikely to explain the MW disc of satellites.

  5. 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

  6. Active galactic nuclei. III - Accretion flow in an externally supplied cluster of black holes

    NASA Technical Reports Server (NTRS)

    Pacholczyk, A. G.; Stoeger, W. R.; Stepinski, T. F.

    1989-01-01

    This third paper in the series modeling QSOs and AGNs as clusters of accreting black holes studies the accretion flow within an externally supplied cluster. Significant radiation will be emitted by the cluster core, but the black holes in the outer halo, where the flow is considered spherically symmetric, will not contribute much to the overall luminosity of the source because of their large velocities relative to the infalling gas and therefore their small accretion radii. As a result, the scenario discussed in Paper I will refer to the cluster cores, rather than to entire clusters. This will steepen the high-frequency region of the spectrum unless inverse Compton scattering is effective. In many cases accretion flow in the central part of the cluster will be optically thick to electron scattering, resulting in a spectrum featuring optically thick radiative component in addition to power-law regimes. The fitting of these spectra to QSO and AGN observations is discussed, and application to 3C 273 is worked out as an example.

  7. Accretion physics: It's not U, it's B

    NASA Astrophysics Data System (ADS)

    Miller, Jon

    2017-03-01

    Black holes grow by accreting mass, but the process is messy and redistributes gas and energy into their environments. New evidence shows that magnetic processes mediate both the accretion and ejection of matter.

  8. Misaligned Accretion and Jet Production

    NASA Astrophysics Data System (ADS)

    King, Andrew; Nixon, Chris

    2018-04-01

    Disk accretion onto a black hole is often misaligned from its spin axis. If the disk maintains a significant magnetic field normal to its local plane, we show that dipole radiation from Lense–Thirring precessing disk annuli can extract a significant fraction of the accretion energy, sharply peaked toward small disk radii R (as R ‑17/2 for fields with constant equipartition ratio). This low-frequency emission is immediately absorbed by surrounding matter or refracted toward the regions of lowest density. The resultant mechanical pressure, dipole angular pattern, and much lower matter density toward the rotational poles create a strong tendency to drive jets along the black hole spin axis, similar to the spin-axis jets of radio pulsars, also strong dipole emitters. The coherent primary emission may explain the high brightness temperatures seen in jets. The intrinsic disk emission is modulated at Lense–Thirring frequencies near the inner edge, providing a physical mechanism for low-frequency quasi-periodic oscillations (QPOs). Dipole emission requires nonzero hole spin, but uses only disk accretion energy. No spin energy is extracted, unlike the Blandford–Znajek process. Magnetohydrodynamic/general-relativistic magnetohydrodynamic (MHD/GRMHD) formulations do not directly give radiation fields, but can be checked post-process for dipole emission and therefore self-consistency, given sufficient resolution. Jets driven by dipole radiation should be more common in active galactic nuclei (AGN) than in X-ray binaries, and in low accretion-rate states than high, agreeing with observation. In non-black hole accretion, misaligned disk annuli precess because of the accretor’s mass quadrupole moment, similarly producing jets and QPOs.

  9. Evolving non-thermal electrons in simulations of black hole accretion

    NASA Astrophysics Data System (ADS)

    Chael, Andrew A.; Narayan, Ramesh; Saḑowski, Aleksander

    2017-09-01

    Current simulations of hot accretion flows around black holes assume either a single-temperature gas or, at best, a two-temperature gas with thermal ions and electrons. However, processes like magnetic reconnection and shocks can accelerate electrons into a non-thermal distribution, which will not quickly thermalize at the very low densities found in many systems. Such non-thermal electrons have been invoked to explain the infrared and X-ray spectra and strong variability of Sagittarius A* (Sgr A*), the black hole at the Galactic Center. We present a method for self-consistent evolution of a non-thermal electron population in the general relativistic magnetohydrodynamic code koral. The electron distribution is tracked across Lorentz factor space and is evolved in space and time, in parallel with thermal electrons, thermal ions and radiation. In this study, for simplicity, energy injection into the non-thermal distribution is taken as a fixed fraction of the local electron viscous heating rate. Numerical results are presented for a model with a low mass accretion rate similar to that of Sgr A*. We find that the presence of a non-thermal population of electrons has negligible effect on the overall dynamics of the system. Due to our simple uniform particle injection prescription, the radiative power in the non-thermal simulation is enhanced at large radii. The energy distribution of the non-thermal electrons shows a synchrotron cooling break, with the break Lorentz factor varying with location and time, reflecting the complex interplay between the local viscous heating rate, magnetic field strength and fluid velocity.

  10. 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.

  11. 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.

  12. Mass-loss rates, ionization fractions, shock velocities, and magnetic fields of stellar jets

    NASA Technical Reports Server (NTRS)

    Hartigan, Patrick; Morse, Jon A.; Raymond, John

    1994-01-01

    In this paper we calculate emission-line ratios from a series of planar radiative shock models that cover a wide range of shock velocities, preshock densities, and magnetic fields. The models cover the initial conditions relevant to stellar jets, and we show how to estimate the ionization fractions and shock velocities in jets directly from observations of the strong emission lines in these flows. The ionization fractions in the HH 34, HH 47, and HH 111 jets are approximately 2%, considerably smaller than previous estimates, and the shock velocities are approximately 30 km/s. For each jet the ionization fractions were found from five different line ratios, and the estimates agree to within a factor of approximately 2. The scatter in the estimates of the shock velocities is also small (+/- 4 km/s). The low ionization fractions of stellar jets imply that the observed electron densities are much lower than the total densities, so the mass-loss rates in these flows are correspondingly higher (approximately greater than 2 x 10(exp -7) solar mass/yr). The mass-loss rates in jets are a significant fraction (1%-10%) of the disk accretion rates onto young stellar objects that drive the outflows. The momentum and energy supplied by the visible portion of a typical stellar jet are sufficient to drive a weak molecular outflow. Magnetic fields in stellar jets are difficult to measure because the line ratios from a radiative shock with a magnetic field resemble those of a lower velocity shock without a field. The observed line fluxes can in principle indicate the strength of the field if the geometry of the shocks in the jet is well known.

  13. Investigation of surface water behavior during glaze ice accretion

    NASA Technical Reports Server (NTRS)

    Hansman, R. John, Jr.; Turnock, Stephen R.

    1988-01-01

    Microvideo observations of glaze ice accretions on 1-in-diameter cylinders in a closed-loop refrigerated wind tunnel were obtained to study factors controlling the behavior of unfrozen surface water during glaze ice accretion. Three zones of surface water behavior were noted, each with a characteristic roughness. The effect of substrate thermal and roughness properties on ice accretions was also studied. The contact angle and hysteresis were found to increase sharply at temperatures just below 0 C, explaining the high resistance to motion of water beads observed on accreting glaze ice surfaces. Based on the results, a simple multizone modification to the current glaze ice accretion model is proposed.

  14. Time-dependent simulation of oblique MHD cosmic-ray shocks using the two-fluid model

    NASA Technical Reports Server (NTRS)

    Frank, Adam; Jones, T. W.; Ryu, Dongsu

    1995-01-01

    Using a new, second-order accurate numerical method we present dynamical simulations of oblique MHD cosmic-ray (CR)-modified plane shock evolution. Most of the calculations are done with a two-fluid model for diffusive shock acceleration, but we provide also comparisons between a typical shock computed that way against calculations carried out using the more complete, momentum-dependent, diffusion-advection equation. We also illustrate a test showing that these simulations evolve to dynamical equilibria consistent with previously published steady state analytic calculations for such shocks. In order to improve understanding of the dynamical role of magnetic fields in shocks modified by CR pressure we have explored for time asymptotic states the parameter space of upstream fast mode Mach number, M(sub f), and plasma beta. We compile the results into maps of dynamical steady state CR acceleration efficiency, epsilon(sub c). We have run simulations using constant, and nonisotropic, obliquity (and hence spatially) dependent forms of the diffusion coefficient kappa. Comparison of the results shows that while the final steady states achieved are the same in each case, the history of CR-MHD shocks can be strongly modified by variations in kappa and, therefore, in the acceleration timescale. Also, the coupling of CR and MHD in low beta, oblique shocks substantially influences the transient density spike that forms in strongly CR-modified shocks. We find that inside the density spike a MHD slow mode wave can be generated that eventually steepens into a shock. A strong layer develops within the density spike, driven by MHD stresses. We conjecture that currents in the shear layer could, in nonplanar flows, results in enhanced particle accretion through drift acceleration.

  15. 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.

  16. The accretion of migrating giant planets

    NASA Astrophysics Data System (ADS)

    Dürmann, Christoph; Kley, Wilhelm

    2017-02-01

    Aims: Most studies concerning the growth and evolution of massive planets focus either on their accretion or their migration only. In this work we study both processes concurrently to investigate how they might mutually affect one another. Methods: We modeled a two-dimensional disk with a steady accretion flow onto the central star and embedded a Jupiter mass planet at 5.2 au. The disk is locally isothermal and viscosity is modeled using a constant α. The planet is held on a fixed orbit for a few hundred orbits to allow the disk to adapt and carve a gap. After this period, the planet is released and free to move according to the gravitational interaction with the gas disk. The mass accretion onto the planet is modeled by removing a fraction of gas from the inner Hill sphere, and the removed mass and momentum can be added to the planet. Results: Our results show that a fast migrating planet is able to accrete more gas than a slower migrating planet. Utilizing a tracer fluid we analyzed the origin of the accreted gas originating predominantly from the inner disk for a fast migrating planet. In the case of slower migration, the fraction of gas from the outer disk increases. We also found that even for very high accretion rates, in some cases gas crosses the planetary gap from the inner to the outer disk. Our simulations show that the crossing of gas changes during the migration process as the migration rate slows down. Therefore, classical type II migration where the planet migrates with the viscous drift rate and no gas crosses the gap is no general process but may only occur for special parameters and at a certain time during the orbital evolution of the planet.

  17. 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.

  18. Structure in Radiative Shocks

    NASA Astrophysics Data System (ADS)

    Drake, R. Paul; Visco, A.; Doss, F.; Reighard, A.; Froula, D.; Glenzer, S.; Knauer, J.

    2008-05-01

    Radiative shocks are shock waves fast enough that radiation from the shock-heated matter alters the structure of the shock. They are of fundamental interest to high-energy-density physics and also have applications throughout astrophysics. This poster will review the dimensionless parameters that determine structure in these shocks and will discuss recent experiments to measure such structure for strongly radiative shocks that are optically thin upstream and optically thick downstream. The shock transition itself heats mainly the ions. Immediately downstream of the shock, the ions heat the electrons and the electrons radiate, producing an optically thin cooling layer, followed by the downstream layer of warm, shocked material. The axial structure of these systems is of interest, because the transition from precursor through the cooling layer to the final state is complex and difficult to calculate. Their lateral structure is also of interest, as they seem likely to be subject to some variation on the Vishniac instability of thin layers. In our experiments to produce such shocks, laser ablation launches a Be plasma into a tube of Xe or Ar gas, at a velocity above 100 km/s. This drives a shock down the tube. Radiography provides fundamental information about the structure and evolution of the shocked material in Xe. Thomson scattering and pyrometry have provided data in Ar. We will summarize the available evidence regarding the properties of these shocks, and will discuss their connections to astrophysical cases. This research was sponsored by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Research Grants DE-FG52-07NA28058, DE-FG52-04NA00064, and other grants and contracts.

  19. Bow shocks as tracers of the environment and stellar outflows near the supermassive black hole.

    NASA Astrophysics Data System (ADS)

    Stofanova, L.; Zajaček, M.; Karas, V.

    2017-10-01

    Bow shocks develop near stars in the supersonic motion with respect to the surrounding interstellar environment. In particular, extended shocks emerge due to the interaction of stars with strong winds. We discuss the expected shape and orientation of bow shocks in the context of fast moving stars near a supermassive black hole (SMBH) embedded within Bondi-type accretion flow (Zajaček et al. 2016, MNRAS; Štofanová 2016, BSc. Thesis). We present models which take into account different velocities of the probe star and also consider various scenarios for the ambient medium near the vicinity of the black hole such as an inflow/outflow of the material towards/outwards SMBH or a model which considers inflow and outflow at the same time. Under suitable circumstances, a bow shock structure can be detected in infrared domain and their properties can trace the environment of the Galactic center. On the other hand, if density of the ambient medium is determined from mm/radio observations, bow shocks can be used to constrain mass-loss rates of massive OB/WR stars. X-rays can supplement the spectral evidence, though, the structures are below the angular resolution of the current instruments even in the most favourable case of the Milky Way's SMBH (Sgr A*).

  20. A Systems-Level Perspective on Engine Ice Accretion

    NASA Technical Reports Server (NTRS)

    May, Ryan D.; Guo, Ten-Huei; Simon, Donald L.

    2013-01-01

    The accretion of ice in the compression system of commercial gas turbine engines operating in high ice water content conditions is a safety issue being studied by the aviation sector. While most of the research focuses on the underlying physics of ice accretion and the meteorological conditions in which accretion can occur, a systems-level perspective on the topic lends itself to potential near-term operational improvements. This work focuses on developing an accurate and reliable algorithm for detecting the accretion of ice in the low pressure compressor of a generic 40,000 lbf thrust class engine. The algorithm uses only the two shaft speed sensors and works regardless of engine age, operating condition, and power level. In a 10,000-case Monte Carlo simulation, the detection approach was found to have excellent capability at determining ice accretion from sensor noise with detection occurring when ice blocks an average of 6.8% of the low pressure compressor area. Finally, an initial study highlights a potential mitigation strategy that uses the existing engine actuators to raise the temperature in the low pressure compressor in an effort to reduce the rate at which ice accretes.

  1. Quantification of Ice Accretions for Icing Scaling Evaluations

    NASA Technical Reports Server (NTRS)

    Ruff, Gary A.; Anderson, David N.

    2003-01-01

    The comparison of ice accretion characteristics is an integral part of aircraft icing research. It is often necessary to compare an ice accretion obtained from a flight test or numerical simulation to one produced in an icing wind tunnel or for validation of an icing scaling method. Traditionally, this has been accomplished by overlaying two-dimensional tracings of ice accretion shapes. This paper addresses the basic question of how to compare ice accretions using more quantitative methods. For simplicity, geometric characteristics of the ice accretions are used for the comparison. One method evaluated is a direct comparison of the percent differences of the geometric measurements. The second method inputs these measurements into a fuzzy inference system to obtain a single measure of the goodness of the comparison. The procedures are demonstrated by comparing ice shapes obtained in the Icing Research Tunnel at NASA Glenn Research Center during recent icing scaling tests. The results demonstrate that this type of analysis is useful in quantifying the similarity of ice accretion shapes and that the procedures should be further developed by expanding the analysis to additional icing data sets.

  2. Accretion in Radiative Equipartition (AiRE) Disks

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

    Yazdi, Yasaman K.; Afshordi, Niayesh, E-mail: yyazdi@pitp.ca, E-mail: nafshordi@pitp.ca

    2017-07-01

    Standard accretion disk theory predicts that the total pressure in disks at typical (sub-)Eddington accretion rates becomes radiation pressure dominated. However, radiation pressure dominated disks are thermally unstable. Since these disks are observed in approximate steady state over the instability timescale, our accretion models in the radiation-pressure-dominated regime (i.e., inner disk) need to be modified. Here, we present a modification to the Shakura and Sunyaev model, where the radiation pressure is in equipartition with the gas pressure in the inner region. We call these flows accretion in radiative equipartition (AiRE) disks. We introduce the basic features of AiRE disks andmore » show how they modify disk properties such as the Toomre parameter and the central temperature. We then show that the accretion rate of AiRE disks is limited from above and below, by Toomre and nodal sonic point instabilities, respectively. The former leads to a strict upper limit on the mass of supermassive black holes as a function of cosmic time (and spin), while the latter could explain the transition between hard and soft states of X-ray binaries.« less

  3. Accretion in Radiative Equipartition (AiRE) Disks

    NASA Astrophysics Data System (ADS)

    Yazdi, Yasaman K.; Afshordi, Niayesh

    2017-07-01

    Standard accretion disk theory predicts that the total pressure in disks at typical (sub-)Eddington accretion rates becomes radiation pressure dominated. However, radiation pressure dominated disks are thermally unstable. Since these disks are observed in approximate steady state over the instability timescale, our accretion models in the radiation-pressure-dominated regime (I.e., inner disk) need to be modified. Here, we present a modification to the Shakura & Sunyaev model, where the radiation pressure is in equipartition with the gas pressure in the inner region. We call these flows accretion in radiative equipartition (AiRE) disks. We introduce the basic features of AiRE disks and show how they modify disk properties such as the Toomre parameter and the central temperature. We then show that the accretion rate of AiRE disks is limited from above and below, by Toomre and nodal sonic point instabilities, respectively. The former leads to a strict upper limit on the mass of supermassive black holes as a function of cosmic time (and spin), while the latter could explain the transition between hard and soft states of X-ray binaries.

  4. An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

    NASA Astrophysics Data System (ADS)

    Gandhi, P.; Bachetti, M.; Dhillon, V. S.; Fender, R. P.; Hardy, L. K.; Harrison, F. A.; Littlefair, S. P.; Malzac, J.; Markoff, S.; Marsh, T. R.; Mooley, K.; Stern, D.; Tomsick, J. A.; Walton, D. J.; Casella, P.; Vincentelli, F.; Altamirano, D.; Casares, J.; Ceccobello, C.; Charles, P. A.; Ferrigno, C.; Hynes, R. I.; Knigge, C.; Kuulkers, E.; Pahari, M.; Rahoui, F.; Russell, D. M.; Shaw, A. W.

    2017-12-01

    Relativistic plasma jets are observed in many systems that host accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched1-3. Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot easily be disentangled from other accreting components. Here, we show that rapid optical flux variations from an accreting Galactic black-hole binary are delayed with respect to X-rays radiated from close to the black hole by about 0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these subsecond optical variations has hitherto been controversial4-8. Not only does our work strongly support a jet origin for the optical variations but it also sets a characteristic elevation of ≲103 Schwarzschild radii for the main inner optical emission zone above the black hole9, constraining both internal shock10 and magnetohydrodynamic11 models. Similarities with blazars12,13 suggest that jet structure and launching physics could potentially be unified under mass-invariant models. Two of the best-studied jetted black-hole binaries show very similar optical lags8,14,15, so this size scale may be a defining feature of such systems.

  5. Application of Micro-ramp Flow Control Devices to an Oblique Shock Interaction

    NASA Technical Reports Server (NTRS)

    Hirt, Stefanie; Anderson, Bernhard

    2007-01-01

    Tests are planned in the 15cm x 15cm supersonic wind tunnel at NASA Glenn to demonstrate the applicability of micro-ramp flow control to the management of shock wave boundary layer interactions. These tests will be used as a database for computational fluid dynamics (CFD) validation and Design of Experiments (DoE) design information. Micro-ramps show potential for mechanically simple and fail-safe boundary layer control.

  6. Stochastic events lead to accretion in Saturn’s rings

    NASA Astrophysics Data System (ADS)

    Esposito, L. W.

    2009-12-01

    UVIS occultations indicate accretion is triggered at the B ring edge, in strong density waves in ring A and in the F ring. Moons may trigger accretion by streamline crowding (Lewis & Stewart); which enhances collisions, leading to accretion; increasing random velocities; leading to more collisions and more accretion. Cassini occultations of these strongly perturbed locations show not only accretion but also disaggregation, with time scales of hours to weeks. The collisions may lead to temporary aggregations via stochastic events: they can compress unconsolidated objects, trigger adhesion or bring small pieces into contact with larger or higher-density seeds. Disaggregation then can follow from disruptive collisions or tidal shedding. In the accretion/disruption balance, increased random motions could eventually give the upper hand to disruption… just as ‘irrational exuberance’ can lead to financial panic in the economy; or the overpopulation of hares can lead to boom-and-bust in the population of foxes. This unstable equilibrium can similarly give rise to episodic cycles in accretion: explaining why the observable ring features that indicate embedded objects have been increasing since the beginning of Cassini’s observations of Saturn in 2004.

  7. Kinetic and radiative power from optically thin accretion flows

    NASA Astrophysics Data System (ADS)

    Sądowski, Aleksander; Gaspari, Massimo

    2017-06-01

    We perform a set of general relativistic, radiative, magneto-hydrodynamical simulations (GR-RMHD) to study the transition from radiatively inefficient to efficient state of accretion on a non-rotating black hole. We study ion to electron temperature ratios ranging from TI/Te = 10 to 100, and simulate flows corresponding to accretion rates as low as 10^{-6}\\dot{M}_Edd, and as high as 10^{-2}\\dot{M}_Edd. We have found that the radiative output of accretion flows increases with accretion rate, and that the transition occurs earlier for hotter electrons (lower TI/Te ratio). At the same time, the mechanical efficiency hardly changes and accounts to ≈3 per cent of the accreted rest mass energy flux, even at the highest simulated accretion rates. This is particularly important for the mechanical active galactic nuclei (AGN) feedback regulating massive galaxies, groups and clusters. Comparison with recent observations of radiative and mechanical AGN luminosities suggests that the ion to electron temperature ratio in the inner, collisionless accretion flow should fall within 10 < TI/Te < 30, I.e. the electron temperature should be several percent of the ion temperature.

  8. Regimes of mini black hole abandoned to accretion

    NASA Astrophysics Data System (ADS)

    Paik, Biplab

    2018-01-01

    Being inspired by the Eddington’s idea, along with other auxiliary arguments, it is unveiled that there exist regimes of a black hole that would prohibit accretion of ordinary energy. In explicit words, there exists a lower bound to black hole mass below which matter accretion process does not run for black holes. Not merely the baryonic matter, but, in regimes, also the massless photons could get prohibited from rushing into a black hole. However, unlike the baryon accretion abandoned black hole regime, the mass-regime of a black hole prohibiting accretion of radiation could vary along with its ambient temperature. For example, we discuss that earlier to 10‑8 s after the big-bang, as the cosmological temperature of the Universe grew above ˜ 1014 K, the mass range of black hole designating the radiation accretion abandoned regime, had to be in varying state being connected with the instantaneous age of the evolving Universe by an “one half” power law. It happens to be a fact that a black hole holding regimes prohibiting accretion of energy is gigantic by its size in comparison to the Planck length-scale. Hence the emergence of these regimes demands mini black holes for not being viable as profound suckers of energy. Consideration of accretion abandoned regimes could be crucial for constraining or judging the evolution of primordial black holes over the age of the Universe.

  9. Geometric figure–ground cues override standard depth from accretion-deletion

    PubMed Central

    Tanrıkulu, Ömer Dağlar; Froyen, Vicky; Feldman, Jacob; Singh, Manish

    2016-01-01

    Accretion-deletion is widely considered a decisive cue to surface depth ordering, with the accreting or deleting surface interpreted as behind an adjoining surface. However, Froyen, Feldman, and Singh (2013) have shown that when accretion-deletion occurs on both sides of a contour, accreting-deleting regions can also be perceived as in front and as self-occluding due to rotation in three dimensions. In this study we ask whether geometric figure–ground cues can override the traditional “depth from accretion-deletion” interpretation even when accretion-deletion takes place only on one side of a contour. We used two tasks: a relative-depth task (front/back), and a motion-classification task (translation/rotation). We conducted two experiments, in which texture in only one set of alternating regions was moving; the other set was static. Contrary to the traditional interpretation of accretion-deletion, the moving convex and symmetric regions were perceived as figural and rotating in three dimensions in roughly half of the trials. In the second experiment, giving different motion directions to the moving regions (thereby weakening motion-based grouping) further weakened the traditional accretion-deletion interpretation. Our results show that the standard “depth from accretion-deletion” interpretation is overridden by static geometric cues to figure–ground. Overall, the results demonstrate a rich interaction between accretion-deletion, figure–ground, and structure from motion that is not captured by existing models of depth from motion. PMID:26982528

  10. 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.

  11. El Nino influence on Holocene reef accretion in Hawai'i

    USGS Publications Warehouse

    Rooney, J.; Fletcher, C.; Grossman, E.; Engels, M.; Field, M.

    2004-01-01

    New observations of reef accretion from several locations show that in Hawai'i accretion during early to middle Holocene time occurred in areas where today it is precluded by the wave regime, suggesting an increase in wave energy. Accretion of coral and coralline algae reefs in the Hawaiian Islands today is largely controlled by wave energy. Many coastal areas in the main Hawaiian Islands are periodically exposed to large waves, in particular from North Pacific swell and hurricanes. These are of sufficient intensity to prevent modern net accretion as evidenced by the antecedent nature of the seafloor. Only in areas sheltered from intense wave energy is active accretion observed. Analysis of reef cores reveals patterns of rapid early Holocene accretion in several locations that terminated by middle Holocene time, ca. 5000 yr ago. Previous analyses have suggested that changes in Holocene accretion were a result of reef growth "catching up" to sea level. New data and interpretations indicate that the end of reef accretion in the middle Holocene may be influenced by factors in addition to sea level. Reef accretion histories from the islands of Kaua'i, O'ahu, and Moloka'i may be interpreted to suggest that a change in wave energy contributed to the reduction or termination of Holocene accretion by 5000 yr ago in some areas. In these cases, the decrease in reef accretion occurred before the best estimates of the decrease in relative sea-level rise during the mid-Holocene high stand of sea level in the main Hawaiian Islands. However, reef accretion should decrease following the termination of relative sea-level rise (ca. 3000 yr ago) if reef growth were "catching up" to sea level. Evidence indicates that rapid accretion occurred at these sites in early Holocene time and that no permanent accretion is occurring at these sites today. This pattern persists despite the availability of hard substrate suitable for colonization at a wide range of depths between -30 m and the

  12. Ultrafast shock-induced orientation of polycrystalline films: Applications to high explosives

    NASA Astrophysics Data System (ADS)

    Franken, Jens; Hambir, Selezion A.; Dlott, Dana D.

    1999-02-01

    Tiny laser-driven shock waves of ˜5 GPa pressure (nanoshocks) are used to study fast mechanical processes occurring in a thin layer of polycrystalline insensitive energetic material, (3-nitro-1,2,4-triazol-5-one) (NTO). Ultrafast coherent Raman spectroscopy of shocked NTO shows the existence of three distinct mechanical processes. Very fast (˜600 ps) changes in intensity and the appearance of new transitions are associated with the uniaxial nature of compression by the shock front. Frequency shifting and broadening processes which track the ˜2 ns duration nanoshock are associated with transient changes in density and temperature. A novel slower process (5-10 ns) starts as the shock begins to unload, and continues for several nanoseconds after the shock is over, resulting in changes of widths and intensities of several vibrational transitions. By comparing ultrafast spectra to static Raman spectra of single NTO crystals in various orientations, it is concluded that this process involves shock-induced partial orientation of the crystals in the NTO layer. The NTO crystals are oriented faster than the time scale for initiating chemical reactions. The sensitivity of explosive crystals to shock initiation may depend dramatically on the orientation of the crystal relative to the direction of shock propagation, so the implications of fast shock-induced orientation for energetic materials initiation are discussed briefly.

  13. 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.

  14. Convection Enhances Magnetic Turbulence in AM CVn Accretion Disks

    NASA Astrophysics Data System (ADS)

    Coleman, Matthew S. B.; Blaes, Omer; Hirose, Shigenobu; Hauschildt, Peter H.

    2018-04-01

    We present the results of local, vertically stratified, radiation magnetohydrodynamic shearing-box simulations of magnetorotational instability (MRI) turbulence for a (hydrogen poor) composition applicable to accretion disks in AM CVn type systems. Many of these accreting white dwarf systems are helium analogs of dwarf novae (DNe). We utilize frequency-integrated opacity and equation-of-state tables appropriate for this regime to accurately portray the relevant thermodynamics. We find bistability of thermal equilibria in the effective-temperature, surface-mass-density plane typically associated with disk instabilities. Along this equilibrium curve (i.e., the S-curve), we find that the stress to thermal pressure ratio α varied with peak values of ∼0.15 near the tip of the upper branch. Similar to DNe, we found enhancement of α near the tip of the upper branch caused by convection; this increase in α occurred despite our choice of zero net vertical magnetic flux. Two notable differences we find between DN and AM CVn accretion disk simulations are that AM CVn disks are capable of exhibiting persistent convection in outburst, and ideal MHD is valid throughout quiescence for AM CVns. In contrast, DNe simulations only show intermittent convection, and nonideal MHD effects are likely important in quiescence. By combining our previous work with these new results, we also find that convective enhancement of the MRI is anticorrelated with mean molecular weight.

  15. 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

  16. Reverberation Mapping of AGN Accretion Disks

    NASA Astrophysics Data System (ADS)

    Fausnaugh, Michael; AGN STORM Collaboration

    2017-01-01

    I will discuss new reverberation mapping results that allow us to investigate the temperature structure of AGN accretion disks. By measuring time-delays between broad-band continuum light curves, we can determine the size of the disk as a function of wavelength. I will discuss the detection of continuum lags in NGC 5548 reported by the AGN STORM project and implications for the accretion disk. I will also present evidence for continuum lags in two other AGN for which we recently measured black hole masses from continuum-Hbeta reverberations. The mass measurements allow us to compare the continuum lags to predictions from standard thin disk theory, and our results indicate that the accretion disks are larger than the simplest expectations.

  17. Simulating a Thin Accretion Disk Using PLUTO

    NASA Astrophysics Data System (ADS)

    Phillipson, Rebecca; Vogeley, Michael S.; Boyd, Patricia T.

    2017-08-01

    Accreting black hole systems such as X-ray binaries and active galactic nuclei exhibit variability in their luminosity on many timescales ranging from milliseconds to tens of days, and even hundreds of days. The mechanism(s) driving this variability and the relationship between short- and long-term variability is poorly understood. Current studies on accretion disks seek to determine how the changes in black hole mass, the rate at which mass accretes onto the central black hole, and the external environment affect the variability on scales ranging from stellar-mass black holes to supermassive black holes. Traditionally, the fluid mechanics equations governing accretion disks have been simplified by considering only the kinematics of the disk, and perhaps magnetic fields, in order for their phenomenological behavior to be predicted analytically. We seek to employ numerical techniques to study accretion disks including more complicated physics traditionally ignored in order to more accurately understand their behavior over time. We present a proof-of-concept three dimensional, global simulation using the astrophysical hydrodynamic code PLUTO of a simplified thin disk model about a central black hole which will serve as the basis for development of more complicated models including external effects such as radiation and magnetic fields. We also develop a tool to generate a synthetic light curve that displays the variability in luminosity of the simulation over time. The preliminary simulation and accompanying synthetic light curve demonstrate that PLUTO is a reliable code to perform sophisticated simulations of accretion disk systems which can then be compared to observational results.

  18. 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.

  19. 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.

  20. Dynamics of core accretion

    DOE PAGES

    Nelson, Andrew F.; Ruffert, Maximilian

    2012-12-21

    defined by locally isothermal or isentropic treatments, any cooling that does affect the envelope material will have limited consequences for the dynamics, since the flow quickly carries cooled material out of the core's environment entirely. The angular momentum of material in the envelope, relative to the core, varies both in magnitude and in sign on time-scales of days to months near the core and on time-scales a few years at distances comparable to the Hill radius. The dynamical activity contrasts with the largely static behaviour typically assumed within the framework of the core accretion model for Jovian planet formation. We show that material entering the dynamically active environment may suffer intense heating and cooling events the durations of which are as short as a few hours to a few days. Shorter durations are not observable in our work due to the limits of our resolution. Peak temperatures in these events range from T ~ 1000 K to as high as T ~ 3–4000 K, with densities ρ ~ 10 -9 to 10 -8 gcm -3. These time-scales, densities and temperatures span a range consistent with those required for chondrule formation in the nebular shock model. Finally, we therefore propose that dynamical activity in the Jovian planet formation environment could be responsible for the production of chondrules and other annealed silicates in the solar nebula.« less

  1. Dynamics of core accretion

    NASA Astrophysics Data System (ADS)

    Nelson, Andrew F.; Ruffert, Maximilian

    2013-02-01

    isentropic treatments, any cooling that does affect the envelope material will have limited consequences for the dynamics, since the flow quickly carries cooled material out of the core's environment entirely. The angular momentum of material in the envelope, relative to the core, varies both in magnitude and in sign on time-scales of days to months near the core and on time-scales a few years at distances comparable to the Hill radius. The dynamical activity contrasts with the largely static behaviour typically assumed within the framework of the core accretion model for Jovian planet formation. We show that material entering the dynamically active environment may suffer intense heating and cooling events the durations of which are as short as a few hours to a few days. Shorter durations are not observable in our work due to the limits of our resolution. Peak temperatures in these events range from T ˜ 1000 K to as high as T ˜ 3-4000 K, with densities ρ ˜ 10-9 to 10-8 g cm-3. These time-scales, densities and temperatures span a range consistent with those required for chondrule formation in the nebular shock model. We therefore propose that dynamical activity in the Jovian planet formation environment could be responsible for the production of chondrules and other annealed silicates in the solar nebula.

  2. Chaotic cold accretion on to black holes

    NASA Astrophysics Data System (ADS)

    Gaspari, M.; Ruszkowski, M.; Oh, S. Peng

    2013-07-01

    Bondi theory is often assumed to adequately describe the mode of accretion in astrophysical environments. However, the Bondi flow must be adiabatic, spherically symmetric, steady, unperturbed, with constant boundary conditions. Using 3D adaptive mesh refinement simulations, linking the 50 kpc to the sub-parsec (sub-pc) scales over the course of 40 Myr, we systematically relax the classic assumptions in a typical galaxy hosting a supermassive black hole. In the more realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the non-linear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when tcool/tff ≲ 10. The clouds decouple from the hot gas, `raining' on to the centre. Subsonic turbulence of just over 100 km s-1 (M > 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (tturb/tcool ≲ 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. In this study, we do not inject a fixed initial angular momentum, though vorticity is later seeded by turbulence. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for the frequent fluctuations. Since our modelling is fairly general (turbulence/heating due to AGN feedback, galaxy motions, mergers, stellar evolution), chaotic cold accretion may be common in

  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. 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.

  5. Thin Disks Gone MAD: Magnetically Arrested Accretion in the Thin Regime

    NASA Astrophysics Data System (ADS)

    Avara, Mark J.; McKinney, Jonathan C.; Reynolds, Christopher S.

    2015-01-01

    The collection and concentration of surrounding large scale magnetic fields by black hole accretion disks may be required for production of powerful, spin driven jets. So far, accretion disks have not been shown to grow sufficient poloidal flux via the turbulent dynamo alone to produce such persistent jets. Also, there have been conflicting answers as to how, or even if, an accretion disk can collect enough magnetic flux from the ambient environment. Extending prior numerical studies of magnetically arrested disks (MAD) in the thick (angular height, H/R~1) and intermediate (H/R~.2-.6) accretion regimes, we present our latest results from fully general relativistic MHD simulations of the thinnest BH (H/R~.1) accretion disks to date exhibiting the MAD mode of accretion. We explore the significant deviations of this accretion mode from the standard picture of thin, MRI-driven accretion, and demonstrate the accumulation of large-scale magnetic flux.

  6. NMR spectroscopy of experimentally shocked single crystal quartz: A reexamination of the NMR shock barometer

    NASA Technical Reports Server (NTRS)

    Fiske, P. S.; Gratz, A. J.; Nellis, W. J.

    1993-01-01

    Cygan and others report a broadening of the Si-29 nuclear magnetic resonance (NMR) peak for synthetic quartz powders with increasing shock pressure which they propose as a shock wave barometer for natural systems. These results are expanded by studying single crystal quartz shocked to 12 and 33 GPa using the 6.5 m two-stage light-gas gun at Lawrence Livermore National Laboratories. Our NMR results differ substantially from those of Cygan and others and suggest that the proposed shock wave barometer may require refinement. The difference in results between this study and that of Cygan and others is most likely caused by different starting materials (single crystal vs. powder) and different shock loading histories. NMR results from single crystal studies may be more applicable to natural systems.

  7. Late accretion to the terrestrial planets

    NASA Astrophysics Data System (ADS)

    Brasser, Ramon; Mojzsis, Stephen; Werner, Stephanie; Matsumura, Soko; Ida, Shigeru

    2017-10-01

    IntroductionIt is generally accepted that silicate-metal (`rocky') planet formation relies on coagulation from a mixture of sub-Mars sized planetary embryos and (smaller) planetesimals that dynamically emerge from the evolving circum-solar disc in the first few million years of our Solar System. Once the planets have, for the most part, assembled after a giant impact phase, they continue to be bombarded by a multitude of planetesimals left over from accretion. Here we place limits on the mass and evolution of these planetesimals based on constraints from the highly siderophile element (HSE) budget of the Moon. The terrestrial and lunar HSE budgets indicate that Earth’s and Moon’s additions through late accretion were 0.7 wt% and 0.02 wt% respectively. The disproportionate high accretion between the Earth and Moon could be explained by stochastic accretion of a few remaining Ceres-sized bodies that preferentially targeted the Earth.ResultsFrom a combination of N-body and Monte Carlo simulations of planet formation we conclude:1) matching the terrestrial to lunar HSE ratio requires that late accretion on Earth mostly consisted of a single lunar-size impactor striking the Earth before 4.45 Ga2) the flux of terrestrial impactors must have been low avoid wholesale melting of Earth's crust after 4.4 Ga[6], and to simultaneously match the number of observed lunar basins3) after the terrestrial planets have fully formed, the mass in remnant planetesimals was ~0.001 Earth mass, lower than most previous models suggest.4) Mars' HSE budget also requires a colossal impact with a Ceres-sized object before 4.43 Ga, whose visible remnant could be the hemispherical dichotomy.These conclusions lead to an Hadean eon which is more clement than assumed previously. In addition, our dynamically and geochemically self-consistent scenario requires that future N-body simulations of rocky planet formation either directly incorporate collisional grinding or rely on pebble accretion.

  8. WIND-DRIVEN ACCRETION IN TRANSITIONAL PROTOSTELLAR DISKS

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

    Wang, Lile; Goodman, Jeremy J.

    Transitional protostellar disks have inner cavities that are heavily depleted in dust and gas, yet most of them show signs of ongoing accretion, often at rates comparable to full disks. We show that recent constraints on the gas surface density in a few well-studied disk cavities suggest that the accretion speed is at least transsonic. We propose that this is the natural result of accretion driven by magnetized winds. Typical physical conditions of the gas inside these cavities are estimated for plausible X-ray and FUV radiation fields. The gas near the midplane is molecular and predominantly neutral, with a dimensionlessmore » ambipolar parameter in the right general range for wind solutions of the type developed by Königl, Wardle, and others. That is to say, the density of ions and electrons is sufficient for moderately good coupling to the magnetic field, but it is not so good that the magnetic flux needs to be dragged inward by the accreting neutrals.« less

  9. Prediction of Shock-Induced Cavitation in Water

    NASA Astrophysics Data System (ADS)

    Brundage, Aaron

    2013-06-01

    Fluid-structure interaction problems that require estimating the response of thin structures within fluids to shock loading has wide applicability. For example, these problems may include underwater explosions and the dynamic response of ships and submarines; and biological applications such as Traumatic Brain Injury (TBI) and wound ballistics. In all of these applications the process of cavitation, where small cavities with dissolved gases or vapor are formed as the local pressure drops below the vapor pressure due to shock hydrodynamics, can cause significant damage to the surrounding thin structures or membranes if these bubbles collapse, generating additional shock loading. Hence, a two-phase equation of state (EOS) with three distinct regions of compression, expansion, and tension was developed to model shock-induced cavitation. This EOS was evaluated by comparing data from pressure and temperature shock Hugoniot measurements for water up to 400 kbar, and data from ultrasonic pressure measurements in tension to -0.3 kbar, to simulated responses from CTH, an Eulerian, finite volume shock code. The new EOS model showed significant improvement over pre-existing CTH models such as the SESAME EOS for capturing cavitation. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy/NNSA under contract DE-AC04-94AL85000.

  10. EARTH, MOON, SUN, AND CV ACCRETION DISKS

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

    Montgomery, M. M.

    2009-11-01

    Net tidal torque by the secondary on a misaligned accretion disk, like the net tidal torque by the Moon and the Sun on the equatorial bulge of the spinning and tilted Earth, is suggested by others to be a source to retrograde precession in non-magnetic, accreting cataclysmic variable (CV) dwarf novae (DN) systems that show negative superhumps in their light curves. We investigate this idea in this work. We generate a generic theoretical expression for retrograde precession in spinning disks that are misaligned with the orbital plane. Our generic theoretical expression matches that which describes the retrograde precession of Earths'more » equinoxes. By making appropriate assumptions, we reduce our generic theoretical expression to those generated by others, or to those used by others, to describe retrograde precession in protostellar, protoplanetary, X-ray binary, non-magnetic CV DN, quasar, and black hole systems. We find that spinning, tilted CV DN systems cannot be described by a precessing ring or by a precessing rigid disk. We find that differential rotation and effects on the disk by the accretion stream must be addressed. Our analysis indicates that the best description of a retrogradely precessing spinning, tilted, CV DN accretion disk is a differentially rotating, tilted disk with an attached rotating, tilted ring located near the innermost disk annuli. In agreement with the observations and numerical simulations by others, we find that our numerically simulated CV DN accretion disks retrogradely precess as a unit. Our final, reduced expression for retrograde precession agrees well with our numerical simulation results and with selective observational systems that seem to have main-sequence secondaries. Our results suggest that a major source to retrograde precession is tidal torques like that by the Moon and the Sun on the Earth. In addition, these tidal torques should be common to a variety of systems where one member is spinning and tilted, regardless

  11. Stochastic events may lead to accretion in Saturn's rings

    NASA Astrophysics Data System (ADS)

    Esposito, Larry W.

    Stochastic events may lead to accretion in Saturn's rings Larry W. Esposito LASP, University of Colorado UVIS occultations indicate accretion is triggered at the B ring edge, in strong density waves in ring A and in the F ring. Moons may trigger accretion by streamline crowding (Lewis & Stewart); which enhances collisions, leading to accretion; increasing random velocities; leading to more collisions and more accretion. Cassini occultations of these strongly perturbed locations show not only accretion but also disaggregation, with time scales of hours to weeks. The collisions may lead to temporary aggregations via stochastic events: collisions can compress unconsolidated objects, trigger adhesion or bring small pieces into contact with larger or higher-density seeds. Disaggregation then can follow from disruptive collisions or tidal shedding. In the accretion/disruption balance, increased random motions could eventually give the upper hand to disruption. . . just as `irrational exuberance' can lead to financial panic in the economy; or the overpopulation of hares can lead to boom-and-bust in the population of foxes. I present a simple predator-prey model. This system's unstable equilibrium can similarly give rise to episodic cycles in accretion: explaining why the observable ring features that indicate embedded objects have been increasing since the beginning of Cassini's observations of Saturn in 2004. Unlike other interpretations of the peculiar events seen near Saturn Equinox, I emphasize the kinetic description of particle interactions rather than a fluid instability approach; and the dominance of stochastic events involving individual aggregates over free and/or driven modes in a flat disk.

  12. 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.

  13. Relativistic dust accretion of charged particles in Kerr-Newman spacetime

    NASA Astrophysics Data System (ADS)

    Schroven, Kris; Hackmann, Eva; Lämmerzahl, Claus

    2017-09-01

    We describe a new analytical model for the accretion of particles from a rotating and charged spherical shell of dilute collisionless plasma onto a rotating and charged black hole. By assuming a continuous injection of particles at the spherical shell and by treating the black hole and a featureless accretion disk located in the equatorial plane as passive sinks of particles, we build a stationary accretion model. This may then serve as a toy model for plasma feeding an accretion disk around a charged and rotating black hole. Therefore, our new model is a direct generalization of the analytical accretion model introduced by E. Tejeda, P. A. Taylor, and J. C. Miller [Mon. Not. R. Astron. Soc. 429, 925 (2013), 10.1093/mnras/sts316]. We use our generalized model to analyze the influence of a net charge of the black hole, which will in general be very small, on the accretion of plasma. Within the assumptions of our model we demonstrate that already a vanishingly small charge of the black hole may in general still have a non-negligible effect on the motion of the plasma, as long as the electromagnetic field of the plasma is still negligible. Furthermore, we argue that the inner and outer edges of the forming accretion disk strongly depend on the charge of the accreted plasma. The resulting possible configurations of accretion disks are analyzed in detail.

  14. Parallel implementation of geometrical shock dynamics for two dimensional converging shock waves

    NASA Astrophysics Data System (ADS)

    Qiu, Shi; Liu, Kuang; Eliasson, Veronica

    2016-10-01

    Geometrical shock dynamics (GSD) theory is an appealing method to predict the shock motion in the sense that it is more computationally efficient than solving the traditional Euler equations, especially for converging shock waves. However, to solve and optimize large scale configurations, the main bottleneck is the computational cost. Among the existing numerical GSD schemes, there is only one that has been implemented on parallel computers, with the purpose to analyze detonation waves. To extend the computational advantage of the GSD theory to more general applications such as converging shock waves, a numerical implementation using a spatial decomposition method has been coupled with a front tracking approach on parallel computers. In addition, an efficient tridiagonal system solver for massively parallel computers has been applied to resolve the most expensive function in this implementation, resulting in an efficiency of 0.93 while using 32 HPCC cores. Moreover, symmetric boundary conditions have been developed to further reduce the computational cost, achieving a speedup of 19.26 for a 12-sided polygonal converging shock.

  15. 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.

  16. INTEGRAL results on supergiant fast X-ray transients and accretion mechanism interpretation: ionization effect and formation of transient accretion discs

    NASA Astrophysics Data System (ADS)

    Ducci, L.; Sidoli, L.; Paizis, A.

    2010-11-01

    We performed a systematic analysis of all INTEGRAL observations from 2003 to 2009 of 14 supergiant fast X-ray transients (SFXTs), implying a net exposure time of about 30 Ms. For each source we obtained light curves and spectra (3-100 keV), discovering several new outbursts. We discuss the X-ray behaviour of SFXTs emerging from our analysis in the framework of the clumpy wind accretion mechanism we proposed. We discuss the effect of X-ray photoionization on accretion in close binary systems such as IGR J16479-4514 and IGR J17544-2619. We show that, because of X-ray photoionization, there is a high probability of an accretion disc forming from the capture of angular momentum in IGR J16479-4514, and we suggest that the formation of transient accretion discs could be partly responsible for the flaring activity in SFXTs with narrow orbits. We also propose an alternative way to explain the origin of flares with peculiar shapes observed in our analysis applying the model of Lamb et al., which is based on accretion via the Rayleigh-Taylor instability and was originally proposed to explain Type II bursts.

  17. A magnetic accretion switch in pre-cataclysmic binaries

    NASA Astrophysics Data System (ADS)

    Drake, Jeremy J.; Garraffo, Cecilia; Takei, Dai; Gaensicke, Boris

    2014-02-01

    We have investigated the mass accretion rate implied by published surface abundances of Si and C in the white dwarf component of the 3.62 h period pre-cataclysmic binary and planet host candidate QS Vir (DA+M2-4). Diffusion time-scales for gravitational settling imply dot{M} ˜ 10^{-16} M_{odot } yr-1 for the 1999 epoch of the observations, which is three orders of magnitude lower than measured from a 2006 XMM-Newton observation. This is the first time that large accretion rate variations have been seen in a detached pre-cataclysmic variable (pre-CV). A third body in a 14 yr eccentric orbit suggested in a recent eclipse timing study is too distant to perturb the central binary sufficiently to influence accretion. A hypothetical coronal mass ejection just prior to the XMM-Newton observation might explain the higher accretion rate, but the implied size and frequency of such events appear too great. We suggest accretion is most likely modulated by a magnetic cycle on the secondary acting as a wind `accretion switch', a mechanism that can be tested by X-ray and ultraviolet monitoring. If so, QS Vir and similar pre-CVs could provide powerful insights into hitherto inscrutable CV and M dwarf magnetospheres, and mass- and angular-momentum-loss rates.

  18. 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.

  19. Accretion of the Earth.

    PubMed

    Canup, Robin M

    2008-11-28

    The origin of the Earth and its Moon has been the focus of an enormous body of research. In this paper I review some of the current models of terrestrial planet accretion, and discuss assumptions common to most works that may require re-examination. Density-wave interactions between growing planets and the gas nebula may help to explain the current near-circular orbits of the Earth and Venus, and may result in large-scale radial migration of proto-planetary embryos. Migration would weaken the link between the present locations of the planets and the original provenance of the material that formed them. Fragmentation can potentially lead to faster accretion and could also damp final planet orbital eccentricities. The Moon-forming impact is believed to be the final major event in the Earth's accretion. Successful simulations of lunar-forming impacts involve a differentiated impactor containing between 0.1 and 0.2 Earth masses, an impact angle near 45 degrees and an impact speed within 10 per cent of the Earth's escape velocity. All successful impacts-with or without pre-impact rotation-imply that the Moon formed primarily from material originating from the impactor rather than from the proto-Earth. This must ultimately be reconciled with compositional similarities between the Earth and the Moon.

  20. 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

  1. Radiation-driven Turbulent Accretion onto Massive Black Holes

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

    Park, KwangHo; Wise, John H.; Bogdanović, Tamara, E-mail: kwangho.park@physics.gatech.edu

    Accretion of gas and interaction of matter and radiation are at the heart of many questions pertaining to black hole (BH) growth and coevolution of massive BHs and their host galaxies. To answer them, it is critical to quantify how the ionizing radiation that emanates from the innermost regions of the BH accretion flow couples to the surrounding medium and how it regulates the BH fueling. In this work, we use high-resolution three-dimensional (3D) radiation-hydrodynamic simulations with the code Enzo , equipped with adaptive ray-tracing module Moray , to investigate radiation-regulated BH accretion of cold gas. Our simulations reproduce findingsmore » from an earlier generation of 1D/2D simulations: the accretion-powered UV and X-ray radiation forms a highly ionized bubble, which leads to suppression of BH accretion rate characterized by quasi-periodic outbursts. A new feature revealed by the 3D simulations is the highly turbulent nature of the gas flow in vicinity of the ionization front. During quiescent periods between accretion outbursts, the ionized bubble shrinks in size and the gas density that precedes the ionization front increases. Consequently, the 3D simulations show oscillations in the accretion rate of only ∼2–3 orders of magnitude, significantly smaller than 1D/2D models. We calculate the energy budget of the gas flow and find that turbulence is the main contributor to the kinetic energy of the gas but corresponds to less than 10% of its thermal energy and thus does not contribute significantly to the pressure support of the gas.« less

  2. 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.

  3. Principles and application of extracorporeal shock wave lithotripsy.

    PubMed

    Robinson, S N; Crane, V S; Jones, D G; Cochran, J S; Williams, O B

    1987-04-01

    The physics, instrumentation, and patient-care aspects of extracorporeal shock wave lithotripsy (ESWL) in the treatment of kidney stone disease are described. The kidney stone is located through the use of two integrated roentgenographic imaging systems. The x-ray tubes, fixed on either side of a tub of water in which the patient is partially immersed, are directed upward. The patient is maneuvered until the imaging systems indicate the kidney stone is within the second focus of the reflector and within the 1.5-cu cm target area. Once within this alignment, the stone is ready for shock wave treatment; general or regional anesthesia is used to immobilize the patient so that the position of the stone can be maintained within the focus of the shock wave. When the stone is repeatedly subjected to this high-energy force, it begins to disintegrate until fragments of less than 1 mm are left. ESWL can (1) disintegrate kidney stones of all types, (2) be efficiently transmitted over distances that allow the shock wave source to be outside the body, (3) safely pass through living tissue, and (4) be precisely controlled and focused into a small target area. ESWL is a safe, effective, and cost-saving treatment that can be used for 90% of all kidney stone disease that previously required surgery.

  4. Influence of accretion on lead in the Earth

    NASA Astrophysics Data System (ADS)

    Galer, Stephen J. G.; Goldstein, Steven L.

    The Pb abundance and isotope composition of the Earth is fundamentally altered from bulk solar system values by the processes occurring during accretion. The most important of the possible processes are volatile element loss and core formation, or some form of inhomogeneous accretion/condensation. The final result is an Earth highly impoverished in 204Pb and other Pb isotopes in primordial abundance. Depending on the exact timing, some radiogenic Pb is also lost either to space or to the core; the degree of loss occurs in the same order as the parent decay constants, namely 207Pb > 206Pb > 208Pb. In this contribution, we explore the likely effects accretion had on the Pb isotope composition of the present day bulk silicate Earth and its secular isotope evolution. This is used to address a number of questions: (1) What can be learned about accretion from the Pb isotope composition of the bulk silicate Earth? (2) Can effects of accretion reconcile the classical "Pb paradox" of a 206Pb-rich bulk silicate Earth? (3) What exactly is the meaning of the "age of the Earth" within the context of Pb isotopes? By consideration of a number of accretion scenarios it is demonstrated that Pb isotopes yield information only on the following two coupled quantities: Firstly, the accretion interval Δ T, the time between initial condensation of the solar nebula (at 4.566Ga) and when accretion-produced U/Pb fractionation (whether loss of Pb to the core or to space) in the silicate Earth ceased. Secondly, the mean 238U/204 Pb ratio μ during accretion—no details of changes in μ during the accretion interval can be resolved. The effects of accretion are thus adequately considered in terms of a simple two-stage model described by μ over ΔT followed by a postaccretion μ. The systematics of μ and ΔT are then examined for the cases of present day terrestrial reservoirs and Archean leads. These estimates of μ and ΔT for the present and past silicate Earth are not compatible with

  5. Aerodynamic Simulation of Ice Accretion on Airfoils

    NASA Technical Reports Server (NTRS)

    Broeren, Andy P.; Addy, Harold E., Jr.; Bragg, Michael B.; Busch, Greg T.; Montreuil, Emmanuel

    2011-01-01

    This report describes recent improvements in aerodynamic scaling and simulation of ice accretion on airfoils. Ice accretions were classified into four types on the basis of aerodynamic effects: roughness, horn, streamwise, and spanwise ridge. The NASA Icing Research Tunnel (IRT) was used to generate ice accretions within these four types using both subscale and full-scale models. Large-scale, pressurized windtunnel testing was performed using a 72-in.- (1.83-m-) chord, NACA 23012 airfoil model with high-fidelity, three-dimensional castings of the IRT ice accretions. Performance data were recorded over Reynolds numbers from 4.5 x 10(exp 6) to 15.9 x 10(exp 6) and Mach numbers from 0.10 to 0.28. Lower fidelity ice-accretion simulation methods were developed and tested on an 18-in.- (0.46-m-) chord NACA 23012 airfoil model in a small-scale wind tunnel at a lower Reynolds number. The aerodynamic accuracy of the lower fidelity, subscale ice simulations was validated against the full-scale results for a factor of 4 reduction in model scale and a factor of 8 reduction in Reynolds number. This research has defined the level of geometric fidelity required for artificial ice shapes to yield aerodynamic performance results to within a known level of uncertainty and has culminated in a proposed methodology for subscale iced-airfoil aerodynamic simulation.

  6. Isothermal Bondi Accretion in Jaffe and Hernquist Galaxies with a Central Black Hole: Fully Analytical Solutions

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

    Ciotti, Luca; Pellegrini, Silvia, E-mail: luca.ciotti@unibo.it

    One of the most active fields of research of modern-day astrophysics is that of massive black hole formation and coevolution with the host galaxy. In these investigations, ranging from cosmological simulations, to semi-analytical modeling, to observational studies, the Bondi solution for accretion on a central point-mass is widely adopted. In this work we generalize the classical Bondi accretion theory to take into account the effects of the gravitational potential of the host galaxy, and of radiation pressure in the optically thin limit. Then, we present the fully analytical solution, in terms of the Lambert–Euler W -function, for isothermal accretion inmore » Jaffe and Hernquist galaxies with a central black hole. The flow structure is found to be sensitive to the shape of the mass profile of the host galaxy. These results and the formulae that are provided, most importantly, the one for the critical accretion parameter, allow for a direct evaluation of all flow properties, and are then useful for the abovementioned studies. As an application, we examine the departure from the true mass accretion rate of estimates obtained using the gas properties at various distances from the black hole, under the hypothesis of classical Bondi accretion. An overestimate is obtained from regions close to the black hole, and an underestimate outside a few Bondi radii; the exact position of the transition between the two kinds of departure depends on the galaxy model.« less

  7. Magnetic dynamos in accreting planetary bodies

    NASA Astrophysics Data System (ADS)

    Golabek, G.; Labrosse, S.; Gerya, T.; Morishima, R.; Tackley, P. J.

    2012-12-01

    Laboratory measurements revealed ancient remanent magnetization in meteorites [1] indicating the activity of magnetic dynamos in the corresponding meteorite parent body. To study under which circumstances dynamo activity is possible, we use a new methodology to simulate the internal evolution of a planetary body during accretion and differentiation. Using the N-body code PKDGRAV [2] we simulate the accretion of planetary embryos from an initial annulus of several thousand planetesimals. The growth history of the largest resulting planetary embryo is used as an input for the thermomechanical 2D code I2ELVIS [3]. The thermomechanical model takes recent parametrizations of impact processes [4] and of the magnetic dynamo [5] into account. It was pointed out that impacts can not only deposit heat deep into the target body, which is later buried by ejecta of further impacts [6], but also that impacts expose in the crater region originally deep-seated layers, thus cooling the interior [7]. This combination of impact effects becomes even more important when we consider that planetesimals of all masses contribute to planetary accretion. This leads occasionally to collisions between bodies with large ratios between impactor and target mass. Thus, all these processes can be expected to have a profound effect on the thermal evolution during the epoch of planetary accretion and may have implications for the magnetic dynamo activity. Results show that late-formed planetesimals do not experience silicate melting and avoid thermal alteration, whereas in early-formed bodies accretion and iron core growth occur almost simultaneously and a highly variable magnetic dynamo can operate in the interior of these bodies.

  8. 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.

  9. Accretion disks around black holes

    NASA Technical Reports Server (NTRS)

    Abramowicz, M. A.

    1994-01-01

    The physics of accretion flow very close to a black hole is dominated by several general relativistic effects. It cannot be described by the standard Shakura Sunyaev model or by its relativistic version developed by Novikov and Thome. The most important of these effects is a dynamical mass loss from the inner edge of the disk (Roche lobe overflow). The relativistic Roche lobe overflow induces a strong advective cooling, which is sufficient to stabilize local, axially symmetric thermal and viscous modes. It also stabilizes the non-axially-symmetric global modes discovered by Papaloizou and Pringle. The Roche lobe overflow, however, destabilizes sufficiently self-gravitating accretion disks with respect to a catastrophic runaway of mass due to minute changes of the gravitational field induced by the changes in the mass and angular momentum of the central black hole. One of the two acoustic modes may become trapped near the inner edge of the disk. All these effects, absent in the standard model, have dramatic implications for time-dependent behavior of the accretion disks around black holes.

  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. Shock Temperatures of Major Silicates in Rocky Planets

    NASA Astrophysics Data System (ADS)

    Davies, E.; Root, S.; Spaulding, D.; Kraus, R. G.; Stewart, S. T.; Jacobsen, S. B.; Mattsson, T. R.

    2016-12-01

    Rocky extra-solar planets have been discovered with very high masses that challenge our theoretical understanding of planetary structures and notions of planet formation. In order to constrain models and understand mechanisms of both the formation and subsequent evolution of these planets, it is imperative to determine the properties of materials within the interiors of large Earth-like planets. The major minerals olivine [(Mg,Fe)2SiO4] and enstatite [(Mg,Fe)SiO3], along with Fe-rich metal (with 5% Ni), are the most abundant solids from which Earth-like planets accrete. These materials are subject to ultra-high pressures and temperatures (approaching 10TPa and 10,000 K) during planetary formation and in the present day interiors of large rocky planets. Here, we present results of shock compression experiments on the Sandia Z machine. Shock compression experiments with the Sandia Z machine use large current and field densities that generate magnetic pressures up to 650 GPa that can accelerate flyer plates up to 40 km/s. We report shock temperatures for pressures greater than 270 GPa for forsterite (Mg2SiO4) and enstatite. Our results, together with prior data, demonstrate discrepancies in shock temperatures on forsterite in the region of possible incongruent melting on the Hugoniot. Key gaps in the Hugoniot contribute to this uncertainty. EOS formalisms such as M-ANEOS, which are commonly used in planetary impact simulations, over predict temperatures above 200 GPa with significant disagreement above 500 GPa. As a result, the amount of material subject to shock-induced vaporization during giant impacts is larger than currently estimated. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. This work was performed under the auspices of the U

  12. 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.

  13. 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.

  14. Accretion Disks in Supersoft X-ray Sources

    NASA Technical Reports Server (NTRS)

    Popham, Robert; DiStefano, Rosanne

    1996-01-01

    We examine the role of the accretion disk in the steady-burning white dwarf model for supersoft sources. The accretion luminosity of the disk is quite small compared to the nuclear burning luminosity of the central source. Thus, in contrast to standard accretion disks, the main role of the disk is to reprocess the radiation from the white dwarf. We calculate models of accretion disks around luminous white dwarfs and compare the resulting disk fluxes to optical and UV observations of the LMC supersoft sources CAL 83, CAL 87, and RX J0513.9-6951. We find that if the white dwarf luminosity is near the upper end of the steady-burning region, and the flaring of the disk is included, then reprocessing by the disk can account for the UV fluxes and a substantial fraction of the optical fluxes of these systems. Reprocessing by the companion star can provide additional optical flux, and here too the disk plays an important role: since the disk is fairly thick, it shadows a significant fraction of the companion's surface.

  15. The Large-scale Magnetic Fields of Thin Accretion Disks

    NASA Astrophysics Data System (ADS)

    Cao, Xinwu; Spruit, Hendrik C.

    2013-03-01

    Large-scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large-scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared with the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number P m is around unity. In this work, we revisit this problem considering the angular momentum of the disk to be removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-beta at the midplane of order several hundred, and one for strong accreted fields, β ~ 1. We surmise that the first is relevant for the accretion of weak, possibly external, fields through the outer parts of the disk, while the latter one could explain the tendency, observed in full three-dimensional numerical simulations, of strong flux bundles at the centers of disk to stay confined in spite of strong magnetororational instability turbulence surrounding them.

  16. Design of compact piezoelectric transducers for shock wave applications

    NASA Astrophysics Data System (ADS)

    Dreyer, Thomas; Liebler, Marko; Riedlinger, Rainer E.; Ginter, Siegfried

    2003-10-01

    The application of focused intense sound pulses to treat several orthopedic diseases has gained in importance during the past years. Self-focusing piezoelectric transducers known from ESWL are not well suited for this purpose due to their size. Therefore compact transducers have to be designed. This implies an increase of the pressure pulse amplitude generated at the radiating surface. A stacked placement of two piezoelectric layers driven by two high-voltage pulses with an adjustable delay accomplishes this. Several designs are presented here representing transducers of different sizes. In principle piezoelectric transducers have the ability to vary the pressure pulse shape to a wider extent than other shock wave sources. Based on FEM simulations of the transducer the influence of some driving parameters, like a variation of the interpulse delay or shape of the driving voltage, on the resulting focal pressure signal is demonstrated. The results show the feasibility to control some parameters of the signal, for example the peak negative pressure amplitude. This possibility could provide new aspects in basic research as well as in clinical applications.

  17. Problem of gas accretion on a gravitational center

    NASA Technical Reports Server (NTRS)

    Ladygin, V. A.

    1980-01-01

    A method of the approximated solution of the problem of accretion on a rapidly moving gravitational center is developed. This solution is obtained in the vicinity of the axis of symmetry in the region of the potential flow. The solution of the problem of stationary gas accretion on a moving gravitational center simulates the movement of a substance in interstellar space in the vicinity of a black hole. A detailed picture of gas accretion on a black hole is of interest in connection with the problem of observation of black holes.

  18. 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.

  19. Radiative Efficiency of Collisionless Accretion

    NASA Astrophysics Data System (ADS)

    Gruzinov, Andrei V.

    1998-07-01

    The radiative efficiency, η≡L/Ṁc2, of a slowly accreting black hole is estimated using a two-temperature model of accretion. The radiative efficiency depends on the magnetic field strength near the Schwarzschild radius. For weak magnetic fields, i.e., β-1 ≡ B2/8πp <~ 10-3, the low efficiency η ~ 10-4 that is assumed in some theoretical models is achieved. For β-1 > 10-3, a significant fraction of viscous heat is dissipated by electrons and radiated away resulting in η > 10-4. At equipartition magnetic fields, β-1 ~ 1, we estimate η ~ 10-1.

  20. Application of CHAD hydrodynamics to shock-wave problems

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

    Trease, H.E.; O`Rourke, P.J.; Sahota, M.S.

    1997-12-31

    CHAD is the latest in a sequence of continually evolving computer codes written to effectively utilize massively parallel computer architectures and the latest grid generators for unstructured meshes. Its applications range from automotive design issues such as in-cylinder and manifold flows of internal combustion engines, vehicle aerodynamics, underhood cooling and passenger compartment heating, ventilation, and air conditioning to shock hydrodynamics and materials modeling. CHAD solves the full unsteady Navier-Stoke equations with the k-epsilon turbulence model in three space dimensions. The code has four major features that distinguish it from the earlier KIVA code, also developed at Los Alamos. First, itmore » is based on a node-centered, finite-volume method in which, like finite element methods, all fluid variables are located at computational nodes. The computational mesh efficiently and accurately handles all element shapes ranging from tetrahedra to hexahedra. Second, it is written in standard Fortran 90 and relies on automatic domain decomposition and a universal communication library written in standard C and MPI for unstructured grids to effectively exploit distributed-memory parallel architectures. Thus the code is fully portable to a variety of computing platforms such as uniprocessor workstations, symmetric multiprocessors, clusters of workstations, and massively parallel platforms. Third, CHAD utilizes a variable explicit/implicit upwind method for convection that improves computational efficiency in flows that have large velocity Courant number variations due to velocity of mesh size variations. Fourth, CHAD is designed to also simulate shock hydrodynamics involving multimaterial anisotropic behavior under high shear. The authors will discuss CHAD capabilities and show several sample calculations showing the strengths and weaknesses of CHAD.« less

  1. DOUBLE BOW SHOCKS AROUND YOUNG, RUNAWAY RED SUPERGIANTS: APPLICATION TO BETELGEUSE

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

    Mackey, Jonathan; Mohamed, Shazrene; Neilson, Hilding R.

    2012-05-20

    A significant fraction of massive stars are moving supersonically through the interstellar medium (ISM), either due to disruption of a binary system or ejection from their parent star cluster. The interaction of their wind with the ISM produces a bow shock. In late evolutionary stages these stars may undergo rapid transitions from red to blue and vice versa on the Hertzsprung-Russell diagram, with accompanying rapid changes to their stellar winds and bow shocks. Recent three-dimensional simulations of the bow shock produced by the nearby runaway red supergiant (RSG) Betelgeuse, under the assumption of a constant wind, indicate that the bowmore » shock is very young (<30, 000 years old), hence Betelgeuse may have only recently become an RSG. To test this possibility, we have calculated stellar evolution models for single stars which match the observed properties of Betelgeuse in the RSG phase. The resulting evolving stellar wind is incorporated into two-dimensional hydrodynamic simulations in which we model a runaway blue supergiant (BSG) as it undergoes the transition to an RSG near the end of its life. We find that the collapsing BSG wind bubble induces a bow shock-shaped inner shell around the RSG wind that resembles Betelgeuse's bow shock, and has a similar mass. Surrounding this is the larger-scale retreating bow shock generated by the now defunct BSG wind's interaction with the ISM. We suggest that this outer shell could explain the bar feature located (at least in projection) just in front of Betelgeuse's bow shock.« less

  2. 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.

  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. Numerical simulation of ice accretion phenomena on rotor blade of axial blower

    NASA Astrophysics Data System (ADS)

    Matsuura, Taiki; Suzuki, Masaya; Yamamoto, Makoto; Shishido, Shinichiro; Murooka, Takeshi; Miyagawa, Hiroshi

    2012-08-01

    Ice accretion is the phenomenon that super-cooled water droplets impinge and accrete on a body. It is well known that ice accretion on blades and airfoils leads to performance degradation and severe accidents. For this reason, experimental investigations have been carried out using flight tests or icing tunnels. However, it is too expensive, dangerous, and difficult to set actual icing conditions. Hence, computational fluid dynamics is useful to predict ice accretion. A rotor blade is one of jet engine components where ice accretes. Therefore, the authors focus on the ice accretion on a rotor blade in this study. Three-dimensional icing phenomena on the rotor blade of a commercial axial blower are computed here, and ice accretion on the rotor blade is numerically investigated.

  5. Hot accretion flow with anisotropic viscosity

    NASA Astrophysics Data System (ADS)

    Wu, Mao-Chun; Bu, De-Fu; Gan, Zhao-Ming; Yuan, Ye-Fei

    2017-12-01

    In extremely low accretion rate systems, the ion mean-free path can be much larger than the gyroradius. Therefore, gas pressure is anisotropic with respect to magnetic field lines. The effects of pressure anisotropy can be modeled by an anisotropic viscosity with respect to magnetic field lines. Angular momentum can be transferred by anisotropic viscosity. In this paper, we investigate hot accretion flow with anisotropic viscosity. We consider the case that anisotropic viscous stress is much larger than Maxwell stress. We find that the flow is convectively unstable. We also find that the mass inflow rate decreases towards a black hole. Wind is very weak; its mass flux is 10-15% of the mass inflow rate. The inward decrease of inflow rate is mainly due to convective motions. This result may be useful to understand the accretion flow in the Galactic Center Sgr A* and M 87 galaxy.

  6. Exciting cell membranes with a blustering heat shock.

    PubMed

    Liu, Qiang; Frerck, Micah J; Holman, Holly A; Jorgensen, Erik M; Rabbitt, Richard D

    2014-04-15

    Brief heat shocks delivered to cells by pulsed laser light can evoke action potentials in neurons and contraction in cardiomyocytes, but the primary biophysical mechanism has been elusive. In this report we show in the neuromuscular junction of Caenorhabditis elegans that application of a 500°C/s heat shock for 500 μs evoked ~35 pA of excitatory current and injected ~23 fC(femtocoulomb) of charge into the cell while raising the temperature only 0.25°C. The key variable driving the current was the rate of change of temperature (dT/dt heat shock), not temperature itself. The photothermal heat shock current was voltage-dependent and was from thermally driven displacement of ions near the plasma membrane. The charge movement was rapid during the heat shock and slow during thermal relaxation, thus leading to an asymmetrical capacitive current that briefly depolarized the cell. A simple quantitative model is introduced to describe modulation of the membrane potential and facilitate practical application of optical heat shock stimuli. Copyright © 2014 Biophysical Society. Published by Elsevier Inc. All rights reserved.

  7. 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.

  8. LAUNCHING AND QUENCHING OF BLACK HOLE RELATIVISTIC JETS AT LOW ACCRETION RATE

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

    Pu, Hung-Yi; Chang, Hsiang-Kuang; Hirotani, Kouichi

    2012-10-20

    Relativistic jets are launched from black hole (BH) X-ray binaries and active galactic nuclei when the disk accretion rate is below a certain limit (i.e., when the ratio of the accretion rate to the Eddingtion accretion rate, m-dot , is below about 0.01) but quenched when above. We propose a new paradigm to explain this observed coupling between the jet and the accretion disk by investigating the extraction of the rotational energy of a BH when it is surrounded by different types of accretion disk. At low accretion rates (e.g., when m-dot {approx}<0.1), the accretion near the event horizon ismore » quasi-spherical. The accreting plasmas fall onto the event horizon in a wide range of latitudes, breaking down the force-free approximation near the horizon. To incorporate the plasma inertia effect, we consider the magnetohydrodynamical (MHD) extraction of the rotational energy from BHs by the accreting MHD fluid, as described by the MHD Penrose process. It is found that the energy extraction operates, and hence a relativistic jet is launched, preferentially when the accretion disk consists of an outer Shakura-Sunyaev disk (SSD) and an inner advection-dominated accretion flow. When the entire accretion disk type changes into an SSD, the jet is quenched because the plasmas bring more rest-mass energy than what is extracted from the hole electromagnetically to stop the extraction. Several other issues related to observed BH disk-jet couplings, such as why the radio luminosity increases with increasing X-ray luminosity until the radio emission drops, are also explained.« less

  9. On the Accretion Rates of SW Sextantis Nova-like Variables

    NASA Astrophysics Data System (ADS)

    Ballouz, Ronald-Louis; Sion, Edward M.

    2009-06-01

    We present accretion rates for selected samples of nova-like variables having IUE archival spectra and distances uniformly determined using an infrared method by Knigge. A comparison with accretion rates derived independently with a multiparametric optimization modeling approach by Puebla et al. is carried out. The accretion rates of SW Sextantis nova-like systems are compared with the accretion rates of non-SW Sextantis systems in the Puebla et al. sample and in our sample, which was selected in the orbital period range of three to four and a half hours, with all systems having distances using the method of Knigge. Based upon the two independent modeling approaches, we find no significant difference between the accretion rates of SW Sextantis systems and non-SW Sextantis nova-like systems insofar as optically thick disk models are appropriate. We find little evidence to suggest that the SW Sex stars have higher accretion rates than other nova-like cataclysmic variables (CVs) above the period gap within the same range of orbital periods.

  10. Gas-rich dwarfs and accretion phenomena in early-type galaxies

    NASA Technical Reports Server (NTRS)

    Silk, J.; Norman, C.

    1979-01-01

    An analysis is presented of the combined effects of cloud accretion and galactic winds and coronae. An accretion model is developed wherein gas-rich dwarf galaxies are accreted into galactic halos, which provides an adequate source of H I to account for observations of neutral gas in early-type galaxies. Accretion is found to fuel the wind, thereby regulating the accretion flow and yielding a time-dependent model for star formation, enrichment, and nuclear activity. The permissible parameter range for intergalactic gas clouds and galaxy groups is discussed, along with the frequency of gas-rich dwarfs and their large ratios of gas mass to luminosity. Also considered is the occurrence of gas stripping and the consequent formation of dwarf spheroidal systems that remain in the halo, and gas clouds that dissipate and suffer further infall. A cosmological implication of the model is that, because the characteristic time scale of a gas-rich dwarf galaxy to be accreted and lose its gas is comparable to a Hubble time, there may have been a far more extensive primordial distribution of such systems at earlier epochs.

  11. 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.

  12. Airfoil Ice-Accretion Aerodynamics Simulation

    NASA Technical Reports Server (NTRS)

    Bragg, Michael B.; Broeren, Andy P.; Addy, Harold E.; Potapczuk, Mark G.; Guffond, Didier; Montreuil, E.

    2007-01-01

    NASA Glenn Research Center, ONERA, and the University of Illinois are conducting a major research program whose goal is to improve our understanding of the aerodynamic scaling of ice accretions on airfoils. The program when it is completed will result in validated scaled simulation methods that produce the essential aerodynamic features of the full-scale iced-airfoil. This research will provide some of the first, high-fidelity, full-scale, iced-airfoil aerodynamic data. An initial study classified ice accretions based on their aerodynamics into four types: roughness, streamwise ice, horn ice, and spanwise-ridge ice. Subscale testing using a NACA 23012 airfoil was performed in the NASA IRT and University of Illinois wind tunnel to better understand the aerodynamics of these ice types and to test various levels of ice simulation fidelity. These studies are briefly reviewed here and have been presented in more detail in other papers. Based on these results, full-scale testing at the ONERA F1 tunnel using cast ice shapes obtained from molds taken in the IRT will provide full-scale iced airfoil data from full-scale ice accretions. Using these data as a baseline, the final step is to validate the simulation methods in scale in the Illinois wind tunnel. Computational ice accretion methods including LEWICE and ONICE have been used to guide the experiments and are briefly described and results shown. When full-scale and simulation aerodynamic results are available, these data will be used to further develop computational tools. Thus the purpose of the paper is to present an overview of the program and key results to date.

  13. Radiatively-suppressed spherical accretion under relativistic radiative transfer

    NASA Astrophysics Data System (ADS)

    Fukue, Jun

    2018-03-01

    We numerically examine radiatively-suppressed relativistic spherical accretion flows on to a central object with mass M under Newtonian gravity and special relativity. We simultaneously solve both the relativistic radiative transfer equation and the relativistic hydrodynamical equations for spherically symmetric flows under the double iteration process in the case of the intermediate optical depth. We find that the accretion flow is suppressed, compared with the freefall case in the nonrelativistic regime. For example, in the case of accretion on to a luminous core with accretion luminosity L*, the freefall velocity v normalized by the speed of light c under the radiative force in the nonrelativistic regime is β (\\hat{r}) = v/c = -√{(1-Γ _*)/(\\hat{r}+1-Γ _*)}, where Γ* (≡ L*/LE, LE being the Eddington luminosity) is the Eddington parameter and \\hat{r} (= r/rS, rS being the Schwarzschild radius) the normalized radius, whereas the infall speed at the central core is ˜0.7β(1), irrespective of the mass-accretion rate. This is due to the relativistic effect; the comoving flux is enhanced by the advective flux. We briefly examine and discuss an isothermal case, where the emission takes place in the entire space.

  14. Effects of Planetesimal Accretion on the Structural Evolution of Sub-Neptunes

    NASA Astrophysics Data System (ADS)

    Chatterjee, Sourav; Chen, Howard

    2018-01-01

    A remarkable discovery of NASA's Kepler mission is the wide diversity in the average densities of planets even when they are of similar mass. After gas disk dissipation, fully formed planets could accrete nearby planetesimals from a remnant planetesimal disk. We present calculations using the open-source stellar evolution toolkit Modules for Experiments in Stellar Astrophysics (MESA) modified to include the deposition of planetesimals into the H/He envelopes of sub-Neptunes. We show that planetesimal accretion can alter the mass-radius isochrones for these planets. The additional energy deposited via planetesimal accretion puffs up the envelopes leading to enhanced gas loss during the phase of rapid accretion. As a result, the same initial planet can evolve to contain very different final envelope-mass fractions. This manifest as differences in the average planet densities long after accretion stops. Differences in the accretion history, total accreted mass, and the inherent stochasticity of the accretion process can bring wide diversity in final average densities even when the initial planets are very similar. These effects are particularly important for planets initially less massive than ~10 MEarth and with envelope mass fraction less than ~10%, thought to be the most common type of planets discovered by Kepler.

  15. Coherent Raman Studies of Shocked Liquids

    NASA Astrophysics Data System (ADS)

    McGrane, Shawn; Brown, Kathryn; Dang, Nhan; Bolme, Cynthia; Moore, David

    2013-06-01

    Transient vibrational spectroscopies offer the potential to directly observe time dependent shock induced chemical reaction kinetics. We report recent experiments that couple a hybrid picosecond/femtosecond coherent anti-Stokes Raman spectroscopy (CARS) diagnostic with our tabletop ultrafast laser driven shock platform. Initial results on liquids shocked to 20 GPa suggest that sub-picosecond dephasing at high pressure and temperature may limit the application of this nonresonant background free version of CARS. Initial results using interferometric CARS to increase sensitivity and overcome these limitations will be presented.

  16. Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion

    PubMed Central

    Johansen, Anders; Low, Mordecai-Mark Mac; Lacerda, Pedro; Bizzarro, Martin

    2015-01-01

    Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas drag–assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo runaway accretion of chondrules within ~3 My, forming planetary embryos up to Mars’s size along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size sorting of chondrules consistent with chondrites. Accretion of millimeter-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disc lifetime outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles in the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts. PMID:26601169

  17. Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion

    NASA Astrophysics Data System (ADS)

    Johansen, Anders; Mac Low, Mordecai-Mark; Lacerda, Pedro; Bizzarro, Martin

    2015-04-01

    Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas drag-assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo runaway accretion of chondrules within ~3 My, forming planetary embryos up to Mars's size along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size sorting of chondrules consistent with chondrites. Accretion of millimeter-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disc lifetime outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles in the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts.

  18. Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion.

    PubMed

    Johansen, Anders; Low, Mordecai-Mark Mac; Lacerda, Pedro; Bizzarro, Martin

    2015-04-01

    Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas drag-assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo runaway accretion of chondrules within ~3 My, forming planetary embryos up to Mars's size along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size sorting of chondrules consistent with chondrites. Accretion of millimeter-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disc lifetime outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles in the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts.

  19. OBSERVATIONAL LIMITS ON TYPE 1 ACTIVE GALACTIC NUCLEUS ACCRETION RATE IN COSMOS

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

    Trump, Jonathan R.; Impey, Chris D.; Gabor, Jared

    2009-07-20

    We present black hole masses and accretion rates for 182 Type 1 active galactic nuclei (AGNs) in COSMOS. We estimate masses using the scaling relations for the broad H {beta}, Mg II, and C IV emission lines in the redshift ranges 0.16 < z < 0.88, 1 < z < 2.4, and 2.7 < z < 4.9. We estimate the accretion rate using an Eddington ratio L{sub I}/L{sub Edd} estimated from optical and X-ray data. We find that very few Type 1 AGNs accrete below L{sub I} /L{sub Edd} {approx} 0.01, despite simulations of synthetic spectra which show that themore » survey is sensitive to such Type 1 AGNs. At lower accretion rates the broad-line region may become obscured, diluted, or nonexistent. We find evidence that Type 1 AGNs at higher accretion rates have higher optical luminosities, as more of their emission comes from the cool (optical) accretion disk with respect to shorter wavelengths. We measure a larger range in accretion rate than previous works, suggesting that COSMOS is more efficient at finding low accretion rate Type 1 AGNs. However, the measured range in accretion rate is still comparable to the intrinsic scatter from the scaling relations, suggesting that Type 1 AGNs accrete at a narrow range of Eddington ratio, with L{sub I} /L{sub Edd} {approx} 0.1.« less

  20. 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.

  1. Supermassive blackholes without super Eddington accretion

    NASA Astrophysics Data System (ADS)

    Christian, Damian Joseph; Kim, Matt I.; Garofalo, David; D'Avanzo, Jaclyn; Torres, John

    2017-08-01

    We explore the X-ray luminosity function at high redshift for active galactic nuclei using an albeit simplified model for mass build-up using a combination of mergers and mass accretion in the gap paradigm (Garofalo et al. 2010). Using a retrograde-dominated configuration we find an interesting low probability channel for the growth of one billion solar mass black holes within hundreds of millions of years of the big bang without appealing to super Eddington accretion (Kim et al. 2016). This result is made more compelling by the connection between this channel and an end product involving active galaxies with FRI radio morphology but weaker jet powers in mildly sub-Eddington accretion regimes. We will discuss our connection between the unexplained paucity of a given family of AGNs and the rapid growth of supermassive black holes, two heretofore seemingly unrelated aspects of the physics of AGNs that will help further understand their properties and evolution.

  2. Highly Accreting Quasars at High Redshift

    NASA Astrophysics Data System (ADS)

    Martínez-Aldama, Mary L.; Del Olmo, Ascensión; Marziani, Paola; Sulentic, Jack W.; Negrete, C. Alenka; Dultzin, Deborah; Perea, Jaime; D'Onofrio, Mauro

    2017-12-01

    We present preliminary results of a spectroscopic analysis for a sample of type 1 highly accreting quasars (LLedd>0.2) at high redshift, z 2-3. The quasars were observed with the OSIRIS spectrograph on the GTC 10.4 m telescope located at the Observatorio del Roque de los Muchachos in La Palma. The highly accreting quasars were identified using the 4D Eigenvector 1 formalism, which is able to organize type 1 quasars over a broad range of redshift and luminosity. The kinematic and physical properties of the broad line region have been derived by fitting the profiles of strong UV emission lines such as AlIII, SiIII and CIII. The majority of our sources show strong blueshifts in the high-ionization lines and high Eddington ratios which are related with the productions of outflows. The importance of highly accreting quasars goes beyond a detailed understanding of their physics: their extreme Eddington ratio makes them candidates standard candles for cosmological studies.

  3. 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.

  4. 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

  5. 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.

  6. Radial accretion flows on static spherically symmetric black holes

    NASA Astrophysics Data System (ADS)

    Chaverra, Eliana; Sarbach, Olivier

    2015-08-01

    We analyze the steady radial accretion of matter into a nonrotating black hole. Neglecting the self-gravity of the accreting matter, we consider a rather general class of static, spherically symmetric and asymptotically flat background spacetimes with a regular horizon. In addition to the Schwarzschild metric, this class contains certain deformation of it, which could arise in alternative gravity theories or from solutions of the classical Einstein equations in the presence of external matter fields. Modeling the ambient matter surrounding the black hole by a relativistic perfect fluid, we reformulate the accretion problem as a dynamical system, and under rather general assumptions on the fluid equation of state, we determine the local and global qualitative behavior of its phase flow. Based on our analysis and generalizing previous work by Michel, we prove that for any given positive particle density number at infinity, there exists a unique radial, steady-state accretion flow which is regular at the horizon. We determine the physical parameters of the flow, including its accretion and compression rates, and discuss their dependency on the background metric.

  7. Swept-Wing Ice Accretion Characterization and Aerodynamics

    NASA Technical Reports Server (NTRS)

    Broeren, Andy P.; Potapczuk, Mark G.; Riley, James T.; Villedieu, Philippe; Moens, Frederic; Bragg, Michael B.

    2013-01-01

    NASA, FAA, ONERA, the University of Illinois and Boeing have embarked on a significant, collaborative research effort to address the technical challenges associated with icing on large-scale, three-dimensional swept wings. The overall goal is to improve the fidelity of experimental and computational simulation methods for swept-wing ice accretion formation and resulting aerodynamic effect. A seven-phase research effort has been designed that incorporates ice-accretion and aerodynamic experiments and computational simulations. As the baseline, full-scale, swept-wing-reference geometry, this research will utilize the 65% scale Common Research Model configuration. Ice-accretion testing will be conducted in the NASA Icing Research Tunnel for three hybrid swept-wing models representing the 20%, 64% and 83% semispan stations of the baseline-reference wing. Three-dimensional measurement techniques are being developed and validated to document the experimental ice-accretion geometries. Artificial ice shapes of varying geometric fidelity will be developed for aerodynamic testing over a large Reynolds number range in the ONERA F1 pressurized wind tunnel and in a smaller-scale atmospheric wind tunnel. Concurrent research will be conducted to explore and further develop the use of computational simulation tools for ice accretion and aerodynamics on swept wings. The combined results of this research effort will result in an improved understanding of the ice formation and aerodynamic effects on swept wings. The purpose of this paper is to describe this research effort in more detail and report on the current results and status to date. 1

  8. Swept-Wing Ice Accretion Characterization and Aerodynamics

    NASA Technical Reports Server (NTRS)

    Broeren, Andy P.; Potapczuk, Mark G.; Riley, James T.; Villedieu, Philippe; Moens, Frederic; Bragg, Michael B.

    2013-01-01

    NASA, FAA, ONERA, the University of Illinois and Boeing have embarked on a significant, collaborative research effort to address the technical challenges associated with icing on large-scale, three-dimensional swept wings. The overall goal is to improve the fidelity of experimental and computational simulation methods for swept-wing ice accretion formation and resulting aerodynamic effect. A seven-phase research effort has been designed that incorporates ice-accretion and aerodynamic experiments and computational simulations. As the baseline, full-scale, swept-wing-reference geometry, this research will utilize the 65 percent scale Common Research Model configuration. Ice-accretion testing will be conducted in the NASA Icing Research Tunnel for three hybrid swept-wing models representing the 20, 64 and 83 percent semispan stations of the baseline-reference wing. Threedimensional measurement techniques are being developed and validated to document the experimental ice-accretion geometries. Artificial ice shapes of varying geometric fidelity will be developed for aerodynamic testing over a large Reynolds number range in the ONERA F1 pressurized wind tunnel and in a smaller-scale atmospheric wind tunnel. Concurrent research will be conducted to explore and further develop the use of computational simulation tools for ice accretion and aerodynamics on swept wings. The combined results of this research effort will result in an improved understanding of the ice formation and aerodynamic effects on swept wings. The purpose of this paper is to describe this research effort in more detail and report on the current results and status to date.

  9. 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.

  10. Episodic accretion: the interplay of infall and disc instabilities

    NASA Astrophysics Data System (ADS)

    Kuffmeier, Michael; Frimann, Søren; Jensen, Sigurd S.; Haugbølle, Troels

    2018-04-01

    Using zoom-simulations carried out with the adaptive mesh-refinement code RAMSES with a dynamic range of up to 227 ≈ 1.34 × 108 we investigate the accretion profiles around six stars embedded in different environments inside a (40 pc)3 giant molecular cloud, the role of mass infall and disc instabilities on the accretion profile, and thus on the luminosity of the forming protostar. Our results show that the environment in which the protostar is embedded determines the overall accretion profile of the protostar. Infall on to the circumstellar disc may trigger gravitational disc instabilities in the disc at distances of around ˜10 to ˜50 au leading to rapid transport of angular momentum and strong accretion bursts. These bursts typically last for about ˜10 to a ˜100 yr, consistent with typical orbital times at the location of the instability, and enhance the luminosity of the protostar. Calculations with the stellar evolution code MESA show that the accretion bursts induce significant changes in the protostellar properties, such as the stellar temperature and radius. We apply the obtained protostellar properties to produce synthetic observables with RADMC3D and predict that accretion bursts lead to observable enhancements around 20 to 200 μm in the spectral energy distribution of Class 0 type young stellar objects.

  11. Application of the Cubed-Sphere Grid to Tilted Black-Hole Accretion Disks

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

    Fragile, P C; Lindner, C C; Anninos, P

    2008-09-24

    In recent work we presented the first results of global general relativistic magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion disks around rotating black holes. The simulated tilted disks showed dramatic differences from comparable untilted disks, such as asymmetrical accretion onto the hole through opposing 'plunging streams' and global precession of the disk powered by a torque provided by the black hole. However, those simulations used a traditional spherical-polar grid that was purposefully underresolved along the pole, which prevented us from assessing the behavior of any jets that may have been associated with the tilted disks. To address this shortcomingmore » we have added a block-structured 'cubed-sphere' grid option to the Cosmos++ GRMHD code, which will allow us to simultaneously resolve the disk and polar regions. Here we present our implementation of this grid and the results of a small suite of validation tests intended to demonstrate that the new grid performs as expected. The most important test in this work is a comparison of identical tilted disks, one evolved using our spherical-polar grid and the other with the cubed-sphere grid. We also demonstrate an interesting dependence of the early-time evolution of our disks on their orientation with respect to the grid alignment. This dependence arises from the differing treatment of current sheets within the disks, especially whether they are aligned with symmetry planes of the grid or not.« less

  12. 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.

  13. Evolution of the luminosity function of quasar accretion disks

    NASA Technical Reports Server (NTRS)

    Caditz, David M.; Petrosian, Vahe; Wandel, Amri

    1991-01-01

    Using an accretion-disk model, accretion disk luminosities are calculated for a grid of black hole masses and accretion rates. It is shown that, as the black-hole mass increases with time, the monochromatic luminosity at a given frequency first increases and then decreases rapidly as this frequency is crossed by the Wien cutoff. The upper limit on the monochromatic luminosity, which is characteristic for a given epoch, constrains the evolution of quasar luminosities and determines the evolultion of the quasar luminosity function.

  14. 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.

  15. Shock waves in strongly coupled plasmas

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

    Khlebnikov, Sergei; Kruczenski, Martin; Michalogiorgakis, Georgios

    2010-12-15

    Shock waves are supersonic disturbances propagating in a fluid and giving rise to dissipation and drag. Weak shocks, i.e., those of small amplitude, can be well described within the hydrodynamic approximation. On the other hand, strong shocks are discontinuous within hydrodynamics and therefore probe the microscopics of the theory. In this paper, we consider the case of the strongly coupled N=4 plasma whose microscopic description, applicable for scales smaller than the inverse temperature, is given in terms of gravity in an asymptotically AdS{sub 5} space. In the gravity approximation, weak and strong shocks should be described by smooth metrics withmore » no discontinuities. For weak shocks, we find the dual metric in a derivative expansion, and for strong shocks we use linearized gravity to find the exponential tail that determines the width of the shock. In particular, we find that, when the velocity of the fluid relative to the shock approaches the speed of light v{yields}1 the penetration depth l scales as l{approx}(1-v{sup 2}){sup 1/4}. We compare the results with second-order hydrodynamics and the Israel-Stewart approximation. Although they all agree in the hydrodynamic regime of weak shocks, we show that there is not even qualitative agreement for strong shocks. For the gravity side, the existence of shock waves implies that there are disturbances of constant shape propagating on the horizon of the dual black holes.« less

  16. 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.

  17. Automated Detection and Analysis of Interplanetary Shocks Running Real-Time on the Web

    NASA Astrophysics Data System (ADS)

    Vorotnikov, V.; Smith, C. W.; Hu, Q.; Szabo, A.; Skoug, R. M.; Cohen, C. M.; Davis, A. J.

    2008-05-01

    The ACE real-time data stream provides web-based now-casting capabilities for solar wind conditions upstream of Earth. We have built a fully automated code that finds and analyzes interplanetary shocks as they occur and posts their solutions on the Web for possible real-time application to space weather nowcasting. Shock analysis algorithms based on the Rankine-Hugoniot jump conditions exist and are in wide-spread use today for the interactive analysis of interplanetary shocks yielding parameters such as shock speed and propagation direction and shock strength in the form of compression ratios. At a previous meeting we reported on efforts to develop a fully automated code that used ACE Level-2 (science quality) data to prove the applicability and correctness of the code and the associated shock-finder. We have since adapted the code to run ACE RTSW data provided by NOAA. This data lacks the full 3-dimensional velocity vector for the solar wind and contains only a single component wind speed. We show that by assuming the wind velocity to be radial strong shock solutions remain essentially unchanged and the analysis performs as well as it would if 3-D velocity components were available. This is due, at least in part, to the fact that strong shocks tend to have nearly radial shock normals and it is the strong shocks that are most effective in space weather applications. Strong shocks are the only shocks that concern us in this application. The code is now running on the Web and the results are available to all.

  18. Accretion of southern Alaska

    USGS Publications Warehouse

    Hillhouse, J.W.

    1987-01-01

    Paleomagnetic data from southern Alaska indicate that the Wrangellia and Peninsular terranes collided with central Alaska probably by 65 Ma ago and certainly no later than 55 Ma ago. The accretion of these terranes to the mainland was followed by the arrival of the Ghost Rocks volcanic assemblage at the southern margin of Kodiak Island. Poleward movement of these terranes can be explained by rapid motion of the Kula oceanic plate, mainly from 85 to 43 Ma ago, according to recent reconstructions derived from the hot-spot reference frame. After accretion, much of southwestern Alaska underwent a counterclockwise rotation of about 50 ?? as indicated by paleomagnetic poles from volcanic rocks of Late Cretaceous and Early Tertiary age. Compression between North America and Asia during opening of the North Atlantic (68-44 Ma ago) may account for the rotation. ?? 1987.

  19. Hydrodynamic simulations of accretion flows with time-varying viscosity

    NASA Astrophysics Data System (ADS)

    Roy, Abhishek; Chakrabarti, Sandip K.

    2017-12-01

    X-ray outbursts of stellar-mass black hole candidates are believed to be due to a sudden rise in viscosity, which transports angular momentum efficiently and increases the accretion rates, causing higher X-ray flux. After the viscosity is reduced, the outburst subsides and the object returns back to the pre-outburst quiescence stage. In the absence of a satisfactory understanding of the physical mechanism leading to such a sharp time dependence of viscous processes, we perform numerical simulations where we include the rise and fall of a viscosity parameter at an outer injection grid, assumed to be located at the accumulation radius where matter from the companion is piled up before being released by enhanced viscosity. We use a power-law radial dependence of the viscosity parameter (α ∼ rε), but the exponent (ε) is allowed to vary with time to mimic a fast rise and decay of the viscosity parameter. Since X-ray spectra of a black hole candidate can be explained by a Keplerian disc component in the presence of a post-shock region of an advective flow, our goal here is also to understand whether the flow configurations required to explain the spectral states of an outbursting source could be obtained by a time-varying viscosity. We present the results of our simulations to prove that low-angular-momentum (sub-Keplerian) advective flows do form a Keplerian disc in the pre-shock region when the viscosity is enhanced, which disappears on a much longer time-scale after the viscosity is withdrawn. From the variation of the Keplerian disc inside an advective halo, we believe that our result, for the first time, is able to simulate the two-component advective flow dynamics during an entire X-ray outburst and explain the observed hysteresis effects in the hardness-intensity diagram.

  20. Ultrafast collisional ion heating by electrostatic shocks.

    PubMed

    Turrell, A E; Sherlock, M; Rose, S J

    2015-11-13

    High-intensity lasers can be used to generate shockwaves, which have found applications in nuclear fusion, proton imaging, cancer therapies and materials science. Collisionless electrostatic shocks are one type of shockwave widely studied for applications involving ion acceleration. Here we show a novel mechanism for collisionless electrostatic shocks to heat small amounts of solid density matter to temperatures of ∼keV in tens of femtoseconds. Unusually, electrons play no direct role in the heating and it is the ions that determine the heating rate. Ions are heated due to an interplay between the electric field of the shock, the local density increase during the passage of the shock and collisions between different species of ion. In simulations, these factors combine to produce rapid, localized heating of the lighter ion species. Although the heated volume is modest, this would be one of the fastest heating mechanisms discovered if demonstrated in the laboratory.

  1. Modeling of Particle Acceleration at Multiple Shocks via Diffusive Shock Acceleration: Preliminary Results

    NASA Technical Reports Server (NTRS)

    Parker, L. Neergaard; Zank, G. P.

    2013-01-01

    Successful forecasting of energetic particle events in space weather models require algorithms for correctly predicting the spectrum of ions accelerated from a background population of charged particles. We present preliminary results from a model that diffusively accelerates particles at multiple shocks. Our basic approach is related to box models in which a distribution of particles is diffusively accelerated inside the box while simultaneously experiencing decompression through adiabatic expansion and losses from the convection and diffusion of particles outside the box. We adiabatically decompress the accelerated particle distribution between each shock by either the method explored in Melrose and Pope (1993) and Pope and Melrose (1994) or by the approach set forth in Zank et al. (2000) where we solve the transport equation by a method analogous to operator splitting. The second method incorporates the additional loss terms of convection and diffusion and allows for the use of a variable time between shocks. We use a maximum injection energy (E(sub max)) appropriate for quasi-parallel and quasi-perpendicular shocks and provide a preliminary application of the diffusive acceleration of particles by multiple shocks with frequencies appropriate for solar maximum (i.e., a non-Markovian process).

  2. Modeling of Particle Acceleration at Multiple Shocks Via Diffusive Shock Acceleration: Preliminary Results

    NASA Technical Reports Server (NTRS)

    Parker, Linda Neergaard; Zank, Gary P.

    2013-01-01

    We present preliminary results from a model that diffusively accelerates particles at multiple shocks. Our basic approach is related to box models (Protheroe and Stanev, 1998; Moraal and Axford, 1983; Ball and Kirk, 1992; Drury et al., 1999) in which a distribution of particles is diffusively accelerated inside the box while simultaneously experiencing decompression through adiabatic expansion and losses from the convection and diffusion of particles outside the box (Melrose and Pope, 1993; Zank et al., 2000). We adiabatically decompress the accelerated particle distribution between each shock by either the method explored in Melrose and Pope (1993) and Pope and Melrose (1994) or by the approach set forth in Zank et al. (2000) where we solve the transport equation by a method analogous to operator splitting. The second method incorporates the additional loss terms of convection and diffusion and allows for the use of a variable time between shocks. We use a maximum injection energy (Emax) appropriate for quasi-parallel and quasi-perpendicular shocks (Zank et al., 2000, 2006; Dosch and Shalchi, 2010) and provide a preliminary application of the diffusive acceleration of particles by multiple shocks with frequencies appropriate for solar maximum (i.e., a non-Markovian process).

  3. Process and application of shock compression by nanosecond pulses of frequency-doubled Nd:YAG laser

    NASA Astrophysics Data System (ADS)

    Sano, Yuji; Kimura, Motohiko; Mukai, Naruhiko; Yoda, Masaki; Obata, Minoru; Ogisu, Tatsuki

    2000-02-01

    The authors have developed a new process of laser-induced shock compression to introduce a residual compressive stress on material surface, which is effective for prevention of stress corrosion cracking (SCC) and enhancement of fatigue strength of metal materials. The process developed is unique and beneficial. It requires no pre-conditioning for the surface, whereas the conventional process requires that the so-called sacrificial layer is made to protect the surface from damage. The new process can be freely applied to water- immersed components, since it uses water-penetrable green light of a frequency-doubled Nd:YAG laser. The process developed has the potential to open up new high-power laser applications in manufacturing and maintenance technologies. The laser-induced shock compression process (LSP) can be used to improve a residual stress field from tensile to compressive. In order to understand the physics and optimize the process, the propagation of a shock wave generated by the impulse of laser irradiation and the dynamic response of the material were analyzed by time-dependent elasto-plastic calculations with a finite element program using laser-induced plasma pressure as an external load. The analysis shows that a permanent strain and a residual compressive stress remain after the passage of the shock wave with amplitude exceeding the yield strength of the material. A practical system materializing the LSP was designed, manufactured, and tested to confirm the applicability to core components of light water reactors (LWRs). The system accesses the target component and remotely irradiates laser pulses to the heat affected zone (HAZ) along weld lines. Various functional tests were conducted using a full-scale mockup facility, in which remote maintenance work in a reactor vessel could be simulated. The results showed that the system remotely accessed the target weld lines and successfully introduced a residual compressive stress. After sufficient training

  4. On a Stochastic Failure Model under Random Shocks

    NASA Astrophysics Data System (ADS)

    Cha, Ji Hwan

    2013-02-01

    In most conventional settings, the events caused by an external shock are initiated at the moments of its occurrence. In this paper, we study a new classes of shock model, where each shock from a nonhomogeneous Poisson processes can trigger a failure of a system not immediately, as in classical extreme shock models, but with delay of some random time. We derive the corresponding survival and failure rate functions. Furthermore, we study the limiting behaviour of the failure rate function where it is applicable.

  5. Heating a plasma by a broadband stream of fast electrons: Fast ignition, shock ignition, and Gbar shock wave applications

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

    Gus’kov, S. Yu., E-mail: guskov@sci.lebedev.ru; Nicolai, Ph.; Ribeyre, X.

    2015-09-15

    An exact analytic solution is found for the steady-state distribution function of fast electrons with an arbitrary initial spectrum irradiating a planar low-Z plasma with an arbitrary density distribution. The solution is applied to study the heating of a material by fast electrons of different spectra such as a monoenergetic spectrum, a step-like distribution in a given energy range, and a Maxwellian spectrum, which is inherent in laser-produced fast electrons. The heating of shock- and fast-ignited precompressed inertial confinement fusion (ICF) targets as well as the heating of a target designed to generate a Gbar shock wave for equation ofmore » state (EOS) experiments by laser-produced fast electrons with a Maxwellian spectrum is investigated. A relation is established between the energies of two groups of Maxwellian fast electrons, which are responsible for generation of a shock wave and heating the upstream material (preheating). The minimum energy of the fast and shock igniting beams as well as of the beam for a Gbar shock wave generation increases with the spectral width of the electron distribution.« less

  6. Shock-induced damage in rocks: Application to impact cratering

    NASA Astrophysics Data System (ADS)

    Ai, Huirong

    Shock-induced damage beneath impact craters is studied in this work. Two representative terrestrial rocks, San Marcos granite and Bedford limestone, are chosen as test target. Impacts into the rock targets with different combinations of projectile material, size, impact angle, and impact velocity are carried out at cm scale in the laboratory. Shock-induced damage and fracturing would cause large-scale compressional wave velocity reduction in the recovered target beneath the impact crater. The shock-induced damage is measured by mapping the compressional wave velocity reduction in the recovered target. A cm scale nondestructive tomography technique is developed for this purpose. This technique is proved to be effective in mapping the damage in San Marcos granite, and the inverted velocity profile is in very good agreement with the result from dicing method and cut open directly. Both compressional velocity and attenuation are measured in three orthogonal directions on cubes prepared from one granite target impacted by a lead bullet at 1200 m/s. Anisotropy is observed from both results, but the attenuation seems to be a more useful parameter than acoustic velocity in studying orientation of cracks. Our experiments indicate that the shock-induced damage is a function of impact conditions including projectile type and size, impact velocity, and target properties. Combined with other crater phenomena such as crater diameter, depth, ejecta, etc., shock-induced damage would be used as an important yet not well recognized constraint for impact history. The shock-induced damage is also calculated numerically to be compared with the experiments for a few representative shots. The Johnson-Holmquist strength and failure model, initially developed for ceramics, is applied to geological materials. Strength is a complicated function of pressure, strain, strain rate, and damage. The JH model, coupled with a crack softening model, is used to describe both the inelastic response of

  7. Intracorporeal pneumatic shock application for the treatment of chronic plantar fasciitis: a randomized, double blind prospective clinical trial.

    PubMed

    Dogramaci, Yunus; Kalaci, Aydiner; Emir, Anil; Yanat, Ahmet Nedim; Gökçe, Ahmet

    2010-04-01

    Plantar fasciitis (PF) is a common clinical condition that usually resolves with non-operative treatments. Extracorporeal shock wave therapy (ESWT) has been used in the treatment of chronic PF not responding to other conservative measures; however, ESWT devices are expensive and available for daily practice in only few centers (In developing countries). A pneumatic lithotripter is a cheap and readily available device which uses pneumatic shock application for the intracorporeal lithotripsy. The aim of this study was to investigate the clinical efficacy of intracorporeal pneumatic shock therapy (IPST) application for the treatment of chronic PF using a cheap and readily available pneumatic lithotripter. A randomized, double-blind, placebo-controlled study was conducted. A total of 50 patients with clinically and radiologically confirmed PF were randomly allocated to either an active- (treatment) (n = 25) or inactive (placebo) (n = 25) group. Under local anesthesia and posterior tibial nerve block, a rigid probe was directly introduced into the calcaneal spur under fluoroscopic control; a standard protocol of 1,000 shock was applied during a single session into the calcaneal spur. The main outcome measure was the patients' subjective assessment of pain by means of a Visual Analog Scale (VAS) and the Roles and Maudsley Score before the treatment and 6 months later. At the 6 months, the rate of successful outcomes (excellent + good results) in the treatment group (92%) were significantly higher comparing to the control group (24%) (P < 0.001). Heel pain measured 6 months after using the VAS were 2.04 +/- 1.67 in treatment group and 7.16 +/- 1.57 in control group as compared to 8.92 +/- 1.22 and 9.12 +/- 1.23 before the commencement of the treatment. No complications attributable to the procedure such as rupture of the planter fascia, hematoma, or infection were observed during the study. This pilot study showed that IPST is an effective and safe method of treatment of

  8. Shock tube and shock wave research; Proceedings of the Eleventh International Symposium, University of Washington, Seattle, Wash., July 11-14, 1977

    NASA Technical Reports Server (NTRS)

    Ahlborn, B. (Editor); Hertzberg, A.; Russell, D.

    1978-01-01

    Papers are presented on the applications of shock-wave technology to the study of hydrodynamics, the use of the pressure-wave machine for charging diesel engines, and measurements of the heat-transfer rate in gas-turbine components. Consideration is given to shock propagation along 90-degree bends, the explosive dissemination of liquids, and rotational and vibrational relaxation behind weak shock waves in water vapor. Shock phenomena associated with expansion flows are described and stratospheric-related research using the shock tube is outlined. Attention is given to shock-wave ignition of magnesium powders, Mach reflection and boundary layers, and transition in the shock-induced unsteady boundary layer on a flat plate. Shock-tube measurements of induction and post-induction rates for low-Btu gas mixtures are presented and shock-initiated ignition in COS-N2O-Ar mixtures is described. Cluster growth rates in supersaturated lead vapor are presented and a study of laser-induced plasma motion in a solenoidal magnetic field is reviewed.

  9. Users Manual for the NASA Lewis Ice Accretion Prediction Code (LEWICE)

    NASA Technical Reports Server (NTRS)

    Ruff, Gary A.; Berkowitz, Brian M.

    1990-01-01

    LEWICE is an ice accretion prediction code that applies a time-stepping procedure to calculate the shape of an ice accretion. The potential flow field is calculated in LEWICE using the Douglas Hess-Smith 2-D panel code (S24Y). This potential flow field is then used to calculate the trajectories of particles and the impingement points on the body. These calculations are performed to determine the distribution of liquid water impinging on the body, which then serves as input to the icing thermodynamic code. The icing thermodynamic model is based on the work of Messinger, but contains several major modifications and improvements. This model is used to calculate the ice growth rate at each point on the surface of the geometry. By specifying an icing time increment, the ice growth rate can be interpreted as an ice thickness which is added to the body, resulting in the generation of new coordinates. This procedure is repeated, beginning with the potential flow calculations, until the desired icing time is reached. The operation of LEWICE is illustrated through the use of five examples. These examples are representative of the types of applications expected for LEWICE. All input and output is discussed, along with many of the diagnostic messages contained in the code. Several error conditions that may occur in the code for certain icing conditions are identified, and a course of action is recommended. LEWICE has been used to calculate a variety of ice shapes, but should still be considered a research code. The code should be exercised further to identify any shortcomings and inadequacies. Any modifications identified as a result of these cases, or of additional experimental results, should be incorporated into the model. Using it as a test bed for improvements to the ice accretion model is one important application of LEWICE.

  10. Andromeda's SMBH Projected Accretion Rate

    NASA Astrophysics Data System (ADS)

    Wilson, John

    2014-03-01

    A formula for calculating the half-life of galaxy clusters is proposed. A galactic half-life is the estimated amount of time that the most massive supermassive black hole (SMBH) in the galaxy cluster will have accreted one half of the mass in the cluster. The calculation is based on a projection of the SMBH continuing its exponentially decreasing rate of accretion that it had in its first 13 billion years. The calculated half-life for the Andromeda SMBH is approximately 1.4327e14 years from the Big Bang. Several proposals have suggested that black holes could be significant factors in the formation of new universes. Part of the verification or falsification of this hypothesis could be done by an N-body simulation. These simulations require an enormous amount of computer power and time. Some plausible projection of the growth of the supermassive black hole is needed to prepare an N-body simulation budget proposal. For now, this method provides an estimate for the growth rate of the Andromeda SMBH and deposition of the outcome of most of the galaxy cluster's mass which is either accreted by the SMBH, lost by ejection from the cluster, or lost in the form of energy.

  11. 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.

  12. Investigation of surface water behavior during glaze ice accretion

    NASA Technical Reports Server (NTRS)

    Hansman, R. John, Jr.; Turnock, Stephen R.

    1990-01-01

    A series of experimental investigations that focused on isolating the primary factors that control the behavior of unfrozen surface water during glaze ice accretion were conducted. Detailed microvideo observations were made of glaze ice accretions on 2.54 cm diam cylinders in a closed-loop refrigerated wind tunnel. Distinct zones of surface water behavior were observed; a smooth wet zone in the stagnation region with a uniform water film, a rough zone where surface tension effects caused coalescence of surface water into stationary beads, and a zone where surface water ran back as rivulets. The location of the transition from the smooth to the rough zone was found to migrate towards the stagnation point with time. Comparative tests were conducted to study the effect of the substrate thermal and roughness properties on ice accretion. The importance of surface water behavior was evaluated by the addition of a surface tension reducing agent to the icing tunnel water supply, which significantly altered the accreted glaze ice shape. Measurements were made to determine the contact angle behavior of water droplets on ice. A simple multizone modification to current glaze ice accretion models was proposed to include the observed surface roughness behavior.

  13. Unveiling slim accretion disc in AGN through X-ray and Infrared observations

    NASA Astrophysics Data System (ADS)

    Castelló-Mor, Núria; Kaspi, Shai; Netzer, Hagai; Du, Pu; Hu, Chen; Ho, Luis C.; Bai, Jin-Ming; Bian, Wei-Hao; Yuan, Ye-Fei; Wang, Jian-Min

    2017-05-01

    In this work, which is a continuation of Castelló-Mor et al., we present new X-ray and infrared (IR) data for a sample of active galactic nuclei (AGN) covering a wide range in Eddington ratio over a small luminosity range. In particular, we rigorously explore the dependence of the optical-to-X-ray spectral index αOX and the IR-to-optical spectral index on the dimensionless accretion rate, \\dot{M} = \\dot{m}/η, where \\dot{m} = LAGN/LEdd and η is the mass-to-radiation conversion efficiency, in low- and high-accretion rate sources. We find that the spectral energy distribution (SED) of the faster accreting sources is surprisingly similar to those from the comparison sample of sources with lower accretion rate. In particular: (I) The optical-to-UV AGN SED of slow and fast accreting AGN can be fitted with thin accretion disc (AD) models. (II) The value of αOX is very similar in slow and fast accreting systems up to a dimensionless accretion rate \\dot{M}c ˜ 10. We only find a correlation between αOX and \\dot{M} for sources with \\dot{M} > \\dot{M}c. In such cases, the faster accreting sources appear to have systematically larger αOX values. (III) We also find that the torus in the faster accreting systems seems to be less efficient in reprocessing the primary AGN radiation having lower IR-to-optical spectral slopes. These findings, failing to recover the predicted differences between the SEDs of slim and thin ADs within the observed spectral window, suggest that additional physical processes or very special geometry act to reduce the extreme-UV radiation in fast accreting AGN. This may be related to photon trapping, strong winds and perhaps other yet unknown physical processes.

  14. Accretion Disk and Dust Emission in Low-Luminosity AGN

    NASA Astrophysics Data System (ADS)

    Biddle, Lauren I.; Mason, Rachel; Alonso-Herrero, Almudena; Colina, Luis; Diaz, Ruben; Flohic, Helene; Gonzalez-Martin, Omaira; Ho, Luis C.; Lira, Paulina; Martins, Lucimara; McDermid, Richard; Perlman, Eric S.; Ramos Almeida, Christina; Riffel, Rogerio; Ardila, Alberto; Ruschel Dutra, Daniel; Schiavon, Ricardo; Thanjavur, Karun; Winge, Claudia

    2015-01-01

    Observations obtained in the near-infrared (near-IR; 0.8 - 2.5 μm) can assist our understanding of the physical and evolutionary processes of galaxies. Using a set of near-IR spectra of nearby galaxies obtained with the cross-dispersed mode of GNIRS on the Gemini North telescope, we investigate how the accretion disk and hot dust emission depend on the luminosity of the active nucleus. We recover faint AGN emission from the starlight-dominated nuclear regions of the galaxies, and measure properties such as the spectral shape and luminosity of the accretion disk and dust. The aim of this work is to establish whether the standard thin accretion disk may be truncated in low-accretion-rate AGN, as well as evaluate whether the torus of the AGN unified model still exists at low luminosities.

  15. Continuum Reverberation Mapping of AGN Accretion Disks

    NASA Astrophysics Data System (ADS)

    Fausnaugh, Michael M.; Peterson, Bradley M.; Starkey, David A.; Horne, Keith; AGN Storm Collaboration

    2017-12-01

    We show recent detections of inter-band continuum lags in three AGN (NGC 5548, NGC 2617, and MCG+08-11-011), which provide new constraints on the temperature profiles and absolute sizes of the accretion disks. We find lags larger than would be predicted for standard geometrically thin, optically thick accretion disks by factors of 2.3 to 3.3. For NGC 5548, the data span UV through optical/near-IR wavelengths, and we are able to discern a steeper temperature profile than the T˜ R^{-3/4} expected for a standard thin disk . Using a physical model, we are also able to estimate the inclinations of the disks for two objects. These results are similar to those found from gravitational microlensing of strongly lensed quasars, and provide a complementary approach for investigating the accretion disk structure in local, low luminsoity AGN.

  16. 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.

  17. Hot Accretion Spots and Nitrogen Enhancement in Algol-Type Interacting Binary Systems

    NASA Astrophysics Data System (ADS)

    Peters, Geraldine J.

    2013-07-01

    The shock from the impact of a gas stream onto the photosphere of a mass gainer in an Algol system is expected to produce a hot spot and also heat circumstellar material that is splashed from the impact site. Furthermore in some cases we expect the stripped-down mass loser (a late-type subgiant) to be nitrogen rich and carbon poor if CNO-processed material is currently being transferred. I will present observational evidence from the Kepler and FUSE spacecrafts for the presence of hot spots and discuss their behavior. FUSE observations of Algols that convincingly show that the gas stream material from some Algol secondaries is N-enhanced and virtually devoid of carbon is also presented. From Kepler data we have identified a long-term phenomenon in which the relative brightness of the quadrature light varies by a few percent and numerically reverses over a time scale of about a 100-400 days. We call the systems T/L (trailing hemisphere/leading hemisphere) variables. Such behavior has never been identified from ground-based photometry. WX Draconis (A8 + K0 IV, P = 1.80 d), a system whose primary displays delta Scuti-like pulsations, is the prototype. The Kepler light curves are being modeled with the latest version of the Wilson-Devinney program that includes the capability of treating migrating hot/cool spots. I will discuss whether the T/L behavior is likely due to a migrating hot accretion spot on the primary or variability in a large cool-spotted area on the secondary. From archival FUSE data we have confirmed the presence of a hot accretion spot in the Algol system U Cephei and present information on the spot parameters/behavior and conditions in a splash plasma from the impact site. Kepler and archival FUSE observations continue to provide important information on the nature of the component stars in Algol systems and the detailed physics of mass transfer, especially the role of accretion hot spots. We are grateful for support from NASA grants NNX11AC78G and

  18. Helicopter rotor noise investigation during ice accretion

    NASA Astrophysics Data System (ADS)

    Cheng, Baofeng

    An investigation of helicopter rotor noise during ice accretion is conducted using experimental, theoretical, and numerical methods. This research is the acoustic part of a joint helicopter rotor icing physics, modeling, and detection project at The Pennsylvania State University Vertical Lift Research Center of Excellence (VLRCOE). The current research aims to provide acoustic insight and understanding of the rotor icing physics and investigate the feasibility of detecting rotor icing through noise measurements, especially at the early stage of ice accretion. All helicopter main rotor noise source mechanisms and their change during ice accretion are discussed. Changes of the thickness noise, steady loading noise, and especially the turbulent boundary layer - trailing edge (TBL-TE) noise due to ice accretion are identified and studied. The change of the discrete frequency noise (thickness noise and steady loading noise) due to ice accretion is calculated by using PSU-WOPWOP, an advanced rotorcraft acoustic prediction code. The change is noticeable, but too small to be used in icing detection. The small thickness noise change is due to the small volume of the accreted ice compared to that of the entire blade, although a large iced airfoil shape is used. For the loading noise calculation, two simplified methods are used to generate the loading on the rotor blades, which is the input for the loading noise calculation: 1) compact loading from blade element momentum theory, icing effects are considered by increasing the drag coefficient; and 2) pressure loading from the 2-D CFD simulation, icing effects are considered by using the iced airfoil shape. Comprehensive rotor broadband noise measurements are carried out on rotor blades with different roughness sizes and rotation speeds in two facilities: the Adverse Environment Rotor Test Stand (AERTS) facility at The Pennsylvania State University, and The University of Maryland Acoustic Chamber (UMAC). In both facilities the

  19. 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.

  20. 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.

  1. Advection-dominated Inflow/Outflows from Evaporating Accretion Disks.

    PubMed

    Turolla; Dullemond

    2000-03-01

    In this Letter we investigate the properties of advection-dominated accretion flows (ADAFs) fed by the evaporation of a Shakura-Sunyaev accretion disk (SSD). In our picture, the ADAF fills the central cavity evacuated by the SSD and extends beyond the transition radius into a coronal region. We find that, because of global angular momentum conservation, a significant fraction of the hot gas flows away from the black hole, forming a transsonic wind, unless the injection rate depends only weakly on radius (if r2sigma&d2;~r-xi, xi<1&solm0;2). The Bernoulli number of the inflowing gas is negative if the transition radius is less, similar100 Schwarzschild radii, so matter falling into the hole is gravitationally bound. The ratio of inflowing to outflowing mass is approximately 1/2, so in these solutions the accretion rate is of the same order as in standard ADAFs and much larger than in advection-dominated inflow/outflow models. The possible relevance of evaporation-fed solutions to accretion flows in black hole X-ray binaries is briefly discussed.

  2. Accretion onto a noncommutative-inspired Schwarzschild black hole

    NASA Astrophysics Data System (ADS)

    Gangopadhyay, Sunandan; Paik, Biplab; Mandal, Rituparna

    2018-05-01

    In this paper, we investigate the problem of ordinary baryonic matter accretion onto the noncommutative (NC) geometry-inspired Schwarzschild black hole. The fundamental equations governing the spherically symmetric steady state matter accretion are deduced. These equations are seen to be modified due to the presence of noncommutativity. The matter accretion rate is computed and is found to increase rapidly with the increase in strength of the NC parameter. The sonic radius reduces while the sound speed at the sonic point increases with the increase in the strength of noncommutativity. The profile of the thermal environment is finally investigated below the sonic radius and at the event horizon and is found to be affected by noncommutativity.

  3. Holocene reef accretion: southwest Molokai, Hawaii, U.S.A.

    USGS Publications Warehouse

    Engels, Mary S.; Fletcher, Charles H.; Field, Michael E.; Storlazzi, Curt D.; Grossman, Eric E.; Rooney, John J.B.; Conger, Christopher L.; Glenn, Craig

    2004-01-01

    Two reef systems off south Molokai, Hale O Lono and Hikauhi (separated by only 10 km), show strong and fundamental differences in modern ecosystem structure and Holocene accretion history that reflect the influence of wave-induced near-bed shear stresses on reef development in Hawaii. Both sites are exposed to similar impacts from south, Kona, and trade-wind swell. However, the Hale O Lono site is exposed to north swell and the Hikuahi site is not. As a result, the reef at Hale O Lono records no late Holocene net accretion while the reef at Hikauhi records consistent and robust accretion over late Holocene time. Analysis and dating of 24 cores from Hale O Lono and Hikauhi reveal the presence of five major lithofacies that reflect paleo-environmental conditions. In order of decreasing depositional energy they are: (1) coral-algal bindstone; (2) mixed skeletal rudstone; (3) massive coral framestone; (4) unconsolidated floatstone; and (5) branching coral framestone-bafflestone. At Hale O Lono, 10 cores document a backstepping reef ranging from ∼ 8,100 cal yr BP (offshore) to ∼ 4,800 cal yr BP (nearshore). A depauperate community of modern coral diminishes shoreward and seaward of ∼ 15 m depth due to wave energy, disrupted recruitment activities, and physical abrasion. Evidence suggests a change from conditions conducive to accretion during the early Holocene to conditions detrimental to accretion in the late Holocene. Reef structure at Hikauhi, reconstructed from 14 cores, reveals a thick, rapidly accreting and young reef (maximum age ∼ 900 cal yr BP). Living coral cover on this reef increases seaward with distance from the reef crest but terminates at a depth of ∼ 20 m where the reef ends in a large sand field. The primary limitation on vertical reef growth is accommodation space under wave base, not recruitment activities or energy conditions. Interpretations of cored lithofacies suggest that modern reef growth on the southwest corner of Molokai, and by

  4. Implementation and Validation of 3-D Ice Accretion Measurement Methodology

    NASA Technical Reports Server (NTRS)

    Lee, Sam; Broeren, Andy P.; Kreeger, Richard E.; Potapczuk, Mark; Utt, Lloyd

    2014-01-01

    A research program has been implemented to develop and validate the use of a commercial 3-D laser scanning system to record ice accretion geometry in the NASA Icing Research Tunnel. A main component of the program was the geometric assessment of the 3- D laser scanning system on a 2-D (straight wing) and a 3-D (swept wing) airfoil geometries. This exercise consisted of comparison of scanned ice accretion to castings of the same ice accretion. The scan data were also used to create rapid prototype artificial ice shapes that were scanned and compared to the original ice accretion. The results from geometric comparisons on the straight wing showed that the ice shape models generated through the scan/rapid prototype process compared reasonably well with the cast shapes. Similar results were obtained with the geometric comparisons on the swept wing. It was difficult to precisely compare the scans of the cast shapes to the original ice accretion scans because the cast shapes appear to have shrunk during the mold/casting process by as much as 0.10-inch. However the comparison of the local ice-shape features were possible and produced better results. The rapid prototype manufacturing process was shown to reproduce the original ice accretion scan normally within 0.01-inch.

  5. Magnetic Field Transport in Accretion Disks

    NASA Astrophysics Data System (ADS)

    Jafari, Amir; Vishniac, Ethan T.

    2018-02-01

    The leading models for launching astrophysical jets rely on strong poloidal magnetic fields threading the central parts of their host accretion disks. Numerical simulations of magneto-rotationally turbulent disks suggest that such fields are actually advected from the environment by the accreting matter rather than generated by internal dynamos. This is puzzling from a theoretical point of view, since the reconnection of the radial field across the midplane should cause an outward drift on timescales much shorter than the accretion time. We suggest that a combination of effects are responsible for reducing the radial field near the midplane, causing efficient inward advection of the poloidal field. Magnetic buoyancy in subsonic turbulence pushes the field lines away from the midplane, decreasing the large-scale radial field in the main body of the disk. In magneto-rotationally driven turbulence, magnetic buoyancy dominates over the effects of turbulent pumping, which works against it, and turbulent diamagnetism, which works with it, in determining the vertical drift of the magnetic field. Balancing buoyancy with diffusion implies that the bending angle of the large-scale poloidal field can be very large near the surface, as required for outflows, but vanishes near the midplane, which impedes turbulent reconnection and outward diffusion. This effect becomes less efficient as the poloidal flux increases. This suggests that accretion disks are less likely to form jets if they have a modest ratio of outer to inner radii or if the ambient field is very weak. The former effect is probably responsible for the scarcity of jets in cataclysmic variable systems.

  6. The Radio Jets and Accretion Disk in NGC 4261

    NASA Astrophysics Data System (ADS)

    Jones, Dayton L.; Wehrle, Ann E.; Meier, David L.; Piner, B. Glenn

    2000-05-01

    .8 pc. Assuming that the accretion disk is geometrically and optically thin and composed of a uniform 104 K plasma, the average electron density in the inner 0.1 pc of the disk is 103-108 cm-3. The mass of ionized gas in the inner pc of the disk is 101-103 Msolar, sufficient to power the radio source for ~104-106 yr. Equating thermal gas pressure and magnetic field strength gives a disk magnetic field of ~10-4 to 10-2 gauss at 0.1 pc. We include an appendix containing expressions for a simple, optically thin, gas-pressure-dominated accretion disk model that may be applicable to other galaxies in addition to NGC 4261.

  7. Deformation and thermal histories of ordinary chondrites: Evidence for post-deformation annealing and syn-metamorphic shock

    NASA Astrophysics Data System (ADS)

    Ruzicka, Alex; Hugo, Richard; Hutson, Melinda

    2015-08-01

    We show that olivine microstructures in seven metamorphosed ordinary chondrites of different groups studied with optical and transmission electron microscopy can be used to evaluate the post-deformation cooling setting of the meteorites, and to discriminate between collisions affecting cold and warm parent bodies. The L6 chondrites Park (shock stage S1), Bruderheim (S4), Leedey (S4), and Morrow County (S5) were affected by variable shock deformation followed by relatively rapid cooling, and probably cooled as fragments liberated by impact in near-surface settings. In contrast, Kernouvé (H6 S1), Portales Valley (H6/7 S1), and MIL 99301 (LL6 S1) appear to have cooled slowly after shock, probably by deep burial in warm materials. In these chondrites, post-deformation annealing lowered apparent optical strain levels in olivine. Additionally, Kernouvé, Morrow County, Park, MIL 99301, and possibly Portales Valley, show evidence for having been deformed at an elevated temperature (⩾800-1000 °C). The high temperatures for Morrow County can be explained by dynamic heating during intense shock, but Kernouvé, Park, and MIL 99301 were probably shocked while the H, L and LL parent bodies were warm, during early, endogenically-driven thermal metamorphism. Thus, whereas the S4 and S5 chondrites experienced purely shock-induced heating and cooling, all the S1 chondrites examined show evidence for static heating consistent with either syn-metamorphic shock (Kernouvé, MIL 99301, Park), post-deformation burial in warm materials (Kernouvé, MIL 99301, Portales Valley), or both. The results show the pitfalls in relying on optical shock classification alone to infer an absence of shock and to construct cooling stratigraphy models for parent bodies. Moreover, they provide support for the idea that "secondary" metamorphic and "tertiary" shock processes overlapped in time shortly after the accretion of chondritic planetesimals, and that impacts into warm asteroidal bodies were

  8. Elemental Fractionation During Rapid Accretion of the Moon Triggered by a Giant Impact

    NASA Technical Reports Server (NTRS)

    Abe, Y.; Zahnle, K. J.; Hashimoto, A.

    1998-01-01

    Recently, Ida et al. made an N-body simulation of lunar accretion from a protolunar disk formed by a giant impact. One of their important conclusions is that the accretion time of the Moon is as short as one month. Such rapid accretion is a necessary consequence of the high surface density of a lunar mass disk accreting just beyond the Roche limit (about 3Re); the Safronov accretion time (a few days) is even shorter. The energy of accretion always exceeds the gravitational binding energy of newly arriving matter. Hence, without an energy sink, the accreting body is thermally unstable. For the Earth and other planets, radiation acts as the sink. However, in such a short accretion time, the Moon cannot radiate the accretional energy. Even radiating at a silicate cloudtop temperature of roughly 2000 K, it would take more than 100 yr to radiatively cool the Moon. The plausible alternative heat sinks are heat capacity, latent heat of vaporization, and thermal escape of the gas to space (i.e., hydrodynamic blowoff). The latter becomes plausible for the Moon because the scale height at 2000 K (about 300 km) is a significant fraction of the lunar radius. The early stages of lunar (or "lunatesimal") growth release relatively little energy and can occur simply by heating the material, especially if the accreting material is originally cold. However, the material is unlikely to be cold, because the disk itself is hot and cooling time is long, while the lunar accretion time iss very short. Therefore, the moon is likely to accrete condensed material just after it condenses. Accordingly, the newly accreted material will be on the verge of vaporization and will have very little heat capacity to spare. The immediate heat sink is the latent heat of vaporization. Most of the vapor will escape from the moon, because the thermal energy in the gas can be used to drive escape. However, vaporization is generally incomplete. the latent heat of vaporization exceeds the energy of accretion

  9. LUNAR ACCRETION FROM A ROCHE-INTERIOR FLUID DISK

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

    Salmon, Julien; Canup, Robin M., E-mail: julien@boulder.swri.edu, E-mail: robin@boulder.swri.edu

    2012-11-20

    We use a hybrid numerical approach to simulate the formation of the Moon from an impact-generated disk, consisting of a fluid model for the disk inside the Roche limit and an N-body code to describe accretion outside the Roche limit. As the inner disk spreads due to a thermally regulated viscosity, material is delivered across the Roche limit and accretes into moonlets that are added to the N-body simulation. Contrary to an accretion timescale of a few months obtained with prior pure N-body codes, here the final stage of the Moon's growth is controlled by the slow spreading of themore » inner disk, resulting in a total lunar accretion timescale of {approx}10{sup 2} years. It has been proposed that the inner disk may compositionally equilibrate with the Earth through diffusive mixing, which offers a potential explanation for the identical oxygen isotope compositions of the Earth and Moon. However, the mass fraction of the final Moon that is derived from the inner disk is limited by resonant torques between the disk and exterior growing moons. For initial disks containing <2.5 lunar masses (M{sub Last-Quarter-Moon }), we find that a final Moon with mass > 0.8 M{sub Last-Quarter-Moon} contains {<=}60% material derived from the inner disk, with this material preferentially delivered to the Moon at the end of its accretion.« less

  10. TESTING THE PROPAGATING FLUCTUATIONS MODEL WITH A LONG, GLOBAL ACCRETION DISK SIMULATION

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

    Hogg, J Drew; Reynolds, Christopher S.

    2016-07-20

    The broadband variability of many accreting systems displays characteristic structures; log-normal flux distributions, root-mean square (rms)-flux relations, and long inter-band lags. These characteristics are usually interpreted as inward propagating fluctuations of the mass accretion rate in an accretion disk driven by stochasticity of the angular momentum transport mechanism. We present the first analysis of propagating fluctuations in a long-duration, high-resolution, global three-dimensional magnetohydrodynamic (MHD) simulation of a geometrically thin ( h / r ≈ 0.1) accretion disk around a black hole. While the dynamical-timescale turbulent fluctuations in the Maxwell stresses are too rapid to drive radially coherent fluctuations in themore » accretion rate, we find that the low-frequency quasi-periodic dynamo action introduces low-frequency fluctuations in the Maxwell stresses, which then drive the propagating fluctuations. Examining both the mass accretion rate and emission proxies, we recover log-normality, linear rms-flux relations, and radial coherence that would produce inter-band lags. Hence, we successfully relate and connect the phenomenology of propagating fluctuations to modern MHD accretion disk theory.« less

  11. GRAVITATIONAL ACCRETION OF PARTICLES ONTO MOONLETS EMBEDDED IN SATURN's RINGS

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

    Yasui, Yuki; Ohtsuki, Keiji; Daisaka, Hiroshi, E-mail: y.yasui@whale.kobe-u.ac.jp, E-mail: ohtsuki@tiger.kobe-u.ac.jp

    2014-12-20

    Using a local N-body simulation, we examine gravitational accretion of ring particles onto moonlet cores in Saturn's rings. We find that gravitational accretion of particles onto moonlet cores is unlikely to occur in the C ring and probably difficult in the inner B ring as well provided that the cores are rigid water ice. Dependence of particle accretion on ring thickness changes when the radial distance from the planet and/or the density of particles is varied: the former determines the size of the core's Hill radius relative to its physical size, while the latter changes the effect of self-gravity ofmore » accreted particles. We find that particle accretion onto high-latitude regions of the core surface can occur even if the rings' vertical thickness is much smaller than the core radius, although redistribution of particles onto the high-latitude regions would not be perfectly efficient in outer regions of the rings such as the outer A ring, where the size of the core's Hill sphere in the vertical direction is significantly larger than the core's physical radius. Our results suggest that large boulders recently inferred from observations of transparent holes in the C ring are not formed locally by gravitational accretion, while propeller moonlets in the A ring would be gravitational aggregates formed by particle accretion onto dense cores. Our results also imply that the main bodies of small satellites near the outer edge of Saturn's rings may have been formed in rather thin rings.« less

  12. 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

  13. Shock waves; Proceedings of the 18th International Symposium, Sendai, Japan, July 21-26, 1991. Vols. 1 & 2

    NASA Astrophysics Data System (ADS)

    Takayama, Kazuyoshi

    Various papers on shock waves are presented. The general topics addressed include: shock wave structure, propagation, and interaction; shock wave reflection, diffraction, refraction, and focusing; shock waves in condensed matter; shock waves in dusty gases and multiphase media; hypersonic flows and shock waves; chemical processes and related combustion phenomena; explosions, blast waves, and laser initiation of shock waves; shock tube technology and instrumentation; CFD of shock wave phenomena; medical applications and biological effects; industrial applications.

  14. Heat shock treatment improves Trametes versicolor laccase production.

    PubMed

    Wang, Feng; Guo, Chen; Wei, Tao; Zhang, Tian; Liu, Chun-Zhao

    2012-09-01

    An efficient heat shock strategy has been developed to improve laccase production in submerged Trametes versicolor cultures. The optimized heat shock strategy consists of subjecting T. versicolor mycelial pellets to three heat shock treatments at 45 °C for 45 min, starting at culture day 0, with a 24-h interval between treatments. Laccase production increased by more than 1.6-fold relative to the control in both flasks and a 5-L bioreactor because the expression of the laccase gene was enhanced by heat shock induction. The present work demonstrates that heat shock induction is a promising method because it both improves fungal laccase production and has a good potential in industrial application.

  15. Migration of accreting planets in radiative discs from dynamical torques

    NASA Astrophysics Data System (ADS)

    Pierens, A.; Raymond, S. N.

    2016-11-01

    We present the results of hydrodynamical simulations of the orbital evolution of planets undergoing runaway gas accretion in radiative discs. We consider accreting disc models with constant mass flux through the disc, and where radiative cooling balances the effect of viscous heating and stellar irradiation. We assume that 20-30 M⊕ giant planet cores are formed in the region where viscous heating dominates and migrate outward under the action of a strong entropy-related corotation torque. In the case where gas accretion is neglected and for an α viscous stress parameter α = 2 × 10-3, we find evidence for strong dynamical torques in accreting discs with accretion rates {dot{M}}≳ 7× 10^{-8} M_{⊙} yr{}^{-1}. Their main effect is to increase outward migration rates by a factor of ˜2 typically. In the presence of gas accretion, however, runaway outward migration is observed with the planet passing through the zero-torque radius and the transition between the viscous heating and stellar heating dominated regimes. The ability for an accreting planet to enter a fast migration regime is found to depend strongly on the planet growth rate, but can occur for values of the mass flux through the disc of {dot{M}}≳ 5× 10^{-8} M_{⊙} yr{}^{-1}. We find that an episode of runaway outward migration can cause an accreting planet formed in the 5-10 au region to temporarily orbit at star-planet separations as large as ˜60-70 au. However, increase in the amplitude of the Lindblad torque associated with planet growth plus change in the streamline topology near the planet systematically cause the direction of migration to be reversed. Subsequent evolution corresponds to the planet migrating inward rapidly until it becomes massive enough to open a gap in the disc and migrate in the type II regime. Our results indicate that a planet can reach large orbital distances under the combined effect of dynamical torques and gas accretion, but an alternative mechanism is required to

  16. Accretion Rate and the Physical Nature of Unobscured Active Galaxies

    NASA Astrophysics Data System (ADS)

    Trump, Jonathan R.; Impey, Christopher D.; Kelly, Brandon C.; Civano, Francesca; Gabor, Jared M.; Diamond-Stanic, Aleksandar M.; Merloni, Andrea; Urry, C. Megan; Hao, Heng; Jahnke, Knud; Nagao, Tohru; Taniguchi, Yoshi; Koekemoer, Anton M.; Lanzuisi, Giorgio; Liu, Charles; Mainieri, Vincenzo; Salvato, Mara; Scoville, Nick Z.

    2011-05-01

    We show how accretion rate governs the physical properties of a sample of unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rates by using accurate intrinsic accretion luminosities (L int) from well-sampled multiwavelength spectral energy distributions from the Cosmic Evolution Survey, and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L int/L Edd > 10-2), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L int/L Edd < 10-2) are unobscured and yet lack a broad-line region. The disappearance of the broad emission lines is caused by an expanding radiatively inefficient accretion flow (RIAF) at the inner radius of the accretion disk. The presence of the RIAF also drives L int/L Edd < 10-2 narrow-line and lineless AGNs to have ratios of radio-to-optical/UV emission that are 10 times higher than L int/L Edd > 10-2 broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L int/L Edd < 10-2 AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together, these results suggest that specific accretion rate is an important physical "axis" of AGN unification, as described by a simple model. Based on observations with the XMM-Newton satellite, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA; the Magellan telescope, operated by the Carnegie Observatories; the ESO Very Large Telescope; and the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian

  17. X-rays from accretion of red giant winds

    NASA Technical Reports Server (NTRS)

    Jura, M.; Helfand, D. J.

    1984-01-01

    X-ray observations of the late-type red giants Mira and R Aqr obtained with the Einstein Observatory are presented, and the general problems of white dwarf accretion from late-type giant winds is considered. The extremely low measured luminosities obtained for the two systems leads to the conclusion that the companions of Mira and R Aqr are most likely low-mass main sequence objects rather than white dwarfs as is usually assumed. The expected X-ray luminosities of true red giant/white dwarf systems are considered, and it is concluded that far too few have been detected if the canonical accretion scenario is adopted. A possible explanation of this situation in terms of grain-dominated Eddington-limited accretion is proposed.

  18. Burnett-Cattaneo continuum theory for shock waves.

    PubMed

    Holian, Brad Lee; Mareschal, Michel; Ravelo, Ramon

    2011-02-01

    We model strong shock-wave propagation, both in the ideal gas and in the dense Lennard-Jones fluid, using a refinement of earlier work, which accounts for the cold compression in the early stages of the shock rise by a nonlinear, Burnett-like, strain-rate dependence of the thermal conductivity, and relaxation of kinetic-temperature components on the hot, compressed side of the shock front. The relaxation of the disequilibrium among the three components of the kinetic temperature, namely, the difference between the component in the direction of a planar shock wave and those in the transverse directions, particularly in the region near the shock front, is accomplished at a much more quantitative level by a rigorous application of the Cattaneo-Maxwell relaxation equation to a reference solution, namely, the steady shock-wave solution of linear Navier-Stokes-Fourier theory, along with the nonlinear Burnett heat-flux term. Our new continuum theory is in nearly quantitative agreement with nonequilibrium molecular-dynamics simulations under strong shock-wave conditions, using relaxation parameters obtained from the reference solution. ©2011 American Physical Society

  19. 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.

  20. Tori sequences as remnants of multiple accreting periods of Kerr SMBHs

    NASA Astrophysics Data System (ADS)

    Pugliese, D.; Stuchlík, Z.

    2018-03-01

    Super-massive black holes (SMBHs) hosted in active galactic nuclei (AGNs) can be characterized by multi-accreting periods as the attractors interact with the environment during their life-time. These multi-accretion episodes should leave traces in the matter orbiting the attractor. Counterrotating and even misaligned structures orbiting around the SMBHs would be consequences of these episodes. Our task in this work is to consider situations where such accretions occur and to trace their remnants represented by several toroidal accreting fluids, corotating or counterrotating relative to the central Kerr attractor, and created in various regimes during the evolution of matter configurations around SMBHs. We focus particularly on the emergence of matter instabilities, i.e., tori collisions, accretion onto the central Kerr black hole, or creation of jet-like structures (proto-jets). Each orbiting configuration is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluid. We prove that sequences of configurations and hot points, where an instability occurs, characterize the Kerr SMBHs, depending mainly on their spin-mass ratios. The occurrence of tori accretion or collision are strongly constrained by the fluid rotation with respect to the central black hole and the relative rotation with respect to each other. Our investigation provides characteristic of attractors where traces of multi-accreting episodes can be found and observed.

  1. Low-density, radiatively inefficient rotating-accretion flow on to a black hole

    NASA Astrophysics Data System (ADS)

    Inayoshi, Kohei; Ostriker, Jeremiah P.; Haiman, Zoltán; Kuiper, Rolf

    2018-05-01

    We study low-density axisymmetric accretion flows on to black holes (BHs) with two-dimensional hydrodynamical simulations, adopting the α-viscosity prescription. When the gas angular momentum is low enough to form a rotationally supported disc within the Bondi radius (RB), we find a global steady accretion solution. The solution consists of a rotational equilibrium distribution around r ˜ RB, where the density follows ρ ∝ (1 + RB/r)3/2, surrounding a geometrically thick and optically thin accretion disc at the centrifugal radius RC(accretion flows (ρ ∝ r-1/2). In the inner solution, the gas inflow rate decreases towards the centre due to convection (\\dot{M}∝ r), and the net accretion rate (including both inflows and outflows) is strongly suppressed by several orders of magnitude from the Bondi accretion rate \\dot{M}_B. The net accretion rate depends on the viscous strength, following \\dot{M}/\\dot{M}_B∝ (α /0.01)^{0.6}. This solution holds for low accretion rates of \\dot{M}_B/\\dot{M}_Edd≲ 10^{-3} having minimal radiation cooling, where \\dot{M}_Edd is the Eddington accretion rate. In a hot plasma at the bottom (r < 10-3 RB), thermal conduction would dominate the convective energy flux. Since suppression of the accretion by convection ceases, the final BH feeding rate is found to be \\dot{M}/\\dot{M}_B˜ 10^{-3}-10-2. This rate is as low as \\dot{M}/\\dot{M}_Edd˜ 10^{-7}-10-6 inferred for SgrA* and the nuclear BHs in M31 and M87, and can explain their low luminosities, without invoking any feedback mechanism.

  2. 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

  3. 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

  4. Numerical studies of asymmetric adiabatic accretion flow - The effect of velocity gradients

    NASA Technical Reports Server (NTRS)

    Taam, Ronald E.; Fryxell, B. A.

    1989-01-01

    A numerical study of the time variation of the angular momentum and mass capture rates for a central object accreting from a uniform medium with a velocity gradient transverse to the direction of the mean flow is presented, covering a range of velocity asymmetries and Mach numbers in the incident flow. It is found that the mass accretion rate in a given evolutionary sequence varies in an irregular manner, with the matter accreting onto the central object from either a continuously moving accretion wake or from an accretion disk. The implications of the results from the study of short-term fluctuations observed in the pulse period and luminosity of X-ray pulsars are discussed.

  5. 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.

  6. 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.

  7. Dynamically important magnetic fields near accreting supermassive black holes.

    PubMed

    Zamaninasab, M; Clausen-Brown, E; Savolainen, T; Tchekhovskoy, A

    2014-06-05

    Accreting supermassive black holes at the centres of active galaxies often produce 'jets'--collimated bipolar outflows of relativistic particles. Magnetic fields probably play a critical role in jet formation and in accretion disk physics. A dynamically important magnetic field was recently found near the Galactic Centre black hole. If this is common and if the field continues to near the black hole event horizon, disk structures will be affected, invalidating assumptions made in standard models. Here we report that jet magnetic field and accretion disk luminosity are tightly correlated over seven orders of magnitude for a sample of 76 radio-loud active galaxies. We conclude that the jet-launching regions of these radio-loud galaxies are threaded by dynamically important fields, which will affect the disk properties. These fields obstruct gas infall, compress the accretion disk vertically, slow down the disk rotation by carrying away its angular momentum in an outflow and determine the directionality of jets.

  8. Criteria for retrograde rotation of accreting black holes

    NASA Astrophysics Data System (ADS)

    Mikhailov, A. G.; Piotrovich, M. Yu; Gnedin, Yu N.; Natsvlishvili, T. M.; Buliga, S. D.

    2018-06-01

    Rotating supermassive black holes produce jets and their origin is connected to the magnetic field that is generated by accreting matter flow. There is a point of view that electromagnetic fields around rotating black holes are brought to the hole by accretion. In this situation the prograde accreting discs produce weaker large-scale black hole threading magnetic fields, implying weaker jets than in retrograde regimes. The basic goal of this paper is to find the best candidates for retrograde accreting systems in observed active galactic nuclei. We show that active galactic nuclei with low Eddington ratio are really the best candidates for retrograde systems. This conclusion is obtained for kinetically dominated Fanaroff-Riley class II radio galaxies, flat-spectrum radio-loud narrow-line Seyfert I galaxies and a number of nearby galaxies. Our conclusion is that the best candidates for retrograde systems are the noticeable population of active galactic nuclei in the Universe. This result corresponds to the conclusion that in the merging process the interaction of merging black holes with a retrograde circumbinary disc is considerably more effective for shrinking the binary system.

  9. Accretion onto a moving Reissner-Nordström black hole

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

    Jiao, Lei; Yang, Rongjia, E-mail: jiaoleizhijia@163.com, E-mail: yangrongjia@tsinghua.org.cn

    We obtain an analytic solution for accretion of a gaseous medium with a adiabatic equation of state ( P =ρ) onto a Reissner-Nordström black hole which moves at a constant velocity through the medium. We obtain the specific expression for each component of the velocity and present the mass accretion rate which depends on the mass and the electric charge. The result we obtained may be helpful to understand the physical mechanism of accretion onto a moving black hole.

  10. Overview of shock waves in medicine

    NASA Astrophysics Data System (ADS)

    Cleveland, Robin O.

    2003-10-01

    A brief overview of three applications of shock waves is presented. Shock wave lithotripsy (SWL) has been in clinical use for more than 20 years. In the United States it is used to treat more than 80% of kidney stone cases and has wide acceptance with patients because it is a noninvasive procedure. Despite SWLs enormous success there is no agreement on how shock waves comminute stones. There is also a general acceptance that shock waves lead to trauma to the soft tissue of the kidney. Yet there has been little forward progress in developing lithotripters which provide comminution with less side-effects, indeed the original machine is still considered the gold standard. The last decade has seen the advent of new shock wave devices for treating principally musculoskeletal indications, such as plantar fasciitis, tennis elbow, and bone fractures that do not heal. This is referred to as shock wave therapy (SWT). The mechanisms by which SWT works are even less well understood than SWL and the consequences of bioeffects have also not been studied in detail. Shock waves have also been shown to be effective at enhancing drug delivery into cells and assisting with gene transfection. [Work partially supported by NIH.

  11. Resolving components of wind accreting systems: a case study of Mira AB

    NASA Astrophysics Data System (ADS)

    Karovska, M.

    2004-12-01

    Mass transfer in many systems occurs by wind interaction rather then by tidal interaction, because the primary does not fill its Roche surface. The nearby detached binary Mira AB provides a unique laboratory for studying wind accretion processes because this system can be resolved and the interacting components can be studied individually, which is not possible in most accreting systems. The study of Mira AB wind accretion and mass transfer may therefore help understand the accretion processes in many other astronomical systems.

  12. Magnetic dynamos in accreting planetary bodies

    NASA Astrophysics Data System (ADS)

    Golabek, Gregor; Labrosse, Stéphane; Gerya, Taras; Morishima, Ryuji; Tackley, Paul

    2013-04-01

    Laboratory measurements revealed ancient remanent magnetization in meteorites [1] indicating the activity of magnetic dynamos in the corresponding meteorite parent body. To study under which circumstances dynamo activity is possible, we use a new methodology to simulate the internal evolution of a planetary body during accretion and differentiation. Using the N-body code PKDGRAV [2] we simulate the accretion of planetary embryos from an initial annulus of several thousand planetesimals. The growth history of the largest resulting planetary embryo is used as an input for the thermomechanical 2D code I2ELVIS [3]. The thermomechanical model takes recent parametrizations of impact processes [4] and of the magnetic dynamo [5] into account. It was pointed out that impacts can not only deposit heat deep into the target body, which is later buried by ejecta of further impacts [6], but also that impacts expose in the crater region originally deep-seated layers, thus cooling the interior [7]. This combination of impact effects becomes even more important when we consider that planetesimals of all masses contribute to planetary accretion. This leads occasionally to collisions between bodies with large ratios between impactor and target mass. Thus, all these processes can be expected to have a profound effect on the thermal evolution during the epoch of planetary accretion and may have implications for the magnetic dynamo activity. Results show that late-formed planetesimals do not experience silicate melting and avoid thermal alteration, whereas in early-formed bodies accretion and iron core growth occur almost simultaneously and a highly variable magnetic dynamo can operate in the interior of these bodies. [1] Weiss, B.P. et al., Science, 322, 713-716, 2008. [2] Richardson, D. C. et al., Icarus, 143, 45-59, 2000. [3] Gerya, T.V and Yuen, D.J., Phys. Earth Planet. Int., 163, 83-105, 2007. [4] Monteux, J. et al., Geophys. Res. Lett., 34, L24201, 2007. [5] Aubert, J. et al

  13. Fallback Accretion in Core-Collapse Supernova Explosions

    NASA Astrophysics Data System (ADS)

    Gerling-Dunsmore, Hannalore J.; Ott, Christian D.

    2015-04-01

    Core-collapse supernovae (CCSNe) are expected to result in one of two kinds remnants: neutron stars (NSs) and black holes (BHs). It is believed that if a CCSN explosion fails, a BH results, and if the explosion is successful, a NS results. This certainly is the case if there is a strong explosion that unbinds the entire stellar mantle. However, in the case of a weak or severely asymmetric explosion, a substantial quantity of material may fall back. This is commonly called fallback accretion, and it is a potential means of BH formation. We study fallback accretion in spherically-symmetric (1D) neutrino-driven CCSNe using the open-source GR1D code. We obtain explosions by artificially enchancing neutrino energy deposition and in this way also control the explosion energy. We present results on the mapping from progenitor structure and explosion energy to amount and rate of fallback accretion. This research was partially supported by NSF Award No. AST-1212170.

  14. FRB as products of accretion disc funnels

    NASA Astrophysics Data System (ADS)

    Katz, J. I.

    2017-10-01

    The repeating FRB 121102, the only fast radio burst (FRB) with an accurately determined position, is associated with a variable persistent radio source. I suggest that an FRB originates in the accretion disc funnels of black holes. Narrowly collimated radiation is emitted along the wandering instantaneous angular momentum axis of accreted matter. This emission is observed as a fast radio burst when it sweeps across the direction to the observer. In this model, in contrast to neutron star (pulsar, RRAT or SGR) models, repeating FRBs do not have underlying periodicity and are co-located with persistent radio sources resulting from their off-axis emission. The model is analogous, on smaller spatial, lower mass and accretion rate and shorter temporal scales, to an active galactic nucleus (AGN), with FRB corresponding to blazars in which the jets point towards us. The small inferred black hole masses imply that FRBs are not associated with galactic nuclei.

  15. 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 $\\times 10^3 R_g$.

  16. Accretion and primary differentiation of Mars

    NASA Technical Reports Server (NTRS)

    Drake, Michael J.

    1988-01-01

    In collecting samples from Mars to address questions such as whether Mars accreted homogeneously or heterogeneously, how Mars segregated into a metallic core and silicate mantle, and whether Mars outgassed catastrophically coincident with accretion or more serenely on a longer timescale, we must be guided by our experience in addressing these questions for the Earth, Moon, and igneous meteorite parent bodies. A key measurement to be made on any sample returned from Mars is its oxygen isotopic composition. A single measurement will suffice to bind the SNC meteorites to Mars or demonstrate that they cannot be samples of that planet. A positive identification of Mars as the SNC parent planet will permit all that has been learned from the SNC meteorites to be applied to Mars with confidence. A negative result will perhaps be more exciting in forcing us to look for another object that has been geologically active in the recent past. If the oxygen isotopic composition of Earth and Mars are established to be distinct, accretion theory must provide for different compositions for two planets now separated by only 0.5 AU.

  17. Accretion and primary differentiation of Mars

    NASA Astrophysics Data System (ADS)

    Drake, Michael J.

    In collecting samples from Mars to address questions such as whether Mars accreted homogeneously or heterogeneously, how Mars segregated into a metallic core and silicate mantle, and whether Mars outgassed catastrophically coincident with accretion or more serenely on a longer timescale, we must be guided by our experience in addressing these questions for the Earth, Moon, and igneous meteorite parent bodies. A key measurement to be made on any sample returned from Mars is its oxygen isotopic composition. A single measurement will suffice to bind the SNC meteorites to Mars or demonstrate that they cannot be samples of that planet. A positive identification of Mars as the SNC parent planet will permit all that has been learned from the SNC meteorites to be applied to Mars with confidence. A negative result will perhaps be more exciting in forcing us to look for another object that has been geologically active in the recent past. If the oxygen isotopic composition of Earth and Mars are established to be distinct, accretion theory must provide for different compositions for two planets now separated by only 0.5 AU.

  18. 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.

  19. 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.

  20. 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

  1. Changes in the metallicity of gas giant planets due to pebble accretion

    NASA Astrophysics Data System (ADS)

    Humphries, R. J.; Nayakshin, S.

    2018-06-01

    We run numerical simulations to study the accretion of gas and dust grains on to gas giant planets embedded into massive protoplanetary discs. The outcome is found to depend on the disc cooling rate, planet mass, grain size, and irradiative feedback from the planet. If radiative cooling is efficient, planets accrete both gas and pebbles rapidly, open a gap, and usually become massive brown dwarfs. In the inefficient cooling case, gas is too hot to accrete on to the planet but pebble accretion continues and the planets migrate inward rapidly. Radiative feedback from the planet tends to suppress gas accretion. Our simulations predict that metal enrichment of planets by dust grain accretion inversely correlates with the final planet mass, in accordance with the observed trend in the inferred bulk composition of Solar system and exosolar giant planets. To account for observations, however, as many as ˜30-50 per cent of the dust mass should be in the form of large grains.

  2. Note: A table-top blast driven shock tube

    NASA Astrophysics Data System (ADS)

    Courtney, Michael W.; Courtney, Amy C.

    2010-12-01

    The prevalence of blast-induced traumatic brain injury in conflicts in Iraq and Afghanistan has motivated laboratory scale experiments on biomedical effects of blast waves and studies of blast wave transmission properties of various materials in hopes of improving armor design to mitigate these injuries. This paper describes the design and performance of a table-top shock tube that is more convenient and widely accessible than traditional compression driven and blast driven shock tubes. The design is simple: it is an explosive driven shock tube employing a rifle primer that explodes when impacted by the firing pin. The firearm barrel acts as the shock tube, and the shock wave emerges from the muzzle. The small size of this shock tube can facilitate localized application of a blast wave to a subject, tissue, or material under test.

  3. Note: A table-top blast driven shock tube.

    PubMed

    Courtney, Michael W; Courtney, Amy C

    2010-12-01

    The prevalence of blast-induced traumatic brain injury in conflicts in Iraq and Afghanistan has motivated laboratory scale experiments on biomedical effects of blast waves and studies of blast wave transmission properties of various materials in hopes of improving armor design to mitigate these injuries. This paper describes the design and performance of a table-top shock tube that is more convenient and widely accessible than traditional compression driven and blast driven shock tubes. The design is simple: it is an explosive driven shock tube employing a rifle primer that explodes when impacted by the firing pin. The firearm barrel acts as the shock tube, and the shock wave emerges from the muzzle. The small size of this shock tube can facilitate localized application of a blast wave to a subject, tissue, or material under test.

  4. 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.

  5. Hyper-Eddington accretion flows on to massive black holes

    NASA Astrophysics Data System (ADS)

    Inayoshi, Kohei; Haiman, Zoltán; Ostriker, Jeremiah P.

    2016-07-01

    We study very high rate, spherically symmetric accretion flows on to massive black holes (BHs; 102 ≲ MBH ≲ 106 M⊙) embedded in dense metal-poor clouds, performing one-dimensional radiation hydrodynamical simulations. We find solutions from outside the Bondi radius at hyper-Eddington rates, unimpeded by radiation feedback when (n∞/105 cm-3) > (MBH/104 M⊙)-1(T∞/104 K)3/2, where n∞ and T∞ are the density and temperature of ambient gas. Accretion rates in this regime are steady, and larger than 5000LEdd/c2, where LEdd is the Eddington luminosity. At lower Bondi rates, the accretion is episodic due to radiative feedback and the average rate is below the Eddington rate. In the hyper-Eddington case, the solution consists of a radiation-dominated central core, where photon trapping due to electron scattering is important, and an accreting envelope which follows a Bondi profile with T ≃ 8000 K. When the emergent luminosity is limited to ≲ LEdd because of photon trapping, radiation from the central region does not affect the gas dynamics at larger scales. We apply our result to the rapid formation of massive BHs in protogalaxies with a virial temperature of Tvir ≳ 104K. Once a seed BH forms at the centre of the galaxy, it can grow to a maximum ˜105(Tvir/104 K) M⊙ via gas accretion independent of the initial BH mass. Finally, we discuss possible observational signatures of rapidly accreting BHs with/without allowance for dust. We suggest that these systems could explain Lyα emitters without X-rays and nearby luminous infrared sources with hot dust emission, respectively.

  6. Asm-Triggered too Observations of Z Sources at Low Accretion Rate

    NASA Astrophysics Data System (ADS)

    van der Klis, Michiel

    We propose to perform a pointed observation if the ASM shows that a Z source has entered a state of low accretion rate. This would provide a unique opportunity to detect millisecond pulsations. In Sco X-1 we would expect to discover beat-frequency QPO, and could perform a unique high count rate study of them. At sufficiently low accretion rate it would be possible to study the accretion flow when the magnetospheric radius approaches the corotation radius. The frequency of the horizontal branch QPO should go to zero here, and centrifugal inhibition of the accretion should set in, providing direct tests of the magnetospheric model of Z sources.

  7. Application of cosmic-ray shock theories to the Cygnus Loop - an alternative model

    NASA Astrophysics Data System (ADS)

    Boulares, Ahmed; Cox, Donald P.

    1988-10-01

    Steady state cosmic-ray shock models are investigated in light of observations of the Cygnus Loop supernova remnant. In this work the authors find that the model of Völk, Drury, and McKenzie, in which the plasma waves are generated by the streaming instability of the cosmic rays and are dissipated into the gas, can be made consistent with some observed characteristics of Cygnus Loop shocks. The waves heat the gas substantially in the cosmic-ray precursor, in addition to the usual heating in the (possibly weak) gas shock. The model is used to deduce upstream densities and shock velocities using known quantities for Cygnus Loop shocks. Compared to the usual pure gas shock interpretation, it is found that lower densities and approximately 3 times higher velocities are required. If the cosmic-ray models are valid, this could significantly alter our understanding of the Cygnus Loop's distance and age and of the energy released during the initial explosion.

  8. Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498-2921

    NASA Astrophysics Data System (ADS)

    Falanga, M.; Kuiper, L.; Poutanen, J.; Galloway, D. K.; Bozzo, E.; Goldwurm, A.; Hermsen, W.; Stella, L.

    2012-09-01

    Context. IGR J17498-2921 is the third X-ray transient accreting millisecond pulsar discovered by INTEGRAL. It was in outburst for about 40 days beginning on August 08, 2011. Aims: We analyze the spectral and timing properties of the object and the characteristics of X-ray bursts to constrain the physical processes responsible for the X-ray production in this class of sources. Methods: We studied the broad-band spectrum of the persistent emission in the 0.6-300 keV energy band using simultaneous INTEGRAL, RXTE, and Swift data obtained in August-September 2011. We also describe the timing properties in the 2-100 keV energy range such as the outburst lightcurve, pulse profile, pulsed fraction, pulsed emission, time lags, and study the properties of X-ray bursts discovered by RXTE, Swift, and INTEGRAL and the recurrence time. Results: The broad-band average spectrum is well-described by thermal Comptonization with an electron temperature of kTe ~ 50 keV, soft seed photons of kTbb ~ 1 keV, and Thomson optical depth τT ~ 1 in a slab geometry. The slab area corresponds to a black body radius of Rbb ~ 9 km. During the outburst, the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scattering optical depth that are constant within the errors. This behavior has been interpreted as indicating that the X-ray emission originates above the neutron star (NS) surface in a hot slab (either the heated NS surface or the accretion shock). The INTEGRAL, RXTE, and Swift data reveal the X-ray pulsation at a period of 2.5 ms up to ~65 keV. The pulsed fraction is consistent with being constant, i.e. energy independent and has a typical value of 6-7%. The nearly sinusoidal pulses show soft lags that seem to saturate near 10 keV at a rather small value of ~-60 μs with those observed in other accreting pulsars. The short burst profiles indicate that there is a hydrogen-poor material at ignition, which suggests either that the accreted material is hydrogen

  9. Spatial derivatives of flow quantities behind curved shocks of all strengths

    NASA Technical Reports Server (NTRS)

    Darden, C. M.

    1984-01-01

    Explicit formulas in terms of shock curvature are developed for spatial derivatives of flow quantities behind a curved shock for two-dimensional inviscid steady flow. Factors which yield the equations indeterminate as the shock strength approaches 0 have been cancelled analytically so that formulas are valid for shocks of any strength. An application for the method is shown in the solution of shock coalescence when nonaxisymmetric effects are felt through derivatives in the circumferential direction. The solution of this problem requires flow derivatives behind the shock in both the axial and radial direction.

  10. 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.

  11. 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.

  12. Observations of Accreting Pulsars with the FERMI-GBM

    NASA Technical Reports Server (NTRS)

    Wilson-Hodge, Colleen

    2012-01-01

    The Gamma-ray Burst Monitor (GBM) on-board Fermi comprises 12 NaI detectors spanning the 8-1000 keV band and 2 BGO detectors spanning the 100 keV to 40 MeV band. These detectors view the entire unocculted sky, providing long (approximately 40 ks/day) observations of accreting pulsars daily, which allow long-term monitoring of spin-frequencies and pulsed uxes via epoch-folded searches plus daily blind searches for new pulsars. Phase averaged uxes can be measured using the Earth occultation technique. In this talk I will present highlights of GBM accretion-powered pulsar monitoring such as the discovery of a torque reversal in 4U1626-67, a high-energy QPO in A0535+26, and evidence for a stable accretion disk in OAO 1657-415.

  13. Baseline Assessment of Net Calcium Carbonate Accretion Rates on U.S. Pacific Reefs.

    PubMed

    Vargas-Ángel, Bernardo; Richards, Cristi L; Vroom, Peter S; Price, Nichole N; Schils, Tom; Young, Charles W; Smith, Jennifer; Johnson, Maggie D; Brainard, Russell E

    2015-01-01

    This paper presents a comprehensive quantitative baseline assessment of in situ net calcium carbonate accretion rates (g CaCO3 cm(-2) yr(-1)) of early successional recruitment communities on Calcification Accretion Unit (CAU) plates deployed on coral reefs at 78 discrete sites, across 11 islands in the central and south Pacific Oceans. Accretion rates varied substantially within and between islands, reef zones, levels of wave exposure, and island geomorphology. For forereef sites, mean accretion rates were the highest at Rose Atoll, Jarvis, and Swains Islands, and the lowest at Johnston Atoll and Tutuila. A comparison between reef zones showed higher accretion rates on forereefs compared to lagoon sites; mean accretion rates were also higher on windward than leeward sites but only for a subset of islands. High levels of spatial variability in net carbonate accretion rates reported herein draw attention to the heterogeneity of the community assemblages. Percent cover of key early successional taxa on CAU plates did not reflect that of the mature communities present on surrounding benthos, possibly due to the short deployment period (2 years) of the experimental units. Yet, net CaCO3 accretion rates were positively correlated with crustose coralline algae (CCA) percent cover on the surrounding benthos and on the CAU plates, which on average represented >70% of the accreted material. For foreeefs and lagoon sites combined CaCO3 accretion rates were statistically correlated with total alkalinity and Chlorophyll-a; a GAM analysis indicated that SiOH and Halimeda were the best predictor variables of accretion rates on lagoon sites, and total alkalinity and Chlorophyll-a for forereef sites, demonstrating the utility of CAUs as a tool to monitor changes in reef accretion rates as they relate to ocean acidification. This study underscores the pivotal role CCA play as a key benthic component and supporting actively calcifying reefs; high Mg-calcite exoskeletons makes CCA

  14. Baseline Assessment of Net Calcium Carbonate Accretion Rates on U.S. Pacific Reefs

    PubMed Central

    Vargas-Ángel, Bernardo; Richards, Cristi L.; Vroom, Peter S.; Price, Nichole N.; Schils, Tom; Young, Charles W.; Smith, Jennifer; Johnson, Maggie D.; Brainard, Russell E.

    2015-01-01

    This paper presents a comprehensive quantitative baseline assessment of in situ net calcium carbonate accretion rates (g CaCO3 cm-2 yr-1) of early successional recruitment communities on Calcification Accretion Unit (CAU) plates deployed on coral reefs at 78 discrete sites, across 11 islands in the central and south Pacific Oceans. Accretion rates varied substantially within and between islands, reef zones, levels of wave exposure, and island geomorphology. For forereef sites, mean accretion rates were the highest at Rose Atoll, Jarvis, and Swains Islands, and the lowest at Johnston Atoll and Tutuila. A comparison between reef zones showed higher accretion rates on forereefs compared to lagoon sites; mean accretion rates were also higher on windward than leeward sites but only for a subset of islands. High levels of spatial variability in net carbonate accretion rates reported herein draw attention to the heterogeneity of the community assemblages. Percent cover of key early successional taxa on CAU plates did not reflect that of the mature communities present on surrounding benthos, possibly due to the short deployment period (2 years) of the experimental units. Yet, net CaCO3 accretion rates were positively correlated with crustose coralline algae (CCA) percent cover on the surrounding benthos and on the CAU plates, which on average represented >70% of the accreted material. For foreeefs and lagoon sites combined CaCO3 accretion rates were statistically correlated with total alkalinity and Chlorophyll-a; a GAM analysis indicated that SiOH and Halimeda were the best predictor variables of accretion rates on lagoon sites, and total alkalinity and Chlorophyll-a for forereef sites, demonstrating the utility of CAUs as a tool to monitor changes in reef accretion rates as they relate to ocean acidification. This study underscores the pivotal role CCA play as a key benthic component and supporting actively calcifying reefs; high Mg-calcite exoskeletons makes CCA

  15. Can a Wind Model Mimic a Convection-Dominated Accretion Flow Model?

    NASA Astrophysics Data System (ADS)

    Chang, Heon-Young

    2001-06-01

    In this paper we investigate the properties of advection-dominated accretion flows(ADAFs) in case that outflows carry away infalling matter with its angular momentum and energy. Positive Bernoulli numbers in ADAFs allow a fraction of the gas to be ex-pelled in a form of outflows. The ADAFs are also unstable to convection. We present self-similar solutions for advection-dominated accretion flows in the presence of out-flows from the accretion flows (ADIOS). The axisymmetric flow is treated in variables integrated over polar sections and the effects of outflows on the accretion rlow are parameterized for possible configurations compatible with the one dimensional self-similar ADAF solution. We explicitly derive self-similar solutions of ADAFs in the presence of outflows and show that the strong outflows in the accretion flows result in a flatter density profile, which is similar to that of the convection-dominated accretion flows (CDAFs) in which convection transports the a! ngular momentum inward and the energy outward. These two different versions of the ADAF model should show similar behaviors in X-ray spectrum to some extent. Even though the two models may show similar behaviors, they should be distinguishable due to different physical properties. We suggest that for a central object of which mass is known these two different accretion flows should have different X-ray flux value due to deficient matter in the wind model.

  16. Accretion of Jupiter-mass planets in the limit of vanishing viscosity

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

    Szulágyi, J.; Morbidelli, A.; Crida, A.

    In the core-accretion model, the nominal runaway gas-accretion phase brings most planets to multiple Jupiter masses. However, known giant planets are predominantly Jupiter mass bodies. Obtaining longer timescales for gas accretion may require using realistic equations of states, or accounting for the dynamics of the circumplanetary disk (CPD) in the low-viscosity regime, or both. Here we explore the second way by using global, three-dimensional isothermal hydrodynamical simulations with eight levels of nested grids around the planet. In our simulations, the vertical inflow from the circumstellar disk (CSD) to the CPD determines the shape of the CPD and its accretion rate.more » Even without a prescribed viscosity, Jupiter's mass-doubling time is ∼10{sup 4} yr, assuming the planet at 5.2 AU and a Minimum Mass Solar Nebula. However, we show that this high accretion rate is due to resolution-dependent numerical viscosity. Furthermore, we consider the scenario of a layered CSD, viscous only in its surface layer, and an inviscid CPD. We identify two planet-accretion mechanisms that are independent of the viscosity in the CPD: (1) the polar inflow—defined as a part of the vertical inflow with a centrifugal radius smaller than two Jupiter radii and (2) the torque exerted by the star on the CPD. In the limit of zero effective viscosity, these two mechanisms would produce an accretion rate 40 times smaller than in the simulation.« less

  17. Shock wave driven microparticles for pharmaceutical applications

    NASA Astrophysics Data System (ADS)

    Menezes, V.; Takayama, K.; Gojani, A.; Hosseini, S. H. R.

    2008-10-01

    Ablation created by a Q-switched Nd:Yttrium Aluminum Garnet (Nd:YAG) laser beam focusing on a thin aluminum foil surface spontaneously generates a shock wave that propagates through the foil and deforms it at a high speed. This high-speed foil deformation can project dry micro- particles deposited on the anterior surface of the foil at high speeds such that the particles have sufficient momentum to penetrate soft targets. We used this method of particle acceleration to develop a drug delivery device to deliver DNA/drug coated microparticles into soft human-body targets for pharmaceutical applications. The device physics has been studied by observing the process of particle acceleration using a high-speed video camera in a shadowgraph system. Though the initial rate of foil deformation is over 5 km/s, the observed particle velocities are in the range of 900-400 m/s over a distance of 1.5-10 mm from the launch pad. The device has been tested by delivering microparticles into liver tissues of experimental rats and artificial soft human-body targets, modeled using gelatin. The penetration depths observed in the experimental targets are quite encouraging to develop a future clinical therapeutic device for treatments such as gene therapy, treatment of cancer and tumor cells, epidermal and mucosal immunizations etc.

  18. Hydrogen and helium shell burning during white dwarf accretion

    NASA Astrophysics Data System (ADS)

    Cui, Xiao; Meng, Xiang-Cun; Han, Zhan-Wen

    2018-05-01

    Type Ia supernovae (SNe Ia) are believed to be thermonuclear explosions of carbon oxygen (CO) white dwarfs (WDs) with masses close to the Chandrasekhar mass limit. How a CO WD accretes matter and grows in mass to this limit is not well understood, hindering our understanding of SN Ia explosions and the reliability of using SNe Ia as a cosmological distance indicator. In this work, we employed the stellar evolution code MESA to simulate the accretion process of hydrogen-rich material onto a 1.0 M ⊙ CO WD at a high rate (over the Eddington limit) of 4.3 × 10‑7 M ⊙ yr‑1. The simulation demonstrates the characteristics of the double shell burning on top of the WD, with a hydrogen shell burning on top of a helium burning shell. The results show that helium shell burning is not steady (i.e. it flashes). Flashes from the helium shell are weaker than those in the case of accretion of helium-rich material onto a CO WD. The carbon to oxygen mass ratio resulting from the helium shell burning is higher than what was previously thought. Interestingly, the CO WD growing due to accretion has an outer part containing a small fraction of helium in addition to carbon and oxygen. The flashes become weaker and weaker as the accretion continues.

  19. 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.

  20. Intermediate shocks in three-dimensional magnetohydrodynamic bow-shock flows with multiple interacting shock fronts

    PubMed

    De Sterck H; Poedts

    2000-06-12

    Simulation results of three-dimensional (3D) stationary magnetohydrodynamic (MHD) bow-shock flows around perfectly conducting spheres are presented. For strong upstream magnetic field a new complex bow-shock flow topology arises consisting of two consecutive interacting shock fronts. It is shown that the leading shock front contains a segment of intermediate 1-3 shock type. This is the first confirmation in 3D that intermediate shocks, which were believed to be unphysical for a long time, can be formed and can persist for small-dissipation MHD in a realistic flow configuration.

  1. He-accreting carbon-oxygen white dwarfs and Type Ia supernovae

    NASA Astrophysics Data System (ADS)

    Wang, Bo; Podsiadlowski, Philipp; Han, Zhanwen

    2017-12-01

    He accretion on to carbon-oxygen white dwarfs (CO WDs) plays a fundamental role when studying the formation of Type Ia supernovae (SNe Ia). Employing the MESA stellar evolution code, we calculated the long-term evolution of He-accreting CO WDs. Previous studies usually supposed that a WD can grow in mass to the Chandrasekhar limit in the stable He burning region and finally produce an SN Ia. However, in this study, we find that off-centre carbon ignition occurs in the stable He burning region if the accretion rate is above a critical value (∼2.05 × 10-6 M⊙ yr-1), resulting in accretion-induced collapse rather than an SN Ia. If the accretion rate is below the critical value, explosive carbon ignition will eventually happen in the centre producing an SN Ia. Taking into account the possibility of off-centre carbon ignition, we have re-determined the initial parameter space that produces SNe Ia in the He star donor channel, one of the promising channels to produce SNe Ia in young populations. Since this parameter space is smaller than was found in the previous study of Wang et al. (2009), the SN Ia rates are also correspondingly smaller. We also determined the chemical abundance profile of the He-accreting WDs at the moment of explosive carbon ignition, which can be used as initial input for SN Ia explosion models.

  2. [Predicting Spectra of Accretion Disks Around Galactic Black Holes

    NASA Technical Reports Server (NTRS)

    Krolik, Julian H.

    2004-01-01

    The purpose of this grant was to construct detailed atmosphere solutions in order to predict the spectra of accretion disks around Galactic black holes. Our plan of action was to take an existing disk atmosphere code (TLUSTY, created by Ivan Hubeny) and introduce those additional physical processes necessary to make it applicable to disks of this variety. These modifications include: treating Comptonization; introducing continuous opacity due to heavy elements; incorporating line opacity due to heavy elements; adopting a disk structure that reflects readjustments due to radiation pressure effects; and injecting heat via a physically-plausible vertical distribution.

  3. Visualizing SPH Cataclysmic Variable Accretion Disk Simulations with Blender

    NASA Astrophysics Data System (ADS)

    Kent, Brian R.; Wood, Matthew A.

    2015-01-01

    We present innovative ways to use Blender, a 3D graphics package, to visualize smoothed particle hydrodynamics particle data of cataclysmic variable accretion disks. We focus on the methods of shape key data constructs to increasedata i/o and manipulation speed. The implementation of the methods outlined allow for compositing of the various visualization layers into a final animation. The viewing of the disk in 3D from different angles can allow for a visual analysisof the physical system and orbits. The techniques have a wide ranging set of applications in astronomical visualization,including both observation and theoretical data.

  4. Evolution of a rotating black hole with a magnetized accretion disk.

    NASA Astrophysics Data System (ADS)

    Lee, H. K.; Kim, H.-K.

    2000-03-01

    The effect of an accretion disk on the Blandford-Znajek process and the evolution of a black hole are discussed using a simplified system for the black hole-accretion disk in which the accretion rate is supposed to be dominated by the strong magnetic field on the disk. The evolution of the mass and the angular momentum of the black hole are formulated and discussed with numerical calculations.

  5. Shock tube Multiphase Experiments

    NASA Astrophysics Data System (ADS)

    Middlebrooks, John; Allen, Roy; Paudel, Manoj; Young, Calvin; Musick, Ben; McFarland, Jacob

    2017-11-01

    Shock driven multiphase instabilities (SDMI) are unique physical phenomena that have far-reaching practical applications in engineering and science. The instability is present in high energy explosions, scramjet combustors, and supernovae events. The SDMI arises when a multiphase interface is impulsively accelerated by the passage of a shockwave. It is similar in development to the Richtmyer-Meshkov (RM) instability however, particle-to-gas coupling is the driving mechanism of the SDMI. As particle effects such as lag and phase change become more prominent, the SDMI's development begins to significantly deviate from the RM instability. We have developed an experiment for studying the SDMI in our shock tube facility. In our experiments, a multiphase interface is created using a laminar jet and flowed into the shock tube where it is accelerated by the passage of a planar shockwave. The interface development is captured using CCD cameras synchronized with planar laser illumination. This talk will give an overview of new experiments conducted to examine the development of a shocked cylindrical multiphase interface. The effects of Atwood number, particle size, and a second acceleration (reshock) of the interface will be discussed.

  6. 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.

  7. Modeling of Particle Acceleration at Multiple Shocks Via Diffusive Shock Acceleration: Preliminary Results

    NASA Astrophysics Data System (ADS)

    Parker, L. N.; Zank, G. P.

    2013-12-01

    Successful forecasting of energetic particle events in space weather models require algorithms for correctly predicting the spectrum of ions accelerated from a background population of charged particles. We present preliminary results from a model that diffusively accelerates particles at multiple shocks. Our basic approach is related to box models (Protheroe and Stanev, 1998; Moraal and Axford, 1983; Ball and Kirk, 1992; Drury et al., 1999) in which a distribution of particles is diffusively accelerated inside the box while simultaneously experiencing decompression through adiabatic expansion and losses from the convection and diffusion of particles outside the box (Melrose and Pope, 1993; Zank et al., 2000). We adiabatically decompress the accelerated particle distribution between each shock by either the method explored in Melrose and Pope (1993) and Pope and Melrose (1994) or by the approach set forth in Zank et al. (2000) where we solve the transport equation by a method analogous to operator splitting. The second method incorporates the additional loss terms of convection and diffusion and allows for the use of a variable time between shocks. We use a maximum injection energy (Emax) appropriate for quasi-parallel and quasi-perpendicular shocks (Zank et al., 2000, 2006; Dosch and Shalchi, 2010) and provide a preliminary application of the diffusive acceleration of particles by multiple shocks with frequencies appropriate for solar maximum (i.e., a non-Markovian process).

  8. 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.

  9. Accretion tori and cones of ionizing radiation in Seyfert galaxies

    NASA Technical Reports Server (NTRS)

    Acosta-Pulido, Jose A.; Perez-Fournon, Ismael; Calvani, Massimo; Wilson, Andrew S.

    1990-01-01

    The photoionization of extended narrow-line regions in Seyfert galaxies by the radiation produced in a thick accretion disk is studied. The emission-line spectrum is calculated for a range of black hole masses, varying the values of the ionization parameter and the disk size. It is found that models with a million solar masses fit observations of very large accretion disk sizes, while models with 10 million solar masses fit them better with smaller disks. The latter models are preferable since they have lower super-Eddington accretion rates.

  10. Accretional evolution of a planetesimal swarm. I - A new simulation

    NASA Technical Reports Server (NTRS)

    Spaute, Dominique; Weidenschilling, Stuart J.; Davis, Donald R.; Marzari, Francesco

    1991-01-01

    This novel simulation of planetary accretion simultaneously treats many interacting heliocentric distance zones and characterizes planetesimals via Keplerian elements. The numerical code employed, in addition to following the size distribution and the orbit-element distribution of a planetesimal swarm from arbitrary size and orbit distributions, treats a small number of the largest bodies as discrete objects with individual orbits. The accretion algorithm used yields good agreement with the analytic solutions; agreement is also obtained with the results of Weatherill and Stewart (1989) for gravitational accretion of planetesimals having equivalent initial conditions.

  11. The reflection of an ionized shock wave

    NASA Astrophysics Data System (ADS)

    Asakura, Fumioki; Corli, Andrea

    2018-03-01

    In a previous paper, we studied the thermodynamic and kinetic theory for an ionized gas, in one space dimension; in this paper, we provide an application of those results to the reflection of a shock wave in an electromagnetic shock tube. Under some reasonable limitations, which fully agree with experimental data, we prove that both the incident and the reflected shock waves satisfy the Lax entropy conditions; this result holds even outside genuinely nonlinear regions, which are present in the model. We show that the temperature increases in a significant way behind the incident shock front but the degree of ionization does not undergo a similar growth. On the contrary, the degree of ionization increases substantially behind the reflected shock front. We explain these phenomena by means of the concavity of the Hugoniot loci. Therefore, our results not only fit perfectly but explain what was remarked in experiments.

  12. 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.

  13. 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.

  14. 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.

  15. More on accreting black hole spacetime in equatorial plane

    NASA Astrophysics Data System (ADS)

    Salahshoor, K.; Nozari, K.; Khesali, A. R.

    2017-02-01

    Spacetime around an accreting black hole is an interesting issue to study. The metric of an isolated black hole (rotating or non-rotating) spacetime has been well-known for decades. Although metrics of some spacetimes containing accreting black holes are known in some situations, the issue has some faces that are not well-known yet and need further investigation. In this paper, we construct a new form of metric which the effect of accretion disk on black hole spacetime is taken into account in the equatorial plane. We study motion and trajectories of massive particles and also photons falling from infinity towards black hole in equatorial plane around the black hole. We use an exponential form for the density profile of the accretion disk in equatorial plane as ρ =ρ0e^{-α r}. We show that with this density profile, the disk is radially stable if α ≤ 3 × 10^{-3} (in units of length inverse). In order to study some important quantities related to the accretion disks such as locations of marginally stable circular orbits (r_{ms} or r_{ISCO}), marginally bounded circular orbits (r_{mb}), and also photon orbits in equatorial plane, we use the effective potential approach. We show that in this spacetime metric the innermost stable circular orbit in equatorial plane is given by r_{ISCO}=4.03 μ (where μ =MG/c 2) which is different, but comparable, with the Schwarzschild spacetime result, r^{(Sch)}_{ISCO}=6 μ . We show that the maximum radiation efficiency of the accretion disk, η , in equatorial plane is 8.6 percent which is greater than the corresponding value for Schwarzschild spacetime. Finally, we show that in this setup photons can have stable circular orbits in equatorial plane unlike the Schwarzschild spacetime.

  16. Parsec-Scale Accretion and Winds Irradiated by a Quasar

    NASA Technical Reports Server (NTRS)

    Dorodnitsyn, A.; Kallman, T.; Proga, D.

    2016-01-01

    We present numerical simulations of properties of a parsec-scale torus exposed to illumination by the central black hole in an active galactic nucleus (AGN). Our physical model allows to investigate the balance between the formation of winds and accretion simultaneously. Radiation-driven winds are allowed by taking into account radiation pressure due to UV and IR radiation along with X-ray heating and dust sublimation. Accretion is allowed through angular momentum transport and the solution of the equations of radiative, viscous radiation hydrodynamics. Our methods adopt flux-limited diffusion radiation hydrodynamics for the dusty, infrared pressure driven part of the flow, along with X-ray heating and cooling. Angular momentum transport in the accreting part of the flow is modeled using effective viscosity. Our results demonstrate that radiation pressure on dust can play an important role in shaping AGN obscuration. For example, when the luminosity illuminating the torus exceeds L greater than 0.01 L(sub Edd), where L(sub Edd) is the Eddington luminosity, we find no episodes of sustained disk accretion because radiation pressure does not allow a disk to form. Despite the absence of the disk accretion, the flow of gas to smaller radii still proceeds at a rate 10(exp -4)-10(exp -1)M dot yr(exp -1) through the capturing of the gas from the hot evaporative flow, thus providing a mechanism to deliver gas from a radiation-pressure dominated torus to the inner accretion disk. As L L(sub edd) increases, larger radiation input leads to larger torus aspect ratios and increased obscuration of the central black hole. We also find the important role of the X-ray heated gas in shaping the obscuring torus.

  17. 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

  18. 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.

  19. Satellite accretion on to massive galaxies with central black holes

    NASA Astrophysics Data System (ADS)

    Boylan-Kolchin, Michael; Ma, Chung-Pei

    2007-02-01

    Minor mergers of galaxies are expected to be common in a hierarchical cosmology such as Λ cold dark matter. Though less disruptive than major mergers, minor mergers are more frequent and thus have the potential to affect galactic structure significantly. In this paper, we dissect the case-by-case outcome from a set of numerical simulations of a single satellite elliptical galaxy accreting on to a massive elliptical galaxy. We take care to explore cosmologically relevant orbital parameters and to set up realistic initial galaxy models that include all three relevant dynamical components: dark matter haloes, stellar bulges, and central massive black holes (BHs). The effects of several different parameters are considered, including orbital energy and angular momentum, satellite density and inner density profile, satellite-to-host mass ratio, and presence of a BH at the centre of the host. BHs play a crucial role in protecting the shallow stellar cores of the hosts, as satellites merging on to a host with a central BH are more strongly disrupted than those merging on to hosts without BHs. Orbital parameters play an important role in determining the degree of disruption: satellites on less-bound or more-eccentric orbits are more easily destroyed than those on more-bound or more-circular orbits as a result of an increased number of pericentric passages and greater cumulative effects of gravitational shocking and tidal stripping. In addition, satellites with densities typical of faint elliptical galaxies are disrupted relatively easily, while denser satellites can survive much better in the tidal field of the host. Over the range of parameters explored, we find that the accretion of a single satellite elliptical galaxy can result in a broad variety of changes, in both signs, in the surface brightness profile and colour of the central part of an elliptical galaxy. Our results show that detailed properties of the stellar components of merging satellites can strongly affect

  20. 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.