The Role of Stellar Feedback on the Structure of the ISM and Star Formation in Galaxies

NASA Astrophysics Data System (ADS)

Grisdale, Kearn Michael

2017-08-01

Stellar feedback refers to the injection of energy, momentum and mass into the interstellar medium (ISM) by massive stars. This feedback owes to a combination of ionising radiation, radiation pressure, stellar winds and supernovae and is likely responsible both for the inefficiency of star formation in galaxies, and the observed super-sonic turbulence of the ISM. In this thesis, I study how stellar feedback shapes the ISM thereby regulating galaxy evolution. In particular, I focus on three key questions: (i) How does stellar feedback shape the gas density distribution of the ISM? (ii) How does feedback change or influence the distribution of the kinetic energy in the ISM? and (iii) What role does feedback play in determining the star formation efficiency of giant molecular clouds (GMCs)? To answer these questions, I run high resolution (Deltax 4.6 pc) numerical simulations of three isolated galaxies, both with and without stellar feedback. I compare these simulations to observations of six galaxies from The HI Nearby Galaxy Survey (THINGS) using power spectra, and I use clump finding techniques to identify GMCs in my simulations and calculate their properties. I find that the kinetic energy power spectra in stellar feedback- regulated galaxies, regardless of the galaxy's mass and size, show scalings in excellent agreement with supersonic turbulence on scales below the thickness of the HI layer. I show that feedback influences the gas density field, and drives gas turbulence, up to large (kiloparsec) scales. This is in stark contrast to the density fields generated by large-scale gravity-only driven turbulence (i.e. without stellar feedback). Simulations with stellar feedback are able to reproduce the internal properties of GMCs such as: mass, size and velocity dispersion. Finally, I demonstrate that my simulations naturally reproduce the observed scatter (3.5-4 dex) in the star formation efficiency per free-fall time of GMCs, despite only employing a simple Schmidt star formation law. I conclude that the neutral gas content of galaxies carries signatures of stellar feedback on all scales and that stellar feedback is, therefore, key to regulating the evolution of galaxies over cosmic time.

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. Conclusions: The initial magnetic field strength and configuration in the region of impact of the stream are expected to influence the chromospheric absorption and, therefore, the observability of the shock-heated plasma in the X-ray band. In addition, the field strength and configuration also influence the energy balance of the shocked plasma with its emission measure at T > 106 K, which is lower than expected for a uniform field. The above effects contribute to underestimating the mass accretion rates derived in the X-ray band. Movies are available in electronic form at http://www.aanda.org

Disruption of circumstellar discs by large-scale stellar magnetic fields

NASA Astrophysics Data System (ADS)

ud-Doula, Asif; Owocki, Stanley P.; Kee, Nathaniel Dylan

2018-05-01

Spectropolarimetric surveys reveal that 8-10% of OBA stars harbor large-scale magnetic fields, but thus far no such fields have been detected in any classical Be stars. Motivated by this, we present here MHD simulations for how a pre-existing Keplerian disc - like that inferred to form from decretion of material from rapidly rotating Be stars - can be disrupted by a rotation-aligned stellar dipole field. For characteristic stellar and disc parameters of a near-critically rotating B2e star, we find that a polar surface field strength of just 10 G can significantly disrupt the disc, while a field of 100 G, near the observational upper limit inferred for most Be stars, completely destroys the disc over just a few days. Our parameter study shows that the efficacy of this magnetic disruption of a disc scales with the characteristic plasma beta (defined as the ratio between thermal and magnetic pressure) in the disc, but is surprisingly insensitive to other variations, e.g. in stellar rotation speed, or the mass loss rate of the star's radiatively driven wind. The disc disruption seen here for even a modest field strength suggests that the presumed formation of such Be discs by decretion of material from the star would likely be strongly inhibited by such fields; this provides an attractive explanation for why no large-scale fields are detected from such Be stars.

Hot interstellar gas and ionization of embedded clouds

NASA Technical Reports Server (NTRS)

Cheng, K.-P.; Bruhweiler, F.

1990-01-01

Researchers present detailed photoionization calculations for the instellar cloud in which the Sun is embedded. They consider the EUV radiation field with contribution from discrete stellar sources and from a thermal bremsstrahlung-radiative recombination spectrum emitted from the surrounding 10 to the 6th power k coronal substrate. They establish lower limits to the fractional ionization of hydrogen and helium of 0.17 and 0.29 respectively. The high He ionization fraction results primarily from very strong line emission below 500 A originating in the surrounding coronal substrate while the H ionization is dominated by the EUV radiation from the discrete stellar sources. The dual effects of thermal conduction and the EUV spectrum of the 10 to the 6th k plasma on ionization in the cloud skin are explored. The EUV radiation field and Auger ionization have insignificant effects on the resulting ionic column densities of Si IV, C IV, N V and O VI through the cloud skin. Calculations show that the abundances of these species are dominated by collisional ionization in the thermal conduction front. Because of a low charge exchange rate with hydrogen, the ionic column density ratios of N (CIII)/N (CII) and N (NII)/N (NI) are dominated by the EUV radiation field in the local interstellar medium. These ratios should be important diagnostics for the EUV radiation field and serve as surrogate indicators of the interstellar He and H ionization fraction respectively. Spacecraft such as Lyman which is designed to obtain high resolution spectral data down to the Lyman limit at 912 A could sample interstellar lines of these ions.

Magnetic braking of stellar cores in red giants and supergiants

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

Maeder, André; Meynet, Georges, E-mail: andre.maeder@unige, E-mail: georges.meynet@unige.ch

2014-10-01

Magnetic configurations, stable on the long term, appear to exist in various evolutionary phases, from main-sequence stars to white dwarfs and neutron stars. The large-scale ordered nature of these fields, often approximately dipolar, and their scaling according to the flux conservation scenario favor a fossil field model. We make some first estimates of the magnetic coupling between the stellar cores and the outer layers in red giants and supergiants. Analytical expressions of the truncation radius of the field coupling are established for a convective envelope and for a rotating radiative zone with horizontal turbulence. The timescales of the internal exchangesmore » of angular momentum are considered. Numerical estimates are made on the basis of recent model grids. The direct magnetic coupling of the core to the extended convective envelope of red giants and supergiants appears unlikely. However, we find that the intermediate radiative zone is fully coupled to the core during the He-burning and later phases. This coupling is able to produce a strong spin down of the core of red giants and supergiants, also leading to relatively slowly rotating stellar remnants such as white dwarfs and pulsars. Some angular momentum is also transferred to the outer convective envelope of red giants and supergiants during the He-burning phase and later.« less

Dynamo action and magnetic activity during the pre-main sequence: Influence of rotation and structural changes

NASA Astrophysics Data System (ADS)

Emeriau-Viard, Constance; Brun, Allan Sacha

2017-10-01

During the PMS, structure and rotation rate of stars evolve significantly. We wish to assess the consequences of these drastic changes on stellar dynamo, internal magnetic field topology and activity level by mean of HPC simulations with the ASH code. To answer this question, we develop 3D MHD simulations that represent specific stages of stellar evolution along the PMS. We choose five different models characterized by the radius of their radiative zone following an evolutionary track, from 1 Myr to 50 Myr, computed by a 1D stellar evolution code. We introduce a seed magnetic field in the youngest model and then we spread it through all simulations. First of all, we study the consequences that the increase of rotation rate and the change of geometry of the convective zone have on the dynamo field that exists in the convective envelop. The magnetic energy increases, the topology of the magnetic field becomes more complex and the axisymmetric magnetic field becomes less predominant as the star ages. The computation of the fully convective MHD model shows that a strong dynamo develops with a ratio of magnetic to kinetic energy reaching equipartition and even super-equipartition states in the faster rotating cases. Magnetic fields resulting from our MHD simulations possess a mixed poloidal-toroidal topology with no obvious dominant component. We also study the relaxation of the vestige dynamo magnetic field within the radiative core and found that it satisfies stability criteria. Hence it does not experience a global reconfiguration and instead slowly relaxes by retaining its mixed poloidal-toroidal topology.

Non-Thermal Spectra from Pulsar Magnetospheres in the Full Electromagnetic Cascade Scenario

NASA Astrophysics Data System (ADS)

Peng, Qi-Yong; Zhang, Li

2008-08-01

We simulated non-thermal emission from a pulsar magnetosphere within the framework of a full polar-cap cascade scenario by taking the acceleration gap into account, using the Monte Carlo method. For a given electric field parallel to open field lines located at some height above the surface of a neutron star, primary electrons were accelerated by parallel electric fields and lost their energies by curvature radiation; these photons were converted to electron-positron pairs, which emitted photons through subsequent quantum synchrotron radiation and inverse Compton scattering, leading to a cascade. In our calculations, the acceleration gap was assumed to be high above the stellar surface (about several stellar radii); the primary and secondary particles and photons emitted during the journey of those particles in the magnetosphere were traced using the Monte Carlo method. In such a scenario, we calculated the non-thermal photon spectra for different pulsar parameters and compared the model results for two normal pulsars and one millisecond pulsar with the observed data.

AR Scorpii and possible gravitational wave radiation from pulsar white dwarfs

NASA Astrophysics Data System (ADS)

Franzon, B.; Schramm, S.

2017-06-01

In view of the new recent observation and measurement of the rotating and highly magnetized white dwarf AR Scorpii, we determine bounds of its moment of inertia, magnetic fields and radius. Moreover, we investigate the possibility of fast rotating and/or magnetized white dwarfs to be sources of detectable gravitational wave (GW) emission. Numerical stellar models at different baryon masses are constructed. For each star configuration, we compute self-consistent relativistic solutions for white dwarfs endowed with poloidal magnetic fields by solving the Einstein-Maxwell field equations in a self-consistent way. The magnetic field supplies an anisotropic pressure, leading to the braking of the spherical symmetry of the star. In this case, we compute the quadrupole moment of the mass distribution. Next, we perform an estimate of the GW of such objects. Finally, we show that the new recent observation and measurement pulsar white dwarf AR Scorpii, as well as other stellar models, might generate GW radiation that lies in the bandwidth of the discussed next generation of space-based GW detectors DECI-hertz Interferometer Gravitational wave Observatory (DECIGO) and Big Bang Observer (BBO).

Stellar model chromospheres. IV - The formation of the H-epsilon feature in the sun /G2 V/ and Arcturus /K2 III/

NASA Technical Reports Server (NTRS)

Ayres, T. R.; Linsky, J. L.

1975-01-01

The formation of the Balmer-series member H-epsilon in the near-red wing of the Ca II H line is discussed for two cases: the sun (H-epsilon absorption profile) and Arcturus (H-epsilon emission profile). It is shown that although the H-epsilon source functions in both stars are dominated by the Balmer-continuum radiation field through photoionizations, the line-formation problems in the two stars are quantitatively different, owing to a substantial difference in the relative importance of the stellar chromosphere temperature inversion as compared with the stellar photosphere.

CAN WE PREDICT THE GLOBAL MAGNETIC TOPOLOGY OF A PRE-MAIN-SEQUENCE STAR FROM ITS POSITION IN THE HERTZSPRUNG-RUSSELL DIAGRAM?

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

Gregory, S. G.; Hillenbrand, L. A.; Donati, J.-F.

2012-08-20

Zeeman-Doppler imaging studies have shown that the magnetic fields of T Tauri stars can be significantly more complex than a simple dipole and can vary markedly between sources. We collect and summarize the magnetic field topology information obtained to date and present Hertzsprung-Russell (H-R) diagrams for the stars in the sample. Intriguingly, the large-scale field topology of a given pre-main-sequence (PMS) star is strongly dependent upon the stellar internal structure, with the strength of the dipole component of its multipolar magnetic field decaying rapidly with the development of a radiative core. Using the observational data as a basis, we arguemore » that the general characteristics of the global magnetic field of a PMS star can be determined from its position in the H-R diagram. Moving from hotter and more luminous to cooler and less luminous stars across the PMS of the H-R diagram, we present evidence for four distinct magnetic topology regimes. Stars with large radiative cores, empirically estimated to be those with a core mass in excess of {approx}40% of the stellar mass, host highly complex and dominantly non-axisymmetric magnetic fields, while those with smaller radiative cores host axisymmetric fields with field modes of higher order than the dipole dominant (typically, but not always, the octupole). Fully convective stars above {approx}> 0.5 M{sub Sun} appear to host dominantly axisymmetric fields with strong (kilo-Gauss) dipole components. Based on similarities between the magnetic properties of PMS stars and main-sequence M-dwarfs with similar internal structures, we speculate that a bistable dynamo process operates for lower mass stars ({approx}< 0.5 M{sub Sun} at an age of a few Myr) and that they will be found to host a variety of magnetic field topologies. If the magnetic topology trends across the H-R diagram are confirmed, they may provide a new method of constraining PMS stellar evolution models.« less

Constraints on Grain Formation Around Carbon Stars from Laboratory Studies of Presolar Graphite

NASA Technical Reports Server (NTRS)

Bernatowicz, T. J.; Akande, O. W.; Croat, T. K.; Cowsik, R.

2005-01-01

We report the results of an investigation into the physical conditions in the mass outflows of asymptotic giant branch (AGB) carbon stars that are required for the formation of micron-sized presolar graphite grains, either with or without internal crystals of titanium carbide (TiC). In addition to providing detailed information about stellar nucleosynthesis, the structure and composition of presolar grains give unique information about the conditions of grain formation. In the present work we use laboratory observations of presolar graphite to gain insight into the physical conditions in circumstellar outflows from carbon AGB stars. The periodic pulsation of AGB stars enhances the gas density through shocks in the stellar atmosphere above the photosphere, promoting the condensation of dust grains. Copious mass outflow occurs largely because grains are coupled to the radiation field of the star, which accelerates them by radiation pressure; momentum is in turn transferred to gas molecules by collisions with grains. The dust/gas mixture is effectively a two-component fluid whose motion depends on atmospheric structure and which, in turn, influences that structure. In particular, the radiation pressure on the grains determines the velocity field of the outflow and thus the density distribution, while the density distribution itself determines the conditions of radiative transfer within the outflow and thus the effective radiation pressure.

GAMA/H-ATLAS: The Dust Opacity-Stellar Mass Surface Density Relation for Spiral Galaxies

NASA Astrophysics Data System (ADS)

Grootes, M. W.; Tuffs, R. J.; Popescu, C. C.; Pastrav, B.; Andrae, E.; Gunawardhana, M.; Kelvin, L. S.; Liske, J.; Seibert, M.; Taylor, E. N.; Graham, Alister W.; Baes, M.; Baldry, I. K.; Bourne, N.; Brough, S.; Cooray, A.; Dariush, A.; De Zotti, G.; Driver, S. P.; Dunne, L.; Gomez, H.; Hopkins, A. M.; Hopwood, R.; Jarvis, M.; Loveday, J.; Maddox, S.; Madore, B. F.; Michałowski, M. J.; Norberg, P.; Parkinson, H. R.; Prescott, M.; Robotham, A. S. G.; Smith, D. J. B.; Thomas, D.; Valiante, E.

2013-03-01

We report the discovery of a well-defined correlation between B-band face-on central optical depth due to dust, τ ^f_B, and the stellar mass surface density, μ*, of nearby (z <= 0.13) spiral galaxies: {log}(τ ^{f}_{B}) = 1.12(+/- 0.11) \\cdot {log}({μ _{*}}/{{M}_{⊙ } {kpc}^{-2}}) - 8.6(+/- 0.8). This relation was derived from a sample of spiral galaxies taken from the Galaxy and Mass Assembly (GAMA) survey, which were detected in the FIR/submillimeter (submm) in the Herschel-ATLAS science demonstration phase field. Using a quantitative analysis of the NUV attenuation-inclination relation for complete samples of GAMA spirals categorized according to stellar mass surface density, we demonstrate that this correlation can be used to statistically correct for dust attenuation purely on the basis of optical photometry and Sérsic-profile morphological fits. Considered together with previously established empirical relationships of stellar mass to metallicity and gas mass, the near linearity and high constant of proportionality of the τ ^f_B - μ_{*} relation disfavors a stellar origin for the bulk of refractory grains in spiral galaxies, instead being consistent with the existence of a ubiquitous and very rapid mechanism for the growth of dust in the interstellar medium. We use the τ ^f_B - μ_{*} relation in conjunction with the radiation transfer model for spiral galaxies of Popescu & Tuffs to derive intrinsic scaling relations between specific star formation rate (SFR), stellar mass, and stellar surface density, in which attenuation of the UV light used for the measurement of SFR is corrected on an object-to-object basis. A marked reduction in scatter in these relations is achieved which we demonstrate is due to correction of both the inclination-dependent and face-on components of attenuation. Our results are consistent with a general picture of spiral galaxies in which most of the submm emission originates from grains residing in translucent structures, exposed to UV in the diffuse interstellar radiation field.

Ionization in the local interstellar and intergalactic media

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

Cheng, K.

1990-01-01

Detailed photoionization calculations for the local interstellar medium (LISM) and the intergalactic medium (IGM) are presented. Constraints in the LISM are imposed by H I column density derived from IUE and Copernicus data toward nearby B stars and hot white dwarfs. The EUV radiation field is modeled including contributions from discrete stellar sources and from a thermal bremsstrahlung-radiative recombination spectrum emitted from the surrounding 10(exp 6) K coronal substrate. Lower limits to the fractional ionization of hydrogen and helium of 0.17 and 0.30 respectively are established. The derived limits have important implications for the interpretation of the H I andmore » He I backscattering results. The high He ionization fraction results primarily from very strong line emission below 500 A originating in the surrounding coronal substrate while the H ionization is dominated by the EUV radiation from the discrete stellar sources. The dual effects of thermal conduction and the EUV spectrum of the 10(exp 6) K plasma on ionization in the cloud skin are explored. The EUV radiation field and Auger ionization have insignificant effects on the resulting ionic column densities of Si IV, C IV, N V and O VI through the cloud skin. Calculations show that the abundances of these species are dominated by collisional ionization in the thermal conduction front. Because of a low charge exchange rate with hydrogen, the ionic column density ratios of N(C III)/N(C II) and N(N II)/N(N I) are dominated by the EUV radiation field in the local interstellar medium. These ratios should be important diagnostics for the EUV radiation field and serve as surrogate indicators of the interstellar He and H ionization fraction respectively. The same photoionization model is applied to the intergalactic medium.« less

Observations from Juno's Radiation Monitoring Investigation during Juno's Early Orbits

NASA Astrophysics Data System (ADS)

Becker, Heidi N.; Jorgensen, John L.; Adriani, Alberto; Mura, Alessandro; Connerney, John E. P.; Santos-Costa, Daniel; Bolton, Scott J.; Levin, Steven M.; Alexander, James W.; Adumitroaie, Virgil; Manor-Chapman, Emily A.; Daubar, Ingrid J.; Lee, Clifford; Benn, Mathias; Denver, Troelz; Sushkova, Julia; Cicchetti, Andrea; Noschese, Raffaella; Thorne, Richard M.

2017-04-01

Juno's Radiation Monitoring (RM) Investigation profiles Jupiter's >10-MeV electron environment throughout unexplored regions of the Jovian magnetosphere. RM's measurement approach involves active retrieval of the characteristic noise signatures from penetrating radiation in images obtained by Juno's heavily shielded star cameras and science instruments. Collaborative observation campaigns of "radiation image" collection and penetrating particle counts are conducted at targeted opportunities within the magnetosphere during each of Juno's perijove passes using the spacecraft Stellar Reference Unit, the Magnetic Field Investigation's Advanced Stellar Compass Imagers, and the JIRAM infrared imager. Simultaneous observations gathered from these very different instruments provide comparative spectral information due to substantial differences in instrument shielding. Juno's orbit provides a unique sampling of energetic particles within Jupiter's innermost radiation belts and polar regions. We present a survey of observations of the high energy radiation environment made by Juno's SRU and ASC star cameras and the JIRAM infrared imager during Juno's early perijove passes on August 27 and December 11, 2016; and February 2 and March 27, 2017. The JPL author's copyright for this publication is held by the California Institute of Technology. Government Sponsorship acknowledged.

Testing Dissipative Magnetosphere Model Light Curves and Spectra with Fermi Pulsars

NASA Technical Reports Server (NTRS)

Brambilla, Gabriele; Kalapotharakos, Constantinos; Harding, Alice K.; Kazanas, Demosthenes

2015-01-01

We explore the emission properties of a dissipative pulsar magnetosphere model introduced by Kalapotharakos et al. comparing its high-energy light curves and spectra, due to curvature radiation, with data collected by the Fermi LAT. The magnetosphere structure is assumed to be near the force-free solution. The accelerating electric field, inside the light cylinder (LC), is assumed to be negligible, while outside the LC it rescales with a finite conductivity (sigma). In our approach we calculate the corresponding high-energy emission by integrating the trajectories of test particles that originate from the stellar surface, taking into account both the accelerating electric field components and the radiation reaction forces. First, we explore the parameter space assuming different value sets for the stellar magnetic field, stellar period, and conductivity. We show that the general properties of the model are in a good agreement with observed emission characteristics of young gamma-ray pulsars, including features of the phase-resolved spectra. Second, we find model parameters that fit each pulsar belonging to a group of eight bright pulsars that have a published phase-resolved spectrum. The sigma values that best describe each of the pulsars in this group show an increase with the spin-down rate (E? ) and a decrease with the pulsar age, expected if pair cascades are providing the magnetospheric conductivity. Finally, we explore the limits of our analysis and suggest future directions for improving such models.

The make-up of stars

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

Asplund, Martin

2014-11-20

The chemical composition of stars contain vital clues not only about the stars themselves but also about the conditions prevailing before their births. As such, stellar spectroscopy plays a key role in contemporary astrophysics and cosmology by probing cosmic, galactic, stellar and planetary evolution. In this review I will describe the theoretical foundations of quantitative stellar spectroscopy: stellar atmosphere models and spectral line formation. I will focus mainly on more recent advances in the field, in particular the advent of realistic time-dependent, 3D, (magneto-)hydrodynamical simulations of stellar surface convection and atmospheres and non-LTE radiative transfer relevant for stars like themore » Sun. I will also discuss some particular applications of this type of modelling which have resulted in some exciting break-throughs in our understanding and with wider implications: the solar chemical composition, the chemical signatures of planet formation imprinted in stellar abundances, the cosmological Li problem(s) and where the first stars may be residing today.« less

Can Flare Loops Contribute to the White-light Emission of Stellar Superflares?

NASA Astrophysics Data System (ADS)

Heinzel, P.; Shibata, K.

2018-06-01

Since the discovery of stellar superflares by the Kepler satellite, these extremely energetic events have been studied in analogy to solar flares. Their white-light (WL) continuum emission has been interpreted as being produced by heated ribbons. In this paper, we compute the WL emission from overlying flare loops depending on their density and temperature and show that, under conditions expected during superflares, the continuum brightening due to extended loop arcades can significantly contribute to stellar flux detected by Kepler. This requires electron densities in the loops of 1012‑1013 cm‑3 or higher. We show that such densities, exceeding those typically present in solar-flare loops, can be reached on M-dwarf and solar-type superflare stars with large starspots and much stronger magnetic fields. Quite importantly, the WL radiation of loops is not very sensitive to their temperature and thus both cool as well as hot loops may contribute. We show that the WL intensity emergent from optically thin loops is lower than the blackbody radiation from flare ribbons, but the contribution of loops to total stellar flux can be quite important due to their significant emitting areas. This new scenario for interpreting superflare emission suggests that the observed WL flux is due to a mixture of the ribbon and loop radiation and can be even loop-dominated during the gradual phase of superflares.

Coupling hydrodynamics and radiation calculations for star-jet interactions in active galactic nuclei

NASA Astrophysics Data System (ADS)

de la Cita, V. M.; Bosch-Ramon, V.; Paredes-Fortuny, X.; Khangulyan, D.; Perucho, M.

2016-06-01

Context. Stars and their winds can contribute to the non-thermal emission in extragalactic jets. Because of the complexity of jet-star interactions, the properties of the resulting emission are closely linked to those of the emitting flows. Aims: We simulate the interaction between a stellar wind and a relativistic extragalactic jet and use the hydrodynamic results to compute the non-thermal emission under different conditions. Methods: We performed relativistic axisymmetric hydrodynamical simulations of a relativistic jet interacting with a supersonic, non-relativistic stellar wind. We computed the corresponding streamlines out of the simulation results and calculated the injection, evolution, and emission of non-thermal particles accelerated in the jet shock, focusing on electrons or e±-pairs. Several cases were explored, considering different jet-star interaction locations, magnetic fields, and observer lines of sight. The jet luminosity and star properties were fixed, but the results are easily scalable when these parameters are changed. Results: Individual jet-star interactions produce synchrotron and inverse Compton emission that peaks from X-rays to MeV energies (depending on the magnetic field), and at ~100-1000 GeV (depending on the stellar type), respectively. The radiation spectrum is hard in the scenarios explored here as a result of non-radiative cooling dominance, as low-energy electrons are efficiently advected even under relatively high magnetic fields. Interactions of jets with cold stars lead to an even harder inverse Compton spectrum because of the Klein-Nishina effect in the cross section. Doppler boosting has a strong effect on the observer luminosity. Conclusions: The emission levels for individual interactions found here are in the line of previous, more approximate, estimates, strengthening the hypothesis that collective jet-star interactions could significantly contribute at high energies under efficient particle acceleration.

Rotation-induced YORP break-up of small bodies to produce post-main-sequence debris

NASA Astrophysics Data System (ADS)

Veras, D.; Jacobson, S. A.; Gänsicke, B. T.

2017-09-01

We hypothesize that the in situ break-up of small bodies such as asteroids spun to fission during the giant branch phases of stellar evolution provides an important contribution to the debris orbiting and ultimately polluting white dwarfs. The YORP (Yarkovsky-O'Keefe-Radviesvki-Paddock) effect, which arises from radiation pressure, accelerates the spin rate of asymmetric asteroids, which can eventually shear themselves apart. This pressure is maintained and enhanced around dying stars because the outward push of an asteroid due to stellar mass loss is insignificant compared to the resulting stellar luminosity increase. Consequently, giant star radiation will destroy nearly all bodies with radii in the range 100 m-10 km that survive their parent star's main-sequence lifetime within a distance of about 7 au; smaller bodies are spun apart to their strongest, competent components. This estimate is conservative and would increase for highly asymmetric shapes or incorporation of the inward drag due to giant star stellar wind. The resulting debris field, which could extend to thousands of au, may be perturbed by remnant planetary systems to reproduce the observed dusty and gaseous discs which accompany polluted white dwarfs.

Radio Emission from Binary Stars

NASA Astrophysics Data System (ADS)

Hjellming, R.; Murdin, P.

2000-11-01

Stellar radio emission is most common in double star systems where each star provides something essential in producing the large amounts of radio radiation needed for it to be detectable by RADIO TELESCOPES. They transfer mass, supply energy or, when one of the stars is a NEUTRON STAR or BLACK HOLE, have the strong gravitational fields needed for the energetic particles and magnetic fields needed...

The influence of continuum radiation fields on hydrogen radio recombination lines

NASA Astrophysics Data System (ADS)

Prozesky, Andri; Smits, Derck P.

2018-05-01

Calculations of hydrogen departure coefficients using a model with the angular momentum quantum levels resolved that includes the effects of external radiation fields are presented. The stimulating processes are important at radio frequencies and can influence level populations. New numerical techniques with a solid mathematical basis have been incorporated into the model to ensure convergence of the solution. Our results differ from previous results by up to 20 per cent. A direct solver with a similar accuracy but more efficient than the iterative method is used to evaluate the influence of continuum radiation on the hydrogen population structure. The effects on departure coefficients of continuum radiation from dust, the cosmic microwave background, the stellar ionising radiation, and free-free radiation are quantified. Tables of emission and absorption coefficients for interpreting observed radio recombination lines are provided.

A close halo of large transparent grains around extreme red giant stars

NASA Astrophysics Data System (ADS)

Norris, Barnaby R. M.; Tuthill, Peter G.; Ireland, Michael J.; Lacour, Sylvestre; Zijlstra, Albert A.; Lykou, Foteini; Evans, Thomas M.; Stewart, Paul; Bedding, Timothy R.

2012-04-01

An intermediate-mass star ends its life by ejecting the bulk of its envelope in a slow, dense wind. Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust, which is then accelerated by radiation pressure, entraining the gas and driving the wind. Explaining the amount of mass loss, however, has been a problem because of the difficulty of observing tenuous gas and dust only tens of milliarcseconds from the star. For this reason, there is no consensus on the way sufficient momentum is transferred from the light from the star to the outflow. Here we report spatially resolved, multiwavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the Hertzsprung-Russell diagram. When imaged in scattered light, dust shells were found at remarkably small radii (less than about two stellar radii) and with unexpectedly large grains (about 300 nanometres in radius). This proximity to the photosphere argues for dust species that are transparent to the light from the star and, therefore, resistant to sublimation by the intense radiation field. Although transparency usually implies insufficient radiative pressure to drive a wind, the radiation field can accelerate these large grains through photon scattering rather than absorption--a plausible mass loss mechanism for lower-amplitude pulsating stars.

Radiative Transfer in Stellar Atmospheres

NASA Astrophysics Data System (ADS)

Rutten, Robert J.

2003-05-01

The main topic treated in these graduate course notes is the classical theory of radiative transfer for explaining stellar spectra. It needs relatively much attention to be mastered. Radiative transfer in gaseous media that are neither optically thin nor fully opaque and scatter to boot is a key part of astrophysics but not a transparent subject. These course notes represent a middle road between Mihalas' "Stellar Atmospheres" (graduate level and up) and the books by Novotny and Boehm-Vitense (undergraduate level). They are at about the level of Gray's "The observation and analysis of stellar photospheres" but emphasize NLTE radiative transfer rather than observational techniques and data interpretation.

Radiation effects on space-based stellar photometry: theoretical models and empirical results for CoRoT Space Telescope

NASA Astrophysics Data System (ADS)

Pinheiro da Silva, L.; Rolland, G.; Lapeyrere, V.; Auvergne, M.

2008-03-01

Convection, Rotation and planetary Transits (CoRoT) is a space mission dedicated to stellar seismology and the search for extrasolar planets. Both scientific programs are based on very high precision photometry and require long, uninterrupted observations. The instrument is based on an afocal telescope and a wide-field camera, consisting of four E2V-4280 CCD devices. This set is mounted on a recurrent platform for insertion in low Earth orbit. The CoRoT satellite has been recently launched for a nominal mission duration of three years. In this work, we discuss the impact of space radiation on CoRoT CCDs, in sight of the in-flight characterization results obtained during the satellite's commissioning phase, as well as the very first observational data. We start by describing the population of trapped particles at the satellite altitude, and by presenting a theoretical prediction for the incoming radiation fluxes seen by the CCDs behind shielding. Empirical results regarding particle impact rates and their geographical distribution are then presented and discussed. The effect of particle impacts is also statistically characterized, with respect to the ionizing energy imparted to the CCDs and the size of impact trails. Based on these results, we discuss the effects of space radiation on precise and time-resolved stellar photometry from space. Finally, we present preliminary results concerning permanent radiation damage on CoRoT CCDs, as extrapolated from the data available at the beginning of the satellite's lifetime.

Simulation of the small-scale magnetism in main-sequence stellar atmospheres

NASA Astrophysics Data System (ADS)

Salhab, R. G.; Steiner, O.; Berdyugina, S. V.; Freytag, B.; Rajaguru, S. P.; Steffen, M.

2018-06-01

Context. Observations of the Sun tell us that its granular and subgranular small-scale magnetism has significant consequences for global quantities such as the total solar irradiance or convective blueshift of spectral lines. Aims: In this paper, properties of the small-scale magnetism of four cool stellar atmospheres, including the Sun, are investigated, and in particular its effects on the radiative intensity and flux. Methods: We carried out three-dimensional radiation magnetohydrodynamic simulations with the CO5BOLD code in two different settings: with and without a magnetic field. These are thought to represent states of high and low small-scale magnetic activity of a stellar magnetic cycle. Results: We find that the presence of small-scale magnetism increases the bolometric intensity and flux in all investigated models. The surplus in radiative flux of the magnetic over the magnetic field-free atmosphere increases with increasing effective temperature, Teff, from 0.47% for spectral type K8V to 1.05% for the solar model, but decreases for higher effective temperatures than solar. The degree of evacuation of the magnetic flux concentrations monotonically increases with Teff as does their depression of the visible optical surface, that is the Wilson depression. Nevertheless, the strength of the field concentrations on this surface stays remarkably unchanged at ≈1560 G throughout the considered range of spectral types. With respect to the surrounding gas pressure, the field strength is close to (thermal) equipartition for the Sun and spectral type F5V but is clearly sub-equipartition for K2V and more so for K8V. The magnetic flux concentrations appear most conspicuous for model K2V owing to their high brightness contrast. Conclusions: For mean magnetic flux densities of approximately 50 G, we expect the small-scale magnetism of stars in the spectral range from F5V to K8V to produce a positive contribution to their bolometric luminosity. The modulation seems to be most effective for early G-type stars.

The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets

NASA Astrophysics Data System (ADS)

Garraffo, Cecilia; Drake, Jeremy J.; Cohen, Ofer; Alvarado-Gómez, Julian D.; Moschou, Sofia P.

2017-07-01

Recently, four additional Earth-mass planets were discovered orbiting the nearby ultracool M8 dwarf, TRAPPIST-1, making a remarkable total of seven planets with equilibrium temperatures compatible with the presence of liquid water on their surface. Temperate terrestrial planets around an M-dwarf orbit close to their parent star, rendering their atmospheres vulnerable to erosion by the stellar wind and energetic electromagnetic and particle radiation. Here, we use state-of-the-art 3D magnetohydrodynamic models to simulate the wind around TRAPPIST-1 and study the conditions at each planetary orbit. All planets experience a stellar wind pressure between 103 and 105 times the solar wind pressure on Earth. All orbits pass through wind pressure changes of an order of magnitude and most planets spend a large fraction of their orbital period in the sub-Alfvénic regime. For plausible planetary magnetic field strengths, all magnetospheres are greatly compressed and undergo much more dynamic change than that of the Earth. The planetary magnetic fields connect with the stellar radial field over much of the planetary surface, allowing the direct flow of stellar wind particles onto the planetary atmosphere. These conditions could result in strong atmospheric stripping and evaporation and should be taken into account for any realistic assessment of the evolution and habitability of the TRAPPIST-1 planets.

Using stellar spectra to illustrate thermal radiation laws

NASA Astrophysics Data System (ADS)

Kaltcheva, N. T.; Pritzl, B. J.

2018-05-01

Stars are point-source emitters that are the closest to the definition of a blackbody in comparison to all other similar sources of radiation found in nature. Existing libraries on stellar spectra are thus a valuable resource that can be used to introduce the laws of thermal radiation in a classroom setting. In this article we briefly describe some of the opportunities that available databases on stellar spectra provide for students to gain a deeper understanding on thermal radiation and spectral line characteristics.

Infrared analysis of LMC superbubbles

NASA Technical Reports Server (NTRS)

Verter, Fran; Dwek, Eli

1990-01-01

Researchers are analyzing three superbubbles in the Large Magellanic Cloud (LMC), cataloged by Meaburn (1980) as LMC-1, LMC-4 (a.k.a. Shapley Constellation III), and LMC-5. Superbubbles are the largest infrared sources in the disks of external galaxies. Their expansion requires multiple supernovae from successive generations of star formation. In LMC superbubbles, the grains swept up by shocks and winds represent an interstellar medium (ISM) whose abundances are quite different from the Galaxy. By applying the Dwek (1986) grain model, we can derive the composition and size spectrum of the grains. The inputs to this model are the dust emission in the four Infrared Astronomy Satellite (IRAS) bands and the interstellar radiation field (ISRF) that provides the heating. The first step in the project is to derive the ISRF for star-forming regions on the periphery of superbubbles. Researchers are doing this by combining observations at several wavelengths to determine the energy budget of the region. They will use a UV image to trace the ionizing stellar radiation that escapes, an H alpha image to trace the ionizing stellar radiation that is absorbed by gas, and the four IRAS images to trace the stellar radiation, both ionizing and non-ionizing, that is absorbed by dust. This multi-wavelength approach has the advantages that we do not have to assume the shape of the IMF or the extinction of the source.

Effects of binary stellar populations on direct collapse black hole formation

NASA Astrophysics Data System (ADS)

Agarwal, Bhaskar; Cullen, Fergus; Khochfar, Sadegh; Klessen, Ralf S.; Glover, Simon C. O.; Johnson, Jarrett

2017-06-01

The critical Lyman-Werner (LW) flux required for direct collapse blackholes (DCBH) formation, or Jcrit, depends on the shape of the irradiating spectral energy distribution (SED). The SEDs employed thus far have been representative of realistic single stellar populations. We study the effect of binary stellar populations on the formation of DCBH, as a result of their contribution to the LW radiation field. Although binary populations with ages > 10 Myr yield a larger LW photon output, we find that the corresponding values of Jcrit can be up to 100 times higher than single stellar populations. We attribute this to the shape of the binary SEDs as they produce a sub-critical rate of H- photodetaching 0.76 eV photons as compared to single stellar populations, reaffirming the role that H- plays in DCBH formation. This further corroborates the idea that DCBH formation is better understood in terms of a critical region in the H2-H- photodestruction rate parameter space, rather than a single value of LW flux.

Center-to-limb polarization in continuum spectra of F, G, K stars

NASA Astrophysics Data System (ADS)

Kostogryz, N. M.; Berdyugina, S. V.

2015-03-01

Context. Scattering and absorption processes in stellar atmosphere affect the center-to-limb variations of the intensity (CLVI) and the linear polarization (CLVP) of stellar radiation. Aims: There are several theoretical and observational studies of CLVI using different stellar models, however, most studies of CLVP have concentrated on the solar atmosphere and have not considered the CLVP in cooler non-gray stellar atmospheres at all. In this paper, we present a theoretical study of the CLV of the intensity and the linear polarization in continuum spectra of different spectral type stars. Methods: We solve the radiative transfer equations for polarized light iteratively assuming no magnetic field and considering a plane-parallel model atmospheres and various opacities. Results: We calculate the CLVI and the CLVP for Phoenix stellar model atmospheres for the range of effective temperatures (4500 K-6900 K), gravities (log g = 3.0-5.0), and wavelengths (4000-7000 Å), which are tabulated and available at the CDS. In addition, we present several tests of our code and compare our results with measurements and calculations of CLVI and the CLVP for the Sun. The resulting CLVI are fitted with polynomials and their coefficients are presented in this paper. Conclusions: For the stellar model atmospheres with lower gravity and effective temperature the CLVP is larger. Full Tables 1 and 2, and coefficients of polynomials are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/575/A89

Circumstellar shells, the formation of grains, and radiation transfer

NASA Technical Reports Server (NTRS)

Lefevre, Jean

1987-01-01

Advances in infrared astronomy during the last decade have firmly established the presence of dust around a large number of cold giant and supergiant stars. To describe the properties of stars and to understand their evolution, it is necessary to know the nature of the giants and their influence on stellar radiation. Two questions are considered: the formation of grains around cold stars and the modification of stellar radiation by the stellar shell.

Post-main-sequence debris from rotation-induced YORP break-up of small bodies

NASA Astrophysics Data System (ADS)

Veras, Dimitri; Jacobson, Seth A.; Gänsicke, Boris T.

2014-12-01

Although discs of dust and gas have been observed orbiting white dwarfs, the origin of this circumstellar matter is uncertain. We hypothesize that the in situ break-up of small bodies such as asteroids spun to fission during the giant branch phases of stellar evolution provides an important contribution to this debris. The YORP (Yarkovsky-O'Keefe-Radviesvki-Paddock) effect, which arises from radiation pressure, accelerates the spin rate of asymmetric asteroids, which can eventually shear themselves apart. This pressure is maintained and enhanced around dying stars because the outward push of an asteroid due to stellar mass loss is insignificant compared to the resulting stellar luminosity increase. Consequently, giant star radiation will destroy nearly all bodies with radii in the range 100 m-10 km that survive their parent star's main-sequence lifetime within a distance of about 7 au; smaller bodies are spun apart to their strongest, competent components. This estimate is conservative and would increase for highly asymmetric shapes or incorporation of the inward drag due to giant star stellar wind. The resulting debris field, which could extend to thousands of au, may be perturbed by remnant planetary systems to reproduce the observed dusty and gaseous discs which accompany polluted white dwarfs.

Gravitational radiation quadrupole formula is valid for gravitationally interacting systems

NASA Technical Reports Server (NTRS)

Walker, M.; Will, C. M.

1980-01-01

An argument is presented for the validity of the quadrupole formula for gravitational radiation energy loss in the far field of nearly Newtonian (e.g., binary stellar) systems. This argument differs from earlier ones in that it determines beforehand the formal accuracy of approximation required to describe gravitationally self-interacting systems, uses the corresponding approximate equation of motion explicitly, and evaluates the appropriate asymptotic quantities by matching along the correct space-time light cones.

Planetary geology, stellar evolution and galactic cosmology

NASA Technical Reports Server (NTRS)

1972-01-01

Field studies of selected basalt flows in the Snake River Plain, Idaho, were made for comparative lunar and Mars geological investigations. Studies of basalt lava tubes were also initiated in Washington, Oregon, Hawaii, and northern California. The main effort in the stellar evolution research is toward the development of a computer code to calculate hydrodynamic flow coupled with radiative energy transport. Estimates of the rotation effects on a collapsing cloud indicate that the total angular momentum is the critical parameter. The study of Paschen and Balmer alpha lines of positronium atoms in the center of a galaxy is mentioned.

The Far Ultraviolet M-dwarf Evolution Survey (FUMES): Overview and Initial Results

NASA Astrophysics Data System (ADS)

Pineda, J. Sebastian; France, Kevin; Youngblood, Allison

2018-01-01

M-dwarf stars are prime targets for exoplanet searches because of their close proximity and favorable properties for both planet detection and characterization, with current searches around these targets having already discovered several Earth-sized planets within their star’s habitable zones. However, the atmospheric characterization and potential habitability of these exoplanetary systems depends critically on the high-energy stellar radiation environment from X-rays to NUV. Strong radiation at these energies can lead to atmospheric mass loss and is a strong driver of photochemistry in planetary atmospheres. Recently, the MUSCLES Treasury Survey provided the first comprehensive assessment of the high-energy radiation field around old, planet hosting M-dwarfs. However, the habitability and potential for such exoplanetary atmospheres to develop life also depends on the evolution of the atmosphere and hence the evolution of the incident radiation field. The strong high-energy spectrum of young M-dwarfs can have devastating consequences for the potential habitability of a given system. We, thus, introduce the Far Ultraviolet M-dwarf Evolution Survey (FUMES), a new HST-STIS observing campaign targeting 10 early-mid M dwarfs with known rotation periods, including 6 targets with known ages, to assess the evolution of the FUV radiation, including Lyα, of M-dwarf stars with stellar rotation period. We present the initial results of our survey characterizing the FUV emission features of our targets and the implications of our measurements for the evolution of the entire high-energy radiation environment around M-dwarfs from youth to old age.

The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets

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

Garraffo, Cecilia; Drake, Jeremy J.; Cohen, Ofer

2017-07-10

Recently, four additional Earth-mass planets were discovered orbiting the nearby ultracool M8 dwarf, TRAPPIST-1, making a remarkable total of seven planets with equilibrium temperatures compatible with the presence of liquid water on their surface. Temperate terrestrial planets around an M-dwarf orbit close to their parent star, rendering their atmospheres vulnerable to erosion by the stellar wind and energetic electromagnetic and particle radiation. Here, we use state-of-the-art 3D magnetohydrodynamic models to simulate the wind around TRAPPIST-1 and study the conditions at each planetary orbit. All planets experience a stellar wind pressure between 10{sup 3} and 10{sup 5} times the solar windmore » pressure on Earth. All orbits pass through wind pressure changes of an order of magnitude and most planets spend a large fraction of their orbital period in the sub-Alfvénic regime. For plausible planetary magnetic field strengths, all magnetospheres are greatly compressed and undergo much more dynamic change than that of the Earth. The planetary magnetic fields connect with the stellar radial field over much of the planetary surface, allowing the direct flow of stellar wind particles onto the planetary atmosphere. These conditions could result in strong atmospheric stripping and evaporation and should be taken into account for any realistic assessment of the evolution and habitability of the TRAPPIST-1 planets.« less

Advances In Understanding Solar And Stellar Flares

NASA Astrophysics Data System (ADS)

Kowalski, Adam F.

2016-07-01

Flares result from the sudden reconnection and relaxation of magnetic fields in the coronae of stellar atmospheres. The highly dynamic atmospheric response produces radiation across the electromagnetic spectrum, from the radio to X-rays, on a range of timescales, from seconds to days. New high resolution data of solar flares have revealed the intrinsic spatial properties of the flaring chromosphere, which is thought to be where the majority of the flare energy is released as radiation in the optical and near-UV continua and emission lines. New data of stellar flares have revealed the detailed properties of the broadband (white-light) continuum emission, which provides straightforward constraints for models of the transformation of stored magnetic energy in the corona into thermal energy of the lower atmosphere. In this talk, we discuss the physical processes that produce several important spectral phenomena in the near-ultraviolet and optical as revealed from new radiative-hydrodynamic models of flares on the Sun and low mass stars. We present recent progress with high-flux nonthermal electron beams in reproducing the observed optical continuum color temperature of T 10,000 K and the Balmer jump properties in the near-ultraviolet. These beams produce dense, heated chromospheric condensations, which can explain the shape and strength of the continuum emission in M dwarf flares and the red-wing asymmetries in the chromospheric emission lines in recent observations of solar flares from the Interface Region Imaging Spectrograph. Current theoretical challenges and future modeling directions will be discussed, as well as observational synergies between solar and stellar flares.

Radiative Hydrodynamic Simulations of In Situ Star Formation in the Galactic Center

NASA Astrophysics Data System (ADS)

Frazer, Chris; Heitsch, Fabian

2018-01-01

Many stars observed in the Galactic Center (GC) orbit the supermassive black hole (SMBH), Sagittarius A*, in a region where the extreme gravitational field is expected to inhibit star formation. Yet, many of these stars are young which favors an in situ formation scenario. Previous numerical work on this topic has focused on two possible solutions. First, the tidal capture of a > 10^4 Msun infalling molecular cloud by an SMBH may result in the formation of a surrounding gas disk which then rapidly cools and forms stars. This process results in stellar populations that are consistent with the observed stellar disk in the GC. Second, dense gas clumps of approximately 100 Msun on highly eccentric orbits about an SMBH can experience sparks of star formation via orbital compressions occurring during pericenter passage. In my dissertation, I build upon these models using a series of grid-based radiative hydrodynamic simulations, including the effects of both ionizing ultraviolet light from existing stars as well as X-ray radiation emanating from the central black hole. Radiation is treated with an adaptive ray-tracing routine, including appropriate heating and cooling for both neutral and ionized gas. These models show that ultraviolet radiation is sufficiently strong to heat low mass gas clouds, thus suppressing star formation from clump compression. Gas disks that form from cloud capture become sufficiently dense to provide shielding from the radiation of existing central stars, thus allowing star formation to continue. Conversely, X-rays easily penetrate and heat the potentially star forming gas. For sufficiently high radiation fields, this provides a mechanism to disrupt star formation for both scenarios considered above.

On the Formation of Massive Stars

NASA Technical Reports Server (NTRS)

Yorke, Harold W.; Sonnhalter, Cordula

2002-01-01

We calculate numerically the collapse of slowly rotating, nonmagnetic, massive molecular clumps of masses 30,60, and 120 Stellar Mass, which conceivably could lead to the formation of massive stars. Because radiative acceleration on dust grains plays a critical role in the clump's dynamical evolution, we have improved the module for continuum radiation transfer in an existing two-dimensional (axial symmetry assumed) radiation hydrodynamic code. In particular, rather than using "gray" dust opacities and "gray" radiation transfer, we calculate the dust's wavelength-dependent absorption and emission simultaneously with the radiation density at each wavelength and the equilibrium temperatures of three grain components: amorphous carbon particles. silicates, and " dirty ice " -coated silicates. Because our simulations cannot spatially resolve the innermost regions of the molecular clump, however, we cannot distinguish between the formation of a dense central cluster or a single massive object. Furthermore, we cannot exclude significant mass loss from the central object(s) that may interact with the inflow into the central grid cell. Thus, with our basic assumption that all material in the innermost grid cell accretes onto a single object. we are able to provide only an upper limit to the mass of stars that could possibly be formed. We introduce a semianalytical scheme for augmenting existing evolutionary tracks of pre-main-sequence protostars by including the effects of accretion. By considering an open outermost boundary, an arbitrary amount of material could, in principal, be accreted onto this central star. However, for the three cases considered (30, 60, and 120 Stellar Mass originally within the computation grid), radiation acceleration limited the final masses to 3 1.6, 33.6, and 42.9 Stellar Mass, respectively, for wavelength-dependent radiation transfer and to 19.1, 20.1, and 22.9 Stellar Mass. for the corresponding simulations with gray radiation transfer. Our calculations demonstrate that massive stars can in principle be formed via accretion through a disk. The accretion rate onto the central source increases rapidly after one initial free-fall time and decreases monotonically afterward. By enhancing the nonisotropic character of the radiation field, the accretion disk reduces the effects of radiative acceleration in the radial direction - a process we call the "flashlight effect." The flashlight effect is further amplified in our case by including the effects of frequency-dependent radiation transfer. We conclude with the warning that a careful treatment of radiation transfer is a mandatory requirement for realistic simulations of the formation of massive stars.

Estimating stellar effective temperatures and detected angular parameters using stochastic particle swarm optimization

NASA Astrophysics Data System (ADS)

Zhang, Chuan-Xin; Yuan, Yuan; Zhang, Hao-Wei; Shuai, Yong; Tan, He-Ping

2016-09-01

Considering features of stellar spectral radiation and sky surveys, we established a computational model for stellar effective temperatures, detected angular parameters and gray rates. Using known stellar flux data in some bands, we estimated stellar effective temperatures and detected angular parameters using stochastic particle swarm optimization (SPSO). We first verified the reliability of SPSO, and then determined reasonable parameters that produced highly accurate estimates under certain gray deviation levels. Finally, we calculated 177 860 stellar effective temperatures and detected angular parameters using data from the Midcourse Space Experiment (MSX) catalog. These derived stellar effective temperatures were accurate when we compared them to known values from literatures. This research makes full use of catalog data and presents an original technique for studying stellar characteristics. It proposes a novel method for calculating stellar effective temperatures and detecting angular parameters, and provides theoretical and practical data for finding information about radiation in any band.

Boundary modelling of the stellarator Wendelstein 7-X

NASA Astrophysics Data System (ADS)

Renner, H.; Strumberger, E.; Kisslinger, J.; Nührenberg, J.; Wobig, H.

1997-02-01

To justify the design of the divertor plates in W7-X the magnetic fields of finite-β HELIAS equilibria for the so-called high-mirror case have been computed for various average β-values up to < β > = 0.04 with the NEMEC free-boundary equilibrium code [S.P. Hirshman, W.I. van Rij and W.I. Merkel, Comput. Phys. Commun. 43 (1986) 143] in combination with the newly developed MFBE (magnetic field solver for finite-beta equilibria) code. In a second study the unloading of the target plates by radiation was investigated. The B2 code [B.J. Braams, Ph.D. Thesis, Rijksuniversiteit Utrecht (1986)] was applied for the first time to stellarators to provide of a self-consistent modelling of the SOL including effects of neutrals and impurities.

Neutron starquakes and the nature of gamma-ray bursts

NASA Technical Reports Server (NTRS)

Madau, P.; Blaes, O.; Blandford, R. D.; Goldreich, P.

1989-01-01

The possibility that gamma-ray bursts originate from quakes deep in the solid crust of a neutron star is investigated. Seismic waves are radiated if shear stress is relieved by brittle fracture. However they cannot propagate directly to the surface but are temporarily trapped below a reflecting layer. The shaking of the stellar surface couples the seismic waves to Alfven waves which propagate out into the magnetosphere. The crust-magnetosphere transmission coefficient strongly increases with wave frequency and magnetic field strength. Alfven wave luminosities sufficient to power galactic gamma-ray bursts are possible if magnetic fields greater than 100 billion G cover at least part of the stellar surface. As the Alfven waves propagate out into the low density magnetosphere, they become increasingly charge starved, thereby accelerating particles to relativistic energies.

HOW CAN NEWLY BORN RAPIDLY ROTATING NEUTRON STARS BECOME MAGNETARS?

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

Cheng, Quan; Yu, Yun-Wei, E-mail: yuyw@mail.ccnu.edu.cn

2014-05-10

In a newly born (high-temperature and Keplerian rotating) neutron star, r-mode instability can lead to stellar differential rotation, which winds the seed poloidal magnetic field (∼10{sup 11} G) to generate an ultra-high (∼10{sup 17} G) toroidal field component. Subsequently, by succumbing to the Tayler instability, the toroidal field could be partially transformed into a new poloidal field. Through such dynamo processes, the newly born neutron star with sufficiently rapid rotation could become a magnetar on a timescale of ∼10{sup 2} {sup –} {sup 3} s, with a surface dipolar magnetic field of ∼10{sup 15} G. Accompanying the field amplification, the star could spinmore » down to a period of ∼5 ms through gravitational wave radiation due to the r-mode instability and, in particular, the non-axisymmetric stellar deformation caused by the toroidal field. This scenario provides a possible explanation for why the remnant neutron stars formed in gamma-ray bursts and superluminous supernovae could be millisecond magnetars.« less

Assessing the Habitability of TRAPPIST-1e: MHD Simulations of Atmospheric Loss Due to CMEs and Stellar Wind

NASA Astrophysics Data System (ADS)

Harbach, Laura Marshall; Drake, Jeremy J.; Garraffo, Cecilia; Alvarado-Gomez, Julian D.; Moschou, Sofia P.; Cohen, Ofer

2018-01-01

Recently, three rocky planets were discovered in the habitable zone of the nearby planetary system TRAPPIST-1. The increasing number of exoplanet detections has led to further research into the planetary requirements for sustaining life. Habitable zone occupants have, in principle, the capacity to retain liquid water, whereas actual habitability might depend on atmospheric retention. However, stellar winds and photon radiation interactions with the planet can lead to severe atmospheric depletion and have a catastrophic impact on a planet’s habitability. While the implications of photoevaporation on atmospheric erosion have been researched to some degree, the influence of stellar winds and Coronal Mass Ejections (CMEs) has yet to be analyzed in detail. Here, we model the effect of the stellar wind and CMEs on the atmospheric envelope of a planet situated in the orbit of TRAPPIST-1e using 3D magnetohydrodynamic (MHD) simulations. In particular, we discuss the atmospheric loss due to the effect of a CME, and the relevance of the stellar and planetary magnetic fields on the sustainability of M-dwarf exoplanetary atmospheres.

The relative impact of photoionizing radiation and stellar winds on different environments

NASA Astrophysics Data System (ADS)

Haid, S.; Walch, S.; Seifried, D.; Wünsch, R.; Dinnbier, F.; Naab, T.

2018-05-01

Photoionizing radiation and stellar winds from massive stars deposit energy and momentum into the interstellar medium (ISM). They might disperse the local ISM, change its turbulent multi-phase structure, and even regulate star formation. Ionizing radiation dominates the massive stars' energy output, but the relative effect of winds might change with stellar mass and the properties of the ambient ISM. We present simulations of the interaction of stellar winds and ionizing radiation of 12, 23, and 60 M⊙ stars within a cold neutral (CNM, n0 = 100 cm-3), warm neutral (WNM, n0 = 1, 10 cm-3) or warm ionized (WIM, n0 = 0.1 cm-3) medium. The FLASH simulations adopt the novel tree-based radiation transfer algorithm TREERAY. With the On-the-Spot approximation and a temperature-dependent recombination coefficient, it is coupled to a chemical network with radiative heating and cooling. In the homogeneous CNM, the total momentum injection ranges from 1.6× 104 to 4× 105 M⊙ km s-1 and is always dominated by the expansion of the ionized HII region. In the WIM, stellar winds dominate (2× 102 to 5× 103 M⊙ km s-1), while the input from radiation is small (˜ 102 M⊙ km s-1). The WNM (n0 = 1 cm-3) is a transition regime. Energetically, stellar winds couple more efficiently to the ISM (˜ 0.1 percent of wind luminosity) than radiation (< 0.001 percent of ionizing luminosity). For estimating the impact of massive stars, the strongly mass-dependent ratios of wind to ionizing luminosity and the properties of the ambient medium have to be considered.

Magnetised winds and their influence in the escaping upper atmosphere of HD 209458b

NASA Astrophysics Data System (ADS)

D'Angelo, Carolina Villarreal; Esquivel, Alejandro; Schneiter, Matías; Sgró, Mario Agustín

2018-06-01

Lyman α observations during an exoplanet transit have proved to be very useful to study the interaction between the stellar wind and the planetary atmosphere. They have been extensively used to constrain planetary system parameters that are not directly observed, such as the planetary mass loss rate. In this way, Ly α observations can be a powerful tool to infer the existence of a planetary magnetic field, since it is expected that the latter will affect the escaping planetary material. To explore the effect that magnetic fields have on the Ly α absorption of HD 209458b, we run a set of 3D MHD simulations including dipolar magnetic fields for the planet and the star. We assume values for the surface magnetic field at the poles of the planet in the range of [0-5] G, and from 1 to 5 G at the poles of the star. Our models also include collisional and photo-ionisation, radiative recombination, and an approximation for the radiation pressure. Our results show that the magnetic field of the planet and the star change the shape of the Ly α absorption profile, since it controls the extent of the planetary magnetosphere and the amount of neutral material inside it. The model that best reproduces the absorption observed in HD 209458b (with canonical values for the stellar wind parameters) corresponds to a dipole planetary field of ≲ 1 gauss at the poles.

Turbulence and wave particle interactions in solar-terrestrial plasmas

NASA Technical Reports Server (NTRS)

Dulk, G. A.; Goldman, M. V.; Toomre, J.

1985-01-01

Activities in the following study areas are reported: (1) particle and wave processes in solar flares; (2) solar convection zone turbulence; and (3) solar radiation emission. To investigate the amplification of cyclotron maser radiation in solar flares, a radio frequency. (RF) heating model was developed for the corona surrounding the energy release site. Then nonlinear simulations of compressible convection display prominent penetration by plumes into regions of stable stratification at the base of the solar convection zone, leading to the excitation of internal gravity waves there. Lastly, linear saturation of electron-beam-driven Langmuir waves by ambient density fluctuations, nonlinear saturation by strong turbulence processes, and radiation emission mechanisms are examined. An additional section discusses solar magnetic fields and hydromagnetic waves in inhomogeneous media, and the effect of magnetic fields on stellar oscillation.

Spectral variability of photospheric radiation due to faculae. I. The Sun and Sun-like stars

NASA Astrophysics Data System (ADS)

Norris, Charlotte M.; Beeck, Benjamin; Unruh, Yvonne C.; Solanki, Sami K.; Krivova, Natalie A.; Yeo, Kok Leng

2017-09-01

Context. Stellar spectral variability on timescales of a day and longer, arising from magnetic surface features such as dark spots and bright faculae, is an important noise source when characterising extra-solar planets. Current 1D models of faculae do not capture the geometric properties and fail to reproduce observed solar facular contrasts. Magnetoconvection simulations provide facular contrasts accounting for geometry. Aims: We calculate facular contrast spectra from magnetoconvection models of the solar photosphere with a view to improve (a) future parameter determinations for planets with early G type host stars and (b) reconstructions of solar spectral variability. Methods: Regions of a solar twin (G2, log g = 4.44) atmosphere with a range of initial average vertical magnetic fields (100 to 500 G) were simulated using a 3D radiation-magnetohydrodynamics code, MURaM, and synthetic intensity spectra were calculated from the ultraviolet (149.5 nm) to the far infrared (160 000 nm) with the ATLAS9 radiative transfer code. Nine viewing angles were investigated to account for facular positions across most of the stellar disc. Results: Contrasts of the radiation from simulation boxes with different levels of magnetic flux relative to an atmosphere with no magnetic field are a complicated function of position, wavelength and magnetic field strength that is not reproduced by 1D facular models. Generally, contrasts increase towards the limb, but at UV wavelengths a saturation and decrease are observed close to the limb. Contrasts also increase strongly from the visible to the UV; there is a rich spectral dependence, with marked peaks in molecular bands and strong spectral lines. At disc centre, a complex relationship with magnetic field was found and areas of strong magnetic field can appear either dark or bright, depending on wavelength. Spectra calculated for a wide variety of magnetic fluxes will also serve to improve total and spectral solar irradiance reconstructions.

Super-Eddington radiation transfer in soft gamma repeaters

NASA Technical Reports Server (NTRS)

Ulmer, Andrew

1994-01-01

Bursts from soft gamma repeaters (SGRs) have been shown to be super-Eddington by a factor of 1000 and have been persuasively associated with compact objects. Super-Eddington radiation transfer on the surface of a strongly magnetic (greater than or equal to 10(exp 13) G) neutron star is studied and related to the observational constraints on SGRs. In strong magnetic fields, Thompson scattering is suppressed in one polarization state, so super-Eddington fluxes can be radiated while the plasma remains in hydrostatic equilibrium. We discuss a model which offers a somewhat natural explanation for the observation that the energy spectra of bursts with varying intensity are similar. The radiation produced is found to be linearly polarized to one part in 1000 in a direction determined by the local magnetic field, and intensity variations between bursts are understood as a change in the radiating area on the source. The net polarization is inversely correlated with burst intensity. Further, it is shown that for radiation transfer calculations in limit of superstrong magnetic fields, it is sufficient to solve the radiation transfer for the low opacity state rather than the coupled equations for both. With this approximation, standard stellar atmosphere techniques are utilized to calculate the model energy spectrum.

The ultraviolet radiation environment in the habitable zones around low-mass exoplanet host stars

NASA Astrophysics Data System (ADS)

France, Kevin; Linsky, Jeffrey L.; Loyd, R. O. Parke

2014-11-01

The EUV (200-911 Å), FUV (912-1750 Å), and NUV (1750-3200 Å) spectral energy distribution of exoplanet host stars has a profound influence on the atmospheres of Earth-like planets in the habitable zone. The stellar EUV radiation drives atmospheric heating, while the FUV (in particular, Ly α) and NUV radiation fields regulate the atmospheric chemistry: the dissociation of H2O and CO2, the production of O2 and O3, and may determine the ultimate habitability of these worlds. Despite the importance of this information for atmospheric modeling of exoplanetary systems, the EUV/FUV/NUV radiation fields of cool (K and M dwarf) exoplanet host stars are almost completely unconstrained by observation or theory. We present observational results from a Hubble Space Telescope survey of M dwarf exoplanet host stars, highlighting the importance of realistic UV radiation fields for the formation of potential biomarker molecules, O2 and O3. We conclude by describing preliminary results on the characterization of the UV time variability of these sources.

Gas Heating, Chemistry and Photoevaporation in Protostellar Disks

NASA Technical Reports Server (NTRS)

Hollenbach, David

2004-01-01

We model the thermal balance, the chemistry, and the radiative transfer in dusty disks orbiting young, low mass stars. These models are motivated by observations of infrared and ultraviolet transitions of H2 from protoplanetary disks, as well as millimeter and submillimeter observations of other molecules such as CO, and infrared continuum observations of the dust. The dust grains are heated primarily by the stellar radiation and the infrared radiation field produced by the dust itself. The gas is heated by collisions with warmer dust grains, X-rays from the region close to the stellar surface, UV pumping of hydrogen molecules, and the grain photoelectric heating mechanism initiated by UV photons from the central star. We treat cases where the gas to dust ratio is high, because the dust has settled to the midplane and coagulated into relatively large objects. We discuss situations in which the infrared emission from H2 can be detected, and how the comparison of the observations with our models can deduce physical parameters such as the mass and the density and temperature distribution of the gas.

PPPC 4 DM secondary: a Poor Particle Physicist Cookbook for secondary radiation from Dark Matter

NASA Astrophysics Data System (ADS)

Buch, Jatan; Cirelli, Marco; Giesen, Gaëlle; Taoso, Marco

2015-09-01

We enlarge the set of recipes and ingredients at disposal of any poor particle physicist eager to cook up signatures from weak-scale Dark Matter models by computing two secondary emissions due to DM particles annihilating or decaying in the galactic halo, namely the radio signals from synchrotron emission and the gamma rays from bremsstrahlung. We consider several magnetic field configurations and propagation scenarios for electrons and positrons. We also provide an improved energy loss function for electrons and positrons in the Galaxy, including synchrotron losses in the different configurations, bremsstrahlung losses, ionization losses and Inverse Compton losses with an updated InterStellar Radiation Field.

The evolution of stable magnetic fields in stars: an analytical approach

NASA Astrophysics Data System (ADS)

Mestel, Leon; Moss, David

2010-07-01

The absence of a rigorous proof of the existence of dynamically stable, large-scale magnetic fields in radiative stars has been for many years a missing element in the fossil field theory for the magnetic Ap/Bp stars. Recent numerical simulations, by Braithwaite & Spruit and Braithwaite & Nordlund, have largely filled this gap, demonstrating convincingly that coherent global scale fields can survive for times of the order of the main-sequence lifetimes of A stars. These dynamically stable configurations take the form of magnetic tori, with linked poloidal and toroidal fields, that slowly rise towards the stellar surface. This paper studies a simple analytical model of such a torus, designed to elucidate the physical processes that govern its evolution. It is found that one-dimensional numerical calculations reproduce some key features of the numerical simulations, with radiative heat transfer, Archimedes' principle, Lorentz force and Ohmic decay all playing significant roles.

Using Stellar Spectra to Illustrate Thermal Radiation Laws

ERIC Educational Resources Information Center

Kaltcheva, N. T.; Pritzl, B. J.

2018-01-01

Stars are point-source emitters that are the closest to the definition of a blackbody in comparison to all other similar sources of radiation found in nature. Existing libraries on stellar spectra are thus a valuable resource that can be used to introduce the laws of thermal radiation in a classroom setting. In this article we briefly describe…

The Little Engines That Could? Globular Clusters Contribute Significantly to Reionization-era Star Formation

NASA Astrophysics Data System (ADS)

Boylan-Kolchin, Michael

2018-06-01

Metal-poor globular clusters (GCs) are both numerous and ancient, which indicates that they may be important contributors to ionizing radiation in the reionization era. Starting from the observed number density and stellar mass function of old GCs at z = 0, I compute the contribution of GCs to ultraviolet luminosity functions (UVLFs) in the high-redshift Universe (10 ≳ z ≳ 4). Even under absolutely minimal assumptions - no disruption of GCs and no reduction in GC stellar mass from early times to the present - GC star formation contributes non-negligibly to the UVLF at luminosities that are accessible to the Hubble Space Telescope (HST; M1500 ≈ -17). If the stellar masses of GCs were significantly higher in the past, as is predicted by most models explaining GC chemical anomalies, then GCs dominate the UV emission from many galaxies in existing deep-field observations. On the other hand, it is difficult to reconcile observed UVLFs with models requiring stellar masses at birth that exceed present-day stellar masses by more than a factor of 5. The James Webb Space Telescope (JWST) will be able to directly detect individual GCs at z ˜ 6 in essentially all bright galaxies, and many galaxies below the knee of the UVLF, for most of the scenarios considered here. The properties of a subset of high-redshift sources with -19 ≲ M_{1500} ≲ -14 in HST lensing fields indicate that they may actually be GCs in formation.

The role of non-ionizing radiation pressure in star formation: the stability of cores and filaments

NASA Astrophysics Data System (ADS)

Seo, Young Min; Youdin, Andrew N.

2016-09-01

Stars form when filaments and dense cores in molecular clouds fragment and collapse due to self-gravity. In the most basic analyses of gravitational stability, the competition between self-gravity and thermal pressure sets the critical (I.e. maximum stable) mass of spheres and the critical line density of cylinders. Previous work has considered additional support from magnetic fields and turbulence. Here, we consider the effects of non-ionizing radiation, specifically the inward radiation pressure force that acts on dense structures embedded in an isotropic radiation field. Using hydrostatic, isothermal models, we find that irradiation lowers the critical mass and line density for gravitational collapse, and can thus act as a trigger for star formation. For structures with moderate central densities, ˜103 cm-3, the interstellar radiation field in the Solar vicinity has an order unity effect on stability thresholds. For more evolved objects with higher central densities, a significant lowering of stability thresholds requires stronger irradiation, as can be found closer to the Galactic centre or near stellar associations. Even when strong sources of ionizing radiation are absent or extincted, our study shows that interstellar irradiation can significantly influence the star formation process.

Distribution of forbidden neutral carbon emission in the ring nebula (NGC 6720)

NASA Technical Reports Server (NTRS)

Jewitt, D. C.; Danielson, G. E.; Kupferman, P. N.; Maran, S. P.

1983-01-01

The spatial distribution of forbidden C I 9823, 9850 A emission in NGC 6720 is reported. Like forbidden O I, the forbidden C I radiation appears enhanced in the region of the bright filaments. A few percent of the carbon atoms in the filaments are neutral. The neutral fraction is consistent with ionization equilibrium calculations made under the assumption of complete shielding of direct stellar radiation by hydrogen. The observed carbon lines are excited by photoelectrons produced from hydrogen by the nebular diffuse radiation field. The forbidden C I observations confirm that the filaments in NGC 6720 are regions of locally enhanced shielding.

Resolving polarized stellar features thanks to polarimetric interferometry

NASA Astrophysics Data System (ADS)

Rousselet-Perraut, Karine; Chesneau, Olivier; Vakili, Farrokh; Mourard, Denis; Janel, Sebastien; Lavaud, Laurent; Crocherie, Axel

2003-02-01

Polarimetry is a powerful means for detecting and constraining various physical phenomena, such as scattering processes or magnetic fields, occuring in a large panel of stellar objects: extended atmospheres of hot stars, CP stars, Young Stellar Objects, Active Galaxy Nuclei, ... However, the lack of angular resolution is generally a strong handicap to drastically constrain the physical parameters and the geometry of the polarizing phenomena because of the cancelling of the polarized signal. In fact, even if stellar features are strongly polarized, the (spectro-)polarimetric signal integrated over the stellar surface rarely exceeds few percents. Coupling polarimetric and interferometric devices allows to resolve these local polarized structures and thus to constrain complex patchy stellar surfaces and/or environments such as disk topology in T Tauri stars, hot stars radiative winds or oscillations in Be star envelopes. In this article, we explain how interfero-polarimetric observables, basically the contrast and the position of the interference fringe patterns versus polarization (and even versus wavelength) are powerful to address the above scientific drivers and we emphasize on the key point of instrumental and data calibrations: since interferometric measurements are differential ones between 2 or more beams, this strongly relaxes the calibration requirements for the fringe phase observable. Prospects induced by the operation of the optical aperture synthesis arrays are also discussed.

On the stability of radiation-pressure-dominated cavities

NASA Astrophysics Data System (ADS)

Kuiper, R.; Klahr, H.; Beuther, H.; Henning, Th.

2012-01-01

Context. When massive stars exert a radiation pressure onto their environment that is higher than their gravitational attraction (super-Eddington condition), they launch a radiation-pressure-driven outflow, which creates cleared cavities. These cavities should prevent any further accretion onto the star from the direction of the bubble, although it has been claimed that a radiative Rayleigh-Taylor instability should lead to the collapse of the outflow cavity and foster the growth of massive stars. Aims: We investigate the stability of idealized radiation-pressure-dominated cavities, focusing on its dependence on the radiation transport approach used in numerical simulations for the stellar radiation feedback. Methods: We compare two different methods for stellar radiation feedback: gray flux-limited diffusion (FLD) and ray-tracing (RT). Both methods are implemented in our self-gravity radiation hydrodynamics simulations for various initial density structures of the collapsing clouds, eventually forming massive stars. We also derive simple analytical models to support our findings. Results: Both methods lead to the launch of a radiation-pressure-dominated outflow cavity. However, only the FLD cases lead to prominent instability in the cavity shell. The RT cases do not show such instability; once the outflow has started, it precedes continuously. The FLD cases display extended epochs of marginal Eddington equilibrium in the cavity shell, making them prone to the radiative Rayleigh-Taylor instability. In the RT cases, the radiation pressure exceeds gravity by 1-2 orders of magnitude. The radiative Rayleigh-Taylor instability is then consequently suppressed. It is a fundamental property of the gray FLD method to neglect the stellar radiation temperature at the location of absorption and thus to underestimate the opacity at the location of the cavity shell. Conclusions: Treating the stellar irradiation in the gray FLD approximation underestimates the radiative forces acting on the cavity shell. This can lead artificially to situations that are affected by the radiative Rayleigh-Taylor instability. The proper treatment of direct stellar irradiation by massive stars is crucial for the stability of radiation-pressure-dominated cavities. Movies are available in electronic form at http://www.aanda.org

Radiation-driven winds of hot stars. V - Wind models for central stars of planetary nebulae

NASA Technical Reports Server (NTRS)

Pauldrach, A.; Puls, J.; Kudritzki, R. P.; Mendez, R. H.; Heap, S. R.

1988-01-01

Wind models using the recent improvements of radiation driven wind theory by Pauldrach et al. (1986) and Pauldrach (1987) are presented for central stars of planetary nebulae. The models are computed along evolutionary tracks evolving with different stellar mass from the Asymptotic Giant Branch. We show that the calculated terminal wind velocities are in agreement with the observations and allow in principle an independent determination of stellar masses and radii. The computed mass-loss rates are in qualitative agreement with the occurrence of spectroscopic stellar wind features as a function of stellar effective temperature and gravity.

An iterative phase-space explicit discontinuous Galerkin method for stellar radiative transfer in extended atmospheres

NASA Astrophysics Data System (ADS)

de Almeida, Valmor F.

2017-07-01

A phase-space discontinuous Galerkin (PSDG) method is presented for the solution of stellar radiative transfer problems. It allows for greater adaptivity than competing methods without sacrificing generality. The method is extensively tested on a spherically symmetric, static, inverse-power-law scattering atmosphere. Results for different sizes of atmospheres and intensities of scattering agreed with asymptotic values. The exponentially decaying behavior of the radiative field in the diffusive-transparent transition region, and the forward peaking behavior at the surface of extended atmospheres were accurately captured. The integrodifferential equation of radiation transfer is solved iteratively by alternating between the radiative pressure equation and the original equation with the integral term treated as an energy density source term. In each iteration, the equations are solved via an explicit, flux-conserving, discontinuous Galerkin method. Finite elements are ordered in wave fronts perpendicular to the characteristic curves so that elemental linear algebraic systems are solved quickly by sweeping the phase space element by element. Two implementations of a diffusive boundary condition at the origin are demonstrated wherein the finite discontinuity in the radiation intensity is accurately captured by the proposed method. This allows for a consistent mechanism to preserve photon luminosity. The method was proved to be robust and fast, and a case is made for the adequacy of parallel processing. In addition to classical two-dimensional plots, results of normalized radiation intensity were mapped onto a log-polar surface exhibiting all distinguishing features of the problem studied.

Radiative Feedback from Primordial Protostars and Final Mass of the First Stars

NASA Technical Reports Server (NTRS)

Hosokawa, Takashi; Omukai, Kazuyuki; Yoshida, Naoki; Yorke, Harold W.

2012-01-01

In this contribution, we review our efforts toward understanding the typical mass-scale of primordial stars. Our direct numerical simulations show that, in both of Population III.1 and III.2 cases, strong UV stellar radiative feedback terminatesmass accretion onto a protostar.AnHII region formed around the protostar very dynamically expands throughout the gas accreting envelope, which cuts off the gas supply to a circumstellar disk. The disk is exposed to the stellar UV radiation and loses its mass by photoevaporation. The derived final masses are 43 Stellar Mass and 17 Stellar Mass in our fiducial Population III.1 and III.2 cases. Much more massive stars should form in other exceptional conditions. In atomic-cooling halos where H2 molecules are dissociated, for instance, a protostar grows via very rapid mass accretion with the rates M* approx. 0.1 - 1 Stellar Mass/yr. Our newstellar evolution calculations show that the protostar significantly inflates and never contracts to reach the ZAMS stage in this case. Such the "supergiant protostars" have very low UV luminosity, which results in weak radiative feedback against the accretion flow. In the early universe, supermassive stars formed through this process might provide massive seeds of supermassive black holes.

YOUNG STELLAR CLUSTERS WITH A SCHUSTER MASS DISTRIBUTION. I. STATIONARY WINDS

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

Palous, Jan; Wuensch, Richard; Hueyotl-Zahuantitla, Filiberto

2013-08-01

Hydrodynamic models for spherically symmetric winds driven by young stellar clusters with a generalized Schuster stellar density profile are explored. For this we use both semi-analytic models and one-dimensional numerical simulations. We determine the properties of quasi-adiabatic and radiative stationary winds and define the radius at which the flow turns from subsonic to supersonic for all stellar density distributions. Strongly radiative winds significantly diminish their terminal speed and thus their mechanical luminosity is strongly reduced. This also reduces their potential negative feedback into their host galaxy interstellar medium. The critical luminosity above which radiative cooling becomes dominant within the clusters,more » leading to thermal instabilities which make the winds non-stationary, is determined, and its dependence on the star cluster density profile, core radius, and half-mass radius is discussed.« less

Variable interstellar radiation fields in simulated dwarf galaxies: supernovae versus photoelectric heating

NASA Astrophysics Data System (ADS)

Hu, Chia-Yu; Naab, Thorsten; Glover, Simon C. O.; Walch, Stefanie; Clark, Paul C.

2017-10-01

We present high-resolution hydrodynamical simulations of isolated dwarf galaxies including self-gravity, non-equilibrium cooling and chemistry, interstellar radiation fields (ISRF) and shielding, star formation, and stellar feedback. This includes spatially and temporally varying photoelectric (PE) heating, photoionization, resolved supernova (SN) blast waves and metal enrichment. A new flexible method to sample the stellar initial mass function allows us to follow the contribution to the ISRF, the metal output and the SN delay times of individual massive stars. We find that SNe play the dominant role in regulating the global star formation rate, shaping the multiphase interstellar medium (ISM) and driving galactic outflows. Outflow rates (with mass-loading factors of a few) and hot gas fractions of the ISM increase with the number of SNe exploding in low-density environments where radiative energy losses are low. While PE heating alone can suppress star formation as efficiently as SNe alone can do, it is unable to drive outflows and reproduce the multiphase ISM that emerges naturally whenever SNe are included. We discuss the potential origins for the discrepancy between our results and another recent study that claimed that PE heating dominates over SNe. In the absence of SNe and photoionization (mechanisms to disperse dense clouds), the impact of PE heating is highly overestimated owing to the (unrealistic) proximity of dense gas to the radiation sources. This leads to a substantial boost of the infrared continuum emission from the UV-irradiated dust and a far-infrared line-to-continuum ratio too low compared to observations.

Basic research in solar physics

NASA Technical Reports Server (NTRS)

Linsky, Jeffrey L.

1991-01-01

This grant, dating back more than 20 years has supported a variety of investigations of the chromospheres and coronae of the Sun and related cool stars by the Principal Investigator, his postdocs and graduate students, and colleagues at other institutions. This work involved studies of radiative transfer and spectral line formation theory, and the application of these techniques to the analysis of spectra obtained from space and ground-based observatories in the optical, ultraviolet, x-ray and radio portions of the spectrum. Space observations have included the analysis of spectra from OSO-7, Skylab, SMM, and the HRTS rocket experiments. Recent work has concentrated on the interaction of magnetic fields, plasma and radiation in the outer atmospheres of the Sun and other magnetically active stars with different fundamental parameters. Our study of phenomena common to the Sun and stars, the 'solar-stellar connection', can elucidate the fundamental physics, because spatially-resolved observations of the Sun provide us with the 'groundtruth,' while interpretation of stellar data permit us to isolate those parameters critical to stellar activity. Recently, we have studied the differences in physical properties between solar regions of high magnetic flux density and the surrounding plasma. High-resolution CN and CO spectroheliograms have been used to model the thermal inhomogeneities driven by unstable CO cooling, and we have analyzed spatially resolved UV spectra from HRTS to model the thermal structure and energy balance of small-scale structures. The study of nonlinear relations between atmospheric radiative losses and the photospheric magnetic flux density has been continued. We have also proposed a new model for the decay of plages by random walk diffusion of magnetic flux. Our analysis of phenomena common to the Sun and stars included the application of available spectroscopic diagnostics, establishing evidence that the atmospheres of the least active stars are heated at a 'basal' rate that is also found in the centers of solar supergranules, and using the Doppler-imaging technique to measure the position, size, and brightness of stellar active regions. We are computing multi-component models for solar and stellar atmospheres, and models for coronal loops and for the transition-region down flows. The study of solar and stellar flares permits us to assess the role of turbulent energy transport, to pinpoint the mechanism behind Type I radio bursts, to determine whether plasma radiation or cyclotron maser is responsible for microwave flares on M dwarfs, and to extend our knowledge of the basic physics pertinent to cyclotron-maser processes operating on the Sun.

NONLINEAR EVOLUTION OF THE RADIATION-DRIVEN MAGNETO-ACOUSTIC INSTABILITY

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

Fernandez, Rodrigo; Socrates, Aristotle

2013-04-20

We examine the nonlinear development of unstable magnetosonic waves driven by a background radiative flux-the radiation-driven magneto-acoustic instability (RMI, a.k.a. the ''photon bubble'' instability). The RMI may serve as a persistent source of density, radiative flux, and magnetic field fluctuations in stably stratified, optically thick media. The conditions for instability are present in a variety of astrophysical environments and do not require the radiation pressure to dominate or the magnetic field to be strong. Here, we numerically study the saturation properties of the RMI, covering three orders of magnitude in the relative strength of radiation, magnetic field, and gas energies.more » Two-dimensional, time-dependent radiation-magnetohydrodynamic simulations of local, stably stratified domains are conducted with Zeus-MP in the optically thick, highly conducting limit. Our results confirm the theoretical expectations of Blaes and Socrates in that the RMI operates even in gas-pressure-dominated environments that are weakly magnetized. The saturation amplitude is a monotonically increasing function of the ratio of radiation to gas pressure. Keeping this ratio constant, we find that the saturation amplitude peaks when the magnetic pressure is comparable to the radiation pressure. We discuss the implications of our results for the dynamics of magnetized stellar envelopes, where the RMI should act as a source of sub-photospheric perturbations.« less

The effect of oblateness and gravity darkening on the radiation driving in winds from rapidly rotating B stars

NASA Technical Reports Server (NTRS)

Cranmer, Steven R.; Owocki, Stanley P.

1995-01-01

We calculate the radiative driving force for winds around rapidly rotating oblate B stars, and we estimate the impact these forces should have on the production of a wind compressed disk. The effects of limb darkening, gravity darkening, oblateness, and an arbitrary wind velocity field are included in the computation of vector 'oblate finite disk' (OFD) factors, which depend on both radius and colatitude in the wind. The impact of limb darkening alone, with or without rotation, can increase the mass loss by as much as 10% over values computed using the standard uniformly bright spherical finite disk factor. For rapidly rotating stars, limb darkening makes 'sub-stellar' gravity darkening the dominant effect in the radial and latitudinal OFD factors, and lessens the impact of gravity darkening at other visible latitudes (nearer to the oblate limb). Thus, the radial radiative driving is generally stronger over the poles and weaker over the equator, following the gravity darkening at these latitudes. The nonradial radiative driving is considerably smaller in magnitude than the radial component, but is directed both away from the equatorial plane and in a retrograde azimuthal direction, acting to decrease the effective stellar rotation velocity. These forces thus weaken the equatorward wind compression compared to wind models computed with nonrotating finite disk factors.

Long-Lived Inverse Chirp Signals from Core-Collapse in Massive Scalar-Tensor Gravity

NASA Astrophysics Data System (ADS)

Sperhake, Ulrich; Moore, Christopher J.; Rosca, Roxana; Agathos, Michalis; Gerosa, Davide; Ott, Christian D.

2017-11-01

This Letter considers stellar core collapse in massive scalar-tensor theories of gravity. The presence of a mass term for the scalar field allows for dramatic increases in the radiated gravitational wave signal. There are several potential smoking gun signatures of a departure from general relativity associated with this process. These signatures could show up within existing LIGO-Virgo searches.

Birth of millisecond pulsars in globular clusters

NASA Technical Reports Server (NTRS)

Grindlay, J. E.; Bailyn, C. D.

1988-01-01

It is argued here that accretion-induced collapse of white dwarfs in binaries can form millisecond pulsars directly without requiring a precursor low-mass X-ray binary stage. Ablation of the precollapse binary companion by the millisecond pulsar's radiation field, a process invoked to explain some of the characteristics of the recently discovered eclipsing millisecond pulsar, can then yield isolated neutron stars witout requiring an additional stellar encounter.

The influence of radiative core growth on coronal X-ray emission from pre-main-sequence stars

NASA Astrophysics Data System (ADS)

Gregory, Scott G.; Adams, Fred C.; Davies, Claire L.

2016-04-01

Pre-main-sequence (PMS) stars of mass ≳0.35 M⊙ transition from hosting fully convective interiors to configurations with a radiative core and outer convective envelope during their gravitational contraction. This stellar structure change influences the external magnetic field topology and, as we demonstrate herein, affects the coronal X-ray emission as a stellar analogue of the solar tachocline develops. We have combined archival X-ray, spectroscopic, and photometric data for ˜1000 PMS stars from five of the best studied star-forming regions: the Orion Nebula Cluster, NGC 2264, IC 348, NGC 2362, and NGC 6530. Using a modern, PMS calibrated, spectral type-to-effective temperature and intrinsic colour scale, we de-redden the photometry using colours appropriate for each spectral type, and determine the stellar mass, age, and internal structure consistently for the entire sample. We find that PMS stars on Henyey tracks have, on average, lower fractional X-ray luminosities (LX/L*) than those on Hayashi tracks, where this effect is driven by changes in LX. X-ray emission decays faster with age for higher mass PMS stars. There is a strong correlation between L* and LX for Hayashi track stars but no correlation for Henyey track stars. There is no correlation between LX and radiative core mass or radius. However, the longer stars have spent with radiative cores, the less X-ray luminous they become. The decay of coronal X-ray emission from young early K to late G-type PMS stars, the progenitors of main-sequence A-type stars, is consistent with the dearth of X-ray detections of the latter.

A Reduced-order NLTE Kinetic Model for Radiating Plasmas of Outer Envelopes of Stellar Atmospheres

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

Munafò, Alessandro; Mansour, Nagi N.; Panesi, Marco, E-mail: munafo@illinois.edu, E-mail: nagi.n.mansour@nasa.gov, E-mail: m.panesi@illinois.edu

The present work proposes a self-consistent reduced-order NLTE kinetic model for radiating plasmas found in the outer layers of stellar atmospheres. A detailed collisional-radiative kinetic mechanism is constructed by leveraging the most up-to-date set of ab initio and experimental data available in the literature. This constitutes the starting point for the derivation of a reduced-order model, obtained by lumping the bound energy states into groups. In order to determine the needed thermo-physical group properties, uniform and Maxwell–Boltzmann energy distributions are used to reconstruct the energy population of each group. Finally, the reduced set of governing equations for the material gasmore » and the radiation field is obtained based on the moment method. Applications consider the steady flow across a shock wave in partially ionized hydrogen. The results clearly demonstrate that adopting a Maxwell–Boltzmann grouping allows, on the one hand, for a substantial reduction of the number of unknowns and, on the other, to maintain accuracy for both gas and radiation quantities. Also, it is observed that, when neglecting line radiation, the use of two groups already leads to a very accurate resolution of the photo-ionization precursor, internal relaxation, and radiative cooling regions. The inclusion of line radiation requires adopting just one additional group to account for optically thin losses in the α , β , and γ lines of the Balmer and Paschen series. This trend has been observed for a wide range of shock wave velocities.« less

A plausible energy source and structure for quasi-stellar objects

NASA Technical Reports Server (NTRS)

Daltabuit, E.; Cox, D.

1972-01-01

If a collision of two large, massive, fast gas clouds occurs, their kinetic energy is converted to radiation in a pair of shock fronts at their interface. The resulting structure is described, and the relevance of this as a radiation source for quasi-stellar objects is considered.

Strong magnetic field generated by the extreme oxygen-rich red supergiant VY Canis Majoris

NASA Astrophysics Data System (ADS)

Shinnaga, Hiroko; Claussen, Mark J.; Yamamoto, Satoshi; Shimojo, Masumi

2017-12-01

Evolved stars experience high mass-loss rates forming thick circumstellar envelopes (CSEs). The circumstellar material is made of the result of stellar nucleosynthesis and, as such, plays a crucial role in the chemical evolution of galaxies and the universe. Since asymmetric geometries of CSEs are common, and with very complex structures for some cases, radiative pressure from the stars can explain only a small portion of the mass-loss processes; thus the essential driving mechanism is still unknown, particularly for high-mass stars. Here we report on magnetic field measurements associated with the well-known extreme red supergiant (RSG) VY Canis Majoris (VY CMa). We measured the linear polarization and the Zeeman splitting of the SiO v = 0, J = 1-0 transition using a sensitive radio interferometer. The measured magnetic field strengths are surprisingly high; their upper limits range between 150 and 650 G within 530 au (˜80 R*) of the star. The lower limit of the field strength is expected to be at least ˜10 G based on the high degree of linear polarization. Since the field strengths are very high, the magnetic field must be a key element in understanding the stellar evolution of VY CMa, as well as the dynamical and chemical evolution of the complex CSE of the star. M-type RSGs, with large stellar surface, were thought to be very slow rotators. This would seem to make a dynamo in operation difficult, and would also dilute any fossil magnetic field. At least for VY CMa, we expect that powerful dynamo processes must still be active to generate the intense magnetic field.

Radiative Processes in Jets

NASA Astrophysics Data System (ADS)

Vila, Gabriela S.

Relativistic jets and collimated outflows are ubiquitous phenomena in astrophysical settings, from young stellar objects up to Active Galactic Nuclei. The observed emission from some of these jets can cover the whole electromagnetic spectrum, from radio to gamma-rays. The relevant features of the spectral energy distributions depend on the nature of the source and on the characteristics of the surrounding environment. Here the author reviews the main physical processes that command the interactions between populations of relativistic particles locally accelerated in the jets, with matter, radiation and magnetic fields. Special attention is given to the conditions that lead to the dominance of the different radiative mechanisms. Examples from various types of sources are used to illustrate these effects.

Evolved Late-Type Star FUV Spectra: Mass Loss and Fluorescence

NASA Technical Reports Server (NTRS)

Harper, Graham M.

2005-01-01

This proposal was for a detailed analysis of the far ultraviolet (FUV) photoionizing radiation that provides crucial input physics for mass loss studies, e.g., observations of the flux below 10448, allow us to constrain the Ca II/Ca III balance and make significant progress beyond previous optical studies on stellar mass loss and circumstellar photochemistry. Our targets selection provided good spectral-type coverage required to help unravel the Ca II/Ca III balance as the mass-loss rates increase by over three orders of magnitude from K5 III to M5 III. We also explored the relationship between the FUV radiation field and other UV diagnostics to allow us to empirically estimate the FUV radiation field for the vast majority of stars which are too faint to be observed with FUSE, and to improve upon their uncertain mass-loss rates.

Post Common Envelope Binaries as probes of M dwarf stellar wind and habitable zone radiation environments

NASA Astrophysics Data System (ADS)

Wilson, David

2017-08-01

M dwarf stars are promising targets in the search for extrasolar habitable planets, as their small size and close-in habitable zones make the detection of Earth-analog planets easier than at Solar-type stars. However, the effects of the high stellar activity of M dwarf hosts has uncertain effects on such planets, and may render them uninhabitable. Studying stellar activity at M dwarfs is hindered by a lack of measurements of high-energy radiation, flare activity and, in particular, stellar wind rates. We propose to rectify this by observing a sample of Post Common Envelope Binaries (PCEBs) with HST and XMM-Newton. PCEBs consist of an M dwarf with a white dwarf companion, which experiences the same stellar wind and radiation environment as a close-in planet. The stellar wind of the M dwarf accretes onto the otherwise pure hydrogen atmosphere white dwarf, producing metal lines detectable with ultraviolet spectroscopy. The metal lines can be used to measure accretion rates onto the white dwarf, from with we can accurately infer the stellar wind mass loss rate of the M dwarf, along with abundances of key elements. Simultaneous observations with XMM-Newton will probe X-ray flare occurrence rate and strength, in addition to coronal temperatures. Performing these measurements over twelve PCEBs will provide a sample of M dwarf stellar wind strengths, flare occurrence and X-ray/UV activity that will finally shed light on the true habitability of planets around small stars.

Astrobiological Effects of Stellar Radiation in Circumstellar Environments

NASA Astrophysics Data System (ADS)

Cuntz, Manfred; Gurdemir, Levent; Guinan, Edward F.; Kurucz, Robert L.

2006-10-01

The centerpiece of all life on Earth is carbon-based biochemistry. Previous scientific research has suggested that biochemistry based on carbon may also play a decisive role in extraterrestrial life forms, i.e., alien life outside of Earth, if existent. In the following, we explore if carbon-based macromolecules (such as DNA) in the environments of stars other than the Sun are able to survive the effects of energetic stellar radiation, such as UV-C in the wavelength band between 200 and 290 nm. We focus on main-sequence stars akin to the Sun, but of hotter (F-type stars) and cooler (K- and M-type stars) surface temperature. Emphasis is placed on investigating the radiative environment in stellar habitable zones (HZs). Stellar habitable zones have an important relevance in astrobiology because they constitute circumstellar regions in which a planet of suitable size can have surface temperatures for water to exist in liquid form.

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

NASA Astrophysics Data System (ADS)

Heays, A. N.; Bosman, A. D.; van Dishoeck, E. F.

2017-06-01

A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic physical processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and for other wavelength-dependent radiation fields, including cool stellar and solar radiation, Lyman-α dominated radiation, and a cosmic-ray induced ultraviolet flux. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. Comparison with other databases such as PHIDRATES is made. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of these shielding types depends on the atom or molecule in question and the assumed dust optical properties. All of the new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of the calculated rates to variation of temperature and isotope, and uncertainties in measured or calculated cross sections, are tested and discussed. Tests were conducted on the new rates with an interstellar-cloud chemical model, and find general agreement (within a factor of two) in abundances obtained with the previous iteration of the Leiden database assuming an ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.

Radiatively driven winds from magnetic, fast-rotating stars

NASA Technical Reports Server (NTRS)

Nerney, S.

1986-01-01

An analytical procedure is developed to solve the magnetohydrodynamic equations for the stellar wind problem in the strong-magnetic field, optically thick limit for hot stars. The slow-mode, Alfven, and fast-mode critical points are modified by the radiation terms in the force equation but in a manner that can be treated relatively easily. Once the velocities at the critical points and the distances to the points are known, the streamline constants are determined in a straight-forward manner. This allows the structure of the wind to be elucidated without recourse to complicated computational schemes.

OUTWARD MOTION OF POROUS DUST AGGREGATES BY STELLAR RADIATION PRESSURE IN PROTOPLANETARY DISKS

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

Tazaki, Ryo; Nomura, Hideko, E-mail: rtazaki@kusastro.kyoto-u.ac.jp

2015-02-01

We study the dust motion at the surface layer of protoplanetary disks. Dust grains in the surface layer migrate outward owing to angular momentum transport via gas-drag force induced by the stellar radiation pressure. In this study we calculate the mass flux of the outward motion of compact grains and porous dust aggregates by the radiation pressure. The radiation pressure force for porous dust aggregates is calculated using the T-Matrix Method for the Clusters of Spheres. First, we confirm that porous dust aggregates are forced by strong radiation pressure even if they grow to be larger aggregates, in contrast tomore » homogeneous and spherical compact grains, for which radiation pressure efficiency becomes lower when their sizes increase. In addition, we find that the outward mass flux of porous dust aggregates with monomer size of 0.1 μm is larger than that of compact grains by an order of magnitude at the disk radius of 1 AU, when their sizes are several microns. This implies that large compact grains like calcium-aluminum-rich inclusions are hardly transported to the outer region by stellar radiation pressure, whereas porous dust aggregates like chondritic-porous interplanetary dust particles are efficiently transported to the comet formation region. Crystalline silicates are possibly transported in porous dust aggregates by stellar radiation pressure from the inner hot region to the outer cold cometary region in the protosolar nebula.« less

Magnetic field structure and evolution features of selected stars. III.

NASA Astrophysics Data System (ADS)

Glagolevskij, Yu. V.

2016-01-01

We present the results of modeling for about a hundred magnetic stars. It is shown that the dipole representation of magnetic field structures describes the distribution of the magnetic field over stellar surfaces fairly well. We analyze some patterns which support the relic hypothesis of magnetic field formation.Arguments are given in favor of the assumption that themain properties ofmagnetic stars—slow rotation, predominant orientation of magnetic field lines along the plane of the rotation equator, complex internal structures of magnetic fields—are acquired in the process of gravitational collapse. There are no conditions for that in the non-stationary Hayashi phase and in the stage of a radiative young star.

Polarized radiation diagnostics of stellar magnetic fields

NASA Astrophysics Data System (ADS)

Mathys, Gautier

The main techniques used to diagnose magnetic fields in stars from polarimetric observations are presented. First, a summary of the physics of spectral line formation in the presence of a magnetic field is given. Departures from the simple case of linear Zeeman effect are briefly considered: partial Paschen-Back effect, contribution of hyperfine structure, and combined Stark and Zeeman effects. Important approximate solutions of the equation of transfer of polarized light in spectral lines are introduced. The procedure for disk-integration of emergent Stokes profiles, which is central to stellar magnetic field studies, is described, with special attention to the treatment of stellar rotation. This formalism is used to discuss the determination of the mean longitudinal magnetic field (through the photographic technique and through Balmer line photopolarimetry). This is done within the specific framework of Ap stars, which, with their unique large-scale organized magnetic fields, are an ideal laboratory for studies of stellar magnetism. Special attention is paid to those Ap stars whose magnetically split line components are resolved in high-dispersion Stokes I spectra, and to the determination of their mean magnetic field modulus. Various techniques of exploitation of the information contained in polarized spectral line profiles are reviewed: the moment technique (in particular, the determination of the crossover and of the mean quadratic field), Zeeman-Doppler imaging, and least-squares deconvolution. The prospects that these methods open for linear polarization studies are sketched. The way in which linear polarization diagnostics complement their Stokes I and V counterparts is emphasized by consideration of the results of broad band linear polarization measurements. Illustrations of the use of various diagnostics to derive properties of the magnetic fields of Ap stars are given. This is used to show the interest of deriving more physically realistic models of the geometric structure of these fields. How this can possibly be achieved is briefly discussed. An overview of the current status of polarimetric studies of magnetic fields in non-degenerate stars of other types is presented. The final section is devoted to magnetic fields of white dwarfs. Current knowledge of magnetic fields of isolated white dwarfs is briefly reviewed. Diagnostic techniques are discussed, with particular emphasis on the variety of physical processes to be considered for understanding of spectral line formation over the broad range of magnetic field strengths encountered in these stars.

Winds of Massive Magnetic Stars: Interacting Fields and Flow

NASA Astrophysics Data System (ADS)

Daley-Yates, S.; Stevens, I. R.

2018-01-01

We present results of 3D numerical simulations of magnetically confined, radiatively driven stellar winds of massive stars, conducted using the astrophysical MHD code Pluto, with a focus on understanding the rotational variability of radio and sub-mm emission. Radiative driving is implemented according to the Castor, Abbott and Klein theory of radiatively driven winds. Many magnetic massive stars posses a magnetic axis which is inclined with respect to the rotational axis. This misalignment leads to a complex wind structure as magnetic confinement, centrifugal acceleration and radiative driving act to channel the circumstellar plasma into a warped disk whose observable properties should be apparent in multiple wavelengths. This structure is analysed to calculate free-free thermal radio emission and determine the characteristic intensity maps and radio light curves.

A MODEL OF WHITE DWARF PULSAR AR SCORPII

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

Geng, Jin-Jun; Huang, Yong-Feng; Zhang, Bing, E-mail: gengjinjun@gmail.com, E-mail: hyf@nju.edu.cn, E-mail: zhang@physics.unlv.edu

2016-11-01

A 3.56 hr white dwarf (WD)–M dwarf (MD) close binary system, AR Scorpii, was recently reported to show pulsating emission in radio, IR, optical, and UV, with a 1.97 minute period, which suggests the existence of a WD with a rotation period of 1.95 minutes. We propose a model to explain the temporal and spectral characteristics of the system. The WD is a nearly perpendicular rotator, with both open field line beams sweeping the MD stellar wind periodically. A bow shock propagating into the stellar wind accelerates electrons in the wind. Synchrotron radiation of these shocked electrons can naturally accountmore » for the broadband (from radio to X-rays) spectral energy distribution of the system.« less

An iterative phase-space explicit discontinuous Galerkin method for stellar radiative transfer in extended atmospheres

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

de Almeida, Valmor F.

In this work, a phase-space discontinuous Galerkin (PSDG) method is presented for the solution of stellar radiative transfer problems. It allows for greater adaptivity than competing methods without sacrificing generality. The method is extensively tested on a spherically symmetric, static, inverse-power-law scattering atmosphere. Results for different sizes of atmospheres and intensities of scattering agreed with asymptotic values. The exponentially decaying behavior of the radiative field in the diffusive-transparent transition region, and the forward peaking behavior at the surface of extended atmospheres were accurately captured. The integrodifferential equation of radiation transfer is solved iteratively by alternating between the radiative pressure equationmore » and the original equation with the integral term treated as an energy density source term. In each iteration, the equations are solved via an explicit, flux-conserving, discontinuous Galerkin method. Finite elements are ordered in wave fronts perpendicular to the characteristic curves so that elemental linear algebraic systems are solved quickly by sweeping the phase space element by element. Two implementations of a diffusive boundary condition at the origin are demonstrated wherein the finite discontinuity in the radiation intensity is accurately captured by the proposed method. This allows for a consistent mechanism to preserve photon luminosity. The method was proved to be robust and fast, and a case is made for the adequacy of parallel processing. In addition to classical two-dimensional plots, results of normalized radiation intensity were mapped onto a log-polar surface exhibiting all distinguishing features of the problem studied.« less

An iterative phase-space explicit discontinuous Galerkin method for stellar radiative transfer in extended atmospheres

DOE PAGES

de Almeida, Valmor F.

2017-04-19

In this work, a phase-space discontinuous Galerkin (PSDG) method is presented for the solution of stellar radiative transfer problems. It allows for greater adaptivity than competing methods without sacrificing generality. The method is extensively tested on a spherically symmetric, static, inverse-power-law scattering atmosphere. Results for different sizes of atmospheres and intensities of scattering agreed with asymptotic values. The exponentially decaying behavior of the radiative field in the diffusive-transparent transition region, and the forward peaking behavior at the surface of extended atmospheres were accurately captured. The integrodifferential equation of radiation transfer is solved iteratively by alternating between the radiative pressure equationmore » and the original equation with the integral term treated as an energy density source term. In each iteration, the equations are solved via an explicit, flux-conserving, discontinuous Galerkin method. Finite elements are ordered in wave fronts perpendicular to the characteristic curves so that elemental linear algebraic systems are solved quickly by sweeping the phase space element by element. Two implementations of a diffusive boundary condition at the origin are demonstrated wherein the finite discontinuity in the radiation intensity is accurately captured by the proposed method. This allows for a consistent mechanism to preserve photon luminosity. The method was proved to be robust and fast, and a case is made for the adequacy of parallel processing. In addition to classical two-dimensional plots, results of normalized radiation intensity were mapped onto a log-polar surface exhibiting all distinguishing features of the problem studied.« less

Radiation pressure in super star cluster formation

NASA Astrophysics Data System (ADS)

Tsang, Benny T.-H.; Milosavljević, Miloš

2018-05-01

The physics of star formation at its extreme, in the nuclei of the densest and the most massive star clusters in the universe—potential massive black hole nurseries—has for decades eluded scrutiny. Spectroscopy of these systems has been scarce, whereas theoretical arguments suggest that radiation pressure on dust grains somehow inhibits star formation. Here, we harness an accelerated Monte Carlo radiation transport scheme to report a radiation hydrodynamical simulation of super star cluster formation in turbulent clouds. We find that radiation pressure reduces the global star formation efficiency by 30-35%, and the star formation rate by 15-50%, both relative to a radiation-free control run. Overall, radiation pressure does not terminate the gas supply for star formation and the final stellar mass of the most massive cluster is ˜1.3 × 106 M⊙. The limited impact as compared to in idealized theoretical models is attributed to a radiation-matter anti-correlation in the supersonically turbulent, gravitationally collapsing medium. In isolated regions outside massive clusters, where the gas distribution is less disturbed, radiation pressure is more effective in limiting star formation. The resulting stellar density at the cluster core is ≥108 M⊙ pc-3, with stellar velocity dispersion ≳ 70 km s-1. We conclude that the super star cluster nucleus is propitious to the formation of very massive stars via dynamical core collapse and stellar merging. We speculate that the very massive star may avoid the claimed catastrophic mass loss by continuing to accrete dense gas condensing from a gravitationally-confined ionized phase.

Simulating Convection in Stellar Envelopes

NASA Astrophysics Data System (ADS)

Tanner, Joel

2014-01-01

Understanding convection in stellar envelopes, and providing a mathematical description of it, would represent a substantial advance in stellar astrophysics. As one of the largest sources of uncertainty in stellar models, existing treatments of convection fail to account for many of the dynamical effects of convection, such as turbulent pressure and asymmetry in the velocity field. To better understand stellar convection, we must be able to study and examine it in detail, and one of the best tools for doing so is numerical simulation. Near the stellar surface, both convective and radiative process play a critical role in determining the structure and gas dynamics. By following these processes from first principles, convection can be simulated self-consistently and accurately, even in regions of inefficient energy transport where existing descriptions of convection fail. Our simulation code includes two radiative transfer solvers that are based on different assumptions and approximations. By comparing simulations that differ only in their respective radiative transfer methods, we are able to isolate the effect that radiative efficiency has on the structure of the superadiabatic layer. We find the simulations to be in good general agreement, but they show distinct differences in the thermal structure in the superadiabatic layer and atmosphere. Using the code to construct a grid of three-dimensional radiation hydrodynamic simulations, we investigate the link between convection and various chemical compositions. The stellar parameters correspond to main-sequence stars at several surface gravities, and span a range in effective temperatures (4500 < Teff < 6400). Different chemical compositions include four metallicities (Z = 0.040, 0.020, 0.010, 0.001), three helium abundances (Y = 0.1, 0.2, 0.3) and several levels of alpha-element enhancement. Our grid of simulations shows that various convective properties, such as velocity and the degree of superadiabaticity, are sensitive to changes in opacity which are in response to adjustments to the metallicity and helium abundance. We find that increasing the metallicity forces the location of the transition region to lower densities and pressures, and results in larger mean and turbulent velocities throughout the superadiabatic region. We also quantify the degree of convective overshoot in the atmosphere, and show that it increases with metallicity as well. The signature of helium differs from that of metallicity in the manner in which the photospheric velocity distribution is affected. We also find that helium abundance and surface gravity behave largely in similar ways, but differ in the way they affect the mean molecular weight. A simple model for spectral line formation suggests that the bisectors and absolute Doppler shifts of spectral lines depend on the helium abundance. We look at the effect of alpha-element enhancement and find that it has a considerably smaller effect on the convective dynamics in the superadiabatic layer compared to that of helium abundance. Improving the treatment of convection in stellar models remains one of the primary applications of RHD simulations. A simple and direct way to introduce the effect of 3D convection into 1D stellar models is through the surface boundary condition. Usually the atmospheric structure of a stellar model is defined beforehand in the form of a T-tau relation, and is kept fixed at chemical compositions and stages of evolution. Extracting mean atmospheric stratifications from simulations provides a means of introducing surface boundary conditions to stellar models that self-consistently include the effects of realistic convection and overshoot. We apply data from simulations to stellar models in this manner to measure how realistic atmospheric stratifications relate to the value of the mixing length parameter in calibrated stellar models. Moving beyond improving the surface boundary condition, we also explore a method for calibrating the mixing length parameter, which is relevant for improving the adiabatic structure of sub-photospheric convection. Since the MLT treatment of convection defines the thermal structure of the atmosphere and SAL arbitrarily, one strategy for calibrating the mixing length parameter is to tune it so that it matches the thermodynamics of the simulations. In particular, we consider adjusting the mixing length parameter such that the specific entropy of the model matches that of an equivalent simulation eliminates the need to arbitrarily set the parameter, and in principle will produce stellar models with more accurate radii. By examining simulations along contours in the log(g)-log(Teff) plane that correspond to the convective envelope adiabats, the variation in convective properties can be reduced to a simplified form that is more convenient for use in stellar models.

POLARIZED LINE FORMATION IN MOVING ATMOSPHERES WITH PARTIAL FREQUENCY REDISTRIBUTION AND A WEAK MAGNETIC FIELD

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

Sampoorna, M.; Nagendra, K. N., E-mail: sampoorna@iiap.res.in, E-mail: knn@iiap.res.in

2015-10-10

The dynamical state of the solar and stellar atmospheres depends on the macroscopic velocity fields prevailing within them. The presence of such velocity fields in the line formation regions strongly affects the polarized radiation field emerging from these atmospheres. Thus it becomes necessary to solve the radiative transfer equation for polarized lines in moving atmospheres. Solutions based on the “observer’s frame method” are computationally expensive to obtain, especially when partial frequency redistribution (PRD) in line scattering and large-amplitude velocity fields are taken into account. In this paper we present an efficient alternative method of solution, namely, the comoving frame technique,more » to solve the polarized PRD line formation problems in the presence of velocity fields. We consider one-dimensional planar isothermal atmospheres with vertical velocity fields. We present a study of the effect of velocity fields on the emergent linear polarization profiles formed in optically thick moving atmospheres. We show that the comoving frame method is far superior when compared to the observer’s frame method in terms of the computational speed and memory requirements.« less

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

NASA Astrophysics Data System (ADS)

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

2017-01-01

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

Theory of Bipolar Outflows from Accreting Hot Stars

NASA Astrophysics Data System (ADS)

Konigl, A.

1996-05-01

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

PANDORA, a new facility for interdisciplinary in-plasma physics

NASA Astrophysics Data System (ADS)

Mascali, D.; Musumarra, A.; Leone, F.; Romano, F. P.; Galatà, A.; Gammino, S.; Massimi, C.

2017-07-01

PANDORA, Plasmas for Astrophysics, Nuclear Decays Observation and Radiation for Archaeometry, is planned as a new facility based on a state-of-the-art plasma trap confining energetic plasma for performing interdisciplinary research in the fields of Nuclear Astrophysics, Astrophysics, Plasma Physics and Applications in Material Science and Archaeometry: the plasmas become the environment for measuring, for the first time, nuclear decay rates in stellar-like condition (such as 7Be decay and beta-decay involved in s-process nucleosynthesis), especially as a function of the ionization state of the plasma ions. These studies will give important contributions for addressing several astrophysical issues in both stellar and primordial nucleosynthesis environment ( e.g., determination of solar neutrino flux and 7Li Cosmological Problem), moreover the confined energetic plasma will be a unique light source for high-performance stellar spectroscopy measurements in the visible, UV and X-ray domains, offering advancements in observational astronomy. As to magnetic fields, the experimental validation of theoretical first- and second-order Landé factors will drive the layout of next-generation polarimetric units for the high-resolution spectrograph of the future giant telescopes. In PANDORA new plasma heating methods will be explored, that will push forward the ion beam output, in terms of extracted intensity and charge states. More, advanced and optimized injection methods of ions in an ECR plasma will be experimented, with the aim to optimize its capture efficiency. This will be applied to the ECR-based Charge Breeding technique, that will improve the performances of the SPES ISOL-facility at Laboratori Nazionali di Legnaro-INFN. Finally, PANDORA will be suitable for energy conversion, making the plasma a source of high-intensity electromagnetic radiation, for applications in material science and archaeometry.

Influence of stellar radiation pressure on flow structure in the envelope of hot-Jupiter HD 209458b

NASA Astrophysics Data System (ADS)

Cherenkov, A. A.; Bisikalo, D. V.; Kosovichev, A. G.

2018-03-01

Close-in exoplanets are subjected to extreme radiation of their host stars. Photometric observations of the hot-Jupiter HD 209458b transit by HST/STIS detected strong absorption in the Ly α line, thus indicating the existence of a hydrogen envelope extending beyond the Roche lobe. The gasdynamic modelling (Bisikalo et al.) showed that the stable structure of this envelope is maintained by the balance between the Roche lobe overfilling and stellar wind pressure. Obviously, the dynamics and stability of the envelope can be affected by stellar radiation pressure. Using 3D gasdynamic simulations, we study the impact of radiation pressure in the Ly α line on the envelope of hot-Jupiter HD 209458b, and show that the effect is not strong enough to significantly affect the gasdynamics in the system. For a detectable radiation pressure effect the intensity of the Ly α line has to be by two orders of magnitude greater.

The low-metallicity starburst NGC346: massive-star population and feedback

NASA Astrophysics Data System (ADS)

Oskinova, Lida

2017-08-01

The Small Magellanic Cloud (SMC) is ideal to study young, massive stars at low metallicity. The compact cluster NGC346 contains about half of all O-type stars in the entire SMC. The massive-star population of this cluster powers N66, the brightest and largest HII region in the SMC. We propose to use HST-STIS to slice NGC346 with 20 long-slit exposures, in order to obtain the UV spectra of most of the massive early-type stars of this cluster. Archival data of 13 exposures that cover already a minor part of this cluster will be included in our analyses. Our aim is to quantitatively analyze virtually the whole massive-star population of NGC346. We have already secured the optical spectra of all massive stars in the field with the integral-field spectrograph MUSE at the ESO-VLT. However, for the determination of the stellar-wind parameters, i.e. the mass-loss rates and the wind velocities, ultraviolet spectra are indispensable. Our advanced Potsdam Wolf-Rayet (PoWR) code will be used for modeling the stellar and wind spectra in the course of the analysis. Finally, we will obtain:(a) the fundamental stellar and wind parameters of all stars brighter than spectral type B2V in the field, which, e,g,, will constrain the initial mass function in this young low-metallicity starburst;(b) mass-loss rates of many more OB-type stars at SMC metallicity than hitherto known, allowing to better constrain their metallicity dependence;(c) the integrated feedback by ionizing radiation and stellar winds of the whole massive-star population of NGC346, which will be used as input to model the ecology of the giant HII region N66.These HST UV data will be of high legacy value.

AK Sco, First Detection of a Highly Disturbed Atmosphere in a Pre-Main-Sequence Close Binary

NASA Astrophysics Data System (ADS)

Gómez de Castro, Ana I.

2009-06-01

AK Sco is a unique source: a ~10 Myr old pre-main-sequence (PMS) spectroscopic binary composed of two nearly equal F5 stars that at periastron are separated by barely 11 stellar radii, so the stellar magnetospheres fill the Roche lobe at periastron. The orbit is not yet circularized (e = 0.47) and very strong tides are expected. This makes AK Sco the ideal laboratory to study the effect of gravitational tides in the stellar magnetic field building up during PMS evolution. In this Letter, the detection of a highly disturbed (σ sime 100 km s-1) and very dense atmosphere (n e = 1.6 × 1010 cm-3) is reported. Significant line broadening blurs any signs of ion belts or bow shocks in the spectrum of the atmospheric plasma. The radiative losses cannot be accounted for solely by the dissipation of energy from the tidal wave propagating in the stellar atmosphere or by the accreting material. The release of internal energy from the star seems to be the most likely source of the plasma heating. This is the first clear indication of a highly disturbed atmosphere surrounding a PMS close binary.

Polarized Continuum Radiation from Stellar Atmospheres

NASA Astrophysics Data System (ADS)

Harrington, J. Patrick

2015-10-01

Continuum scattering by free electrons can be significant in early type stars, while in late type stars Rayleigh scattering by hydrogen atoms or molecules may be important. Computer programs used to construct models of stellar atmospheres generally treat the scattering of the continuum radiation as isotropic and unpolarized, but this scattering has a dipole angular dependence and will produce polarization. We review an accurate method for evaluating the polarization and limb darkening of the radiation from model stellar atmospheres. We use this method to obtain results for: (i) Late type stars, based on the MARCS code models (Gustafsson et al. 2008), and (ii) Early type stars, based on the NLTE code TLUSTY (Lanz and Hubeny 2003). These results are tabulated at http://www.astro.umd.edu/~jph/Stellar_Polarization.html. While the net polarization vanishes for an unresolved spherical star, this symmetry is broken by rapid rotation or by the masking of part of the star by a binary companion or during the transit of an exoplanet. We give some numerical results for these last cases.

Non-LTE analysis of the Ofpe/WN9 star HDE 269227 (R84)

NASA Technical Reports Server (NTRS)

Schmutz, Werner; Leitherer, Claus; Hubeny, Ivan; Vogel, Manfred; Hamann, Wolf-Rainer

1991-01-01

The paper presents the results of a spectral analysis of the Ofpe/WN9 star HD 269227 (R84), which assumes a spherically expanding atmosphere to find solutions for equations of radiative transfer. The spectra of hydrogen and helium were predicted with a non-LTE model. Six stellar parameters were determined for R84. The shape of the velocity law is empirically found, since it can be probed from the terminal velocity of the wind. The six stellar parameters are further employed in a hydrodynamic model where stellar wind is assumed to be directed by radiation pressure, duplicating the mass-loss rate and the terminal wind velocity. The velocity laws found by computation and analysis are found to agree, supporting the theory of radiation-driven stellar wind. R84 is surmised to be a post-red supergiant which lost half of its initial mass, possibly during the red-supergiant phase. This mass loss is also suggested by its spectroscopic similarity to S Doradus.

The collapse of a molecular cloud core to stellar densities using radiation non-ideal magnetohydrodynamics

NASA Astrophysics Data System (ADS)

Wurster, James; Bate, Matthew R.; Price, Daniel J.

2018-04-01

We present results from radiation non-ideal magnetohydrodynamics (MHD) calculations that follow the collapse of rotating, magnetized, molecular cloud cores to stellar densities. These are the first such calculations to include all three non-ideal effects: ambipolar diffusion, Ohmic resistivity, and the Hall effect. We employ an ionization model in which cosmic ray ionization dominates at low temperatures and thermal ionization takes over at high temperatures. We explore the effects of varying the cosmic ray ionization rate from ζcr = 10-10 to 10-16 s-1. Models with ionization rates ≳10-12 s-1 produce results that are indistinguishable from ideal MHD. Decreasing the cosmic ray ionization rate extends the lifetime of the first hydrostatic core up to a factor of 2, but the lifetimes are still substantially shorter than those obtained without magnetic fields. Outflows from the first hydrostatic core phase are launched in all models, but the outflows become broader and slower as the ionization rate is reduced. The outflow morphology following stellar core formation is complex and strongly dependent on the cosmic ray ionization rate. Calculations with high ionization rates quickly produce a fast (≈14 km s-1) bipolar outflow that is distinct from the first core outflow, but with the lowest ionization rate, a slower (≈3-4 km s-1) conical outflow develops gradually and seamlessly merges into the first core outflow.

Subphotospheric fluctuations in magnetized radiative envelopes: contribution from unstable magnetosonic waves

NASA Astrophysics Data System (ADS)

Sen, Koushik; Fernández, Rodrigo; Socrates, Aristotle

2018-06-01

We examine the excitation of unstable magnetosonic waves in the radiative envelopes of intermediate- and high-mass stars with a magnetic field of ˜kG strength. Wind clumping close to the star and microturbulence can often be accounted for when including small-scale, subphotospheric density or velocity perturbations. Compressional waves - with wavelengths comparable to or shorter than the gas pressure scale height - can be destabilized by the radiative flux in optically thick media when a magnetic field is present, in a process called the radiation-driven magneto-acoustic instability (RMI). The instability does not require radiation or magnetic pressure to dominate over gas pressure, and acts independently of subsurface convection zones. Here we evaluate the conditions for the RMI to operate on a grid of stellar models covering a mass range 3-40 M⊙ at solar metallicity. For a uniform 1 kG magnetic field, fast magnetosonic modes are unstable down to an optical depth of a few tens, while unstable slow modes extend beyond the depth of the iron convection zone. The qualitative behaviour is robust to magnetic field strength variations by a factor of a few. When combining our findings with previous results for the saturation amplitude of the RMI, we predict velocity fluctuations in the range ˜0.1-10 km s-1. These amplitudes are a monotonically increasing function of the ratio of radiation to gas pressure, or alternatively, of the zero-age main sequence mass.

The violent interstellar medium in Milky-Way like disk galaxies

NASA Astrophysics Data System (ADS)

Karoline Walch, Stefanie

2015-08-01

Molecular clouds are cold, dense, and turbulent filamentary structures that condense out of the multi-phase interstellar medium. They are also the sites of star formation. The minority of new-born stars is massive, but these stars are particularly important for the fate of their parental molecular clouds as their feedback drives turbulence and regulates star formation.I will present results from the SILCC project (SImulating the Life Cycle of molecular Clouds), in which we study the formation and dispersal of molecular clouds within the multi-phase ISM using high-performance, three-dimensional simulations of representative pieces of disk galaxies. Apart from stellar feedback, self-gravity, an external stellar potential, and magnetic fields, we employ an accurate description of gas heating and cooling as well as a small chemical network including molecule formation and (self-)shielding from the interstellar radiation field. We study the impact of the supernova rate and the positioning of the supernova explosions with respect to the molecular gas in a well defined set of simulations. This allows us to draw conclusions on structure of the multi-phase ISM, the amount of molecular gas formed, and the onset of galactic outflows. Furthermore, we show how important stellar wind feedback is for regulating star formation in these disks.

A Phenomenological Two-Ribbon Model for Spatially Unresolved Observations of Stellar Flares

NASA Astrophysics Data System (ADS)

Kowalski, Adam

2018-06-01

Solar flares and flares that occur in much more magnetically active stars share some striking properties, such as the observed Neupert effect. However, stellar flares with the most impressive multi-wavelength data sets are typically much more energetic than solar flares, thus making robust connections difficult to establish. Whereas solar data have the advantage of high spatial resolution providing critical information about the development of flare ribbons, the major advantage of stellar flare data is the readily available broad-wavelength coverage of the white-light radiation and the Balmer jump spectral region. Due to the lack of direct spatial resolution for stellar flares and rarely coverage of the Balmer jump region for solar flares, it is not clear how to make a direct comparison. I will present a new method for modeling stellar flares based on high spatial resolution information of solar flare two-ribbon development for comparisons of the physics of their observed phenomena, such as the red-wing asymmetries in chromospheric lines and the white-light continuum radiation. The new modeling method combines aspects of "multi-thread" modeling and 1D radiative-hydrodynamic modeling. Our algorithm is important for interpreting the impulsive phase of superflares in young G dwarfs in Kepler and understanding how hour-long decay timescales are attained in the gradual phase of some very energetic stellar flares.

Young stellar objects & photoevaporating protoplanetary disks in the Orion's sibling NGC 1977.

NASA Astrophysics Data System (ADS)

Kim, J. S.; Fang, M.; Clarke, C. J.; Facchini, S.; Pascucci, I.; Apai, D.; Bally, J.

We present young stellar population in NGC 1977, Orion Nebula's sibling, and the discovery of new photoevaporating protoplanetary disks (proplyds) around a B star, 42 Ori. NGC 1977 (age≲2 Myr) is located at ˜30arcmin north of the Orion Nebula at a distance of ˜400 pc, but it lacks high mass O stars unlike in Orion Nebula Cluster (ONC). Nevertheless, we have identified seven proplyds in vicinity of its most massive star, 42 Ori (B1V). The proplyds show cometary Halpha emission in HST images, with clear ionization front and tails evaporating away from 42 Ori. These are the first proplyds to be found around a B star, while previously known proplyds were found near O stars. The FUV radiation impinging on these proplyds is 10-30 times weaker than that on the proplyds in ONC. We find that observed proplyd sizes are consistent with a model for photoevaporation in weak FUV radiation field. We briefly discuss one of the interesting YSOs found in this lesser-known star forming region in Orion, NGC 1977.

A cocoon of freshly accelerated cosmic rays detected by Fermi in the Cygnus superbubble.

PubMed

Ackermann, M; Ajello, M; Allafort, A; Baldini, L; Ballet, J; Barbiellini, G; Bastieri, D; Belfiore, A; Bellazzini, R; Berenji, B; Blandford, R D; Bloom, E D; Bonamente, E; Borgland, A W; Bottacini, E; Brigida, M; Bruel, P; Buehler, R; Buson, S; Caliandro, G A; Cameron, R A; Caraveo, P A; Casandjian, J M; Cecchi, C; Chekhtman, A; Cheung, C C; Chiang, J; Ciprini, S; Claus, R; Cohen-Tanugi, J; de Angelis, A; de Palma, F; Dermer, C D; do Couto E Silva, E; Drell, P S; Dumora, D; Favuzzi, C; Fegan, S J; Focke, W B; Fortin, P; Fukazawa, Y; Fusco, P; Gargano, F; Germani, S; Giglietto, N; Giordano, F; Giroletti, M; Glanzman, T; Godfrey, G; Grenier, I A; Guillemot, L; Guiriec, S; Hadasch, D; Hanabata, Y; Harding, A K; Hayashida, M; Hayashi, K; Hays, E; Jóhannesson, G; Johnson, A S; Kamae, T; Katagiri, H; Kataoka, J; Kerr, M; Knödlseder, J; Kuss, M; Lande, J; Latronico, L; Lee, S-H; Longo, F; Loparco, F; Lott, B; Lovellette, M N; Lubrano, P; Martin, P; Mazziotta, M N; McEnery, J E; Mehault, J; Michelson, P F; Mitthumsiri, W; Mizuno, T; Monte, C; Monzani, M E; Morselli, A; Moskalenko, I V; Murgia, S; Naumann-Godo, M; Nolan, P L; Norris, J P; Nuss, E; Ohsugi, T; Okumura, A; Orlando, E; Ormes, J F; Ozaki, M; Paneque, D; Parent, D; Pesce-Rollins, M; Pierbattista, M; Piron, F; Pohl, M; Prokhorov, D; Rainò, S; Rando, R; Razzano, M; Reposeur, T; Ritz, S; Parkinson, P M Saz; Sgrò, C; Siskind, E J; Smith, P D; Spinelli, P; Strong, A W; Takahashi, H; Tanaka, T; Thayer, J G; Thayer, J B; Thompson, D J; Tibaldo, L; Torres, D F; Tosti, G; Tramacere, A; Troja, E; Uchiyama, Y; Vandenbroucke, J; Vasileiou, V; Vianello, G; Vitale, V; Waite, A P; Wang, P; Winer, B L; Wood, K S; Yang, Z; Zimmer, S; Bontemps, S

2011-11-25

The origin of Galactic cosmic rays is a century-long puzzle. Indirect evidence points to their acceleration by supernova shockwaves, but we know little of their escape from the shock and their evolution through the turbulent medium surrounding massive stars. Gamma rays can probe their spreading through the ambient gas and radiation fields. The Fermi Large Area Telescope (LAT) has observed the star-forming region of Cygnus X. The 1- to 100-gigaelectronvolt images reveal a 50-parsec-wide cocoon of freshly accelerated cosmic rays that flood the cavities carved by the stellar winds and ionization fronts from young stellar clusters. It provides an example to study the youth of cosmic rays in a superbubble environment before they merge into the older Galactic population.

V471 Tauri, ballerina of the Hyades

NASA Astrophysics Data System (ADS)

Skillman, David R.; Patterson, Joseph

1988-09-01

Orbital light curves for V471 Tauri, the red dwarf-white dwarf binary in the Hyades, were obtained for the 1980-1983 observing seasons based on photometric and spectroscopic data. The results reveal the effects of tidal distortion of the secondary and a slow, transient wave which may originate from darker areas on the star's surface. A consistent ephemeris is derived. A Ca II line emission similar to that of rapidly rotating late-type stars and an additional component arising from the stellar region bathed in the white dwarf's UV-radiation field are found. An overall orbital-period decrease is noted which may be due to the strong braking of the K star's rotation by its own stellar wind, coupled with the enforcement of synchronous rotation by the tidal interaction with the white dwarf.

The Coupled Physical Structure of Gas and Dust in the IM Lup Protoplanetary Disk

NASA Astrophysics Data System (ADS)

Cleeves, L. Ilsedore; Öberg, Karin I.; Wilner, David J.; Huang, Jane; Loomis, Ryan A.; Andrews, Sean M.; Czekala, Ian

2016-12-01

The spatial distribution of gas and solids in protoplanetary disks determines the composition and formation efficiency of planetary systems. A number of disks show starkly different distributions for the gas and small grains compared to millimeter-centimeter-sized dust. We present new Atacama Large Millimeter/Submillimeter Array observations of the dust continuum, CO, 13CO, and C18O in the IM Lup protoplanetary disk, one of the first systems where this dust-gas dichotomy was clearly seen. The 12CO is detected out to a radius of 970 au, while the millimeter continuum emission is truncated at just 313 au. Based upon these data, we have built a comprehensive physical and chemical model for the disk structure, which takes into account the complex, coupled nature of the gas and dust and the interplay between the local and external environment. We constrain the distributions of gas and dust, the gas temperatures, the CO abundances, the CO optical depths, and the incident external radiation field. We find that the reduction/removal of dust from the outer disk exposes this region to higher stellar and external radiation and decreases the rate of freeze-out, allowing CO to remain in the gas out to large radial distances. We estimate a gas-phase CO abundance of 5% of the interstellar medium value and a low external radiation field (G 0 ≲ 4). The latter is consistent with that expected from the local stellar population. We additionally find tentative evidence for ring-like continuum substructure, suggestions of isotope-selective photodissociation, and a diffuse gas halo.

Radiation Pressure-Driven Magnetic Disk Winds in Broad Absorption Line Quasi-Stellar Objects

NASA Technical Reports Server (NTRS)

DeKool, Martin; Begelman, Mitchell C.

1995-01-01

We explore a model in which QSO broad absorption lines (BALS) are formed in a radiation pressure-driven wind emerging from a magnetized accretion disk. The magnetic field threading the disk material is dragged by the flow and is compressed by the radiation pressure until it is dynamically important and strong enough to contribute to the confinement of the BAL clouds. We construct a simple self-similar model for such radiatively driven magnetized disk winds, in order to explore their properties. It is found that solutions exist for which the entire magnetized flow is confined to a thin wedge over the surface of the disk. For reasonable values of the mass-loss rate, a typical magnetic field strength such that the magnetic pressure is comparable to the inferred gas pressure in BAL clouds, and a moderate amount of internal soft X-ray absorption, we find that the opening angle of the flow is approximately 0.1 rad, in good agreement with the observed covering factor of the broad absorption line region.

A MODEL FOR (QUASI-)PERIODIC MULTIWAVELENGTH PHOTOMETRIC VARIABILITY IN YOUNG STELLAR OBJECTS

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

Kesseli, Aurora Y.; Petkova, Maya A.; Wood, Kenneth

We present radiation transfer models of rotating young stellar objects (YSOs) with hot spots in their atmospheres, inner disk warps, and other three-dimensional effects in the nearby circumstellar environment. Our models are based on the geometry expected from magneto-accretion theory, where material moving inward in the disk flows along magnetic field lines to the star and creates stellar hot spots upon impact. Due to rotation of the star and magnetosphere, the disk is variably illuminated. We compare our model light curves to data from the Spitzer YSOVAR project to determine if these processes can explain the variability observed at opticalmore » and mid-infrared wavelengths in young stars. We focus on those variables exhibiting “dipper” behavior that may be periodic, quasi-periodic, or aperiodic. We find that the stellar hot-spot size and temperature affects the optical and near-infrared light curves, while the shape and vertical extent of the inner disk warp affects the mid-IR light curve variations. Clumpy disk distributions with non-uniform fractal density structure produce more stochastic light curves. We conclude that magneto-accretion theory is consistent with certain aspects of the multiwavelength photometric variability exhibited by low-mass YSOs. More detailed modeling of individual sources can be used to better determine the stellar hot-spot and inner disk geometries of particular sources.« less

New measurements of photospheric magnetic fields in late-type stars and emerging trends

NASA Technical Reports Server (NTRS)

Saar, S. H.; Linsky, J. L.

1986-01-01

The magnetic fields of late-type stars are measured using the method of Saar et al. (1986). The method includes radiative transfer effects and compensation for line blending; the photospheric magnetic field parameters are derived by comparing observed and theoretical line profiles using an LTE code that includes line saturation and full Zeeman pattern. The preliminary mean active region magnetic field strengths (B) and surface area coverages for 20 stars are discussed. It is observed that there is a trend of increasing B towards the cooler dwarfs stars, and the linear correlation between B and the equipartition value of the magnetic field strength suggests that the photospheric gas pressure determines the photospheric magnetic field strengths. A tendency toward larger filling factors at larger stellar angular velocities is also detected.

Compact Starburst Galaxies with Fast Outflows: Spatially Resolved Stellar Mass Profiles

NASA Astrophysics Data System (ADS)

Gottlieb, Sophia; Diamond-Stanic, Aleksandar; Lipscomb, Charles; Ohene, Senyo; Rines, Josh; Moustakas, John; Sell, Paul; Tremonti, Christy; Coil, Alison; Rudnick, Gregory; Hickox, Ryan C.; Geach, James; Kepley, Amanda

2018-01-01

Powerful galactic winds driven by stellar feedback and black hole accretion are thought to play an important role in regulating star formation in galaxies. In particular, strong stellar feedback from supernovae, stellar winds, radiation pressure, and cosmic rays is required by simulations of star-forming galaxies to prevent the vast majority of baryons from cooling and collapsing to form stars. However, it remains unclear whether these stellar processes play a significant role in expelling gas and shutting down star formation in massive progenitors of quiescent galaxies. What are the limits of stellar feedback? We present multi-band photometry with HST/WFC3 (F475W, F814W, F160W) for a dozen compact starburst galaxies at z~0.6 with half-light radii that suggest incredibly large central escape velocities. These massive galaxies are driving fast (>1000 km/s) outflows that have been previously attributed to stellar feedback associated with the compact (r~100 pc) starburst. But how compact is the stellar mass? In the context of the stellar feedback hypothesis, it is unclear whether these fast outflows are being driven at velocities comparable to the escape velocity of an incredibly dense stellar system (as predicted by some models of radiation-pressure winds) or at velocities that exceed the central escape velocity by large factor. Our spatially resolved measurements with HST show that the stellar mass is more extended than the light, and this requires that the physical mechanism responsible for driving the winds must be able to launch gas at velocities that are factors of 5-10 beyond the central escape velocity.

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

Szulagyi, Judit; Pascucci, Ilaria; Abraham, Peter

Mid-infrared atomic and ionic line ratios measured in spectra of pre-main-sequence stars are sensitive indicators of the hardness of the radiation field impinging on the disk surface. We present a low-resolution Spitzer IRS search for [Ar II] at 6.98 {mu}m, [Ne II] at 12.81 {mu}m, and [Ne III] 15.55 {mu}m lines in 56 transitional disks. These objects, characterized by reduced near-infrared but strong far-infrared excess emission, are ideal targets to set constraints on the stellar radiation field onto the disk, because their spectra are not contaminated by shock emission from jets/outflows or by molecular emission lines. After demonstrating that wemore » can detect [Ne II] lines and recover their fluxes from the low-resolution spectra, here we report the first detections of [Ar II] lines toward protoplanetary disks. We did not detect [Ne III] emission in any of our sources. Our [Ne II]/[Ne III] line flux ratios combined with literature data suggest that a soft-EUV or X-ray spectrum produces these gas lines. Furthermore, the [Ar II]/[Ne II] line flux ratios point to a soft X-ray and/or soft-EUV stellar spectrum as the ionization source of the [Ar II] and [Ne II] emitting layer of the disk. If the soft X-ray component dominates over the EUV, then we would expect larger photoevaporation rates and, hence, a reduction of the time available to form planets.« less

Biological effects of stellar collapse neutrinos

PubMed

Collar, J I

1996-02-05

Massive stars in their final stages of collapse radiate most of their binding energy in the form of MeV neutrinos. The recoil atoms that they produce in elastic scattering off nuclei in organic tissue create radiation damage which is highly effective in the production of irreparable DNA harm, leading to cellular mutation, neoplasia, and oncogenesis. Using a conventional model of the galaxy and of the collapse mechanism, the periodicity of nearby stellar collapses and the radiation dose are calculated. The possible contribution of this process to the paleontological record of mass extinctions is examined.

Non-ionising electromagnetic environments on manned spacecraft.

PubMed

Murphy, J R

1989-08-01

Future space travellers and settlers will be exposed to a variety of electromagnetic fields (EMFs). Extrinsic sources will include solar and stellar fluxes, planetary fluxes, and supernovae. Intrinsic sources may include fusion and ion engines, EMFs from electrical equipment, radar, lighting, superconduction energy storage systems, magnetic bearings on gyroscopic control and orientation systems, and magnetic rail microprobe launch systems. Communication sources may include radio and microwave frequencies, and laser generating systems. Magnetic fields may also be used for deflection of radiation. There is also a loss of the normal Geomagnetic field (GMF) which includes static, alternating, and time-varying components. This paper reviews exposure limits and the biological effects of EMFs, and evidence for an electromagnetic sense organ and a relationship between man and the Geomagnetic field.

Magnetic field-related heating instabilities in the surface layers of the sun and stars

NASA Technical Reports Server (NTRS)

Ferrari, A.; Rosner, R.; Vaiana, G. S.

1982-01-01

The stability of a magnetized low-density plasma to current-driven filamentation instabilities is investigated and the results are applied to the surface layers of stars. Unlike previous studies, the initial (i.e., precoronal) state of the stellar surface atmosphere is taken to be a low-density, optically thin magnetized plasma in radiative equilibrium. The linear analysis shows that the surface layers of main-sequence stars (including the sun) which are threaded by magnetic fields are unstable; the instabilities considered lead to structuring perpendicular to the ambient magnetic fields. These results suggest that relatively modest surface motions, in conjunction with the presence of magnetic fields, suffice to account for the presence of inhomogeneous chromospheric and coronal plasma overlying a star's surface.

Star-disk interaction in Herbig Ae/Be stars

NASA Astrophysics Data System (ADS)

Speights, Christa Marie

2012-09-01

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

Radio wavelength observations of magnetic fields on active dwarf M, RS CVn and magnetic stars

NASA Technical Reports Server (NTRS)

Lang, Kenneth R.

1986-01-01

The dwarf M stars, YZ Canis Minoris and AD Leonis, exhibit narrow-band, slowly varying (hours) microwave emission that cannot be explained by conventional thermal radiation mechanisms. The dwarf M stars, AD Leonis and Wolf 424, emit rapid spikes whose high brightness temperatures similarly require a nonthermal radiation process. They are attributed to coherent mechanisms such as an electron-cyclotron maser or coherent plasma radiation. If the electron-cyclotron maser emits at the second or third harmonic gyrofrequency, the coronal magnetic field strength equals 250 G or 167 G, and constraints on the plasma frequency imply an electron density of 6 x 10 to the 9th/cu cm. Radio spikes from AD Leonis and Wolf 424 have rise times less than or equal to 5 ms, indicating a linear size of less than or equal to 1.5 x 10 to the 8th cm, or less than 0.005 of the stellar radius. Although Ap magnetic stars have strong dipole magnetic fields, they exhibit no detectable gyroresonant radiation, suggesting that these stars do not have hot, dense coronae. The binary RS CVn star UX Arietis exhibits variable emission at 6 cm wavelength on time scales ranging from 30 s to more than one hour.

Quantum frictionless trajectories versus geodesics

NASA Astrophysics Data System (ADS)

Barbado, Luis C.; Barceló, Carlos; Garay, Luis J.

2015-10-01

Moving particles outside a star will generally experience quantum friction caused by the Unruh radiation reaction. There exist however radial trajectories that lack this effect (in the outgoing radiation sector, and ignoring backscattering). Along these trajectories, observers perceive just stellar emission, without further contribution from the Unruh effect. They turn out to have the property that the variations of the Doppler and the gravitational shifts compensate each other. They are not geodesics, and their proper acceleration obeys an inverse square law, which means that it could in principle be generated by outgoing stellar radiation. In the case of a black hole emitting Hawking radiation, this may lead to a buoyancy scenario. The ingoing radiation sector has little effect and seems to slow down the fall even further.

Stellar MHD and Nuclear Physics Coupled Together Solve the Puzzle of Oxide Grain Composition

NASA Astrophysics Data System (ADS)

Palmerini, Sara; Trippella, Oscar; Busso, Maurizio; La Cognata, Marco; Petrelli, Maurizio; Zucchini, Azzurra

Oxide grains, enclosed in meteorites, give us very precise information about the stars in which they formed. Grains belonging to group 1 and 2 are characterized by values of 17O/16O and 18O/16O inconsistent with explosive nucleosynthesis scenarios, and are then believed to form in low mass stars. Nowadays, models of non convective mixing coupled with nuclear burning succeed in reproducing the oxygen isotopic mix found in these ancient solids thanks to the more accurate nuclear physics inputs employed in calculations. However, a large part of oxide grains shows values of the 26Al/27Al isotopic ratio too high to be accounted for by the mixing models mentioned above. Recently, [1] demonstrated that the stellar magnetic field might promote the transport of material across the stellar radiative layers. We apply this magnetic mixing model to a 1.2M ⊙ AGB star of solar metallicity. It turns out that the oxygen and aluminum isotopic ratios shown by group 1 and 2 grains are perfectly reproduced.

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

Chen, Xian; Amaro-Seoane, Pau, E-mail: xian.chen@pku.edu.cn, E-mail: pau@ice.cat

The formation of compact stellar-mass binaries is a difficult, but interesting problem in astrophysics. There are two main formation channels: in the field via binary star evolution, or in dense stellar systems via dynamical interactions. The Laser Interferometer Gravitational-wave Observatory (LIGO) has detected black hole binaries (BHBs) via their gravitational radiation. These detections provide us with information about the physical parameters of the system. It has been claimed that when the Laser Interferometer Space Antenna (LISA) is operating, the joint observation of these binaries with LIGO will allow us to derive the channels that lead to their formation. However, wemore » show that for BHBs in dense stellar systems dynamical interactions could lead to high eccentricities such that a fraction of the relativistic mergers are not audible to LISA. A non-detection by LISA puts a lower limit of about 0.005 on the eccentricity of a BHB entering the LIGO band. On the other hand, a deci-Hertz observatory, like DECIGO or Tian Qin, would significantly enhance the chances of a joint detection and shed light on the formation channels of these binaries.« less

Correlation between diffuse interstellar bands (DIBs) and interstellar extinction using data from Bosscha Compact Spectrograph

NASA Astrophysics Data System (ADS)

Puspitarini, L.; Malasan, H. L.; Aprilia; Arifyanto, M. I.; Lallement, R.; Irfan, M.; Puspitaningrum, E.

2018-04-01

A longstanding challenge in astronomical spectroscopy is to uncover the carriers of diffuse interstellar bands (DIBs). They are broad absorption features due to the interstellar matter (ISM). They are seen in stellar spectra of background stars or other astronomical objects. Although we do not know utterly the carriers of the DIBs, they can be a promising tracer of the ISM. One of the interesting properties is their correlations with the interstellar (IS) extinction. For each band, the correlation has considerable dispersion and differences that possibly due to the IS physical conditions. Some DIBs are sensitive to the stellar radiation field, and some are not. To study the effect, we measured the DIB observed in Be/B stars spectra. The stars were observed by using Bosscha Compact Spectrograph at the Bosscha Observatory, Lembang, Indonesia. We performed an automated fitting of a combination of a smooth stellar continuum, the DIB profile, and a synthetic telluric transmission to the spectrum. The DIB measurements were compared to the general DIBs-extinction relationship. The correlation is found to be in good agreement with previous determinations.

Detection of a stellar flare at extreme ultraviolet wavelengths

NASA Technical Reports Server (NTRS)

Barstow, M. A.; Denby, M.; Pye, J. P.; Pankiewicz, G. S.; Bromage, G. E.; Gonzalez-Riestra, R.

1991-01-01

During the all-sky survey conducted by the Rosat Wide Field Camera, the binary flare star system BY Draconis was monitored with coverage by the IUE satellite far-UV and optical observations and by the Rosat X-ray telescope for part of the time. A stellar flare was detected in all four wavebands. This is the first unambiguous EUV detection of a flare and one of the widest simultaneous wavelength-range coverages obtained. The peak luminosity and total energy of this flare in the photon energy range 0.08-0.18 keV are comparable with the values obtained for a number of flares integrated over a larger energy range by Exosat satellite observations in 1983-86. It is concluded that radiation in the EUV carries away a substantial fraction of the total flare energy.

Impulsive solar X-ray bursts. III - Polarization, directivity, and spectrum of the reflected and total bremsstrahlung radiation from a beam of electrons directed toward the photosphere

NASA Technical Reports Server (NTRS)

Langer, S. H.; Petrosian, V.

1977-01-01

The paper presents the spectrum, directivity, and state of polarization of the bremsstrahlung radiation expected from a beam of high-energy electrons spiraling along radial magnetic field lines toward the photosphere. A Monte Carlo method is then described for evaluation of the spectrum, directivity, and polarization of X-rays diffusely reflected from stellar photospheres. The accuracy of the technique is evaluated through comparison with analytic results. The calculated characteristics of the incident X-rays are used to evaluate the spectrum, directivity, and polarization of the reflected and total X-ray fluxes. The results are compared with observations.

Applicability of steady models for hot-star winds

NASA Technical Reports Server (NTRS)

Owocki, Stanley P.; Poe, Clint H.; Castor, John I.

1990-01-01

Non-Sobolev models of radiatively driven stellar winds based on a pure-absorption approximation do not have a well-defined steady state. Here the implications of this for flow time-dependence are examined, showing that, under such circumstances, instabilities in the flow attain an absolute character that leads to intrinsic variability. In this case, steady solutions are inherently inapplicable because they do not represent physically realizable states. However, for actual hot-star winds, driving is principally by scattering, not pure absorption. In practice, the relatively weak force associated with slight asymmetries in the diffuse, scattered radiation field may play a crucial role in breaking the solution degeneracy and reducing the instability from an absolute to an advective character.

How much can we trust high-resolution spectroscopic stellar chemical abundances?

NASA Astrophysics Data System (ADS)

Blanco-Cuaresma, S.; Nordlander, T.; Heiter, U.; Jofré, P.; Masseron, T.; Casamiquela, L.; Tabernero, H. M.; Bhat, S. S.; Casey, A. R.; Meléndez, J.; Ramírez, I.

2017-03-01

To study stellar populations, it is common to combine chemical abundances from different spectroscopic surveys/studies where different setups were used. These inhomogeneities can lead us to inaccurate scientific conclusions. In this work, we studied one aspect of the problem: When deriving chemical abundances from high-resolution stellar spectra, what differences originate from the use of different radiative transfer codes?

Radio emission from an ultraluminous x-ray source.

PubMed

Kaaret, Philip; Corbel, Stephane; Prestwich, Andrea H; Zezas, Andreas

2003-01-17

The physical nature of ultraluminous x-ray sources is uncertain. Stellar-mass black holes with beamed radiation and intermediate black holes with isotropic radiation are two plausible explanations. We discovered radio emission from an ultraluminous x-ray source in the dwarf irregular galaxy NGC 5408. The x-ray, radio, and optical fluxes as well as the x-ray spectral shape are consistent with beamed relativistic jet emission from an accreting stellar black hole. If confirmed, this would suggest that the ultraluminous x-ray sources may be stellar-mass rather than intermediate-mass black holes. However, interpretation of the source as a jet-producing intermediate-mass black hole cannot be ruled out at this time.

Study of the Evolutions of the Universe, Stars and the Solar System ---Personal Retrospect and Prospect---

NASA Astrophysics Data System (ADS)

Hayakawa, S.

The universe, stars and the solar system belong to different hierarchies but have been studied in Jpn. by a common group with close mutual contact. Collaborative effort is not restricted within theoretical astrophysics but is extended to a variety of fields of science. In the present article the author describes on the basis of his personal recollection how the collaborate research has stimulated new fields of observational astronomy using infrared radiation and X-rays. A fee-back from these observational studies has given several inputs to the theory of stellar evolution and will provide important means in the study of cosmology.

THE COUPLED PHYSICAL STRUCTURE OF GAS AND DUST IN THE IM Lup PROTOPLANETARY DISK

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

Cleeves, L. Ilsedore; Öberg, Karin I.; Wilner, David J.

The spatial distribution of gas and solids in protoplanetary disks determines the composition and formation efficiency of planetary systems. A number of disks show starkly different distributions for the gas and small grains compared to millimeter–centimeter-sized dust. We present new Atacama Large Millimeter/Submillimeter Array observations of the dust continuum, CO, {sup 13}CO, and C{sup 18}O in the IM Lup protoplanetary disk, one of the first systems where this dust–gas dichotomy was clearly seen. The {sup 12}CO is detected out to a radius of 970 au, while the millimeter continuum emission is truncated at just 313 au. Based upon these data,more » we have built a comprehensive physical and chemical model for the disk structure, which takes into account the complex, coupled nature of the gas and dust and the interplay between the local and external environment. We constrain the distributions of gas and dust, the gas temperatures, the CO abundances, the CO optical depths, and the incident external radiation field. We find that the reduction/removal of dust from the outer disk exposes this region to higher stellar and external radiation and decreases the rate of freeze-out, allowing CO to remain in the gas out to large radial distances. We estimate a gas-phase CO abundance of 5% of the interstellar medium value and a low external radiation field ( G {sub 0} ≲ 4). The latter is consistent with that expected from the local stellar population. We additionally find tentative evidence for ring-like continuum substructure, suggestions of isotope-selective photodissociation, and a diffuse gas halo.« less

Stellar and wind parameters of massive stars from spectral analysis

NASA Astrophysics Data System (ADS)

Araya, I.; Curé, M.

2017-07-01

The only way to deduce information from stars is to decode the radiation it emits in an appropriate way. Spectroscopy can solve this and derive many properties of stars. In this work we seek to derive simultaneously the stellar and wind characteristics of A and B supergiant stars. Our stellar properties encompass the effective temperature, the surface gravity, the stellar radius, the micro-turbulence velocity, the rotational velocity and, finally, the chemical composition. For wind properties we consider the mass-loss rate, the terminal velocity and the line-force parameters (α, k and δ) obtained from the standard line-driven wind theory. To model the data we use the radiative transport code Fastwind considering the newest hydrodynamical solutions derived with Hydwind code, which needs stellar and line-force parameters to obtain a wind solution. A grid of spectral models of massive stars is created and together with the observed spectra their physical properties are determined through spectral line fittings. These fittings provide an estimation about the line-force parameters, whose theoretical calculations are extremely complex. Furthermore, we expect to confirm that the hydrodynamical solutions obtained with a value of δ slightly larger than ˜ 0.25, called δ-slow solutions, describe quite reliable the radiation line-driven winds of A and late B supergiant stars and at the same time explain disagreements between observational data and theoretical models for the Wind-Momentum Luminosity Relationship (WLR).

Stellar and wind parameters of massive stars from spectral analysis

NASA Astrophysics Data System (ADS)

Araya, Ignacio; Curé, Michel

2017-11-01

The only way to deduce information from stars is to decode the radiation it emits in an appropriate way. Spectroscopy can solve this and derive many properties of stars. In this work we seek to derive simultaneously the stellar and wind characteristics of a wide range of massive stars. Our stellar properties encompass the effective temperature, the surface gravity, the stellar radius, the micro-turbulence velocity, the rotational velocity and the Si abundance. For wind properties we consider the mass-loss rate, the terminal velocity and the line-force parameters α, k and δ (from the line-driven wind theory). To model the data we use the radiative transport code Fastwind considering the newest hydrodynamical solutions derived with Hydwind code, which needs stellar and line-force parameters to obtain a wind solution. A grid of spectral models of massive stars is created and together with the observed spectra their physical properties are determined through spectral line fittings. These fittings provide an estimation about the line-force parameters, whose theoretical calculations are extremely complex. Furthermore, we expect to confirm that the hydrodynamical solutions obtained with a value of δ slightly larger than ~ 0.25, called δ-slow solutions, describe quite reliable the radiation line-driven winds of A and late B supergiant stars and at the same time explain disagreements between observational data and theoretical models for the Wind-Momentum Luminosity Relationship (WLR).

A Blind Search for Magnetospheric Emissions from Planetary Companions to Nearby Solar-type Stars

NASA Astrophysics Data System (ADS)

Lazio, T. Joseph W.; Carmichael, S.; Clark, J.; Elkins, E.; Gudmundsen, P.; Mott, Z.; Szwajkowski, M.; Hennig, L. A.

2010-01-01

This paper reports a blind search for planetary magnetospheric emissions from planets around nearby stars. Young stars are likely to have much stronger stellar winds than the Sun, and because planetary magnetospheric emissions are powered by stellar winds, stronger stellar winds may enhance the radio luminosity of any orbiting planets. Using various stellar catalogs, we selected nearby stars (< 30 pc) with relatively young age estimates (< 3 Gyr), finding between 100 and several hundred stars. We stacked images from the 74-MHz (4-m wavelength) VLA Low-frequency Sky Survey, obtaining 3\\sigma limits on planetary emission of between 10 and 33 mJy. These flux density limits correspond to average planetary luminosities less than 5--10 x 1023erg/s. Using models for the scaling of stellar wind velocity, density, and magnetic field with stellar age, we estimate scaling factors for the strength of stellar winds, relative to the Sun, in our samples. The typical kinetic (magnetic) energy carried by the stellar winds in our samples is 15--50 (5--10) times larger than that of the solar wind. If we assume that every star is orbited by a Jupiter-like planet with a luminosity larger than that of the Jovian decametric radiation by the above factors, our limits on planetary luminosities from the stacking analysis are likely to be a factor of 300 above what would be required to detect the planets in a statistical sense. Similar statistical analyses with observations by future instruments, such as the Low Frequency Array (LOFAR) and the Long Wavelength Array (LWA), offer the promise of improvements by factors of 10--100. Basic research in radio astronomy at NRL is supported by 6.1 Base funding. The LUNAR consortium, is funded by the NASA Lunar Science Institute (Cooperative Agreement NNA09DB30A) to investigate concepts for astrophysical observatories on the Moon.

On neoclassical impurity transport in stellarator geometry

NASA Astrophysics Data System (ADS)

García-Regaña, J. M.; Kleiber, R.; Beidler, C. D.; Turkin, Y.; Maaßberg, H.; Helander, P.

2013-07-01

The impurity dynamics in stellarators has become an issue of moderate concern due to the inherent tendency of the impurities to accumulate in the core when the neoclassical ambipolar radial electric field points radially inwards (ion root regime). This accumulation can lead to collapse of the plasma due to radiative losses, and thus limit high performance plasma discharges in non-axisymmetric devices. A quantitative description of the neoclassical impurity transport is complicated by the breakdown of the assumption of small E × B drift and trapping due to the electrostatic potential variation on a flux surface \\tilde{\\Phi} compared with those due to the magnetic field gradient. This work examines the impact of this potential variation on neoclassical impurity transport in the Large Helical Device heliotron. It shows that the neoclassical impurity transport can be strongly affected by \\tilde{\\Phi} . The central numerical tool used is the δf particle in cell Monte Carlo code EUTERPE. The \\tilde{\\Phi} used in the calculations is provided by the neoclassical code GSRAKE. The possibility of obtaining a more general \\tilde{\\Phi} self-consistently with EUTERPE is also addressed and a preliminary calculation is presented.

The VIMOS Ultra Deep Survey: Nature, ISM properties, and ionizing spectra of CIII]λ1909 emitters at z = 2-4

NASA Astrophysics Data System (ADS)

Nakajima, K.; Schaerer, D.; Le Fèvre, O.; Amorín, R.; Talia, M.; Lemaux, B. C.; Tasca, L. A. M.; Vanzella, E.; Zamorani, G.; Bardelli, S.; Grazian, A.; Guaita, L.; Hathi, N. P.; Pentericci, L.; Zucca, E.

2018-05-01

Context. Ultraviolet (UV) emission-line spectra are used to spectroscopically confirm high-z galaxies and increasingly also to determine their physical properties. Aims: We construct photoionization models to interpret the observed UV spectra of distant galaxies in terms of the dominant radiation field and the physical condition of the interstellar medium (ISM). These models are applied to new spectroscopic observations from the VIMOS Ultra Deep Survey (VUDS). Methods: We construct a large grid of photoionization models, which use several incident radiation fields (stellar populations, active galactic nuclei (AGNs), mix of stars and AGNs, blackbodies, and others), and cover a wide range of metallicities and ionization parameters. From these models we derive new spectral UV line diagnostics using equivalent widths (EWs) of [CIII]λ1909 doublet, CIVλ1549 doublet and the line ratios of [CIII], CIV, and He IIλ1640 recombination lines. We apply these diagnostics to a sample of 450 [CIII]-emitting galaxies at redshifts z = 2-4 previously identified in VUDS. Results: We demonstrate that our photoionization models successfully reproduce observations of nearby and high-redshift sources with known radiation field and/or metallicity. For star-forming galaxies our models predict that [CIII] EW peaks at sub-solar metallicities, whereas CIV EW peaks at even lower metallicity. Using the UV diagnostics, we show that the average star-forming galaxy (EW([CIII]) 2 Å) based on the composite of the 450 UV-selected galaxies' spectra The inferred metallicity and ionization parameter is typically Z = 0.3-0.5 Z⊙ and logU = -2.7 to - 3, in agreement with earlier works at similar redshifts. The models also indicate an average age of 50-200 Myr since the beginning of the current star-formation, and an ionizing photon production rate, ξion, of logξion/erg-1 Hz = 25.3-25.4. Among the sources with EW([CIII]) >= 10 Å, approximately 30% are likely dominated by AGNs. The metallicity derived for galaxies with EW(CIII) = 10-20 Å is low, Z = 0.02-0.2 Z⊙, and the ionization parameter higher (logU -1.7) than the average star-forming galaxy. To explain the average UV observations of the strongest but rarest [CIII] emitters (EW([CIII]) > 20 Å), we find that stellar photoionization is clearly insufficient. A radiation field consisting of a mix of a young stellar population (logξion/erg-1 Hz 25.7) plus an AGN component is required. Furthermore an enhanced C/O abundance ratio (up to the solar value) is needed for metallicities Z = 0.1-0.2 Z⊙ and logU = -1.7 to - 1.5. Conclusions: A large grid of photoionization models has allowed us to propose new diagnostic diagrams to classify the nature of the ionizing radiation field (star formation or AGN) of distant galaxies using UV emission lines, and to constrain their ISM properties. We have applied this grid to a sample of [CIII]-emitting galaxies at z = 2-4 detected in VUDS, finding a range of physical properties and clear evidence for significant AGN contribution in rare sources with very strong [CIII] emission. The UV diagnostics we propose should also serve as an important basis for the interpretation of upcoming observations of high-redshift galaxies. Based on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile, under Large Program 185.A-0791.JSPS Overseas Research Fellow.

The Influence of Atomic Diffusion on Stellar Ages and Chemical Tagging

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

Dotter, Aaron; Conroy, Charlie; Cargile, Phillip

2017-05-10

In the era of large stellar spectroscopic surveys, there is an emphasis on deriving not only stellar abundances but also the ages for millions of stars. In the context of Galactic archeology, stellar ages provide a direct probe of the formation history of the Galaxy. We use the stellar evolution code MESA to compute models with atomic diffusion—with and without radiative acceleration—and extra mixing in the surface layers. The extra mixing consists of both density-dependent turbulent mixing and envelope overshoot mixing. Based on these models we argue that it is important to distinguish between initial, bulk abundances (parameters) and current,more » surface abundances (variables) in the analysis of individual stellar ages. In stars that maintain radiative regions on evolutionary timescales, atomic diffusion modifies the surface abundances. We show that when initial, bulk metallicity is equated with current, surface metallicity in isochrone age analysis, the resulting stellar ages can be systematically overestimated by up to 20%. The change of surface abundances with evolutionary phase also complicates chemical tagging, which is the concept that dispersed star clusters can be identified through unique, high-dimensional chemical signatures. Stars from the same cluster, but in different evolutionary phases, will show different surface abundances. We speculate that calibration of stellar models may allow us to estimate not only stellar ages but also initial abundances for individual stars. In the meantime, analyzing the chemical properties of stars in similar evolutionary phases is essential to minimize the effects of atomic diffusion in the context of chemical tagging.« less

Models for X-Ray Emission from Isolated Pulsars

NASA Technical Reports Server (NTRS)

Wang, F. Y.-H.; Ruderman, M.; Halpern, Jules P.; Zhu, T.; Oliversen, Ronald (Technical Monitor)

2001-01-01

A model is proposed for the observed combination of power-law and thermal X-rays from rotationally powered pulsars. For gamma-ray pulsars with accelerators very many stellar radii above the neutron star surface, 100 MeV curvature gamma-rays from e(-) or e(+) flowing starward out of such accelerators are converted to e1 pairs on closed field lines all around the star. These pairs strongly affect X-ray emission from near the star in two ways. (1) The pairs are a source of synchrotron emission immediately following their creation in regions where B approx. 10(exp 10) G. This emission, in the photon energy range 0.1 keV less than E(sub X) less than 5 MeV, has a power-law spectrum with energy index 0.5 and X-ray luminosity that depends on the back-flow current, and is typically approx. 10(exp 33) ergs/ s. (2) The pairs ultimately a cyclotron resonance "blanket" surrounding the star except for two holes along the open field line bundles which pass through it. In such a blanket the gravitational pull on e(+,-) pairs toward the star is balanced by the hugely amplified push of outflowing surface emitted X-rays wherever cyclotron resonance occurs. Because of it the neutron star is surrounded by a leaky "hohlraum" of hot blackbody radiation with two small holes, which prevents direct X-ray observation of a heated polar cap of a gamma-ray pulsar. Weakly spin modulated radiation from the blanket together with more strongly spin-modulated radiation from the holes through it would then dominate observed low energy (0.1-10 keV) emission. For non-y-ray pulsars, in which no such accelerators with their accompanying extreme relativistic back-flow toward the star are expected, optically thick e1 resonance blankets should not form (except in special cases very close to the open field line bundle). From such pulsars blackbody radiation from both the warm stellar surface and the heated polar caps should be directly observable. In these pulsars, details of the surface magnetic field evolution, especially of polar cap areas, become relevant to observations. The models are compared to X-ray data from Geminga, PSR 1055-52, PSR 0656+14, PSR 1929+10, and PSR 0950+08.

The Magnetic Field of the Class I Protostar WL 17

NASA Astrophysics Data System (ADS)

Johns-Krull, Christopher M.; Greene, T. P.; Doppmann, G.; Covey, K. R.

2007-12-01

Strong stellar magnetic fields are believed to truncate the inner accretion disks around young stars, redirecting the accreting material to the high latitude regions of the stellar surface. In the past few years, observations of strong stellar fields on Classical T Tauri stars [class II young stellar objects (YSOs)] with field strengths in general agreement with the predictions of magnetopsheric accretion theory have bolstered this picture. Currently, nothing is known about the magnetic field properties of younger, more embedded class I YSOs. It is during this protostellar evolutionary phase that stars accrete most of their final mass, but the physics governing this process remains poorly understood. Here, we use high resolution near infrared spectra obtained with NIRSPEC on Keck and with PHOENIX on Gemini South to measure the magnetic field properties of the class I protostar WL 17. We find clear signatures of a strong stellar magnetic field. Initial analysis of this data suggests a surface average field strength of 3.6 kG on the surface of WL 17. This is the highest mean surface field detected to date on any YSO. We present our field measurements and discuss how they fit with the general model of magnetospheric accretion in young stars.

Binary stellar winds. [flow and magnetic field geometry

NASA Technical Reports Server (NTRS)

Siscoe, G. L.; Heinemann, M. A.

1974-01-01

Stellar winds from a binary star pair will interact with each other along a contact discontinuity. We discuss qualitatively the geometry of the flow and field resulting from this interaction in the simplest case where the stars and winds are identical. We consider the shape of the critical surface (defined as the surface where the flow speed is equal to the sound speed) as a function of stellar separation and the role of shock waves in the flow field. The effect of stellar spin and magnetic sectors on the field configuration is given. The relative roles of mass loss and magnetic torque in the evolution of orbital parameters is discussed.

Binary stellar winds. [flow and magnetic field interactions

NASA Technical Reports Server (NTRS)

Siscoe, G. L.; Heinemann, M. A.

1974-01-01

Stellar winds from a binary star will interact with each other along a contact discontinuity. We discuss qualitatively the geometry of the flow and field resulting from this interaction in the simplest case where the stars and winds are identical. We consider the shape of the critical surface (defined as the surface where the flow speed is equal to the sound speed) as a function of stellar separation and the role of shock waves in the flow field. The effect of stellar spin and magnetic sectors on the field configuration is given. The relative roles of mass loss and magnetic torque in the evolution of orbital parameters are discussed.

The High-Excitation Planetary Nebula NCG 246: Optical and Near-Ultraviolet Observations and Two-dimensional Numerical Models

NASA Technical Reports Server (NTRS)

Szentgyorgyi, Andrew; Raymond, John; Franco, Jose; Villaver, Eva; Lopez-Martin, Luis

2003-01-01

We have imaged the planetary nebula (PN) NGC 246 in the near-ultraviolet wavelengths [Ne v] 342.6 nm, the Bowen fluorescence line of 0 111 at 344.4 nm, and a nearby line-free region centered on 338.6 nm, as well as H(alpha), [O III] 500.7 nm, and [S II] 673.0 and 671.5 nm. Imaging in the 344.4 nm line is necessary to deconvolve contamination of the [Ne v] images by O III 342.9 nm. The emission from the shell and inner parts of the nebula is detected in [Ne v]. The radial profiles of the [Ne v] brightness decrease with radius from the exciting star, indicating that the bulk of the emission from this ion is due to the hard UV stellar radiation field, with a (probably) marginal contribution from collisional ionization in a shock between the PN shell and the interstellar medium (ISM). In contrast, the radial profiles of the emission in H(alpha), [0 III] 500.7 nm, and [S II] are flatter and peak at the location of the shell. The emission of [S II] probably traces the interaction of the PN with the ambient ISM. We also present two-dimensional numerical simulations for this PN-ISM interaction. The simulations consider the stellar motion with respect to the ambient ISM, with a velocity of 85 km/s , and include the time evolution of the wind parameters and UV radiation field from the progenitor star.

KSC-08pd2456

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, a technicians clean the radiator on the Wide Field Camera 3, or WFC3,that will be installed on NASA's Hubble Space Telescope. The radiator is the "outside" of WFC3 that will be exposed to space. It will expel heat out of Hubble and into space through black body radiation. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

The Direct Effect of Toroidal Magnetic Fields on Stellar Oscillations: An Analytical Expression for the General Matrix Element

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

Kiefer, René; Schad, Ariane; Roth, Markus

2017-09-10

Where is the solar dynamo located and what is its modus operandi? These are still open questions in solar physics. Helio- and asteroseismology can help answer them by enabling us to study solar and stellar internal structures through global oscillations. The properties of solar and stellar acoustic modes are changing with the level of magnetic activity. However, until now, the inference on subsurface magnetic fields with seismic measures has been very limited. The aim of this paper is to develop a formalism to calculate the effect of large-scale toroidal magnetic fields on solar and stellar global oscillation eigenfunctions and eigenfrequencies.more » If the Lorentz force is added to the equilibrium equation of motion, stellar eigenmodes can couple. In quasi-degenerate perturbation theory, this coupling, also known as the direct effect, can be quantified by the general matrix element. We present the analytical expression of the matrix element for a superposition of subsurface zonal toroidal magnetic field configurations. The matrix element is important for forward calculations of perturbed solar and stellar eigenfunctions and frequency perturbations. The results presented here will help to ascertain solar and stellar large-scale subsurface magnetic fields, and their geometric configuration, strength, and change over the course of activity cycles.« less

The Direct Effect of Toroidal Magnetic Fields on Stellar Oscillations: An Analytical Expression for the General Matrix Element

NASA Astrophysics Data System (ADS)

Kiefer, René; Schad, Ariane; Roth, Markus

2017-09-01

Where is the solar dynamo located and what is its modus operandi? These are still open questions in solar physics. Helio- and asteroseismology can help answer them by enabling us to study solar and stellar internal structures through global oscillations. The properties of solar and stellar acoustic modes are changing with the level of magnetic activity. However, until now, the inference on subsurface magnetic fields with seismic measures has been very limited. The aim of this paper is to develop a formalism to calculate the effect of large-scale toroidal magnetic fields on solar and stellar global oscillation eigenfunctions and eigenfrequencies. If the Lorentz force is added to the equilibrium equation of motion, stellar eigenmodes can couple. In quasi-degenerate perturbation theory, this coupling, also known as the direct effect, can be quantified by the general matrix element. We present the analytical expression of the matrix element for a superposition of subsurface zonal toroidal magnetic field configurations. The matrix element is important for forward calculations of perturbed solar and stellar eigenfunctions and frequency perturbations. The results presented here will help to ascertain solar and stellar large-scale subsurface magnetic fields, and their geometric configuration, strength, and change over the course of activity cycles.

Deriving Physical Properties from Broadband Photometry with Prospector: Description of the Model and a Demonstration of its Accuracy Using 129 Galaxies in the Local Universe

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

Leja, Joel; Johnson, Benjamin D.; Conroy, Charlie

2017-03-10

Broadband photometry of galaxies measures an unresolved mix of complex stellar populations, gas, and dust. Interpreting these data is a challenge for models: many studies have shown that properties derived from modeling galaxy photometry are uncertain by a factor of two or more, and yet answering key questions in the field now requires higher accuracy than this. Here, we present a new model framework specifically designed for these complexities. Our model, Prospector- α , includes dust attenuation and re-radiation, a flexible attenuation curve, nebular emission, stellar metallicity, and a six-component nonparametric star formation history. The flexibility and range of themore » parameter space, coupled with Monte Carlo Markov chain sampling within the Prospector inference framework, is designed to provide unbiased parameters and realistic error bars. We assess the accuracy of the model with aperture-matched optical spectroscopy, which was excluded from the fits. We compare spectral features predicted solely from fits to the broadband photometry to the observed spectral features. Our model predicts H α luminosities with a scatter of ∼0.18 dex and an offset of ∼0.1 dex across a wide range of morphological types and stellar masses. This agreement is remarkable, as the H α luminosity is dependent on accurate star formation rates, dust attenuation, and stellar metallicities. The model also accurately predicts dust-sensitive Balmer decrements, spectroscopic stellar metallicities, polycyclic aromatic hydrocarbon mass fractions, and the age- and metallicity-sensitive features D{sub n}4000 and H δ . Although the model passes all these tests, we caution that we have not yet assessed its performance at higher redshift or the accuracy of recovered stellar masses.« less

Constraining the Radiation and Plasma Environment of the Kepler Circumbinary Habitable-zone Planets

NASA Astrophysics Data System (ADS)

Zuluaga, Jorge I.; Mason, Paul A.; Cuartas-Restrepo, Pablo A.

2016-02-01

The discovery of many planets using the Kepler telescope includes 10 planets orbiting eight binary stars. Three binaries, Kepler-16, Kepler-47, and Kepler-453, have at least one planet in the circumbinary habitable zone (BHZ). We constrain the level of high-energy radiation and the plasma environment in the BHZ of these systems. With this aim, BHZ limits in these Kepler binaries are calculated as a function of time, and the habitability lifetimes are estimated for hypothetical terrestrial planets and/or moons within the BHZ. With the time-dependent BHZ limits established, a self-consistent model is developed describing the evolution of stellar activity and radiation properties as proxies for stellar aggression toward planetary atmospheres. Modeling binary stellar rotation evolution, including the effect of tidal interaction between stars in binaries, is key to establishing the environment around these systems. We find that Kepler-16 and its binary analogs provide a plasma environment favorable for the survival of atmospheres of putative Mars-sized planets and exomoons. Tides have modified the rotation of the stars in Kepler-47, making its radiation environment less harsh in comparison to the solar system. This is a good example of the mechanism first proposed by Mason et al. Kepler-453 has an environment similar to that of the solar system with slightly better than Earth radiation conditions at the inner edge of the BHZ. These results can be reproduced and even reparameterized as stellar evolution and binary tidal models progress, using our online tool http://bhmcalc.net.

CONSTRAINING THE RADIATION AND PLASMA ENVIRONMENT OF THE KEPLER CIRCUMBINARY HABITABLE-ZONE PLANETS

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

Zuluaga, Jorge I.; Mason, Paul A.; Cuartas-Restrepo, Pablo A.

The discovery of many planets using the Kepler telescope includes 10 planets orbiting eight binary stars. Three binaries, Kepler-16, Kepler-47, and Kepler-453, have at least one planet in the circumbinary habitable zone (BHZ). We constrain the level of high-energy radiation and the plasma environment in the BHZ of these systems. With this aim, BHZ limits in these Kepler binaries are calculated as a function of time, and the habitability lifetimes are estimated for hypothetical terrestrial planets and/or moons within the BHZ. With the time-dependent BHZ limits established, a self-consistent model is developed describing the evolution of stellar activity and radiation propertiesmore » as proxies for stellar aggression toward planetary atmospheres. Modeling binary stellar rotation evolution, including the effect of tidal interaction between stars in binaries, is key to establishing the environment around these systems. We find that Kepler-16 and its binary analogs provide a plasma environment favorable for the survival of atmospheres of putative Mars-sized planets and exomoons. Tides have modified the rotation of the stars in Kepler-47, making its radiation environment less harsh in comparison to the solar system. This is a good example of the mechanism first proposed by Mason et al. Kepler-453 has an environment similar to that of the solar system with slightly better than Earth radiation conditions at the inner edge of the BHZ. These results can be reproduced and even reparameterized as stellar evolution and binary tidal models progress, using our online tool http://bhmcalc.net.« less

On the spottedness, magnetism and internal structure of stars

NASA Astrophysics Data System (ADS)

Gershberg, R. E.

Kinematical structures within stellar interiors that are the result of a self-organization of these interiors as thermodynamically open nonlinear systems are proposed as the physical basis for stellar magnetism. It is noted that the ubiquitousness of stellar magnetism that follows from the hypothesis is not in contradiction with observations. These kinematical structures may be energy reservoirs, and changes in these structures may be connected with variations of an energy flux emergent from a stellar surface, while its internal energy sources remain constant, explaining the radiation deficit from sunspots and starspots.

Optical and infrared spectrophotometry of 18 Markarian galaxies

NASA Technical Reports Server (NTRS)

Becklin, E. E.; Neugebauer, G.; Oke, J. B.; Searle, L.

1975-01-01

Slit spectra, spectrophotometric scans and infrared broad band observations are presented. Eight of the program galaxies can be classified as Seyfert galaxies. Arguments are given that thermal, nonthermal and stellar radiation components were present. One group of Seyfert galaxies was characterized both by the presence of a high density region of gas and by a continuum dominated by nonthermal radiation. The continua of the remaining program Seyferts, which did not have a high density region of gas, were dominated by thermal radiation from dust and a stellar continuum. Ten of the galaxies, which are not Seyfert galaxies, are shown to be examples of extragalactic H 2 regions.

Radiation transfer and stellar atmospheres

NASA Astrophysics Data System (ADS)

Swihart, T. L.

This is a revised and expanded version of the author's Basic Physics of Stellar Atmospheres, published in 1971. The equation of transfer is considered, taking into account the intensity and derived quantities, the absorption coefficient, the emission coefficient, the source function, and special integrals for plane media. The gray atmosphere is discussed along with the nongray atmosphere, and aspects of line formation. Topics related to polarization are explored, giving attention to pure polarized radiation, general polarized radiation, transfer in a magnetic plasma, and Rayleigh scattering and the sunlit sky. Physical and astronomical constants, and a number of problems related to the subjects of the book are presented in an appendix.

Dynamic collapses of relativistic degenerate stellar cores and radiation pressure dominated stellar interiors

NASA Astrophysics Data System (ADS)

Shi, Chun-Hui; Lou, Yu-Qing

2018-04-01

We investigate and explore self-similar dynamic radial collapses of relativistic degenerate stellar cores (RDSCs) and radiation pressure dominated stellar interiors (RPDSIs) of spherical symmetry by invoking a conventional polytropic (CP) equation of state (EoS) with a constant polytropic index γ = 4 / 3 and by allowing free-fall non-zero RDSC or RPDSI surface mass density and pressure due to their sustained physical contact with the outer surrounding stellar envelopes also in contraction. Irrespective of the physical triggering mechanisms (including, e.g., photodissociation, electron-positron pair instability, general relativistic instability etc.) for initiating such a self-similar dynamically collapsing RDSC or RPDSI embedded within a massive star, a very massive star (VMS) or a supermassive star (SMS) in contraction and by comparing with the Schwarzschild radii associated with their corresponding RDSC/RPDSI masses, the emergence of central black holes in a wide mass range appears inevitable during such RDSC/RPDSI dynamic collapses inside massive stars, VMSs, and SMSs, respectively. Radial pulsations of progenitor cores or during a stellar core collapse may well leave imprints onto collapsing RDSCs/RPDSIs towards their self-similar dynamic evolution. Massive neutron stars may form during dynamic collapses of RDSC inside massive stars in contraction under proper conditions.

Radiative acceleration in Schwarzschild space-times

NASA Astrophysics Data System (ADS)

Keane, A. J.; Barrett, R. K.; Simmons, J. F. L.

2001-03-01

We examine the radial motion of a material particle in the intense radiation field of a static spherically symmetric compact object with spherical emitting surface outside the Schwarzschild radius. This paper generalizes previous work which dealt with radial motion in the Thomson limit, where the radiation force is simply proportional to the radiative flux. In the general case the average time component of the 4-momentum transferred to the particle is not negligible compared with its rest mass. Consequently, we find that the frequency dependence of the radiation force owing to Compton scattering for highly energetic photons gives rise to an increase in the effective mass of the test particle. In this work we outline the effects of this frequency dependence and compare these with the results in the Thomson limit. We present the frequency dependent saturation velocity curves for a range of stellar luminosities and radiation frequencies and present the resulting phase-space diagrams corresponding to the radial test particle trajectories. In particular, the stable equilibrium points which exist in the Thomson limit are found to be absent in the general case.

Magnetic heating of stellar chromospheres and coronae

NASA Astrophysics Data System (ADS)

van Ballegooijen, A. A.

The theoretical discussion of magnetic heating focuses on heating by dissipation of field-aligned electric currents. Several mechanisms are set forth to account for the very high current densities needed to generate the heat, but observed radiative losses do not justify the resultant Ohmic heating rate. Tearing modes, 'turbulent resistivity', and 'hyper-resistivity' are considered to resolve the implied inefficiency of coronal heating. Because the mechanisms are not readily applicable to the sun, transverse magnetic energy flows and magnetic flare release are considered to account for the magnitude of observed radiative loss. High-resolution observations of the sun are concluded to be an efficient way to examine the issues of magnetic heating in spite of the very small spatial scales of the heating processes.

Radiation hydrodynamics of super star cluster formation

NASA Astrophysics Data System (ADS)

Tsang, Benny Tsz Ho; Milos Milosavljevic

2018-01-01

Throughout the history of the Universe, the nuclei of super star clusters represent the most active sites for star formation. The high densities of massive stars within the clusters produce intense radiation that imparts both energy and momentum on the surrounding star-forming gas. Theoretical claims based on idealized geometries have claimed the dominant role of radiation pressure in controlling the star formation activity within the clusters. In order for cluster formation simulations to be reliable, numerical schemes have to be able to model accurately the radiation flows through the gas clumps at the cluster nuclei with high density contrasts. With a hybrid Monte Carlo radiation transport module we developed, we performed 3D radiation hydrodynamical simulations of super star cluster formation in turbulent clouds. Furthermore, our Monte Carlo radiation treatment provides a native capability to produce synthetic observations, which allows us to predict observational indicators and to inform future observations. We found that radiation pressure has definite, but minor effects on limiting the gas supply for star formation, and the final mass of the most massive cluster is about one million solar masses. The ineffective forcing was due to the density variations inside the clusters, i.e. radiation takes the paths of low densities and avoids forcing on dense clumps. Compared to a radiation-free control run, we further found that the presence of radiation amplifies the density variations. The core of the resulting cluster has a high stellar density, about the threshold required for stellar collisions and merging. The very massive star that form from the stellar merging could continue to gain mass from the surrounding gas reservoir that is gravitationally confined by the deep potential of the cluster, seeding the potential formation of a massive black hole.

Modelling ultraviolet-line diagnostics of stars, the ionized and the neutral interstellar medium in star-forming galaxies

NASA Astrophysics Data System (ADS)

Vidal-García, A.; Charlot, S.; Bruzual, G.; Hubeny, I.

2017-09-01

We combine state-of-the-art models for the production of stellar radiation and its transfer through the interstellar medium (ISM) to investigate ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in star-forming galaxies. We start by assessing the reliability of our stellar population synthesis modelling by fitting absorption-line indices in the ISM-free ultraviolet spectra of 10 Large Magellanic Cloud clusters. In doing so, we find that neglecting stochastic sampling of the stellar initial mass function in these young (∼10-100 Myr), low-mass clusters affects negligibly ultraviolet-based age and metallicity estimates but can lead to significant overestimates of stellar mass. Then, we proceed and develop a simple approach, based on an idealized description of the main features of the ISM, to compute in a physically consistent way the combined influence of nebular emission and interstellar absorption on ultraviolet spectra of star-forming galaxies. Our model accounts for the transfer of radiation through the ionized interiors and outer neutral envelopes of short-lived stellar birth clouds, as well as for radiative transfer through a diffuse intercloud medium. We use this approach to explore the entangled signatures of stars, the ionized and the neutral ISM in ultraviolet spectra of star-forming galaxies. We find that, aside from a few notable exceptions, most standard ultraviolet indices defined in the spectra of ISM-free stellar populations are prone to significant contamination by the ISM, which increases with metallicity. We also identify several nebular-emission and interstellar-absorption features, which stand out as particularly clean tracers of the different phases of the ISM.

Two-Dimensional Study of Mass Outflow from Central Gravitational Astrophysical Object. Analytical 2-D solutions for thermo-radiatively driven stellar winds.

NASA Astrophysics Data System (ADS)

Kakouris, A.

The present PhD Thesis deals with the two-dimensional description of the plasma outflow from central astrophysical objects. The concept of stellar winds was originated by Eugene Parker 1958, and has become a very hot area of research the last decade. Mass outflow from all types of stars, as well as AGNs, quasars or planetary nebulae are observed in all astrophysical scales indicating at least two-dimensional (2-D) features (e.g. Hughes (editor), 1991, Beams and jets in astrophysics, Cambridge University Press). In a first stage, the flows are modeled empirically but their origin has to be in accordance with the fluid mechanics and the conservation laws. So, self-consistent 2-D models are needed (i.e. full solutions of the total set of equations which conserve mass, momentum and energy). The main mechanisms of ejecting plasma from an astrophysical object are the thermal (similar to solar wind), the radiative and the magnetic. Self consistent analytical 2-D steady hydrodynamic (HD) solutions for stellar winds have been presented by Tsinganos & Vlastou 1988, Tsinganos & Trussoni 1990, Tsinganos & Sauty 1992 and Lima & Priest 1993. Following their description we derive a new set of solutions in the present work. Our main assumptions are steady state (\\partial/\\partial t = 0), axisymmetry to the rotational axis (\\partial/\\partial \\phi = 0) and helicoidal geometry for the streamlines (meridional velocity {\\vec u}_{\\theta} = {\\vec 0} ). Besides, the fluid is assumed to be a nonmagnetized fully ionized hydrogen. The model could be named as non polytropic since we do not follow the polytropic assumption with a constant polytropic exponent but we evaluate the total external energy needed by the 1st law of Thermodynamics. Also, the solutions are \\theta-self similar since the dependence to the colatitude is given from the beginning. The generalized differential rotation of the fluid is taken into account considering a dependence of the rotational velocity of (V\\phi \\propto \\sin\\mu \\theta / R ) where \\mu is a parameter and R the radial distance. Using these assumptions we derive fully analytical (only a Simpson integration is needed) 2-D solutions of four types (with velocity maximum either along the equator or the polar axis of the central astrophysical object). One of them (named as solution in Range I) exhibits suitable features for stellar wind interpretation with velocity maximum along the equator because the outflow starts subsonic at the stellar surface and terminates supersonic at infinity. The other solutions are subsonic (breeze) or they could be examined only as inflows. The Range I solution is applied to real astrophysical objects. Moreover, the thermally driven 2 - D solutions are extended including the radiative force due to the absorption of the stellar light in the fluid. So, the 2-D solutions represent thermally and radiatively driven flows. The assumptions for the radiative force inclusion are that the radiative acceleration is radial and it is a function of radial distance solely (i.e. it is independent of the velocity). The first radiatively driven wind model was presented in 1975 by Castor, Abbott & Klein and was applied to O5f main sequence stars. In order to describe the radiative origin of the massive winds from early and late spectral type stars, the radiative force is separated into its continuum, thick lines and thin lines parts. The mechanism of the continuous absorption is the Thomson scattering of the photons by the free plasma electrons and it is always present. If the line contribution corresponds to the thick absorption spectral lines the model is named as 'thick line driven' otherwise the atmosphere is thought 'optically thin'. In this Thesis we consider an optically thin atmosphere and in this case the radiative force is written as a power law of distance (Chen & Marlborough 1994, Lamers 1986). Moreover, we examine the exponential dependence of the radiative acceleration upon the radial distance and exponential deviations from power laws. We apply to supergiant B stars and we obtain results in agreement with observations (Underhill & i oazan 1982). In the first chapter of the Thesis, the reader is introduced in the concept of the astrophysical flows. I show some observational data for outflows and the basic mechanisms of the outflows are reported. In chapter 2, the basic hydrodynamic equations are presented. In chapter 3, some 1-D or 2-D models (relevant to this Thesis) are reported. The new results appear in chapters 4, 5, 6 which posses the 3/4 of the Thesis. In chapter 4, the basic assumptions are presented and the full mathematical derivation and deduction of the solutions are given. The inclusion of the radiative force is also given. In chapter 5, the thermally driven solution is applied to astrophysical objects. We first apply to Sun and to young T Tauri stars and to late type supergiant stars. The 2-D nature of the solutions is presented. We note that the model fails to describe the outflow at the stellar surface because it needs relatively high initial velocities. In that area the magnetic field plays probably an important role. I deduce the role of the centrifugal force in the solutions comparing it with the thermal pressure force, the radiative force and gravity. The result is that the influence of the centrifugal force is negligible. Moreover, I apply the thermally and radiatively driven solution in Range I to B type supergiants. The problem of the high initial velocity at the stellar surface is waved when the radiative force is important. The results coincide with observations. In chapter 6, the haracteristics of the model are summarized and compared with previous models.

Theory for planetary exospheres: III. Radiation pressure effect on the Circular Restricted Three Body Problem and its implication on planetary atmospheres

NASA Astrophysics Data System (ADS)

Beth, A.; Garnier, P.; Toublanc, D.; Dandouras, I.; Mazelle, C.

2016-12-01

The planetary exospheres are poorly known in their outer parts, since the neutral densities are low compared with the instruments detection capabilities. The exospheric models are thus often the main source of information at such high altitudes. We present a new way to take into account analytically the additional effect of the stellar radiation pressure on planetary exospheres. In a series of papers, we present with a Hamiltonian approach the effect of the radiation pressure on dynamical trajectories, density profiles and escaping thermal flux. Our work is a generalization of the study by Bishop and Chamberlain [1989] Icarus, 81, 145-163. In this third paper, we investigate the effect of the stellar radiation pressure on the Circular Restricted Three Body Problem (CR3BP), called also the photogravitational CR3BP, and its implication on the escape and the stability of planetary exospheres, especially for hot Jupiters. In particular, we describe the transformation of the equipotentials and the location of the Lagrange points, and we provide a modified equation for the Hill sphere radius that includes the influence of the radiation pressure. Finally, an application to the hot Jupiter HD 209458b and hot Neptune GJ 436b reveals the existence of a blow-off escape regime induced by the stellar radiation pressure.

O-star parameters from line profiles of wind-blanketed model atmospheres

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

Voels, S.A.

1989-01-01

The basic stellar parameters (i.e. effective temperature, gravity, helium content, bolometric correction, etc...) of several O-stars are determined by matching high signal-to-noise observed line profiles of optical hydrogen and helium line transitions with theoretical line profiles from a core-halo model of the stellar atmosphere. The core-halo atmosphere includes the effect of radiation backscattered from a stellar wind by incorporating the stellar wind model of Abbott and Lucy as a reflective upper boundary condition in the Mihalas atmosphere model. Three of the four supergiants analyzed showed an enhanced surface abundance of helium. Using a large sample of equivalent width data frommore » Conti a simple argument is made that surface enhancement of helium may be a common property of the most luminous supergiants. The stellar atmosphere theory is sufficient to determine the stellar parameters only if careful attention is paid to the detection and exclusion of lines which are not accurately modeled by the physical processes included. It was found that some strong lines which form entirely below the sonic point are not well modeled due to effects of atmospheric extension. For spectral class 09.5, one of these lines is the classification line He I {lambda}4471{angstrom}. For supergiant, the gravity determined could be systematically low by up to 0.05 dex as the radiation pressure due to lines is neglected. Within the error ranges, the stellar parameters determined, including helium abundance, agree with those from the stellar evolution calculations of Maeder and Maynet.« less

Radiative capture reactions in astrophysics

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

Brune, Carl R.; Davids, Barry

Here, the radiative capture reactions of greatest importance in nuclear astrophysics are identified and placed in their stellar contexts. Recent experimental efforts to estimate their thermally averaged rates are surveyed.

Radiative capture reactions in astrophysics

DOE PAGES

Brune, Carl R.; Davids, Barry

2015-08-07

Here, the radiative capture reactions of greatest importance in nuclear astrophysics are identified and placed in their stellar contexts. Recent experimental efforts to estimate their thermally averaged rates are surveyed.

ON THE FATE OF THE MATTER REINSERTED WITHIN YOUNG NUCLEAR STELLAR CLUSTERS

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

Hueyotl-Zahuantitla, Filiberto; Palous, Jan; Wuensch, Richard

2013-04-01

This paper presents a hydrodynamical model describing the evolution of the gas reinserted by stars within a rotating young nuclear star cluster (NSC). We explicitly consider the impact of the stellar component on the flow by means of a uniform insertion of mass and energy within the stellar cluster. The model includes the gravity force of the stellar component and a central supermassive black hole (SMBH), and accounts for the heating from the central source of radiation and the radiative cooling of the thermalized gas. By using a set of parameters typical for NSCs and SMBHs in Seyfert galaxies, ourmore » simulations show that a filamentary/clumpy structure is formed in the inner part of the cluster. This 'torus' is Compton-thick and covers a large fraction of the sky (as seen from the SMBH). In the outer parts of the cluster a powerful wind is produced that inhibits the infall of matter from larger scales and thus the NSC-SMBH interplay occurs in isolation.« less

NESSY: NLTE spectral synthesis code for solar and stellar atmospheres

NASA Astrophysics Data System (ADS)

Tagirov, R. V.; Shapiro, A. I.; Schmutz, W.

2017-07-01

Context. Physics-based models of solar and stellar magnetically-driven variability are based on the calculation of synthetic spectra for various surface magnetic features as well as quiet regions, which are a function of their position on the solar or stellar disc. Such calculations are performed with radiative transfer codes tailored for modeling broad spectral intervals. Aims: We aim to present the NLTE Spectral SYnthesis code (NESSY), which can be used for modeling of the entire (UV-visible-IR and radio) spectra of solar and stellar magnetic features and quiet regions. Methods: NESSY is a further development of the COde for Solar Irradiance (COSI), in which we have implemented an accelerated Λ-iteration (ALI) scheme for co-moving frame (CMF) line radiation transfer based on a new estimate of the local approximate Λ-operator. Results: We show that the new version of the code performs substantially faster than the previous one and yields a reliable calculation of the entire solar spectrum. This calculation is in a good agreement with the available observations.

Impurities in a non-axisymmetric plasma. Transport and effect on bootstrap current

DOE PAGES

Mollén, A.; Landreman, M.; Smith, H. M.; ...

2015-11-20

Impurities cause radiation losses and plasma dilution, and in stellarator plasmas the neoclassical ambipolar radial electric field is often unfavorable for avoiding strong impurity peaking. In this work we use a new continuum drift-kinetic solver, the SFINCS code (the Stellarator Fokker-Planck Iterative Neoclassical Conservative Solver) [M. Landreman et al., Phys. Plasmas 21 (2014) 042503] which employs the full linearized Fokker-Planck-Landau operator, to calculate neoclassical impurity transport coefficients for a Wendelstein 7-X (W7-X) magnetic configuration. We compare SFINCS calculations with theoretical asymptotes in the high collisionality limit. We observe and explain a 1/nu-scaling of the inter-species radial transport coefficient at lowmore » collisionality, arising due to the field term in the inter-species collision operator, and which is not found with simplified collision models even when momentum correction is applied. However, this type of scaling disappears if a radial electric field is present. We use SFINCS to analyze how the impurity content affects the neoclassical impurity dynamics and the bootstrap current. We show that a change in plasma effective charge Z eff of order unity can affect the bootstrap current enough to cause a deviation in the divertor strike point locations.« less

Neoclassical impurity transport in stellarator geometry

NASA Astrophysics Data System (ADS)

García-Regaña, J. M.; Beidler, C. D.; Kleiber, R.; Turkin, Y.; Maaßberg, H.; Helander, P.; Kauffmann, K.

2012-03-01

The appearance of a (neoclassical) inward radial electric field in stellarators is known to cause, under certain plasma conditions, the accumulation of impurities in the core, and sometimes the subsequent plasma radiative collapse. Quantitatively neoclassical theory has barely covered the impurity transport due to the conventional neglect of the assumed first order electrostatic potential and density, φ1 and n1 respectively, in the drift kinetic ordering. This practice, which ignores the fulfilment of the quasi-neutrality condition, carries intrinsically the assumption Z|e|φ1/kBT1, with Z the atomic number, |e| the unit charge, kB the Boltzmann constant and T the temperature. This inequality, valid for the bulk plasma, is violated by high Z impurities. In this work the δf PIC Monte Carlo code EUTERPE [1] together with the GSRAKE code [2] are used to obtain the first numerical output of neoclassical impurity dynamics retaining φ1 and n1 in the drift kinetic equation. The case of the LHD stellarator is considered.[4pt] [1] V. Kornilov et al, Nucl. Fusion 45 238, 2005.[0pt] [2] D. Beidler and W. D. D'haeseleer, Plasma Phys. Control. Fusion 37 463, 1995.

Formation of stellar clusters in magnetized, filamentary infrared dark clouds

NASA Astrophysics Data System (ADS)

Li, Pak Shing; Klein, Richard I.; McKee, Christopher F.

2018-01-01

Star formation in a filamentary infrared dark cloud (IRDC) is simulated over the dynamic range of 4.2 pc to 28 au for a period of 3.5 × 105 yr, including magnetic fields and both radiative and outflow feedback from the protostars. At the end of the simulation, the star formation efficiency is 4.3 per cent and the star formation rate per free-fall time is εff ≃ 0.04, within the range of observed values. The total stellar mass increases as ∼t2, whereas the number of protostars increases as ∼t1.5. We find that the density profile around most of the simulated protostars is ∼ρ ∝ r-1.5. At the end of the simulation, the protostellar mass function approaches the Chabrier stellar initial mass function. We infer that the time to form a star of median mass 0.2 M⊙ is about 1.4 × 105 yr from the median mass accretion rate. We find good agreement among the protostellar luminosities observed in the large sample of Dunham et al., our simulation and a theoretical estimate, and we conclude that the classical protostellar luminosity problem is resolved. The multiplicity of the stellar systems in the simulation agrees, to within a factor of 2, with observations of Class I young stellar objects; most of the simulated multiple systems are unbound. Bipolar protostellar outflows are launched using a subgrid model, and extend up to 1 pc from their host star. The mass-velocity relation of the simulated outflows is consistent with both observation and theory.

A meeting with the universe: Science discoveries from the space program

NASA Technical Reports Server (NTRS)

French, B. M. (Editor); Maran, S. P. (Editor)

1981-01-01

A general history of space exploration is presented. The solar system is discussed. The Sun-Earth relationship is considered, including magnetic fields, solar wind, the magnetosphere, and the Sun-weather relationship. The universe beyond the solar system is discussed. Topics include stellar and galactic evolution, quasars and intergalactic space. The effects of weightlessness and ionizing radiation on human beings are considered. The possibility of extraterrestrial life is discussed. Lunar and planetary exploration, solar-terrestrial physics, astrophysics, biomedical research and exobiology are reviewed. Numerons color illustrations are included.

A meeting with the universe: Science discoveries from the space program

NASA Astrophysics Data System (ADS)

French, Bevan M.; Maran, Stephen P.; Chipman, Eric G.

A general history of space exploration is presented. The solar system is discussed. The Sun-Earth relationship is considered, including magnetic fields, solar wind, the magnetosphere, and the Sun-weather relationship. The universe beyond the solar system is discussed. Topics include stellar and galactic evolution, quasars and intergalactic space. The effects of weightlessness and ionizing radiation on human beings are considered. The possibility of extraterrestrial life is discussed. Lunar and planetary exploration, solar-terrestrial physics, astrophysics, biomedical research and exobiology are reviewed. Numerous color illustrations are included.

Stellar feedback strongly alters the amplification and morphology of galactic magnetic fields

NASA Astrophysics Data System (ADS)

Su, Kung-Yi; Hayward, Christopher C.; Hopkins, Philip F.; Quataert, Eliot; Faucher-Giguère, Claude-André; Kereš, Dušan

2018-01-01

Using high-resolution magnetohydrodynamic simulations of idealized, non-cosmological galaxies, we investigate how cooling, star formation and stellar feedback affect galactic magnetic fields. We find that the amplification histories, saturation values and morphologies of the magnetic fields vary considerably depending on the baryonic physics employed, primarily because of differences in the gas density distribution. In particular, adiabatic runs and runs with a subgrid (effective equation of state) stellar feedback model yield lower saturation values and morphologies that exhibit greater large-scale order compared with runs that adopt explicit stellar feedback and runs with cooling and star formation but no feedback. The discrepancies mostly lie in gas denser than the galactic average, which requires cooling and explicit fragmentation to capture. Independent of the baryonic physics included, the magnetic field strength scales with gas density as B ∝ n2/3, suggesting isotropic flux freezing or equipartition between the magnetic and gravitational energies during the field amplification. We conclude that accurate treatments of cooling, star formation and stellar feedback are crucial for obtaining the correct magnetic field strength and morphology in dense gas, which, in turn, is essential for properly modelling other physical processes that depend on the magnetic field, such as cosmic ray feedback.

The effect of an offset polar cap dipolar magnetic field on the modeling of the Vela pulsar’s γ-ray light curves

PubMed Central

Barnard, M.; Venter, C.; Harding, A. K.

2018-01-01

We performed geometric pulsar light curve modeling using static, retarded vacuum, and offset polar cap (PC) dipole B-fields (the latter is characterized by a parameter ε), in conjunction with standard two-pole caustic (TPC) and outer gap (OG) emission geometries. The offset-PC dipole B-field mimics deviations from the static dipole (which corresponds to ε = 0). In addition to constant-emissivity geometric models, we also considered a slot gap (SG) E-field associated with the offset-PC dipole B-field and found that its inclusion leads to qualitatively different light curves. Solving the particle transport equation shows that the particle energy only becomes large enough to yield significant curvature radiation at large altitudes above the stellar surface, given this relatively low E-field. Therefore, particles do not always attain the radiation-reaction limit. Our overall optimal light curve fit is for the retarded vacuum dipole field and OG model, at an inclination angle α=78−1+1° and observer angle ζ=69−1+2°. For this B-field, the TPC model is statistically disfavored compared to the OG model. For the static dipole field, neither model is significantly preferred. We found that smaller values of ε are favored for the offset-PC dipole field when assuming constant emissivity, and larger ε values favored for variable emissivity, but not significantly so. When multiplying the SG E-field by a factor of 100, we found improved light curve fits, with α and ζ being closer to best fits from independent studies, as well as curvature radiation reaction at lower altitudes. PMID:29681648

The Effect of an Offset Polar Cap Dipolar Magnetic Field on the Modeling of the Vela Pulsar's Gamma-Ray Light Curves

NASA Technical Reports Server (NTRS)

Barnard, M.; Venter, C.; Harding, A. K.

2016-01-01

We performed geometric pulsar light curve modeling using static, retarded vacuum, and offset polar cap (PC) dipole B-fields (the latter is characterized by a parameter epsilon), in conjunction with standard two-pole caustic (TPC) and outer gap (OG) emission geometries. The offset-PC dipole B-field mimics deviations from the static dipole (which corresponds to epsilon equals 0). In addition to constant-emissivity geometric models, we also considered a slot gap (SG) E-field associated with the offset-PC dipole B-field and found that its inclusion leads to qualitatively different light curves. Solving the particle transport equation shows that the particle energy only becomes large enough to yield significant curvature radiation at large altitudes above the stellar surface, given this relatively low E-field. Therefore, particles do not always attain the radiation-reaction limit. Our overall optimal light curve fit is for the retarded vacuum dipole field and OG model, at an inclination angle alpha equals 78 plus or minus 1 degree and observer angle zeta equals 69 plus 2 degrees or minus 1 degree. For this B-field, the TPC model is statistically disfavored compared to the OG model. For the static dipole field, neither model is significantly preferred. We found that smaller values of epsilon are favored for the offset-PC dipole field when assuming constant emissivity, and larger epsilon values favored for variable emissivity, but not significantly so. When multiplying the SG E-field by a factor of 100, we found improved light curve fits, with alpha and zeta being closer to best fits from independent studies, as well as curvature radiation reaction at lower altitudes.

Subresolution Activity in Solar and Stellar Coronae from Magnetic Field Line Tangling

NASA Astrophysics Data System (ADS)

Rappazzo, A. F.; Dahlburg, R. B.; Einaudi, G.; Velli, M.

2018-05-01

The heating of coronal loops is investigated to understand the observational consequences in terms of the thermodynamics and radiative losses from the Sun as well as the magnetized coronae of stars with an outer convective envelope. The dynamics of the Parker coronal heating model are studied for different ratios of the photospheric forcing velocity timescale tp to the Alfvén crossing time along a loop tA. It is shown that for tp/tA ≳ 10-24 the heating rate and maximum temperature are largest and approximately independent of tp/tA, leading to a strong emission in X-rays and EUV. On the opposite decreasing tp/tA to smaller values leads to lower heating rates and plasma temperatures, and consequently fading high-energy radiative emission once tp/tA ≲ 1-3. The average volumetric loop heating rate is shown to scale as ℓ _p u_p B_0^2/4π L^2, where ℓp and up are respectively the convective granule length-scale and velocity, B0 is the intensity of the strong magnetic field threading the loop, and L the loop length. These findings support a recent hypothesis explaining ultracool dwarf observations of stars with similar magnetic field strength but radically different topologies displaying different radiative emission.

Young Stellar Object Candidates in the Aquila Rift Region

NASA Astrophysics Data System (ADS)

Zhang, Miao-miao; Wang, Hong-chi; Stecklum, B.

2010-10-01

Using the 2m telescope of the Turingia State Observatory at Tauten-berg (TLS), imaging observations in 3 wavebands (H α, R and I) are performed in the 16 fields in the Aquila Rift region. The observed fields cover about 7 square degrees. Excluding the 3 fields with unqualified data, the photometrical analysis is made for the remaining 13 fields, from which point sources are identified, and finally 7 H α emission-line star candidates are identified by color-color diagrams. The 7 candidates are located in five fields. Three of them are located near the Galactic plane, while the galactic latitudes of the rest are greater than 4°. The 2 M ASS counterparts of the point sources are identified, and the properties of the 7 H α emission-line star candidates are further analyzed by using the two-color diagrams. It is found that the near-infrared radiation from these H α emission-line star candidates has no obvious infrared excess, one of them even falls on the main-sequence branch. This indicates that the H α-emissive young stellar objects (YSOs) are not always accompanied with the infrared excess, and that the results of the H α emission line observation and the infrared excess observation are mutually supplemented. If the 7 H α emission-line star candidates are regarded as YSO candidates, then the number of YSOs in the Aquila Rift region is quite small. The further confirmation of these candidates needs subsequent spectral observations.

A chirp, a roar and a whisper

NASA Astrophysics Data System (ADS)

Wijers, Ralph

2018-02-01

In 2017, gravitational waves and electromagnetic radiation were detected from the merger of two stellar remnants called neutron stars. An observational analysis reveals how this radiation was released from the merger.

Robust GRMHD Evolutions of Merging Black-Hole Binaries in Magnetized Plasma

NASA Astrophysics Data System (ADS)

Kelly, Bernard; Etienne, Zachariah; Giacomazzo, Bruno; Baker, John

2016-03-01

Black-hole binary (BHB) mergers are expected to be powerful sources of gravitational radiation at stellar and galactic scales. A typical astrophysical environment for these mergers will involve magnetized plasmas accreting onto each hole; the strong-field gravitational dynamics of the merger may churn this plasma in ways that produce characteristic electromagnetic radiation visible to high-energy EM detectors on and above the Earth. Here we return to a cutting-edge GRMHD simulation of equal-mass BHBs in a uniform plasma, originally performed with the Whisky code. Our new tool is the recently released IllinoisGRMHD, a compact, highly-optimized ideal GRMHD code that meshes with the Einstein Toolkit. We establish consistency of IllinoisGRMHD results with the older Whisky results, and investigate the robustness of these results to changes in initial configuration of the BHB and the plasma magnetic field, and discuss the interpretation of the ``jet-like'' features seen in the Poynting flux post-merger. Work supported in part by NASA Grant 13-ATP13-0077.

Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited).

PubMed

Krychowiak, M; Adnan, A; Alonso, A; Andreeva, T; Baldzuhn, J; Barbui, T; Beurskens, M; Biel, W; Biedermann, C; Blackwell, B D; Bosch, H S; Bozhenkov, S; Brakel, R; Bräuer, T; Brotas de Carvalho, B; Burhenn, R; Buttenschön, B; Cappa, A; Cseh, G; Czarnecka, A; Dinklage, A; Drews, P; Dzikowicka, A; Effenberg, F; Endler, M; Erckmann, V; Estrada, T; Ford, O; Fornal, T; Frerichs, H; Fuchert, G; Geiger, J; Grulke, O; Harris, J H; Hartfuß, H J; Hartmann, D; Hathiramani, D; Hirsch, M; Höfel, U; Jabłoński, S; Jakubowski, M W; Kaczmarczyk, J; Klinger, T; Klose, S; Knauer, J; Kocsis, G; König, R; Kornejew, P; Krämer-Flecken, A; Krawczyk, N; Kremeyer, T; Książek, I; Kubkowska, M; Langenberg, A; Laqua, H P; Laux, M; Lazerson, S; Liang, Y; Liu, S C; Lorenz, A; Marchuk, A O; Marsen, S; Moncada, V; Naujoks, D; Neilson, H; Neubauer, O; Neuner, U; Niemann, H; Oosterbeek, J W; Otte, M; Pablant, N; Pasch, E; Sunn Pedersen, T; Pisano, F; Rahbarnia, K; Ryć, L; Schmitz, O; Schmuck, S; Schneider, W; Schröder, T; Schuhmacher, H; Schweer, B; Standley, B; Stange, T; Stephey, L; Svensson, J; Szabolics, T; Szepesi, T; Thomsen, H; Travere, J-M; Trimino Mora, H; Tsuchiya, H; Weir, G M; Wenzel, U; Werner, A; Wiegel, B; Windisch, T; Wolf, R; Wurden, G A; Zhang, D; Zimbal, A; Zoletnik, S

2016-11-01

Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters in December 2015. At the end of the 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were in operation, allowing detailed studies of many interesting plasma phenomena. For example, fast neutral gas manometers supported by video cameras (including one fast-frame camera with frame rates of tens of kHz) as well as visible cameras with different interference filters, with field of views covering all ten half-modules of the stellarator, discovered a MARFE-like radiation zone on the inboard side of machine module 4. This structure is presumably triggered by an inadvertent plasma-wall interaction in module 4 resulting in a high impurity influx that terminates some discharges by radiation cooling. The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s. Although OP1.1 is characterized by short pulses, many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH. An overview of diagnostic performance for OP1.1 is given, including some highlights from the physics campaigns.

High-resolution ultraviolet radiation fields of classical T Tauri stars

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

France, Kevin; Schindhelm, Eric; Bergin, Edwin A.

2014-04-01

The far-ultraviolet (FUV; 912-1700 Å) radiation field from accreting central stars in classical T Tauri systems influences the disk chemistry during the period of giant planet formation. The FUV field may also play a critical role in determining the evolution of the inner disk (r < 10 AU), from a gas- and dust-rich primordial disk to a transitional system where the optically thick warm dust distribution has been depleted. Previous efforts to measure the true stellar+accretion-generated FUV luminosity (both hot gas emission lines and continua) have been complicated by a combination of low-sensitivity and/or low-spectral resolution and did not includemore » the contribution from the bright Lyα emission line. In this work, we present a high-resolution spectroscopic study of the FUV radiation fields of 16 T Tauri stars whose dust disks display a range of evolutionary states. We include reconstructed Lyα line profiles and remove atomic and molecular disk emission (from H{sub 2} and CO fluorescence) to provide robust measurements of both the FUV continuum and hot gas lines (e.g., Lyα, N V, C IV, He II) for an appreciable sample of T Tauri stars for the first time. We find that the flux of the typical classical T Tauri star FUV radiation field at 1 AU from the central star is ∼10{sup 7} times the average interstellar radiation field. The Lyα emission line contributes an average of 88% of the total FUV flux, with the FUV continuum accounting for an average of 8%. Both the FUV continuum and Lyα flux are strongly correlated with C IV flux, suggesting that accretion processes dominate the production of both of these components. On average, only ∼0.5% of the total FUV flux is emitted between the Lyman limit (912 Å) and the H{sub 2} (0-0) absorption band at 1110 Å. The total and component-level high-resolution radiation fields are made publicly available in machine-readable format.« less

Radiation Hydrodynamics with GIZMO: The Disruption of Giant Molecular Clouds by Stellar Radiation Pressure

NASA Astrophysics Data System (ADS)

Khatami, David; Hopkins, Philip F.

2016-01-01

We present a numerical implementation of radiation hydrodynamics for the meshless code GIZMO. The radiation transport is treated as an anisotropic diffusion process combined with radiation pressure effects, photoionization with heating and cooling routines, and a multifrequency treatment of an arbitrary number of sources. As a first application of the method, we investigate the disruption of giant molecular clouds by stellar radiative feedback. Specifically, what fraction of the gas must a GMC convert into stars to cause self-disruption? We test a range of cloud masses and sizes with several source luminosities to probe the effects of photoheating and radiation pressure on timescales shorter than the onset of the first supernovae. Observationally, only ~1-10% of gas is converted into stars, an inefficiency that is likely the result of feedback from newly formed stars. Whether photoheating or radiation pressure dominates is dependent on the given cloud properties. For denser clouds, we expect photoheating to play a negligible role with most of the feedback driven by radiation pressure. This work explores the necessary parameters a GMC must have in order for radiation pressure to be the main disruption process.

Recent advances in non-LTE stellar atmosphere models

NASA Astrophysics Data System (ADS)

Sander, Andreas A. C.

2017-11-01

In the last decades, stellar atmosphere models have become a key tool in understanding massive stars. Applied for spectroscopic analysis, these models provide quantitative information on stellar wind properties as well as fundamental stellar parameters. The intricate non-LTE conditions in stellar winds dictate the development of adequate sophisticated model atmosphere codes. The increase in both, the computational power and our understanding of physical processes in stellar atmospheres, led to an increasing complexity in the models. As a result, codes emerged that can tackle a wide range of stellar and wind parameters. After a brief address of the fundamentals of stellar atmosphere modeling, the current stage of clumped and line-blanketed model atmospheres will be discussed. Finally, the path for the next generation of stellar atmosphere models will be outlined. Apart from discussing multi-dimensional approaches, I will emphasize on the coupling of hydrodynamics with a sophisticated treatment of the radiative transfer. This next generation of models will be able to predict wind parameters from first principles, which could open new doors for our understanding of the various facets of massive star physics, evolution, and death.

Significant and variable linear polarization during the prompt optical flash of GRB 160625B.

PubMed

Troja, E; Lipunov, V M; Mundell, C G; Butler, N R; Watson, A M; Kobayashi, S; Cenko, S B; Marshall, F E; Ricci, R; Fruchter, A; Wieringa, M H; Gorbovskoy, E S; Kornilov, V; Kutyrev, A; Lee, W H; Toy, V; Tyurina, N V; Budnev, N M; Buckley, D A H; González, J; Gress, O; Horesh, A; Panasyuk, M I; Prochaska, J X; Ramirez-Ruiz, E; Lopez, R Rebolo; Richer, M G; Román-Zúñiga, C; Serra-Ricart, M; Yurkov, V; Gehrels, N

2017-07-26

Newly formed black holes of stellar mass launch collimated outflows (jets) of ionized matter that approach the speed of light. These outflows power prompt, brief and intense flashes of γ-rays known as γ-ray bursts (GRBs), followed by longer-lived afterglow radiation that is detected across the electromagnetic spectrum. Measuring the polarization of the observed GRB radiation provides a direct probe of the magnetic fields in the collimated jets. Rapid-response polarimetric observations of newly discovered bursts have probed the initial afterglow phase, and show that, minutes after the prompt emission has ended, the degree of linear polarization can be as high as 30 per cent-consistent with the idea that a stable, globally ordered magnetic field permeates the jet at large distances from the central source. By contrast, optical and γ-ray observations during the prompt phase have led to discordant and often controversial results, and no definitive conclusions have been reached regarding the origin of the prompt radiation or the configuration of the magnetic field. Here we report the detection of substantial (8.3 ± 0.8 per cent from our most conservative simulation), variable linear polarization of a prompt optical flash that accompanied the extremely energetic and long-lived prompt γ-ray emission from GRB 160625B. Our measurements probe the structure of the magnetic field at an early stage of the jet, closer to its central black hole, and show that the prompt phase is produced via fast-cooling synchrotron radiation in a large-scale magnetic field that is advected from the black hole and distorted by dissipation processes within the jet.

Significant and variable linear polarization during the prompt optical flash of GRB 160625B.

NASA Astrophysics Data System (ADS)

Troja, E.; Lipunov, V. M.; Mundell, C. G.; Butler, N. R.; Watson, A. M.; Kobayashi, S.; Cenko, S. B.; Marshall, F. E.; Ricci, R.; Fruchter, A.; Wieringa, M. H.; Gorbovskoy, E. S.; Kornilov, V.; Kutyrev, A.; Lee, W. H.; Toy, V.; Tyurina, N. V.; Budnev, N. M.; Buckley, D. A. H.; González, J.; Gress, O.; Horesh, A.; Panasyuk, M. I.; Prochaska, J. X.; Ramirez-Ruiz, E.; Rebolo Lopez, R.; Richer, M. G.; Roman-Zuniga, C.; Serra-Ricart, M.; Yurkov, V.; Gehrels, N.

2017-07-01

Newly formed black holes of stellar mass launch collimated outflows (jets) of ionized matter that approach the speed of light. These outflows power prompt, brief and intense flashes of γ-rays known as γ-ray bursts (GRBs), followed by longer-lived afterglow radiation that is detected across the electromagnetic spectrum. Measuring the polarization of the observed GRB radiation provides a direct probe of the magnetic fields in the collimated jets. Rapid-response polarimetric observations of newly discovered bursts have probed the initial afterglow phase, and show that, minutes after the prompt emission has ended, the degree of linear polarization can be as high as 30 per cent - consistent with the idea that a stable, globally ordered magnetic field permeates the jet at large distances from the central source. By contrast, optical and γ-ray observations during the prompt phase have led to discordant and often controversial results, and no definitive conclusions have been reached regarding the origin of the prompt radiation or the configuration of the magnetic field. Here we report the detection of substantial (8.3 ± 0.8 per cent from our most conservative simulation), variable linear polarization of a prompt optical flash that accompanied the extremely energetic and long-lived prompt γ-ray emission from GRB 160625B. Our measurements probe the structure of the magnetic field at an early stage of the jet, closer to its central black hole, and show that the prompt phase is produced via fast-cooling synchrotron radiation in a large-scale magnetic field that is advected from the black hole and distorted by dissipation processes within the jet.

Investigating the FUV Emission of Young M dwarfs with FUMES: the Far Ultraviolet M-dwarf Evolution Survey

NASA Astrophysics Data System (ADS)

Pineda, John

2016-10-01

M dwarf stars have become attractive candidates for exoplanet searches and will be a main focus of the upcoming TESS mission, with the continued search for nearby potentially habitable worlds. However, the atmospheric characterization of these exoplanetary systems depends critically on the high energy stellar radiation environment from X-ray to NUV. Strong radiation at these energies can lead to atmospheric mass loss and is a strong driver of photochemistry in planetary atmospheres. Recently, the MUSCLES Treasury Survey (Cycles 19, 22) provided the first comprehensive assessment of the high energy radiation field around old, planet hosting M dwarfs. However, the habitability and potential for such exoplanetary atmospheres to develop life also depends on the evolution of the atmosphere and hence the evolution of the incident radiation field. The strong high energy spectrum of young M dwarfs can have devastating consequences for the potential habitability of a given system. We, thus, propose the Far Ultraviolet M-dwarf Evolution Survey (FUMES) to measure the strong FUV coronal/chromospheric emission features of young M dwarfs (12 - 650 Myr), e.g. He II, C IV, and S IV. FUMES will observe objects with a wide range of rotation rates to directly connect the emission features to the evolution of coronal heating and upper atmospheric structure, and provide observational benchmarks at young ages for models of M dwarf upper atmospheres. Building on results from MUSCLES, we will be able to estimate the whole high energy radiation field and establish the evolutionary picture of the incident radiation throughout the lifetime of exoplanetary systems around early-mid M dwarf hosts.

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

KSC-08pd2455

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, a technician cleans the edge of the radiator on the Wide Field Camera 3, or WFC3,that will be installed on NASA's Hubble Space Telescope. The radiator is the "outside" of WFC3 that will be exposed to space. It will expel heat out of Hubble and into space through black body radiation. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

KSC-08pd2468

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, a technician guides a crane for attachment to the radiator on the Wide Field Camera 3, or WFC3. The WFC3 will be transferred to the Super Lightweight Interchangeable Carrier. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. The radiator is the "outside" of WFC3 that will be exposed to space and will expel heat out of Hubble and into space through black body radiation. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

A two-column formalism for time-dependent modelling of stellar convection. I. Description of the method

NASA Astrophysics Data System (ADS)

Stökl, A.

2008-11-01

Context: In spite of all the advances in multi-dimensional hydrodynamics, investigations of stellar evolution and stellar pulsations still depend on one-dimensional computations. This paper devises an alternative to the mixing-length theory or turbulence models usually adopted in modelling convective transport in such studies. Aims: The present work attempts to develop a time-dependent description of convection, which reflects the essential physics of convection and that is only moderately dependent on numerical parameters and far less time consuming than existing multi-dimensional hydrodynamics computations. Methods: Assuming that the most extensive convective patterns generate the majority of convective transport, the convective velocity field is described using two parallel, radial columns to represent up- and downstream flows. Horizontal exchange, in the form of fluid flow and radiation, over their connecting interface couples the two columns and allows a simple circulating motion. The main parameters of this convective description have straightforward geometrical meanings, namely the diameter of the columns (corresponding to the size of the convective cells) and the ratio of the cross-section between up- and downdrafts. For this geometrical setup, the time-dependent solution of the equations of radiation hydrodynamics is computed from an implicit scheme that has the advantage of being unaffected by the Courant-Friedrichs-Lewy time-step limit. This implementation is part of the TAPIR-Code (short for The adaptive, implicit RHD-Code). Results: To demonstrate the approach, results for convection zones in Cepheids are presented. The convective energy transport and convective velocities agree with expectations for Cepheids and the scheme reproduces both the kinetic energy flux and convective overshoot. A study of the parameter influence shows that the type of solution derived for these stars is in fact fairly robust with respect to the constitutive numerical parameters.

The dependence of stellar properties on initial cloud density

NASA Astrophysics Data System (ADS)

Jones, Michael O.; Bate, Matthew R.

2018-05-01

We investigate the dependence of stellar properties on the initial mean density of the molecular cloud in which stellar clusters form using radiation hydrodynamical simulations that resolve the opacity limit for fragmentation. We have simulated the formation of three star clusters from the gravitational collapse of molecular clouds whose densities vary by a factor of a hundred. As with previous calculations including radiative feedback, we find that the dependence of the characteristic stellar mass, Mc, on the initial mean density of the cloud, ρ, is weaker than the dependence of the thermal Jeans mass. However, unlike previous calculations, which found no statistically significant variation in the median mass with density, we find a weak dependence approximately of the form Mc∝ρ-1/5. The distributions of properties of multiple systems do not vary significantly between the calculations. We compare our results to the result of observational surveys of star-forming regions, and suggest that the similarities between the properties of our lowest density calculation and the nearby Taurus-Auriga region indicate that the apparent excess of solar-type stars observed may be due to the region's low density.

The Resolved Stellar Populations Early Release Science Program

NASA Astrophysics Data System (ADS)

Gilbert, Karoline; Weisz, Daniel; Resolved Stellar Populations ERS Program Team

2018-06-01

The Resolved Stellar Populations Early Release Science Program (PI D. Weisz) will observe Local Group targets covering a range of stellar density and star formation histories, including a globular cluster, and ultra-faint dwarf galaxy, and a star-forming dwarf galaxy. Using observations of these diverse targets we will explore a broad science program: we will measure star formation histories, the sub-solar stellar initial mass function, and proper motions, perform studies of evolved stars, and map extinction in the target fields. Our observations will be of high archival value for other science such as calibrating stellar evolution models, studying variable stars, and searching for metal-poor stars. We will determine optimal observational setups and develop data reduction techniques that will be common to JWST studies of resolved stellar populations. We will also design, test, and release point spread function (PSF) fitting software specific to NIRCam and NIRISS, required for the crowded stellar regime. Prior to the Cycle 2 Call for Proposals, we will release PSF fitting software, matched HST and JWST catalogs, and clear documentation and step-by-step tutorials (such as Jupyter notebooks) for reducing crowded stellar field data and producing resolved stellar photometry catalogs, as well as for specific resolved stellar photometry science applications.

Stellar Models of Rotating, PMS Stars with Magnetic Fields

NASA Astrophysics Data System (ADS)

Mendes, L. T. S.; Landin, N. R.; Vaz, L. P. R.

2014-10-01

We report our ongoing studies of the magnetic field effects on the structure and evolution of low-mass stars, using a method first proposed by Lydon & Sofia (1995, ApJS 101, 357) which treats the magnetic field as a perturbation on the stellar structure equations. The ATON 2.3 stellar evolution code (Ventura et al. 1998, A&A 334, 953) now includes, via this method, the effects of an imposed, parametric magnetic field whose surface strength scales throughout the stellar interior according to one of the three following laws: (a) the ratio between the magnetic and gas energy densities, β_{mg}, is kept at its surface value across the stellar interior, (b) β_{mg} has a shallower decrease in deeper layers, or (c) β_{mg} decays as [m(r)/M_{*}]^{2/3}. We then computed rotating stellar models, starting at the pre-main sequence phase, of 0.4, 0.6, 0.8 and 1.0 M_{odot} with solar chemical composition, mixing-length convection treatment with &alpha=λ/H_{P}=1.5 and surface magnetic field strength of 50 G. Summarizing our main findings: (1) we confirm that the magnetic field inhibits convection and so reduces the convective envelope; (2) the magnetic perturbation effect dominates over that of rotation for 0.8 and 1.0 M_{odot} masses, but their relative impact shows a reversal during the Hayashi tracks at lower masses (0.4 and 0.6 M_{odot}); in any case, the magnetic perturbation makes the tracks cooler; and (3) the magnetic field contributes to higher surface lithium abundances.

Asymmetric MHD outflows/jets from accreting T Tauri stars

NASA Astrophysics Data System (ADS)

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

2015-06-01

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

Photospheres of hot stars. IV - Spectral type O4

NASA Technical Reports Server (NTRS)

Bohannan, Bruce; Abbott, David C.; Voels, Stephen A.; Hummer, David G.

1990-01-01

The basic stellar parameters of a supergiant (Zeta Pup) and two main-sequence stars, 9 Sgr and HD 46223, at spectral class O4 are determined using line profile analysis. The stellar parameters are determined by comparing high signal-to-noise hydrogen and helium line profiles with those from stellar atmosphere models which include the effect of radiation scattered back onto the photosphere from an overlying stellar wind, an effect referred to as wind blanketing. At spectral class O4, the inclusion of wind-blanketing in the model atmosphere reduces the effective temperature by an average of 10 percent. This shift in effective temperature is also reflected by shifts in several other stellar parameters relative to previous O4 spectral-type calibrations. It is also shown through the analysis of the two O4 V stars that scatter in spectral type calibrations is introduced by assuming that the observed line profile reflects the photospheric stellar parameters.

Tracing the First Stars with Fluctuations of the Cosmic Infrared Background

NASA Technical Reports Server (NTRS)

Kashlinsky, A.; Arendt, R. G.; Mather, J.; Moseley, S. H.

2005-01-01

The deepest space- and ground-based observations find metal-enriched galaxies at cosmic times when the Universe was less than 1 Gyr old. These stellar populations had to be preceded by the metal-free first stars, known as 'population III'. Recent cosmic microwave background polarization measurements indicate that stars started forming early-when the Universe was 5200 Myr old. It is now thought that population III stars were significantly more massive than the present metal-rich stellar populations. Although such sources will not be individually detectable by existing or planned telescopes, they would have produced significant cosmic infrared background radiation in the near-infrared, whose fluctuations reflect the conditions in the primordial density field. Here we report a measurement of diffuse flux fluctuations after removing foreground stars and galaxies. The anisotropies exceed the instrument noise and the more local foregrounds; they can be attributed to emission from population III stars, at an era dominated by these objects.

A GRID OF THREE-DIMENSIONAL STELLAR ATMOSPHERE MODELS OF SOLAR METALLICITY. I. GENERAL PROPERTIES, GRANULATION, AND ATMOSPHERIC EXPANSION

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

Trampedach, Regner; Asplund, Martin; Collet, Remo

2013-05-20

Present grids of stellar atmosphere models are the workhorses in interpreting stellar observations and determining their fundamental parameters. These models rely on greatly simplified models of convection, however, lending less predictive power to such models of late-type stars. We present a grid of improved and more reliable stellar atmosphere models of late-type stars, based on deep, three-dimensional (3D), convective, stellar atmosphere simulations. This grid is to be used in general for interpreting observations and improving stellar and asteroseismic modeling. We solve the Navier Stokes equations in 3D and concurrent with the radiative transfer equation, for a range of atmospheric parameters,more » covering most of stellar evolution with convection at the surface. We emphasize the use of the best available atomic physics for quantitative predictions and comparisons with observations. We present granulation size, convective expansion of the acoustic cavity, and asymptotic adiabat as functions of atmospheric parameters.« less

POET: Planetary Orbital Evolution due to Tides

NASA Astrophysics Data System (ADS)

Penev, Kaloyan

2014-08-01

POET (Planetary Orbital Evolution due to Tides) calculates the orbital evolution of a system consisting of a single star with a single planet in orbit under the influence of tides. The following effects are The evolutions of the semimajor axis of the orbit due to the tidal dissipation in the star and the angular momentum of the stellar convective envelope by the tidal coupling are taken into account. In addition, the evolution includes the transfer of angular momentum between the stellar convective and radiative zones, effect of the stellar evolution on the tidal dissipation efficiency, and stellar core and envelope spins and loss of stellar convective zone angular momentum to a magnetically launched wind. POET can be used out of the box, and can also be extended and modified.

Polycyclic aromatic hydrocarbon optical properties and contribution to the acceleration of stellar outflows

NASA Technical Reports Server (NTRS)

Cherchneff, Isabelle; Barker, John R.; Tielens, Alexander G. G. M.

1991-01-01

The optical constants of four polycyclic aromatic hydrocarbon (PAH) molecules (benzene, pyrene, pentacene, and coronene) are determined from their measured laboratory absorption spectra. The Planck mean of the radiation pressure cross section is computed for each molecule and for amorphous carbon (AC) grains, and semiempirically estimated for large PAH molecules up to 400 carbon atoms. Assuming that PAHs are present in carbon-rich stellar outflows, the radiation pressure forces acting on them are calculated and compared with the radiation forces on AC particles. The results show that PAHs possess very different optical properties from AC grains. Small PAHs may experience an 'inverse greenhouse' effect in the inner part of the envelope, as they decouple from the gas close to the photosphere. The radiation pressure force on PAHs is always much less than the force at work on AC grains, and PAH molecules do not affect significantly the dynamics of the outflow.

MERLIN observations of water maser proper motions in VY Canis Majoris

NASA Astrophysics Data System (ADS)

Richards, A. M. S.; Yates, J. A.; Cohen, R. J.

1998-09-01

MERLIN observations of the 22-GHz water masers in the circumstellar envelope of the supergiant VY CMa show an ellipsoidal distribution with a maximum extent of 700 mas east-west and 400 mas north-south. Comparison with observations made nine years earlier shows that the majority of maser features have survived and show proper motions throughout the region. The mean change in position is 28 mas and the proper motions are generally directed away from the assumed stellar position, and tend to be larger for features at greater projected distances. If the H_2O maser region is modelled as a partially filled thick spherical shell, and VY CMa is at a distance of 1.5 kpc, then the proper motion velocities in the direction of expansion are between 8kms^-1 at a distance of 75 mas from the assumed stellar position and 32kms^-1 at 360 mas. These velocities are consistent with the H_2O maser spectral line velocities which correspond to a maximum expansion velocity of 36kms^-1 at 400 mas from the assumed stellar position. These observations are consistent with radiation pressure on dust providing the force to accelerate the stellar wind as it passes through the H_2O maser shell. The H_2O maser region is elongated in the same direction as the dusty nebula around VY CMa. The water masers illuminate the small-scale dynamics and clumpiness which show the role of dust in driving the outflow. The overall ellipsoidal shape may be due to properties of the dust, such as its behaviour in the stellar magnetic field, or to interaction between the wind and circumstellar material. Maser monitoring also shows the difference between changes on the time-scale of stellar variability (a few years) and possible stages in the evolution of VY CMa to its likely fate as a supernova.

Narrow Radiative Recombination Continua: A Signature of Ions Crossing the Contact Discontinuity of Astrophysical Shocks

NASA Technical Reports Server (NTRS)

Behar, Ehud; Nordon, Raanan; Soker, Noam; Kastner, Joel H.; Yu, Young Sam

2009-01-01

X-rays from planetary nebulae (PNs) are believed to originate from a shock driven into the fast stellar wind (v 1000 kilometers per second) as it collides with an earlier circumstellar slow wind (v 10 kilometers per second). In theory, the shocked fast wind (hot hubble) and the ambient cold nebula can remain separated by magnetic fields along a surface referred to as the contact discontinuity (CD) that inhibits diffusion and heat conduction. The CD region is extremely difficult to probe directly owing to its small size and faint emission. This has largely left the study of CDs, stellar-shocks, and the associated micro-physics in the realm of theory. This paper presents spectroscopic evidence for ions from the hot bubble (kT approximately equal to 100 eV) crossing the CD and penetrating the cold nebular gas (kT approximately equal to 1 eV). Specifically, a narrow radiative recombination continuum (RRC) emission feature is identified in the high resolution X-ray spectrum of the PN BD+30degree3639 indicating bare C VII ions are recombining with cool electrons at kT(sub e) = 1.7 plus or minus 1.3 eV. An upper limit to the flux of the narrow RRC of H-like C VI is obtained as well. The RRCs are interpreted as due to C ions from the hot bubble of BD+30degree3639 crossing the CD into the cold nebula, where they ultimately recombine with its cool electrons. The RRC flux ratio of C VII to C VI constrains the temperature jump across the CD to deltakT greater than 80 eV, providing for the first time direct evidence for the stark temperature disparity between the two sides of an astrophysical CD, and constraining the role of magnetic fields and heat conduction accordingly. Two colliding-wind binaries are noted to have similar RRCs suggesting a temperature jump and CD crossing by ions may be common feature of stellar wind shocks.

STELLAR SURFACE MAGNETO-CONVECTION AS A SOURCE OF ASTROPHYSICAL NOISE. I. MULTI-COMPONENT PARAMETERIZATION OF ABSORPTION LINE PROFILES

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

Cegla, H. M.; Shelyag, S.; Watson, C. A.

2013-02-15

We outline our techniques to characterize photospheric granulation as an astrophysical noise source. A four-component parameterization of granulation is developed that can be used to reconstruct stellar line asymmetries and radial velocity shifts due to photospheric convective motions. The four components are made up of absorption line profiles calculated for granules, magnetic intergranular lanes, non-magnetic intergranular lanes, and magnetic bright points at disk center. These components are constructed by averaging Fe I 6302 A magnetically sensitive absorption line profiles output from detailed radiative transport calculations of the solar photosphere. Each of the four categories adopted is based on magnetic fieldmore » and continuum intensity limits determined from examining three-dimensional magnetohydrodynamic simulations with an average magnetic flux of 200 G. Using these four-component line profiles we accurately reconstruct granulation profiles, produced from modeling 12 Multiplication-Sign 12 Mm{sup 2} areas on the solar surface, to within {approx} {+-}20 cm s{sup -1} on a {approx}100 m s{sup -1} granulation signal. We have also successfully reconstructed granulation profiles from a 50 G simulation using the parameterized line profiles from the 200 G average magnetic field simulation. This test demonstrates applicability of the characterization to a range of magnetic stellar activity levels.« less

The Young Massive Stellar Cluster Sandage-96 after the Explosion of SN 2004DJ in NGC 2403

NASA Technical Reports Server (NTRS)

Vinko, J.; Sarneczky, K.; Balog, Z.; Immler, S.; Sugerman, B.; Brown, P. J.; Misselt, K.; Szabo, Gy. M.; Klagyivik, P.; Kun, M.;

Experimental design to understand the interaction of stellar radiation with molecular clouds

NASA Astrophysics Data System (ADS)

VanDervort, Robert; Davis, Josh; Trantham, Matt; Klein, Sallee; Frank, Yechiel; Raicher, Erez; Fraenkel, Moshe; Shvarts, Dov; Keiter, Paul; Drake, R. Paul

2017-06-01

Enhanced star formation triggered by local O and B type stars is an astrophysical problem of interest. O and B type stars are massive, hot stars that emit an enormous amount of radiation. This radiation acts to either compress or blow apart clumps of gas in the interstellar media. For example, in the optically thick limit, when the x-ray radiation in the gas clump has a short mean free path length the x-ray radiation is absorbed near the clump edge and compresses the clump. In the optically thin limit, when the mean free path is long, the radiation is absorbed throughout acting to heat the clump. This heating explodes the gas clump. Careful selection of parameters, such as foam density or source temperature, allow the experimental platform to access different hydrodynamic regimes. The stellar radiation source is mimicked by a laser irradiated thin gold foil. This will provide a source of thermal x-rays (around ~100 eV). The gas clump is mimicked by a low-density foam around 0.150 g/cc. Simulations were done using radiation hydrodynamics codes to tune the experimental parameters. The experiment will be carried out at the Omega laser facility on OMEGA 60.

Maser emission from planetary and stellar magnetospheres

NASA Astrophysics Data System (ADS)

Speirs, David

2012-07-01

A variety of astrophysical radio emissions have been identified to date in association with non-uniform magnetic fields and charged particle streams. From terrestrial auroral kilometric radiation (AKR) to observations of auroral radio emission from the flare star UV Ceti and CU Virginis, there are numerous examples of this intense, highly polarised magnetospheric radio signature [1][2]. Characterised by discrete spectral components at ~300kHz in the terrestrial auroral case, the radiation is clearly non-thermal and there is a strong belief that such emissions are generated by an electron cyclotron maser instability [1]. Previous work has focussed on a loss cone generation mechanism and cavity ducting model for radiation beaming, however recent theory and simulations suggest an alternative model comprising emission driven by an electron horseshoe distribution [1]. Such distributions are formed when particles descend into the increasing magnetic field of planetary / stellar auroral magnetospheres, where conservation of the magnetic moment results in conversion of axial momentum into rotational momentum. Theory has demonstrated that such distributions are highly unstable to cyclotron emission in the X-mode [3], and that these emissions when propagating tangential to the plasma cavity boundary may refract upwards due to plasma density inhomogeneity [4]. Scaled experiments have been conducted at the University of Strathclyde to study the emission process under controlled laboratory conditions [5]. In addition, numerical models have simulated the emission mechanism in the presence of a background plasma and in the absence of radiation boundaries [6]. Here we present the results of beam-plasma simulations that confirm the radiation model for tangential growth and upward refraction [4] and agree with recent Jodrell Bank observations of pulsed, narrowly beamed radio emission from the oblique rotator star CU Virginis [2]. [1] R. Bingham and R. A. Cairns, Phys. Plasmas, 7, 3089 (2000). [2] B.J. Kellett, V. Graffagnino, R. Bingham et al., ArXiv Astrophysics, 0701214 (2007). [3] R.A. Cairns, I. Vorgul, R. Bingham et al., Phys. Plasmas 18, 022902 (2011). [4] J.D. Menietti, R.L. Mutel, I.W. Christopher et al., J. Geophys. Res., 116, A12219 (2011). [5] S.L. McConville, M.E. Koepke, K.M. Gillespie et al., Plasma Phys. Control. Fusion, 53, 124020 (2011). [6] D.C. Speirs, K. Ronald, S.L. McConville, Phys. Plasmas, 17, 056501 (2010).

THE FORMATION OF SECONDARY STELLAR GENERATIONS IN MASSIVE YOUNG STAR CLUSTERS FROM RAPIDLY COOLING SHOCKED STELLAR WINDS

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

Wünsch, R.; Palouš, J.; Ehlerová, S.

We study a model of rapidly cooling shocked stellar winds in young massive clusters and estimate the circumstances under which secondary star formation, out of the reinserted winds from a first stellar generation (1G), is possible. We have used two implementations of the model: a highly idealized, computationally inexpensive, spherically symmetric semi-analytic model, and a complex, three-dimensional radiation-hydrodynamic, simulation; they are in a good mutual agreement. The results confirm our previous findings that, in a cluster with 1G mass 10{sup 7} M {sub ⊙} and half-mass–radius 2.38 pc, the shocked stellar winds become thermally unstable, collapse into dense gaseous structuresmore » that partially accumulate inside the cluster, self-shield against ionizing stellar radiation, and form the second generation (2G) of stars. We have used the semi-analytic model to explore a subset of the parameter space covering a wide range of the observationally poorly constrained parameters: the heating efficiency, η {sub he}, and the mass loading, η {sub ml}. The results show that the fraction of the 1G stellar winds accumulating inside the cluster can be larger than 50% if η {sub he} ≲ 10%, which is suggested by the observations. Furthermore, for low η {sub he}, the model provides a self-consistent mechanism predicting 2G stars forming only in the central zones of the cluster. Finally, we have calculated the accumulated warm gas emission in the H30 α recombination line, analyzed its velocity profile, and estimated its intensity for super star clusters in interacting galaxies NGC4038/9 (Antennae) showing that the warm gas should be detectable with ALMA.« less

Chandra LETGS observation of the active binary Algol

NASA Astrophysics Data System (ADS)

Ness, J.-U.; Schmitt, J. H. M. M.; Burwitz, V.; Mewe, R.; Predehl, P.

2002-06-01

A high-resolution spectrum obtained with the low-energy transmission grating onboard the Chandra observatory is presented and analyzed. Our analysis indicates very hot plasma with temperatures up to T~ 15-20 MK from the continuum and from ratios of hydrogen-like and helium-like ions of Si, Mg, and Ne. In addition lower temperature material is present since O VII and N VI are detected. Two methods for density diagnostics are applied. The He-like triplets from N VII to Si XIII are used and densities around 1011 cm-3 are found for the low temperature ions. Taking the UV radiation field from the B star companion into account, we find that the low-Z ions can be affected by the radiation field quite strongly, such that densities of 3x 1010 cm-3 are also possible, but only assuming that the emitting plasma is immersed in the radiation field. For the high temperature He-like ions only low density limits are found. Using ratios of Fe XXI lines produced at similar temperatures are sensitive to lower densities but again yield only low density limits. We thus conclude that the hot plasma has densities below 1012 cm-3. Assuming a constant pressure corona we show that the characteristic loop sizes must be small compared to the stellar radius and that filling factors below 0.1 are unlikely.

General-relativistic pulsar magnetospheric emission

NASA Astrophysics Data System (ADS)

Pétri, J.

2018-06-01

Most current pulsar emission models assume photon production and emission within the magnetosphere. Low-frequency radiation is preferentially produced in the vicinity of the polar caps, whereas the high-energy tail is shifted to regions closer but still inside the light cylinder. We conducted a systematic study of the merit of several popular radiation sites like the polar cap, the outer gap, and the slot gap. We computed sky maps emanating from each emission site according to a prescribed distribution function for the emitting particles made of an electron/positron mixture. Calculations are performed using a three-dimensional integration of the plasma emissivity in the vacuum electromagnetic field of a rotating and centred general-relativistic dipole. We compare Newtonian electromagnetic fields to their general-relativistic counterpart. In the latter case, light bending is also taken into account. As a typical example, light curves and sky maps are plotted for several power-law indices of the particle distribution function. The detailed pulse profiles strongly depend on the underlying assumption about the fluid motion subject to strong electromagnetic fields. This electromagnetic topology enforces the photon propagation direction directly, or indirectly, from aberration effects. We also discuss the implication of a net stellar electric charge on to sky maps. Taking into account, the electric field strongly affects the light curves originating close to the light cylinder, where the electric field strength becomes comparable to the magnetic field strength.

Strong stellar winds.

PubMed

Conti, P S; McCray, R

1980-04-04

The hottest and most luminous stars lose a substantial fraction of their mass in strong stellar winds. These winds not only affect the evolution of the star, they also carve huge expanding cavities in the surrounding interstellar medium, possibly affecting star formation. The winds are probably driven by radiation pressure, but uncertainties persist in their theoretical description. Strong x-ray sources associated with a few of these hot stars may be used to probe the stellar winds. The nature of the weak x-ray sources recently observed to be associated with many of these stars is uncertain. It is suggested that roughly 10 percent of the luminous hot stars may have as companions neutron stars or black holes orbiting within the stellar winds.

Line overlap and self-shielding of molecular hydrogen in galaxies

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

Gnedin, Nickolay Y.; Draine, Bruce T., E-mail: gnedin@fnal.gov, E-mail: andrey@oddjob.uchicago.edu, E-mail: draine@astro.princeton.edu

2014-11-01

The effect of line overlap in the Lyman and Werner bands, often ignored in galactic studies of the atomic-to-molecular transition, greatly enhances molecular hydrogen self-shielding in low metallicity environments and dominates over dust shielding for metallicities below about 10% solar. We implement that effect in cosmological hydrodynamics simulations with an empirical model, calibrated against the observational data, and provide fitting formulae for the molecular hydrogen fraction as a function of gas density on various spatial scales and in environments with varied dust abundance and interstellar radiation field. We find that line overlap, while important for detailed radiative transfer in themore » Lyman and Werner bands, has only a minor effect on star formation on galactic scales, which, to a much larger degree, is regulated by stellar feedback.« less

Investigating the 3D Structure of the Winds of Hot Supergiants

NASA Astrophysics Data System (ADS)

Klement, Robert

2018-04-01

An observational effort targeting supergiant stars of spectral classes B and A has been started using the VEGA high spectral resolution visible beam combiner at the CHARA array. The H-alpha emission from the structured stellar winds was resolved with respect to the surrounding continuum, showing signs of inhomogenities in the circumstellar environments as well as temporal variability on different time scales. We have begun a radiative transfer modelling effort to investigate the clumpy structure of the stellar winds and the origin of the inhomogenities, probably linked to the stellar photosphere features.

Relaxational effects in radiating stellar collapse

NASA Astrophysics Data System (ADS)

Govender, Megan; Maartens, Roy; Maharaj, Sunil D.

1999-12-01

Relaxational effects in stellar heat transport can in many cases be significant. Relativistic Fourier-Eckart theory is inherently quasi-stationary, and cannot incorporate these effects. The effects are naturally accounted for in causal relativistic thermodynamics, which provides an improved approximation to kinetic theory. Recent results, based on perturbations of a static star, show that relaxation effects can produce a significant increase in the central temperature and temperature gradient for a given luminosity. We use a simple stellar model that allows for non-perturbative deviations from staticity, and confirms qualitatively the predictions of the perturbative models.

The detection of gravitational waves using electrodynamic system of Earth

NASA Astrophysics Data System (ADS)

Grunskaya, Lubov; Isakevich, Valiriy

There is studied the interconnection of tide processes of geophysical and astrophysical origin with the Earth electromagnetic fields. There has been developed a programme-analytical system (PAS) to investigate signal structures in spectral and time series, caused by geophysical and astrophysical processes based on the method of eigen vectors. There were discovered frequencies in the electrical and geomagnetical field of ELF range with PAS, which coincide with the frequency of gravitational -wave radiation of a number of double stellar systems. In the electrical and geomagnetic field there was discovered a specific axion frequency VA=0.5*10-5 Hz belonging to the ELF range which was predicted by the theory. The problem of the anomalous behavior of the electrodynamic system response to the gravitational - wave affect is being discussed. On the basis of the rich experimental material have been investigated the frequencies of gravitational-wave radiation of a number of binary systems: J0700+6418, J1012+5307, J1537+1155, J1959+2048, J2130+1210, J1915+1606. The work is carried out with supporting of RFFI No. 14-07-97510, State Task to Universities on 2014-2016.

Heliophysics 3 Volume Set

NASA Astrophysics Data System (ADS)

Schrijver, Carolus J.; Siscoe, George L.

2010-11-01

Volume 1: Preface; 1. Prologue Carolus J. Schrijver and George L. Siscoe; 2. Introduction to heliophysics Thomas J. Bogdan; 3. Creation and destruction of magnetic field Matthias Rempel; 4. Magnetic field topology Dana W. Longcope; 5. Magnetic reconnection Terry G. Forbes; 6. Structures of the magnetic field Mark B. Moldwin, George L. Siscoe and Carolus J. Schrijver; 7. Turbulence in space plasmas Charles W. Smith; 8. The solar atmosphere Viggo H. Hansteen; 9. Stellar winds and magnetic fields Viggo H. Hansteen; 10. Fundamentals of planetary magnetospheres Vytenis M. Vasyliūnas; 11. Solar-wind magnetosphere coupling: an MHD perspective Frank R. Toffoletto and George L. Siscoe; 12. On the ionosphere and chromosphere Tim Fuller-Rowell and Carolus J. Schrijver; 13. Comparative planetary environments Frances Bagenal; Bibliography; Index. Volume 2: Preface; 1. Perspective on heliophysics George L. Siscoe and Carolus J. Schrijver; 2. Introduction to space storms and radiation Sten Odenwald; 3. In-situ detection of energetic particles George Gloeckler; 4. Radiative signatures of energetic particles Tim Bastian; 5. Observations of solar and stellar eruptions, flares, and jets Hugh Hudson; 6. Models of coronal mass ejections and flares Terry Forbes; 7. Shocks in heliophysics Merav Opher; 8. Particle acceleration in shocks Dietmar Krauss-Varban; 9. Energetic particle transport Joe Giacalone; 10. Energy conversion in planetary magnetospheres Vytenis Vasyliūnas; 11. Energization of trapped particles Janet Green; 12. Flares, CMEs, and atmospheric responses Tim Fuller-Rowell and Stanley C. Solomon; 13. Energetic particles and manned spaceflight 358 Stephen Guetersloh and Neal Zapp; 14. Energetic particles and technology Alan Tribble; Appendix I. Authors and editors; List of illustrations; List of tables; Bibliography; Index. Volume 3: Preface; 1. Interconnectedness in heliophysics Carolus J. Schrijver and George L. Siscoe; 2. Long-term evolution of magnetic activity of Sun-like stars Carolus J. Schrijver; 3. Formation and early evolution of stars and proto-planetary disks Lee W. Hartmann; 4. Planetary habitability on astronomical time scales Donald E. Brownlee; 5. Solar internal flows and dynamo action Mark S. Miesch; 6. Modeling solar and stellar dynamos Paul Charbonneau; 7. Planetary fields and dynamos Ulrich R. Christensen; 8. The structure and evolution of the 3D solar wind John T. Gosling; 9. The heliosphere and cosmic rays J. Randy Jokipii; 10. Solar spectral irradiance: measurements and models Judith L. Lean and Thomas N. Woods; 11. Astrophysical influences on planetary climate systems Juerg Beer; 12. Evaluating the drivers of Earth's climate system Thomas J. Crowley; 13. Ionospheres of the terrestrial planets Stanley C. Solomon; 14. Long-term evolution of the geospace climate Jan J. Sojka; 15. Waves and transport processes in atmospheres and oceans Richard L. Walterscheid; 16. Solar variability, climate, and atmospheric photochemistry Guy P. Brasseur, Daniel Marsch and Hauke Schmidt; Appendix I. Authors and editors; List of illustrations; List of tables; Bibliography; Index.

Heliophysics 3 Volume Paperback Set

NASA Astrophysics Data System (ADS)

Schrijver, Carolus J.; Siscoe, George L.

2013-03-01

Volume 1: Preface; 1. Prologue Carolus J. Schrijver and George L. Siscoe; 2. Introduction to heliophysics Thomas J. Bogdan; 3. Creation and destruction of magnetic field Matthias Rempel; 4. Magnetic field topology Dana W. Longcope; 5. Magnetic reconnection Terry G. Forbes; 6. Structures of the magnetic field Mark B. Moldwin, George L. Siscoe and Carolus J. Schrijver; 7. Turbulence in space plasmas Charles W. Smith; 8. The solar atmosphere Viggo H. Hansteen; 9. Stellar winds and magnetic fields Viggo H. Hansteen; 10. Fundamentals of planetary magnetospheres Vytenis M. Vasyliunas; 11. Solar-wind magnetosphere coupling: an MHD perspective Frank R. Toffoletto and George L. Siscoe; 12. On the ionosphere and chromosphere Tim Fuller-Rowell and Carolus J. Schrijver; 13. Comparative planetary environments Frances Bagenal; Bibliography; Index. Volume 2: Preface; 1. Perspective on heliophysics George L. Siscoe and Carolus J. Schrijver; 2. Introduction to space storms and radiation Sten Odenwald; 3. In-situ detection of energetic particles George Gloeckler; 4. Radiative signatures of energetic particles Tim Bastian; 5. Observations of solar and stellar eruptions, flares, and jets Hugh Hudson; 6. Models of coronal mass ejections and flares Terry Forbes; 7. Shocks in heliophysics Merav Opher; 8. Particle acceleration in shocks Dietmar Krauss-Varban; 9. Energetic particle transport Joe Giacalone; 10. Energy conversion in planetary magnetospheres Vytenis Vasyliunas; 11. Energization of trapped particles Janet Green; 12. Flares, CMEs, and atmospheric responses Tim Fuller-Rowell and Stanley C. Solomon; 13. Energetic particles and manned spaceflight Stephen Guetersloh and Neal Zapp; 14. Energetic particles and technology Alan Tribble; Appendix I. Authors and editors; List of illustrations; List of tables; Bibliography; Index. Volume 3: Preface; 1. Interconnectedness in heliophysics Carolus J. Schrijver and George L. Siscoe; 2. Long-term evolution of magnetic activity of Sun-like stars Carolus J. Schrijver; 3. Formation and early evolution of stars and proto-planetary disks Lee W. Hartmann; 4. Planetary habitability on astronomical time scales Donald E. Brownlee; 5. Solar internal flows and dynamo action Mark S. Miesch; 6. Modeling solar and stellar dynamos Paul Charbonneau; 7. Planetary fields and dynamos Ulrich R. Christensen; 8. The structure and evolution of the 3D solar wind John T. Gosling; 9. The heliosphere and cosmic rays J. Randy Jokipii; 10. Solar spectral irradiance: measurements and models Judith L. Lean and Thomas N. Woods; 11. Astrophysical influences on planetary climate systems Juerg Beer; 12. Evaluating the drivers of Earth's climate system Thomas J. Crowley; 13. Ionospheres of the terrestrial planets Stanley C. Solomon; 14. Long-term evolution of the geospace climate Jan J. Sojka; 15. Waves and transport processes in atmospheres and oceans Richard L. Walterscheid; 16. Solar variability, climate, and atmospheric photochemistry Guy P. Brasseur, Daniel Marsch and Hauke Schmidt; Appendix I. Authors and editors; List of illustrations; List of tables; Bibliography; Index.

Modeling Gravitational Radiation Waveforms from Black Hole Mergers

NASA Technical Reports Server (NTRS)

Baker, J. G.; Centrelia, J. M.; Choi, D.; Koppitz, M.; VanMeter, J.

2006-01-01

Gravitational radiation from merging binary black hole systems is anticipated as a key source for gravitational wave observations. Ground-based instruments, such as the Laser Interferometer Gravitational-wave Observatory (LIGO) may observe mergers of stellar-scale black holes, while the space-based Laser Interferometer Space Antenna (LISA) observatory will be sensitive to mergers of massive galactic-center black holes over a broad range of mass scales. These cataclysmic events may emit an enormous amount of energy in a brief time. Gravitational waves from comparable mass mergers carry away a few percent of the system's mass-energy in just a few wave cycles, with peak gravitational wave luminosities on the order of 10^23 L_Sun. Optimal analysis and interpretation of merger observation data will depend on developing a detailed understanding, based on general relativistic modeling, of the radiation waveforms. We discuss recent progress in modeling radiation from equal mass mergers using numerical simulations of Einstein's gravitational field equations, known as numerical relativity. Our simulations utilize Adaptive Mesh Refinement (AMR) to allow high-resolution near the black holes while simultaneously keeping the outer boundary of the computational domain far from the black holes, and making it possible to read out gravitational radiation waveforms in the weak-field wave zone. We discuss the results from simulations beginning with the black holes orbiting near the system's innermost stable orbit, comparing the recent simulations with earlier "Lazarus" waveform estimates based on an approximate hybrid numerical/perturbative technique.

Growing massive black holes through supercritical accretion of stellar-mass seeds

NASA Astrophysics Data System (ADS)

Lupi, A.; Haardt, F.; Dotti, M.; Fiacconi, D.; Mayer, L.; Madau, P.

2016-03-01

The rapid assembly of the massive black holes that power the luminous quasars observed at z ˜ 6-7 remains a puzzle. Various direct collapse models have been proposed to head-start black hole growth from initial seeds with masses ˜105 M⊙, which can then reach a billion solar mass while accreting at the Eddington limit. Here, we propose an alternative scenario based on radiatively inefficient supercritical accretion of stellar-mass holes embedded in the gaseous circumnuclear discs (CNDs) expected to exist in the cores of high-redshift galaxies. Our sub-pc resolution hydrodynamical simulations show that stellar-mass holes orbiting within the central 100 pc of the CND bind to very high density gas clumps that arise from the fragmentation of the surrounding gas. Owing to the large reservoir of dense cold gas available, a stellar-mass black hole allowed to grow at super-Eddington rates according to the `slim-disc' solution can increase its mass by three orders of magnitudes within a few million years. These findings are supported by simulations run with two different hydro codes, RAMSES based on the Adaptive Mesh Refinement technique and GIZMO based on a new Lagrangian Godunov-type method, and with similar, but not identical, sub-grid recipes for star formation, supernova feedback, black hole accretion and feedback. The low radiative efficiency of supercritical accretion flows are instrumental to the rapid mass growth of our black holes, as they imply modest radiative heating of the surrounding nuclear environment.

Dust ring formation due to sublimation of dust grains drifting radially inward by the Poynting-Robertson drag: An analytical model

NASA Astrophysics Data System (ADS)

Kobayashi, Hiroshi; Watanabe, Sei-ichiro; Kimura, Hiroshi; Yamamoto, Tetsuo

2009-05-01

Dust particles exposed to the stellar radiation and wind drift radially inward by the Poynting-Robertson (P-R) drag and pile up at the zone where they begin to sublime substantially. The reason they pile up or form a ring is that their inward drifts due to the P-R drag are suppressed by stellar radiation pressure when the ratio of radiation pressure to stellar gravity on them increases during their sublimation phases. We present analytic solutions to the orbital and mass evolution of such subliming dust particles, and find their drift velocities at the pileup zone are almost independent of their initial semimajor axes and masses. We derive analytically an enhancement factor of the number density of the particles at the outer edge of the sublimation zone from the solutions. We show that the formula of the enhancement factor reproduces well numerical simulations in the previous studies. The enhancement factor for spherical dust particles of silicate and carbon extends from 3 to more than 20 at stellar luminosities L=0.8-500L, where L is solar luminosity. Although the enhancement factor for fluffy dust particles is smaller than that for spherical particles, sublimating particles inevitably form a dust ring as long as their masses decrease faster than their surface areas during sublimation. The formulation is applicable to dust ring formation for arbitrary shape and material of dust in dust-debris disks as well as in the Solar System.

[Clinical application of high-pitch excretory phase images during dual-source CT urography with stellar photon detector].

PubMed

Sun, Hao; Xue, Hua-dan; Jin, Zheng-yu; Wang, Xuan; Chen, Yu; He, Yong-lan; Zhang, Da-ming; Zhu, Liang; Wang, Yun; Qi, Bing; Xu, Kai; Wang, Ming

2014-10-01

To retrospectively evaluate the clinical feasibility of high-pitch excretory phase images during dual-source CT urography with Stellar photon detector. Totally 100 patients received dual-source CT high-pitch urinary excretory phase scanning with Stellar photon detector [80 kV, ref.92 mAs, CARE Dose 4D and CARE kV, pitch of 3.0, filter back projection reconstruction algorithm (FBP)] (group A). Another 100 patients received dual-source CT high-pitch urinary excretory phase scanning with common detector(100 kV, ref.140 mAs, CARE Dose 4D, pitch of 3.0, FBP) (group B). Quantitative measurement of CT value of urinary segments (Hounsfield units), image noise (Hounsfield units), and effective radiation dose (millisievert) were compared using independent-samples t test between two groups. Urinary system subjective opacification scores were compared using Mann-Whitney U test between two groups. There was no significant difference in subjective opacification score of intrarenal collecting system and ureters between two groups (all P>0.05). The group A images yielded significantly higher CT values of all urinary segments (all P<0.01). There was no significant difference in image noise (P>0.05). The effective radiation dose of group A (1.1 mSv) was significantly lower than that of group B (3.79 mSv) (P<0.01). High-pitch low-tube-voltage during excretory phase dual-source CT urography with Stellar photon detector is feasible, with acceptable image noise and lower radiation dose.

First Magnetic Field Detection on a Class I Protostar

NASA Astrophysics Data System (ADS)

Johns-Krull, Christopher M.; Greene, Thomas P.; Doppmann, Greg W.; Covey, Kevin R.

2009-08-01

Strong stellar magnetic fields are believed to truncate the inner accretion disks around young stars, redirecting the accreting material to the high latitude regions of the stellar surface. In the past few years, observations of strong stellar fields on T Tauri stars with field strengths in general agreement with the predictions of magnetospheric accretion theory have bolstered this picture. Currently, nothing is known about the magnetic field properties of younger, more embedded Class I young stellar objects. It is believed that protostars accrete much of their final mass during the Class I phase, but the physics governing this process remains poorly understood. Here, we use high-resolution near-infrared spectra obtained with NIRSPEC on Keck and with Phoenix on Gemini South to measure the magnetic field properties of the Class I protostar WL 17. We find clear signatures of a strong stellar magnetic field. Analysis of this data suggests a surface average field strength of 2.9 ± 0.43 kG on WL 17. We present our field measurements and discuss how they fit with the general model of magnetospheric accretion in young stars. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil), and SECYT (Argentina). The Phoenix data were obtained under the program: GS-2006A-C-12.

Modeling CO, CO2, and H2O Ice Abundances in the Envelopes of Young Stellar Objects in the Magellanic Clouds

NASA Astrophysics Data System (ADS)

Pauly, Tyler; Garrod, Robin T.

2018-02-01

Massive young stellar objects (MYSOs) in the Magellanic Clouds show infrared absorption features corresponding to significant abundances of CO, CO2, and H2O ice along the line of sight, with the relative abundances of these ices differing between the Magellanic Clouds and the Milky Way. CO ice is not detected toward sources in the Small Magellanic Cloud, and upper limits put its relative abundance well below sources in the Large Magellanic Cloud and the Milky Way. We use our gas-grain chemical code MAGICKAL, with multiple grain sizes and grain temperatures, and further expand it with a treatment for increased interstellar radiation field intensity to model the elevated dust temperatures observed in the MCs. We also adjust the elemental abundances used in the chemical models, guided by observations of H II regions in these metal-poor satellite galaxies. With a grid of models, we are able to reproduce the relative ice fractions observed in MC MYSOs, indicating that metal depletion and elevated grain temperature are important drivers of the MYSO envelope ice composition. Magellanic Cloud elemental abundances have a subgalactic C/O ratio, increasing H2O ice abundances relative to the other ices; elevated grain temperatures favor CO2 production over H2O and CO. The observed shortfall in CO in the Small Magellanic Cloud can be explained by a combination of reduced carbon abundance and increased grain temperatures. The models indicate that a large variation in radiation field strength is required to match the range of observed LMC abundances. CH3OH abundance is found to be enhanced in low-metallicity models, providing seed material for complex organic molecule formation in the Magellanic Clouds.

Investigating the Magnetospheres of Rapidly Rotating B-type Stars

NASA Astrophysics Data System (ADS)

Fletcher, C. L.; Petit, V.; Nazé, Y.; Wade, G. A.; Townsend, R. H.; Owocki, S. P.; Cohen, D. H.; David-Uraz, A.; Shultz, M.

2017-11-01

Recent spectropolarimetric surveys of bright, hot stars have found that ~10% of OB-type stars contain strong (mostly dipolar) surface magnetic fields (~kG). The prominent paradigm describing the interaction between the stellar winds and the surface magnetic field is the magnetically confined wind shock (MCWS) model. In this model, the stellar wind plasma is forced to move along the closed field loops of the magnetic field, colliding at the magnetic equator, and creating a shock. As the shocked material cools radiatively it will emit X-rays. Therefore, X-ray spectroscopy is a key tool in detecting and characterizing the hot wind material confined by the magnetic fields of these stars. Some B-type stars are found to have very short rotational periods. The effects of the rapid rotation on the X-ray production within the magnetosphere have yet to be explored in detail. The added centrifugal force due to rapid rotation is predicted to cause faster wind outflows along the field lines, leading to higher shock temperatures and harder X-rays. However, this is not observed in all rapidly rotating magnetic B-type stars. In order to address this from a theoretical point of view, we use the X-ray Analytical Dynamical Magnetosphere (XADM) model, originally developed for slow rotators, with an implementation of new rapid rotational physics. Using X-ray spectroscopy from ESA's XMM-Newton space telescope, we observed 5 rapidly rotating B-types stars to add to the previous list of observations. Comparing the observed X-ray luminosity and hardness ratio to that predicted by the XADM allows us to determine the role the added centrifugal force plays in the magnetospheric X-ray emission of these stars.

TESTING GALAXY FORMATION MODELS WITH THE GHOSTS SURVEY: THE COLOR PROFILE OF M81's STELLAR HALO

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

Monachesi, Antonela; Bell, Eric F.; Bailin, Jeremy

2013-04-01

We study the properties of the stellar populations in M81's outermost part, which hereafter we will call the stellar halo, using Hubble Space Telescope (HST) Advanced Camera for Surveys observations of 19 fields from the GHOSTS survey. The observed fields probe the stellar halo out to a projected distance of {approx}50 kpc from the galactic center. Each field was observed in both F606W and F814W filters. The 50% completeness levels of the color-magnitude diagrams (CMDs) are typically at 2 mag below the tip of the red giant branch (TRGB). Fields at distances closer than 15 kpc show evidence of disk-dominatedmore » populations whereas fields at larger distances are mostly populated by halo stars. The red giant branch (RGB) of the M81's halo CMDs is well matched with isochrones of {approx}10 Gyr and metallicities [Fe/H] {approx} - 1.2 dex, suggesting that the dominant stellar population of M81's halo has a similar age and metallicity. The halo of M81 is characterized by a color distribution of width {approx}0.4 mag and an approximately constant median value of (F606W - F814W) {approx}1 mag measured using stars within the magnitude range 23.7 {approx}< F814W {approx}< 25.5. When considering only fields located at galactocentric radius R > 15 kpc, we detect no color gradient in the stellar halo of M81. We place a limit of 0.03 {+-} 0.11 mag difference between the median color of RGB M81 halo stars at {approx}15 and at 50 kpc, corresponding to a metallicity difference of 0.08 {+-} 0.35 dex over that radial range for an assumed constant age of 10 Gyr. We compare these results with model predictions for the colors of stellar halos formed purely via accretion of satellite galaxies. When we analyze the cosmologically motivated models in the same way as the HST data, we find that they predict no color gradient for the stellar halos, in good agreement with the observations.« less

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

NASA Technical Reports Server (NTRS)

Kenyon, S. J.; Hartmann, L.

1987-01-01

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

Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions

PubMed Central

Barnes, R.

2015-01-01

Abstract The internal thermal and magnetic evolution of rocky exoplanets is critical to their habitability. We focus on the thermal-orbital evolution of Earth-mass planets around low-mass M stars whose radiative habitable zone overlaps with the “tidal zone,” where tidal dissipation is expected to be a significant heat source in the interior. We develop a thermal-orbital evolution model calibrated to Earth that couples tidal dissipation, with a temperature-dependent Maxwell rheology, to orbital circularization and migration. We illustrate thermal-orbital steady states where surface heat flow is balanced by tidal dissipation and cooling can be stalled for billions of years until circularization occurs. Orbital energy dissipated as tidal heat in the interior drives both inward migration and circularization, with a circularization time that is inversely proportional to the dissipation rate. We identify a peak in the internal dissipation rate as the mantle passes through a viscoelastic state at mantle temperatures near 1800 K. Planets orbiting a 0.1 solar-mass star within 0.07 AU circularize before 10 Gyr, independent of initial eccentricity. Once circular, these planets cool monotonically and maintain dynamos similar to that of Earth. Planets forced into eccentric orbits can experience a super-cooling of the core and rapid core solidification, inhibiting dynamo action for planets in the habitable zone. We find that tidal heating is insignificant in the habitable zone around 0.45 (or larger) solar-mass stars because tidal dissipation is a stronger function of orbital distance than stellar mass, and the habitable zone is farther from larger stars. Suppression of the planetary magnetic field exposes the atmosphere to stellar wind erosion and the surface to harmful radiation. In addition to weak magnetic fields, massive melt eruption rates and prolonged magma oceans may render eccentric planets in the habitable zone of low-mass stars inhospitable for life. Key Words: Tidal dissipation—Thermal history—Planetary interiors—Magnetic field. Astrobiology 15, 739–760. PMID:26393398

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

Heller, René; Zuluaga, Jorge I., E-mail: rheller@physics.mcmaster.ca, E-mail: jzuluaga@fisica.udea.edu.co

With most planets and planetary candidates detected in the stellar habitable zone (HZ) being super-Earths and gas giants rather than Earth-like planets, we naturally wonder if their moons could be habitable. The first detection of such an exomoon has now become feasible, and due to observational biases it will be at least twice as massive as Mars. However, formation models predict that moons can hardly be as massive as Earth. Hence, a giant planet's magnetosphere could be the only possibility for such a moon to be shielded from cosmic and stellar high-energy radiation. Yet, the planetary radiation belt could alsomore » have detrimental effects on exomoon habitability. Here we synthesize models for the evolution of the magnetic environment of giant planets with thresholds from the runaway greenhouse (RG) effect to assess the habitability of exomoons. For modest eccentricities, we find that satellites around Neptune-sized planets in the center of the HZ around K dwarf stars will either be in an RG state and not be habitable, or they will be in wide orbits where they will not be affected by the planetary magnetosphere. Saturn-like planets have stronger fields, and Jupiter-like planets could coat close-in habitable moons soon after formation. Moons at distances between about 5 and 20 planetary radii from a giant planet can be habitable from an illumination and tidal heating point of view, but still the planetary magnetosphere would critically influence their habitability.« less

Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited)

DOE PAGES

Krychowiak, M.

2016-10-27

Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters in December 2015. At the end of the 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were in operation, allowing detailed studies of many interesting plasma phenomena. For example, fast neutral gas manometers supported by video cameras (including one fast-frame camera with frame rates of tens of kHz) as well as visible cameras with different interference filters, with field of views covering all ten half-modules of the stellarator, discovered amore » MARFE-like radiation zone on the inboard side of machine module 4. This structure is presumably triggered by an inadvertent plasma-wall interaction in module 4 resulting in a high impurity influx that terminates some discharges by radiation cooling. The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s. Although OP1.1 is characterized by short pulses, many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH. Finally, an overview of diagnostic performance for OP1.1 is given, including some highlights from the physics campaigns.« less

Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited)

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

Krychowiak, M.

Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters in December 2015. At the end of the 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were in operation, allowing detailed studies of many interesting plasma phenomena. For example, fast neutral gas manometers supported by video cameras (including one fast-frame camera with frame rates of tens of kHz) as well as visible cameras with different interference filters, with field of views covering all ten half-modules of the stellarator, discovered amore » MARFE-like radiation zone on the inboard side of machine module 4. This structure is presumably triggered by an inadvertent plasma-wall interaction in module 4 resulting in a high impurity influx that terminates some discharges by radiation cooling. The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s. Although OP1.1 is characterized by short pulses, many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH. Finally, an overview of diagnostic performance for OP1.1 is given, including some highlights from the physics campaigns.« less

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

NASA Astrophysics Data System (ADS)

Parfrey, K.; Tchekhovskoy, A.

2017-10-01

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

Understanding the Early Evolution of M dwarf Extreme Ultraviolet Radiation

NASA Astrophysics Data System (ADS)

Peacock, Sarah; Barman, Travis; Shkolnik, Evgenya

2015-11-01

The chemistry and evolution of planetary atmospheres depends on the evolution of high-energy radiation emitted by its host star. High levels of extreme ultraviolet (EUV) radiation can drastically alter the atmospheres of terrestrial planets through ionizing, heating, expanding, chemically modifying and eroding them during the first few billion years of a planetary lifetime. While there is evidence that stars emit their highest levels of far and near ultraviolet (FUV; NUV) radiation in the earliest stages of their evolution, we are currently unable to directly measure the EUV radiation. Most previous stellar atmosphere models under-predict FUV and EUV emission from M dwarfs; here we present new models for M stars that include prescriptions for the hot, lowest density atmospheric layers (chromosphere, transition region and corona), from which this radiation is emitted. By comparing our model spectra to GALEX near and far ultraviolet fluxes, we are able to predict the evolution of EUV radiation for M dwarfs from 10 Myr to a few Gyr. This research is the next major step in the HAZMAT (HAbitable Zones and M dwarf Activity across Time) project to analyze how the habitable zone evolves with the evolving properties of stellar and planetary atmospheres.

PULSATION-TRIGGERED MASS LOSS FROM AGB STARS: THE 60 DAY CRITICAL PERIOD

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

McDonald, I.; Zijlstra, A. A., E-mail: iain.mcdonald-2@jb.man.ac.uk, E-mail: albert.zijlstra@manchester.ac.uk

2016-06-01

Low- and intermediate-mass stars eject much of their mass during the late, red giant branch (RGB) phase of evolution. The physics of their strong stellar winds is still poorly understood. In the standard model, stellar pulsations extend the atmosphere, allowing a wind to be driven through radiation pressure on condensing dust particles. Here, we investigate the onset of the wind, using nearby RGB stars drawn from the Hipparcos catalog. We find a sharp onset of dust production when the star first reaches a pulsation period of 60 days. This approximately coincides with the point where the star transitions to themore » first overtone pulsation mode. Models of the spectral energy distributions show stellar mass-loss rate suddenly increasing at this point, by a factor of ∼10 over the existing (chromospherically driven) wind. The dust emission is strongly correlated with both pulsation period and amplitude, indicating stellar pulsation is the main trigger for the strong mass loss, and determines the mass-loss rate. Dust emission does not strongly correlate with stellar luminosity, indicating radiation pressure on dust has little effect on the mass-loss rate. RGB stars do not normally appear to produce dust, whereas dust production by asymptotic giant branch stars appears commonplace, and is probably ubiquitous above the RGB-tip luminosity. We conclude that the strong wind begins with a step change in mass-loss rate and is triggered by stellar pulsations. A second rapid mass-loss-rate enhancement is suggested when the star transitions to the fundamental pulsation mode at a period of ∼300 days.« less

The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations

NASA Astrophysics Data System (ADS)

Wise, John H.; Turk, Matthew J.; Norman, Michael L.; Abel, Tom

2012-01-01

By definition, Population III stars are metal-free, and their protostellar collapse is driven by molecular hydrogen cooling in the gas phase, leading to large characteristic masses. Population II stars with lower characteristic masses form when the star-forming gas reaches a critical metallicity of 10-6-10-3.5 Z ⊙. We present an adaptive mesh refinement radiation hydrodynamics simulation that follows the transition from Population III to Population II star formation. The maximum spatial resolution of 1 comoving parsec allows for individual molecular clouds to be well resolved and their stellar associations to be studied in detail. We model stellar radiative feedback with adaptive ray tracing. A top-heavy initial mass function for the Population III stars is considered, resulting in a plausible distribution of pair-instability supernovae and associated metal enrichment. We find that the gas fraction recovers from 5% to nearly the cosmic fraction in halos with merger histories rich in halos above 107 M ⊙. A single pair-instability supernova is sufficient to enrich the host halo to a metallicity floor of 10-3 Z ⊙ and to transition to Population II star formation. This provides a natural explanation for the observed floor on damped Lyα systems metallicities reported in the literature, which is of this order. We find that stellar metallicities do not necessarily trace stellar ages, as mergers of halos with established stellar populations can create superpositions of t-Z evolutionary tracks. A bimodal metallicity distribution is created after a starburst occurs when the halo can cool efficiently through atomic line cooling.

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

NASA Technical Reports Server (NTRS)

Hartmann, Lee; Hewett, Robert; Calvet, Nuria

1994-01-01

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

The VLT Interferometer and its AMBER Instrument: Simulations of Interferometric Imaging in the Wide-Field Mode

NASA Astrophysics Data System (ADS)

Blöcker, T.; Hofmann, K.-H.; Przygodda, F.; Weigelt, G.

We present computer simulations of interferometric imaging with the VLT interferometer and the AMBER instrument. These simulations include both the astrophysical modelling of a stellar object by radiative transfer calculations and the simulation of light propagation from the object to the detector (through atmosphere, telescopes, and the AMBER instrument), simulation of photon noise and detector read-out noise, and finally data processing of the interferograms. The results show the dependence of the visibility error bars on the following observational parameters: different seeing during the observation of object and reference star (Fried parameters r0,object and r0,ref. ranging between 0.9 m and 1.2 m), different residual tip-tilt error (δtt,object and δtt,ref. ranging between 0.1% and 20% of the Airy disk diameter), and object brightness (Kobject=3.5 mag to 13 mag, Kref.=3.5 mag). Exemplarily, we focus on stars in late stages of stellar evolution and study one of its key objects, the dusty supergiant IRC +10 420 that is rapidly evolving on human timescales. We show computer simulations of VLT interferometry of IRC +10 420 with two ATs (wide-field mode, i.e. without fiber optics spatial filters) and discuss whether the visibility accuracy is sufficient to distinguish between different theoretical model predictions.

A New Two-fluid Radiation-hydrodynamical Model for X-Ray Pulsar Accretion Columns

NASA Astrophysics Data System (ADS)

West, Brent F.; Wolfram, Kenneth D.; Becker, Peter A.

2017-02-01

Previous research centered on the hydrodynamics in X-ray pulsar accretion columns has largely focused on the single-fluid model, in which the super-Eddington luminosity inside the column decelerates the flow to rest at the stellar surface. This type of model has been relatively successful in describing the overall properties of the accretion flows, but it does not account for the possible dynamical effect of the gas pressure. On the other hand, the most successful radiative transport models for pulsars generally do not include a rigorous treatment of the dynamical structure of the column, instead assuming an ad hoc velocity profile. In this paper, we explore the structure of X-ray pulsar accretion columns using a new, self-consistent, “two-fluid” model, which incorporates the dynamical effect of the gas and radiation pressures, the dipole variation of the magnetic field, the thermodynamic effect of all of the relevant coupling and cooling processes, and a rigorous set of physical boundary conditions. The model has six free parameters, which we vary in order to approximately fit the phase-averaged spectra in Her X-1, Cen X-3, and LMC X-4. In this paper, we focus on the dynamical results, which shed new light on the surface magnetic field strength, the inclination of the magnetic field axis relative to the rotation axis, the relative importance of gas and radiation pressures, and the radial variation of the ion, electron, and inverse-Compton temperatures. The results obtained for the X-ray spectra are presented in a separate paper.

Search for Artificial Stellar Sources of Infrared Radiation.

PubMed

Dyson, F J

1960-06-03

If extraterrestrial intelligent beings exist and have reached a high level of technical development, one by-product of their energy metabolism is likely to be the large-scale conversion of starlight into far-infrared radiation. It is proposed that a search for sources of infrared radiation should accompany the recently initiated search for interstellar radio communications.

Extreme value statistics for two-dimensional convective penetration in a pre-main sequence star

NASA Astrophysics Data System (ADS)

Pratt, J.; Baraffe, I.; Goffrey, T.; Constantino, T.; Viallet, M.; Popov, M. V.; Walder, R.; Folini, D.

2017-08-01

Context. In the interior of stars, a convectively unstable zone typically borders a zone that is stable to convection. Convective motions can penetrate the boundary between these zones, creating a layer characterized by intermittent convective mixing, and gradual erosion of the density and temperature stratification. Aims: We examine a penetration layer formed between a central radiative zone and a large convection zone in the deep interior of a young low-mass star. Using the Multidimensional Stellar Implicit Code (MUSIC) to simulate two-dimensional compressible stellar convection in a spherical geometry over long times, we produce statistics that characterize the extent and impact of convective penetration in this layer. Methods: We apply extreme value theory to the maximal extent of convective penetration at any time. We compare statistical results from simulations which treat non-local convection, throughout a large portion of the stellar radius, with simulations designed to treat local convection in a small region surrounding the penetration layer. For each of these situations, we compare simulations of different resolution, which have different velocity magnitudes. We also compare statistical results between simulations that radiate energy at a constant rate to those that allow energy to radiate from the stellar surface according to the local surface temperature. Results: Based on the frequency and depth of penetrating convective structures, we observe two distinct layers that form between the convection zone and the stable radiative zone. We show that the probability density function of the maximal depth of convective penetration at any time corresponds closely in space with the radial position where internal waves are excited. We find that the maximal penetration depth can be modeled by a Weibull distribution with a small shape parameter. Using these results, and building on established scalings for diffusion enhanced by large-scale convective motions, we propose a new form for the diffusion coefficient that may be used for one-dimensional stellar evolution calculations in the large Péclet number regime. These results should contribute to the 321D link.

A substantial amount of hidden magnetic energy in the quiet Sun.

PubMed

Bueno, J Trujillo; Shchukina, N; Ramos, A Asensio

2004-07-15

Deciphering and understanding the small-scale magnetic activity of the quiet solar photosphere should help to solve many of the key problems of solar and stellar physics, such as the magnetic coupling to the outer atmosphere and the coronal heating. At present, we can see only approximately 1 per cent of the complex magnetism of the quiet Sun, which highlights the need to develop a reliable way to investigate the remaining 99 per cent. Here we report three-dimensional radiative transfer modelling of scattering polarization in atomic and molecular lines that indicates the presence of hidden, mixed-polarity fields on subresolution scales. Combining this modelling with recent observational data, we find a ubiquitous tangled magnetic field with an average strength of approximately 130 G, which is much stronger in the intergranular regions of solar surface convection than in the granular regions. So the average magnetic energy density in the quiet solar photosphere is at least two orders of magnitude greater than that derived from simplistic one-dimensional investigations, and sufficient to balance radiative energy losses from the solar chromosphere.

KSC-08pd2460

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, the Wide Field Camera 3, or WFC3, waits to be transferred to the Super Lightweight Interchangeable Carrier. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. The part shown here is the radiator, the "outside" of WFC3 that will be exposed to space and will expel heat out of Hubble and into space through black body radiation. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

A Stellar Mass Threshold for Quenching of Field Galaxies

NASA Astrophysics Data System (ADS)

Geha, M.; Blanton, M. R.; Yan, R.; Tinker, J. L.

2012-09-01

We demonstrate that dwarf galaxies (107 < M stellar < 109 M ⊙, -12 > Mr > -18) with no active star formation are extremely rare (<0.06%) in the field. Our sample is based on the NASA-Sloan Atlas which is a reanalysis of the Sloan Digital Sky Survey Data Release 8. We examine the relative number of quenched versus star-forming dwarf galaxies, defining quenched galaxies as having no Hα emission (EWHα < 2 Å) and a strong 4000 Å break. The fraction of quenched dwarf galaxies decreases rapidly with increasing distance from a massive host, leveling off for distances beyond 1.5 Mpc. We define galaxies beyond 1.5 Mpc of a massive host galaxy to be in the field. We demonstrate that there is a stellar mass threshold of M stellar < 1.0 × 109 M ⊙ below which quenched galaxies do not exist in the field. Below this threshold, we find that none of the 2951 field dwarf galaxies are quenched; all field dwarf galaxies show evidence for recent star formation. Correcting for volume effects, this corresponds to a 1σ upper limit on the quenched fraction of 0.06%. In more dense environments, quenched galaxies account for 23% of the dwarf population over the same stellar mass range. The majority of quenched dwarf galaxies (often classified as dwarf elliptical galaxies) are within 2 virial radii of a massive galaxy, and only a few percent of quenched dwarf galaxies exist beyond 4 virial radii. Thus, for galaxies with stellar mass less than 1.0 × 109 M ⊙, ending star formation requires the presence of a more massive neighbor, providing a stringent constraint on models of star formation feedback.

Astronomy In Denver: Polarization of Stellar Wind Bow Shocks

NASA Astrophysics Data System (ADS)

Lin, Austin A.; Shrestha, Manisha; Wolfe, Tristan; Stencel, Robert E.; Hoffman, Jennifer L.

2018-06-01

When a star with stellar wind moves through the interstellar medium (ISM) at a relative supersonic velocity, an arch like structure known as a stellar wind bow shock is formed. Studying the characteristics of these structures can further our understanding of evolved stellar winds and the composition of the ISM. Observations of these structures have been performed for some time, but the recent discovery of many bow shock structures have opened more ways to study them. These stellar wind bow shocks display aspherical shapes, which cause light scattering through the dense shock material to become polarized. We selected a target star for observation using a catalog compiled from previous studies and observed it in polarized light with the University of Denver’s DUSTPol instrument. Our group has also simulated the polarization of stellar wind bow shocks using a Monte Carlo radiative transfer code. We present the data from our observations and compare them with the simulations. We also discuss the contribution of interstellar polarization to the data.

A relation between the characteristic stellar ages of galaxies and their intrinsic shapes

NASA Astrophysics Data System (ADS)

van de Sande, Jesse; Scott, Nicholas; Bland-Hawthorn, Joss; Brough, Sarah; Bryant, Julia J.; Colless, Matthew; Cortese, Luca; Croom, Scott M.; d'Eugenio, Francesco; Foster, Caroline; Goodwin, Michael; Konstantopoulos, Iraklis S.; Lawrence, Jon S.; McDermid, Richard M.; Medling, Anne M.; Owers, Matt S.; Richards, Samuel N.; Sharp, Rob

2018-06-01

Stellar population and stellar kinematic studies provide unique but complementary insights into how galaxies build-up their stellar mass and angular momentum1-3. A galaxy's mean stellar age reveals when stars were formed, but provides little constraint on how the galaxy's mass was assembled. Resolved stellar dynamics4 trace the change in angular momentum due to mergers, but major mergers tend to obscure the effect of earlier interactions5. With the rise of large multi-object integral field spectroscopic surveys, such as SAMI6 and MaNGA7, and single-object integral field spectroscopic surveys (for example, ATLAS3D (ref. 8), CALIFA9, MASSIVE10), it is now feasible to connect a galaxy's star formation and merger history on the same resolved physical scales, over a large range in galaxy mass, morphology and environment4,11,12. Using the SAMI Galaxy Survey, here we present a combined study of spatially resolved stellar kinematics and global stellar populations. We find a strong correlation of stellar population age with location in the (V/σ, ɛe) diagram that links the ratio of ordered rotation to random motions in a galaxy to its observed ellipticity. For the large majority of galaxies that are oblate rotating spheroids, we find that characteristic stellar age follows the intrinsic ellipticity of galaxies remarkably well.

A relation between the characteristic stellar ages of galaxies and their intrinsic shapes

NASA Astrophysics Data System (ADS)

van de Sande, Jesse; Scott, Nicholas; Bland-Hawthorn, Joss; Brough, Sarah; Bryant, Julia J.; Colless, Matthew; Cortese, Luca; Croom, Scott M.; d'Eugenio, Francesco; Foster, Caroline; Goodwin, Michael; Konstantopoulos, Iraklis S.; Lawrence, Jon S.; McDermid, Richard M.; Medling, Anne M.; Owers, Matt S.; Richards, Samuel N.; Sharp, Rob

2018-04-01

Stellar population and stellar kinematic studies provide unique but complementary insights into how galaxies build-up their stellar mass and angular momentum1-3. A galaxy's mean stellar age reveals when stars were formed, but provides little constraint on how the galaxy's mass was assembled. Resolved stellar dynamics4 trace the change in angular momentum due to mergers, but major mergers tend to obscure the effect of earlier interactions5. With the rise of large multi-object integral field spectroscopic surveys, such as SAMI6 and MaNGA7, and single-object integral field spectroscopic surveys (for example, ATLAS3D (ref. 8), CALIFA9, MASSIVE10), it is now feasible to connect a galaxy's star formation and merger history on the same resolved physical scales, over a large range in galaxy mass, morphology and environment4,11,12. Using the SAMI Galaxy Survey, here we present a combined study of spatially resolved stellar kinematics and global stellar populations. We find a strong correlation of stellar population age with location in the (V/σ, ɛe) diagram that links the ratio of ordered rotation to random motions in a galaxy to its observed ellipticity. For the large majority of galaxies that are oblate rotating spheroids, we find that characteristic stellar age follows the intrinsic ellipticity of galaxies remarkably well.

Three-dimensional photogrammetric measurement of magnetic field lines in the WEGA stellarator

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

Drewelow, Peter; Braeuer, Torsten; Otte, Matthias

2009-12-15

The magnetic confinement of plasmas in fusion experiments can significantly degrade due to perturbations of the magnetic field. A precise analysis of the magnetic field in a stellarator-type experiment utilizes electrons as test particles following the magnetic field line. The usual fluorescent detector for this electron beam limits the provided information to two-dimensional cut views at certain toroidal positions. However, the technique described in this article allows measuring the three-dimensional structure of the magnetic field by means of close-range photogrammetry. After testing and optimizing the main diagnostic components, measurements of the magnetic field lines were accomplished with a spatial resolutionmore » of 5 mm. The results agree with numeric calculations, qualifying this technique as an additional tool to investigate magnetic field configurations in a stellarator. For a possible future application, ways are indicated on how to reduce experimental error sources.« less

Quasi-Axially Symmetric Stellarators with 3 Field Periods

NASA Astrophysics Data System (ADS)

Garabedian, Paul; Ku, Long-Poe

1998-11-01

Compact hybrid configurations with 2 field periods have been studied recently as candidates for a proof of principle experiment at PPPL, cf. A. Reiman et al., Physics design of a high beta quasi-axially symmetric stellarator, J. Plas. Fus. Res. SERIES 1, 429(1998). This enterprise has led us to the discovery of a family of quasi-axially symmetric stellarators with 3 field periods that seem to have significant advantages, although their aspect ratios are a little larger. They have reversed shear and perform better in a local analysis of ballooning modes. Nonlinear equilibrium and stability calculations predict that the average beta limit may be as high as 6% if the bootstrap current turns out to be as big as that expected in comparable tokamaks. The concept relies on a combination of helical fields and bootstrap current to achieve adequate rotational transform at low aspect ratio. A detailed manuscript describing some of this work will be published soon, cf. P.R. Garabedian, Quasi-axially symmetric stellarators, Proc. Natl. Acad. Sci. USA 95 (1998).

Magnetic inhibition of convection and the fundamental properties of low-mass stars. I. Stars with a radiative core

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

Feiden, Gregory A.; Chaboyer, Brian, E-mail: gregory.a.feiden.gr@dartmouth.edu, E-mail: brian.chaboyer@dartmouth.edu

2013-12-20

Magnetic fields are hypothesized to inflate the radii of low-mass stars—defined as less massive than 0.8 M {sub ☉}—in detached eclipsing binaries (DEBs). We investigate this hypothesis using the recently introduced magnetic Dartmouth stellar evolution code. In particular, we focus on stars thought to have a radiative core and convective outer envelope by studying in detail three individual DEBs: UV Psc, YY Gem, and CU Cnc. Our results suggest that the stabilization of thermal convection by a magnetic field is a plausible explanation for the observed model-radius discrepancies. However, surface magnetic field strengths required by the models are significantly strongermore » than those estimated from observed coronal X-ray emission. Agreement between model predicted surface magnetic field strengths and those inferred from X-ray observations can be found by assuming that the magnetic field sources its energy from convection. This approach makes the transport of heat by convection less efficient and is akin to reduced convective mixing length methods used in other studies. Predictions for the metallicity and magnetic field strengths of the aforementioned systems are reported. We also develop an expression relating a reduction in the convective mixing length to a magnetic field strength in units of the equipartition value. Our results are compared with those from previous investigations to incorporate magnetic fields to explain the low-mass DEB radius inflation. Finally, we explore how the effects of magnetic fields might affect mass determinations using asteroseismic data and the implication of magnetic fields on exoplanet studies.« less

sunstardb: A Database for the Study of Stellar Magnetism and the Solar-stellar Connection

NASA Astrophysics Data System (ADS)

Egeland, Ricky

2018-05-01

The “solar-stellar connection” began as a relatively small field of research focused on understanding the processes that generate magnetic fields in stars and sometimes lead to a cyclic pattern of long-term variability in activity, as demonstrated by our Sun. This area of study has recently become more broadly pertinent to questions of exoplanet habitability and exo-space weather, as well as stellar evolution. In contrast to other areas of stellar research, individual stars in the solar-stellar connection often have a distinct identity and character in the literature, due primarily to the rarity of the decades-long time-series that are necessary for studying stellar activity cycles. Furthermore, the underlying stellar dynamo is not well understood theoretically, and is thought to be sensitive to several stellar properties, e.g., luminosity, differential rotation, and the depth of the convection zone, which in turn are often parameterized by other more readily available properties. Relevant observations are scattered throughout the literature and existing stellar databases, and consolidating information for new studies is a tedious and laborious exercise. To accelerate research in this area I developed sunstardb, a relational database of stellar properties and magnetic activity proxy time-series keyed by individual named stars. The organization of the data eliminates the need for the problematic catalog cross-matching operations inherent when building an analysis data set from heterogeneous sources. In this article I describe the principles behind sunstardb, the data structures and programming interfaces, as well as use cases from solar-stellar connection research.

Evaluating Stellarator Divertor Designs with EMC3

NASA Astrophysics Data System (ADS)

Bader, Aaron; Anderson, D. T.; Feng, Y.; Hegna, C. C.; Talmadge, J. N.

2013-10-01

In this paper various improvements of stellarator divertor design are explored. Next step stellarator devices require innovative divertor solutions to handle heat flux loads and impurity control. One avenue is to enhance magnetic flux expansion near strike points, somewhat akin to the X-Divertor concept in Tokamaks. The effect of judiciously placed external coils on flux deposition is calculated for configurations based on the HSX stellarator. In addition, we attempt to optimize divertor plate location to facilitate the external coil placement. Alternate areas of focus involve altering edge island size to elucidate the driving physics in the edge. The 3-D nature of stellarators complicates design and necessitates analysis of new divertor structures with appropriate simulation tools. We evaluate the various configurations with the coupled codes EMC3-EIRENE, allowing us to benchmark configurations based on target heat flux, impurity behavior, radiated power, and transitions to high recycling and detached regimes. Work supported by DOE-SC0006103.

Stellar photometry with the Wide Field/Planetary Camera of the Hubble Space Telescope

NASA Astrophysics Data System (ADS)

Holtzman, Jon A.

1990-07-01

Simulations of Wide Field/Planetary Camera (WF/PC) images are analyzed in order to discover the most effective techniques for stellar photometry and to evaluate the accuracy and limitations of these techniques. The capabilities and operation of the WF/PC and the simulations employed in the study are described. The basic techniques of stellar photometry and methods to improve these techniques for the WF/PC are discussed. The correct parameters for star detection, aperture photometry, and point-spread function (PSF) fitting with the DAOPHOT software of Stetson (1987) are determined. Consideration is given to undersampling of the stellar images by the detector; variations in the PSF; and the crowding of the stellar images. It is noted that, with some changes DAOPHOT, is able to generate photometry almost to the level of photon statistics.

How Extreme is TRAPPIST-1? A look into the planetary system’s extreme-UV radiation environment

NASA Astrophysics Data System (ADS)

Peacock, Sarah; Barman, Travis; Shkolnik, Evgenya L.

2018-01-01

The ultracool dwarf star TRAPPIST-1 hosts three earth-sized planets at orbital distances where water has the potential to exist in liquid form on the planets’ surface. Close-in exoplanets, such as these, become vulnerable to water loss as stellar XUV radiation heats and expands their upper atmospheres. Currently, little is known about the high-energy radiation environment around TRAPPIST-1. Recent efforts to quantify the XUV radiation rely on empirical relationships based on X-ray or Lyman alpha line observations and yield very different results. The scaling relations used between the X-ray and EUV emission result in high-energy irradiation of the planets 10-1000x greater than present day Earth, stripping atmospheres and oceans in 1 Gyr, while EUV estimated from Lyman alpha flux is much lower. Here we present upper-atmosphere PHOENIX models representing the minimum and maximum potential EUV stellar flux from TRAPPIST-1. We use GALEX FUV and NUV photometry for similar aged M stars to determine the UV flux extrema in an effort to better constrain the high-energy radiation environment around TRAPPIST-1.

Radiative feedback and cosmic molecular gas: the role of different radiative sources

NASA Astrophysics Data System (ADS)

Maio, Umberto; Petkova, Margarita; De Lucia, Gabriella; Borgani, Stefano

2016-08-01

We present results from multifrequency radiative hydrodynamical chemistry simulations addressing primordial star formation and related stellar feedback from various populations of stars, stellar spectral energy distributions (SEDs) and initial mass functions. Spectra for massive stars, intermediate-mass stars and regular solar-like stars are adopted over a grid of 150 frequency bins and consistently coupled with hydrodynamics, heavy-element pollution and non-equilibrium species calculations. Powerful massive Population III stars are found to be able to largely ionize H and, subsequently, He and He+, causing an inversion of the equation of state and a boost of the Jeans masses in the early intergalactic medium. Radiative effects on star formation rates are between a factor of a few and 1 dex, depending on the SED. Radiative processes are responsible for gas heating and photoevaporation, although emission from soft SEDs has minor impacts. These findings have implications for cosmic gas preheating, primordial direct-collapse black holes, the build-up of `cosmic fossils' such as low-mass dwarf galaxies, the role of active galactic nuclei during reionization, the early formation of extended discs and angular-momentum catastrophe.

Photometric detection of high proper motions in dense stellar fields using difference image analysis

NASA Astrophysics Data System (ADS)

Eyer, L.; Woźniak, P. R.

2001-10-01

The difference image analysis (DIA) of the images obtained by the Optical Gravitational Lensing Experiment (OGLE-II) revealed a peculiar artefact in the sample of stars proposed as variable by Woźniak in one of the Galactic bulge fields: the occurrence of pairs of candidate variables showing anti-correlated light curves monotonic over a period of 3yr. This effect can be understood, quantified and related to the stellar proper motions. DIA photometry supplemented with a simple model offers an effective and easy way to detect high proper motion stars in very dense stellar fields, where conventional astrometric searches are extremely inefficient.

Multimessenger Astronomy

NASA Astrophysics Data System (ADS)

Bartos, Imre; Kowalski, Marek

2017-04-01

With the recent discovery of gravitational waves and high-energy cosmic neutrinos, we are witnessing the beginning of a new era in multimessenger astronomy. The exploration of the Universe through these new messengers, along with electromagnetic radiation and cosmic rays, gives us new insights into the most extreme energetic cosmic events, environments and particle accelerators. The objects of interest range from galaxies with accreting supermassive black holes in their centre to collapsing stars and coalescing stellar black holes. In this ebook we provide an introduction to the scientific questions surrounding these new messengers and the detectors and observational techniques used to study them, together with an overview of current and future directions in the field.

Scaling Stellar Mass Estimates of Dwarf Galaxies

NASA Astrophysics Data System (ADS)

Carr, Brandon Michael; McQuinn, Kristen B.; Cannon, John M.; Dalcanton, Julianne; Dolphin, Andrew E.; Skillman, Evan D.; Williams, Benjamin F.; van Zee, Liese

2017-01-01

Hubble Space Telescope (HST) optical imaging of resolved stellar populations has been used to constrain the star formation history (SFH) and chemical evolution of many nearby dwarf galaxies. However, even for dwarf galaxies, the angle subtended by nearby systems can be greater than the HST field of view. Thus, estimates of stellar mass from the HST footprint do not accurately represent the total mass of the system, impacting how SFH results can be used in holistic comparisons of galaxy properties. Here, we use the SFHs of dwarfs combined with stellar population synthesis models to determine mass-to-light ratios for individual galaxies, and compare these values with measured infrared luminosities from Spitzer IRAC data. In this way, we determine what fraction of mass is not included in the HST field of view. To test our methodology, we focus on dwarfs whose stellar disks are contained within the HST observations. Then, we also apply this method to galaxies with larger angular sizes to scale the stellar masses accordingly.

Research at the Institute of Astronomy and Astrophysics of the Université Libre de Bruxelles

NASA Astrophysics Data System (ADS)

Karinkuzhi, Drisya; Chamel, Nicolas; Goriely, Stéphane; Jorissen, Alain; Pourbaix, Dimitri; Siess, Lionel; Van Eck, Sophie

2018-04-01

Over the years, a coherent research strategy has developed in the field of stellar physics at the Institute of Astronomy and Astrophysics (IAA). It involves observational studies (chemical composition of giant stars, binary properties, tomography of stellar atmospheres) that make use of the large ESO telescopes as well as of other major instruments. The presence of a high-resolution spectrograph on the 3.6-m Devasthal Optical Telescope (DOT) would therefore be highly beneficial to IAA research. These observations are complemented and supported by theoretical studies of mass transfer in binary systems, of standard and non-standard stellar evolution (including the modelling of stellar hydrodynamical nuclear burning for application to certain thermonuclear supernovae) and of nuclear astrophysics (a field in which IAA has been recognized for a long time as an international centre of excellence), including the theory of nucleosynthesis. IAA also addresses the end-points of stellar evolution as it is carrying out research on the compact remnants of stellar evolution of massive stars: neutron stars.

Near infrared photographic sky survey. 1: Catalog of red stellar objects

NASA Technical Reports Server (NTRS)

Craine, E. R.; Duerr, R. E.; Horner, V. M.; Imhoff, C. L.; Routsis, D. E.; Swihart, D. L.; Turnshek, D. A.

1979-01-01

Red stellar objects for which V-1 was greater than a value of about 2 (supm). 5 were extracted from photographs of 23 program fields. Tabular data for each field show the object name; the 1950 epoch right ascension, declination, galactic longitude, galactic latitude; radial distance from field venter in decimal degrees; color classes; and objects ordered by redness.

Second Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, volume 1

NASA Technical Reports Server (NTRS)

Giampapa, M. S. (Editor); Golub, L. (Editor)

1981-01-01

Solar and stellar atmospheric phenomena and their fundamental physical properties such as gravity, effective temperature and rotation rate, which provides the range in parameter space required to test various theoretical models were investigated. The similarity between solar activity and stellar activity is documented. Some of the topics discussed are: atmospheric structure, magnetic fields, solar and stellar activity, and evolution.

Momentum and energy balance in late-type stellar winds

NASA Technical Reports Server (NTRS)

Macgregor, K. B.

1981-01-01

Observations at ultraviolet and X-ray wavelengths indicate that the classical picture of a static stellar atmosphere containing a radiative equilibrium temperature distribution is inapplicable to the majority of late type stars. Mass loss and the presence of atmospheric regions characterized by gas temperatures in excess of the stellar effective temperature appear to be almost ubiquitous throughout the HR diagram. Evidence pertaining to the thermal and dynamical structure of the outer envelopes of cool stars is summarized. These results are compared with the predictions of several theoretical models which were proposed to account for mass loss from latetype stars. Models in which the outflow is thermally radiatively, or wave driven are considered for identification of the physical processes responsible for the observed wind properties. The observed variation of both the wind, thermal and dynamical structure as one proceeds from the supergiant branch toward the main sequence in the cool portion of the HR diagram give consideration to potential mechanisms for heating and cooling the flow from low gravity stars.

The BRAVE Program. I. Improved Bulge Stellar Velocity Dispersion Estimates for a Sample of Active Galaxies

NASA Astrophysics Data System (ADS)

Batiste, Merida; Bentz, Misty C.; Manne-Nicholas, Emily R.; Onken, Christopher A.; Bershady, Matthew A.

2017-02-01

We present new bulge stellar velocity dispersion measurements for 10 active galaxies with secure MBH determinations from reverberation mapping. These new velocity dispersion measurements are based on spatially resolved kinematics from integral-field (IFU) spectroscopy. In all but one case, the field of view of the IFU extends beyond the effective radius of the galaxy, and in the case of Mrk 79 it extends to almost one half the effective radius. This combination of spatial resolution and field of view allows for secure determinations of stellar velocity dispersion within the effective radius for all 10 target galaxies. Spatially resolved maps of the first (V) and second (σ⋆) moments of the line of sight velocity distribution indicate the presence of kinematic substructure in most cases. In future projects we plan to explore methods of correcting for the effects of kinematic substructure in the derived bulge stellar velocity dispersion measurements.

SORCE: Solar Radiation and Climate Experiment

NASA Technical Reports Server (NTRS)

Cahalan, Robert; Rottman, Gary; Lau, William K. M. (Technical Monitor)

2002-01-01

Contents include the following: Understanding the Sun's influence on the Earth; How the Sun affect Earth's climate; By how much does the Sun's radiation very; Understanding Solar irradiance; History of Solar irradiance observations; The SORCE mission; How do the SORCE instruments measure solar radiation; Total irradiance monitor (TIM); Spectral irradiance monitor (SIM); Solar stellar irradiance comparison experiment (SOLSTICE); XUV photometer system (XPS).

HOW SIGNIFICANT IS RADIATION PRESSURE IN THE DYNAMICS OF THE GAS AROUND YOUNG STELLAR CLUSTERS?

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

Silich, Sergiy; Tenorio-Tagle, Guillermo, E-mail: silich@inaoep.mx

2013-03-01

The impact of radiation pressure on the dynamics of the gas in the vicinity of young stellar clusters is thoroughly discussed. The radiation over the thermal/ram pressure ratio time evolution is calculated explicitly and the crucial roles of the cluster mechanical power, the strong time evolution of the ionizing photon flux, and the bolometric luminosity of the exciting cluster are stressed. It is shown that radiation has only a narrow window of opportunity to dominate the wind-driven shell dynamics. This may occur only at early stages of the bubble evolution and if the shell expands into a dusty and/or amore » very dense proto-cluster medium. The impact of radiation pressure on the wind-driven shell always becomes negligible after about 3 Myr. Finally, the wind-driven model results allow one to compare the model predictions with the distribution of thermal pressure derived from X-ray observations. The shape of the thermal pressure profile then allows us to distinguish between the energy and the momentum-dominated regimes of expansion and thus conclude whether radiative losses of energy or the leakage of hot gas from the bubble interior have been significant during bubble evolution.« less

Temporal variability of the wind from the star τ Boötis

NASA Astrophysics Data System (ADS)

Nicholson, B. A.; Vidotto, A. A.; Mengel, M.; Brookshaw, L.; Carter, B.; Petit, P.; Marsden, S. C.; Jeffers, S. V.; Fares, R.; BCool Collaboration

2016-06-01

We present new wind models for τ Boötis (τ Boo), a hot-Jupiter-host-star whose observable magnetic cycles makes it a uniquely useful target for our goal of monitoring the temporal variability of stellar winds and their exoplanetary impacts. Using spectropolarimetric observations from May 2009 to January 2015, the most extensive information of this type yet available, to reconstruct the stellar magnetic field, we produce multiple 3D magnetohydrodynamic stellar wind models. Our results show that characteristic changes in the large-scale magnetic field as the star undergoes magnetic cycles produce changes in the wind properties, both globally and locally at the position of the orbiting planet. Whilst the mass loss rate of the star varies by only a minimal amount (˜4 per cent), the rates of angular momentum loss and associated spin-down time-scales are seen to vary widely (up to ˜140 per cent), findings consistent with and extending previous research. In addition, we find that temporal variation in the global wind is governed mainly by changes in total magnetic flux rather than changes in wind plasma properties. The magnetic pressure varies with time and location and dominates the stellar wind pressure at the planetary orbit. By assuming a Jovian planetary magnetic field for τ Boo b, we nevertheless conclude that the planetary magnetosphere can remain stable in size for all observed stellar cycle epochs, despite significant changes in the stellar field and the resulting local space weather environment.

CHARACTERIZATION OF CoRoT TARGET FIELDS WITH BERLIN EXOPLANET SEARCH TELESCOPE. II. IDENTIFICATION OF PERIODIC VARIABLE STARS IN THE LRc2 FIELD

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

Kabath, P.; Fruth, T.; Rauer, H.

2009-04-15

We report on photometric observations of the CoRoT LRc2 field with the new robotic Berlin Exoplanet Search Telescope II (BEST II). The telescope system was installed and commissioned at the Observatorio Cerro Armazones, Chile, in 2007. BEST II is a small aperture telescope with a wide field of view dedicated to the characterization of the stellar variability primarily in CoRoT target fields with high stellar densities. The CoRoT stellar field LRc2 was observed with BEST II up to 20 nights in 2007 July and August. From the acquired data containing about 100,000 stars, 426 new periodic variable stars were identifiedmore » and 90% of them are located within the CoRoT exoplanetary CCD segments and may be of further interest for CoRoT additional science programs.« less

A grid of MHD models for stellar mass loss and spin-down rates of solar analogs

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

Cohen, O.; Drake, J. J.

2014-03-01

Stellar winds are believed to be the dominant factor in the spin-down of stars over time. However, stellar winds of solar analogs are poorly constrained due to observational challenges. In this paper, we present a grid of magnetohydrodynamic models to study and quantify the values of stellar mass loss and angular momentum loss rates as a function of the stellar rotation period, magnetic dipole component, and coronal base density. We derive simple scaling laws for the loss rates as a function of these parameters, and constrain the possible mass loss rate of stars with thermally driven winds. Despite the successmore » of our scaling law in matching the results of the model, we find a deviation between the 'solar dipole' case and a real case based on solar observations that overestimates the actual solar mass loss rate by a factor of three. This implies that the model for stellar fields might require a further investigation with additional complexity. Mass loss rates in general are largely controlled by the magnetic field strength, with the wind density varying in proportion to the confining magnetic pressure B {sup 2}. We also find that the mass loss rates obtained using our grid models drop much faster with the increase in rotation period than scaling laws derived using observed stellar activity. For main-sequence solar-like stars, our scaling law for angular momentum loss versus poloidal magnetic field strength retrieves the well-known Skumanich decline of angular velocity with time, Ω{sub *}∝t {sup –1/2}, if the large-scale poloidal magnetic field scales with rotation rate as B{sub p}∝Ω{sub ⋆}{sup 2}.« less

STELLAR MEMBERSHIP AND DUSTY DEBRIS DISKS IN THE {alpha} PERSEI CLUSTER

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

Zuckerman, B.; Melis, Carl; Rhee, Joseph H.

2012-06-10

Because of its proximity to the Galactic plane, reliable identification of members of the {alpha} Persei cluster is often problematic. Based primarily on membership evaluations contained in six published papers, we constructed a mostly complete list of high-fidelity members of spectral type G and earlier that lie within 3 arc degrees of the cluster center. {alpha} Persei was the one nearby, rich, young open cluster not surveyed with the Spitzer Space Telescope. We examined the first and final data releases of the Wide-field Infrared Survey Explorer and found 11, or perhaps 12, {alpha} Per cluster members that have excess mid-infraredmore » emission above the stellar photosphere attributable to an orbiting dusty debris disk. The most unusual of these is V488 Per, a K-type star with an excess IR luminosity 16% (or more) of the stellar luminosity; this is a larger excess fraction than that of any other known dusty main-sequence star. Much of the dust that orbits V488 Per is at a temperature of {approx}800 K; if these grains radiate like blackbodies, then they lie only {approx}0.06 AU from the star. The dust is probably the aftermath of a collision of two planetary embryos or planets with small semimajor axes; such orbital radii are similar to those of many of the transiting planets discovered by the Kepler satellite.« less

Ultraviolet spectrometer experiment for the Voyager mission

NASA Technical Reports Server (NTRS)

Broadfoot, A. L.; Sandel, B. R.; Shemansky, D. E.; Atreya, S. K.; Donahue, T. M.; Moos, H. W.; Bertaux, J. L.; Blamont, J. E.; Ajello, J. M.; Strobel, D. F.

1977-01-01

An objective grating spectrometer covering the wavelength range of 500 to 1700 A with a 10-A resolution is employed for the Voyager ultraviolet spectrometer experiment. In determining the composition and structure of the atmospheres of Saturn, Jupiter and several satellites, the ultraviolet spectrometer will rely on airglow mode observations to measure radiation from the atmospheres due to resonant scattering of solar flux, and the occultation mode for assessments of the atmospheric extinction of solar or stellar radiation as the spacecraft enters shadow zones. Since it is capable of prolonged stellar observations in the 500 to 1000 A wavelength range, the spectrometer is expected to make important contributions to exploratory studies of UV sources.

Uv-Optical Spectra and Imagery of the Bubble Nebula NGC 7635

NASA Astrophysics Data System (ADS)

Walter, Donald

1997-07-01

We propose to acquire UV-optical STIS spectra and WFPC2 imagery of the wind-blown Bubble Nebula NGC 7635. This object is significant to our understanding of galactic chemical evolution, star formation {possibly triggered by radiative implosion}, the mass-loss history of precursors to supernovae, the effect of wind-driven shocks on the ISM and the process of ionization and photoevaporation of high density knots {possibly HH objects} in the presence of an intense stellar wind and radiation field. The ener getic environment of NGC 7635 is more extreme and its features have evolved on a different time scale than in more quiescent objects studied with HST {e.g. Orion and M16}. HST is essential to our study in order to achieve high spatial resolution and ac cess to the UV region of the spectrum. The nebula's nearly spherical shell is the result of a recent { < 10^6 years} stellar mass-loss event and is the best young, clearly observed bubble available for study. We will exam in e the ionization front at the r im of the bubble, the extent to which it is shock-driven and the scale of the photoevaporative flow off the face of the molecular cloud. We will resolve high density knots down to a size of 2.1 x 10^15 cm {140 au}, searching for protostellar objects. STIS U V spectra will allow us to calculate the first accurate C/H abundance in the Perseus arm and test for the presence of a galactic abundance gradient. Finally, with our HST data we will compare our observational results with our radiative shock-model predi ctions.

A Simple Evolutional Model of the UV Habitable Zone and the Possibility of the Persistent Life Existence: The Effects of Mass and Metallicity

NASA Astrophysics Data System (ADS)

Oishi, Midori; Kamaya, Hideyuki

2016-12-01

In addition to the habitable zone (HZ), the UV habitable zone (UV-HZ) is important when considering the existence of persistent life in the universe. The UV-HZ is defined as the area where the UV radiation field from a host star is moderate for persistent life existence. This is because UV is necessary for the synthesis of biochemical compounds. The UV-HZ must overlap the HZ when life appears and evolves. In this paper, following our previous study of the HZ, we examine the UV-HZ in cases with a stellar mass range from 0.08 to 4.00 M ⊙ with various metallicities during the main sequence phase. This mass range was chosen because we are interested in an environment similar to that of Earth. The effect of metallicity is reflected in the spectrum of the host stars, and we reexamine it in the context of the UV-HZ. The present work shows the effect of metallicity when that in the UV-HZ is less than that in the HZ. Furthermore, we find that the chance of persistent life existence declines as the metallicity decreases, as long as the UV radiation is not protected and/or boosted by any mechanisms. This is because the overlapped region of a persistent HZ and UV-HZ decreases. We find that the most appropriate stellar mass for the persistence of life existence is from 1.0 to 1.5 M ⊙ with metallicity Z = 0.02, and only about 1.2 M ⊙ with Z = 0.002. When Z = 0.0002, the chance of persistent life existence is very low, assuming that the ocean does not protect the life from UV radiation.

On the Origin and Evolution of Stellar Chromospheres, Coronae and Winds

NASA Technical Reports Server (NTRS)

Musielak, Z. E.

1997-01-01

The final report discusses work completed on proposals to construct state-of-the-art, theoretical, two-component, chromospheric models for single stars of different spectral types and different evolutionary status. We suggested to use these models to predict the level of the "basal flux", the observed range of variation of chromospheric activity for a given spectral type, and the decrease of this activity with stellar age. In addition, for red giants and supergiants, we also proposed to construct self-consistent, purely theoretical, chromosphere-wind models, and investigate the origin of "dividing lines" in the H-R diagram. In the report, we list the following six specific goals for the first and second year of the proposed research and then describe the completed work: (1) To calculate the acoustic and magnetic wave energy fluxes for stars located in different regions of the H-R diagram; (2) To investigate the transfer of this non-radiative energy through stellar photospheres and to estimate the amount of energy that reaches the chromosphere; (3) To identify major sources of radiative losses in stellar chromospheres and calculate the amount of emitted energy; (4) To use (1) through (3) to construct purely theoretical, two-component, chromospheric models based on the local energy balance. The models will be constructed for stars of different spectral types and different evolutionary status; (5) To explain theoretically the "basal flux", the location of stellar temperature minima and the observed range of chromospheric activity for stars of the same spectral type; and (6) To construct self-consistent, time-dependent stellar wind models based on the momentum deposition by finite amplitude Alfven waves.

Star-forming galaxies in intermediate-redshift clusters: stellar versus dynamical masses of luminous compact blue galaxies

NASA Astrophysics Data System (ADS)

Randriamampandry, S. M.; Crawford, S. M.; Bershady, M. A.; Wirth, G. D.; Cress, C. M.

2017-10-01

We investigate the stellar masses of the class of star-forming objects known as luminous compact blue galaxies (LCBGs) by studying a sample of galaxies in the distant cluster MS 0451.6-0305 at z ≈ 0.54 with ground-based multicolour imaging and spectroscopy. For a sample of 16 spectroscopically confirmed cluster LCBGs (colour B - V < 0.5, surface brightness μB < 21 mag arcsec-2 and magnitude MB < -18.5), we measure stellar masses by fitting spectral energy distribution (SED) models to multiband photometry, and compare with dynamical masses [determined from velocity dispersion in the range 10 < σv(km s- 1) < 80] we previously obtained from their emission-line spectra. We compare two different stellar population models that measure stellar mass in star-bursting galaxies, indicating correlations between the stellar age, extinction and stellar mass derived from the two different SED models. The stellar masses of cluster LCBGs are distributed similarly to those of field LCBGs, but the cluster LCBGs show lower dynamical-to-stellar mass ratios (Mdyn/M⋆ = 2.6) than their field LCBG counterparts (Mdyn/M⋆ = 4.8), echoing trends noted previously in low-redshift dwarf elliptical galaxies. Within this limited sample, the specific star formation rate declines steeply with increasing mass, suggesting that these cluster LCBGs have undergone vigorous star formation.

Fingering convection induced by atomic diffusion in stars: 3D numerical computations and applications to stellar models

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

Zemskova, Varvara; Garaud, Pascale; Deal, Morgan

2014-11-10

Iron-rich layers are known to form in the stellar subsurface through a combination of gravitational settling and radiative levitation. Their presence, nature, and detailed structure can affect the excitation process of various stellar pulsation modes and must therefore be modeled carefully in order to better interpret Kepler asteroseismic data. In this paper, we study the interplay between atomic diffusion and fingering convection in A-type stars, as well as its role in the establishment and evolution of iron accumulation layers. To do so, we use a combination of three-dimensional idealized numerical simulations of fingering convection (which neglect radiative transfer and complexmore » opacity effects) and one-dimensional realistic stellar models. Using the three-dimensional simulations, we first validate the mixing prescription for fingering convection recently proposed by Brown et al. (within the scope of the aforementioned approximation) and identify what system parameters (total mass of iron, iron diffusivity, thermal diffusivity, etc.) play a role in the overall evolution of the layer. We then implement the Brown et al. prescription in the Toulouse-Geneva Evolution Code to study the evolution of the iron abundance profile beneath the stellar surface. We find, as first discussed by Théado et al., that when the concurrent settling of helium is ignored, this accumulation rapidly causes an inversion in the mean molecular weight profile, which then drives fingering convection. The latter mixes iron with the surrounding material very efficiently, and the resulting iron layer is very weak. However, taking helium settling into account partially stabilizes the iron profile against fingering convection, and a large iron overabundance can accumulate. The opacity also increases significantly as a result, and in some cases it ultimately triggers dynamical convection. The direct effects of radiative acceleration on the dynamics of fingering convection (especially in the nonlinear regime) remain to be added in the future to improve the quantitative predictions of the model.« less

Hybrid metric-Palatini stars

NASA Astrophysics Data System (ADS)

Danilǎ, Bogdan; Harko, Tiberiu; Lobo, Francisco S. N.; Mak, M. K.

2017-02-01

We consider the internal structure and the physical properties of specific classes of neutron, quark and Bose-Einstein condensate stars in the recently proposed hybrid metric-Palatini gravity theory, which is a combination of the metric and Palatini f (R ) formalisms. It turns out that the theory is very successful in accounting for the observed phenomenology, since it unifies local constraints at the Solar System level and the late-time cosmic acceleration, even if the scalar field is very light. In this paper, we derive the equilibrium equations for a spherically symmetric configuration (mass continuity and Tolman-Oppenheimer-Volkoff) in the framework of the scalar-tensor representation of the hybrid metric-Palatini theory, and we investigate their solutions numerically for different equations of state of neutron and quark matter, by adopting for the scalar field potential a Higgs-type form. It turns out that the scalar-tensor definition of the potential can be represented as an Clairaut differential equation, and provides an explicit form for f (R ) given by f (R )˜R +Λeff, where Λeff is an effective cosmological constant. Furthermore, stellar models, described by the stiff fluid, radiation-like, bag model and the Bose-Einstein condensate equations of state are explicitly constructed in both general relativity and hybrid metric-Palatini gravity, thus allowing an in-depth comparison between the predictions of these two gravitational theories. As a general result it turns out that for all the considered equations of state, hybrid gravity stars are more massive than their general relativistic counterparts. Furthermore, two classes of stellar models corresponding to two particular choices of the functional form of the scalar field (constant value, and logarithmic form, respectively) are also investigated. Interestingly enough, in the case of a constant scalar field the equation of state of the matter takes the form of the bag model equation of state describing quark matter. As a possible astrophysical application of the obtained results, we suggest that stellar mass black holes, with masses in the range of 3.8 and 6 M⊙ , respectively, could be in fact hybrid metric-Palatini gravity neutron or quark stars.

Gas-Grain Chemical Models: Inclusion of a Grain Size Distribution and a Study Of Young Stellar Objects in the Magellanic Clouds

NASA Astrophysics Data System (ADS)

Pauly, Tyler Andrew

2017-06-01

Computational models of interstellar gas-grain chemistry have aided in our understanding of star-forming regions. Chemical kinetics models rely on a network of chemical reactions and a set of physical conditions in which atomic and molecular species are allowed to form and react. We replace the canonical single grain-size in our chemical model MAGICKAL with a grain size distribution and analyze the effects on the chemical composition of the gas and grain surface in quiescent and collapsing dark cloud models. We find that a grain size distribution coupled with a temperature distribution across grain sizes can significantly affect the bulk ice composition when dust temperatures fall near critical values related to the surface binding energies of common interstellar chemical species. We then apply the updated model to a study of ice formation in the cold envelopes surrounding massive young stellar objects in the Magellanic Clouds. The Magellanic Clouds are local satellite galaxies of the Milky Way, and they provide nearby environments to study star formation at low metallicity. We expand the model calculation of dust temperature to include a treatment for increased interstellar radiation field intensity; we vary the radiation field to model the elevated dust temperatures observed in the Magellanic Clouds. We also adjust the initial elemental abundances used in the model, guided by observations of Magellanic Cloud HII regions. We are able to reproduce the relative ice fractions observed, indicating that metal depletion and elevated grain temperature are important drivers of the envelope ice composition. The observed shortfall in CO in Small Magellanic Cloud sources can be explained by a combination of reduced carbon abundance and increased grain temperatures. The models indicate that a large variation in radiation field strength is required to match the range of observed LMC abundances. CH 3OH abundance is found to be enhanced (relative to total carbon abundance) in low-metallicity models, providing seed material for complex organic molecule formation. We conclude with a preliminary study of the recently discovered hot core in the Large Magellanic Cloud; we create a grid of models to simulate hot core formation in Magellanic Cloud environments, comparing them to models and observations of well-characterized galactic counterparts.

The MUSE Hubble Ultra Deep Field Survey. V. Spatially resolved stellar kinematics of galaxies at redshift 0.2 ≲ z ≲ 0.8

NASA Astrophysics Data System (ADS)

Guérou, Adrien; Krajnović, Davor; Epinat, Benoit; Contini, Thierry; Emsellem, Eric; Bouché, Nicolas; Bacon, Roland; Michel-Dansac, Leo; Richard, Johan; Weilbacher, Peter M.; Schaye, Joop; Marino, Raffaella Anna; den Brok, Mark; Erroz-Ferrer, Santiago

2017-11-01

We present spatially resolved stellar kinematic maps, for the first time, for a sample of 17 intermediate redshift galaxies (0.2 ≲ z ≲ 0.8). We used deep MUSE/VLT integral field spectroscopic observations in the Hubble Deep Field South (HDFS) and Hubble Ultra Deep Field (HUDF), resulting from ≈30 h integration time per field, each covering 1' × 1' field of view, with ≈ 0.̋65 spatial resolution. We selected all galaxies brighter than 25 mag in the I band and for which the stellar continuum is detected over an area that is at least two times larger than the spatial resolution. The resulting sample contains mostly late-type disk, main-sequence star-forming galaxies with 108.5 M⊙ ≲ M∗ ≲ 1010.5 M⊙. Using a full-spectrum fitting technique, we derive two-dimensional maps of the stellar and gas kinematics, including the radial velocity V and velocity dispersion σ. We find that most galaxies in the sample are consistent with having rotating stellar disks with roughly constant velocity dispersions and that the second order velocity moments Vrms = √V2+σ2 of the gas and stars, a scaling proxy for the galaxy gravitational potential, compare well to each other. These spatially resolved observations of the stellar kinematics of intermediate redshift galaxies suggest that the regular stellar kinematics of disk galaxies that is observed in the local Universe was already in place 4-7 Gyr ago and that their gas kinematics traces the gravitational potential of the galaxy, thus is not dominated by shocks and turbulent motions. Finally, we build dynamical axisymmetric Jeans models constrained by the derived stellar kinematics for two specific galaxies and derive their dynamical masses. These are in good agreement (within 25%) with those derived from simple exponential disk models based on the gas kinematics. The obtained mass-to-light ratios hint towards dark matter dominated systems within a few effective radii. Based on observations made with ESO telescopes at the La Silla-Paranal Observatory under programmes 094.A-0289(B), 095.A-0010(A), 096.A-0045(A) and 096.A-0045(B).

Lyman alpha initiated winds in late-type stars

NASA Technical Reports Server (NTRS)

Haisch, B. M.; Linsky, J. L.; Vanderhucht, K. A.

1979-01-01

The IUE survey of late-type stars revealed a sharp division in the HR diagram between stars with solar type spectra (chromosphere and transition region lines) and those with non-solar type spectra (only chromosphere lines). Models of both hot coronae and cool wind flows were calculated using stellar model chromospheres as starting points for stellar wind calculations in order to investigate the possibility of having a supersonic transition locus in the HR diagram dividing hot coronae from cool winds. From these models, it is concluded that the Lyman alpha flux may play an important role in determining the location of a stellar wind critical point. The interaction of Lyman alpha radiation pressure with Alfven waves in producing strong, low temperature stellar winds in the star Arcturus is examined.

Magnetic moment and plasma environment of HD 209458b as determined from Lyα observations.

PubMed

Kislyakova, Kristina G; Holmström, Mats; Lammer, Helmut; Odert, Petra; Khodachenko, Maxim L

2014-11-21

Transit observations of HD 209458b in the stellar Lyman-α(Lyα) line revealed strong absorption in both blue and red wings of the line interpreted as hydrogen atoms escaping from the planet's exosphere at high velocities. The following sources for the absorption were suggested: acceleration by the stellar radiation pressure, natural spectral line broadening, or charge exchange with the stellar wind. We reproduced the observation by means of modeling that includes all aforementioned processes. Our results support a stellar wind with a velocity of ≈400 kilometers per second at the time of the observation and a planetary magnetic moment of ≈1.6 × 10(26) amperes per square meter. Copyright © 2014, American Association for the Advancement of Science.

Critical radiation fluxes and luminosities of black holes and relativistic stars

NASA Technical Reports Server (NTRS)

Lamb, Frederick K.; Miller, M. Coleman

1995-01-01

The critial luminosity at which the outward force of radiation balances the inward force of gravity plays an important role in many astrophysical systems. We present expressions for the radiation force on particles with arbitrary cross sections and analyze the radiation field produced by radiating matter, such as a disk, ring, boundary layer, or stellar surface, that rotates slowly around a slowly rotating gravitating mass. We then use these results to investigate the critical radiation flux and, where possible, the critical luminosity of such a system in genral relativity. We demonstrate that if the radiation source is axisymmetric and emission is back-front symmetric with repect to the local direction of motion of the radiating matter, as seen in the comoving frame, then the radial component of the radiation flux and the diagonal components of the radiation stress-energy tensor outside the source are the same, to first order in the rotation rates, as they would be if the radiation source and gravitating mass were not rotating. We argue that the critical radiation flux for matter at rest in the locally nonrotating frame is often satisfactory as an astrophysical benchmark flux and show that if this benchmark is adopted, many of the complications potentially introduced by rotation of the radiation source and the gravitating mass are avoided. We show that if the radiation field in the absence of rotation would be spherically symmetric and the opacity is independent of frequency and direction, one can define a critical luminosity for the system that is independent of frequency and direction, one can define a critical luminosity for the system that is independent of the spectrum and angular size of the radiation source and is unaffected by rotation of the source and mass and orbital motion of the matter, to first order. Finally, we analyze the conditions under which the maximum possible luminosity of a star or black hole powered by steady spherically symmetric radial accretion is the same in general relativity as in the Newtonian limit.

Stellar Explosions: Hydrodynamics and Nucleosynthesis

NASA Astrophysics Data System (ADS)

Jose, Jordi

2016-01-01

Stars are the main factories of element production in the universe through a suite of complex and intertwined physical processes. Such stellar alchemy is driven by multiple nuclear interactions that through eons have transformed the pristine, metal-poor ashes leftover by the Big Bang into a cosmos with 100 distinct chemical species. The products of stellar nucleosynthesis frequently get mixed inside stars by convective transport or through hydrodynamic instabilities, and a fraction of them is eventually ejected into the interstellar medium, thus polluting the cosmos with gas and dust. The study of the physics of the stars and their role as nucleosynthesis factories owes much to cross-fertilization of different, somehow disconnected fields, ranging from observational astronomy, computational astrophysics, and cosmochemistry to experimental and theoretical nuclear physics. Few books have simultaneously addressed the multidisciplinary nature of this field in an engaging way suitable for students and young scientists. Providing the required multidisciplinary background in a coherent way has been the driving force for Stellar Explosions: Hydrodynamics and Nucleosynthesis. Written by a specialist in stellar astrophysics, this book presents a rigorous but accessible treatment of the physics of stellar explosions from a multidisciplinary perspective at the crossroads of computational astrophysics, observational astronomy, cosmochemistry, and nuclear physics. Basic concepts from all these different fields are applied to the study of classical and recurrent novae, type I and II supernovae, X-ray bursts and superbursts, and stellar mergers. The book shows how a multidisciplinary approach has been instrumental in our understanding of nucleosynthesis in stars, particularly during explosive events.

Stellar Explosions: Hydrodynamics and Nucleosynthesis

NASA Astrophysics Data System (ADS)

José, Jordi

2015-12-01

Stars are the main factories of element production in the universe through a suite of complex and intertwined physical processes. Such stellar alchemy is driven by multiple nuclear interactions that through eons have transformed the pristine, metal-poor ashes leftover by the Big Bang into a cosmos with 100 distinct chemical species. The products of stellar nucleosynthesis frequently get mixed inside stars by convective transport or through hydrodynamic instabilities, and a fraction of them is eventually ejected into the interstellar medium, thus polluting the cosmos with gas and dust. The study of the physics of the stars and their role as nucleosynthesis factories owes much to cross-fertilization of different, somehow disconnected fields, ranging from observational astronomy, computational astrophysics, and cosmochemistry to experimental and theoretical nuclear physics. Few books have simultaneously addressed the multidisciplinary nature of this field in an engaging way suitable for students and young scientists. Providing the required multidisciplinary background in a coherent way has been the driving force for Stellar Explosions: Hydrodynamics and Nucleosynthesis. Written by a specialist in stellar astrophysics, this book presents a rigorous but accessible treatment of the physics of stellar explosions from a multidisciplinary perspective at the crossroads of computational astrophysics, observational astronomy, cosmochemistry, and nuclear physics. Basic concepts from all these different fields are applied to the study of classical and recurrent novae, type I and II supernovae, X-ray bursts and superbursts, and stellar mergers. The book shows how a multidisciplinary approach has been instrumental in our understanding of nucleosynthesis in stars, particularly during explosive events.

Probing stellar mass build-up in galaxies at z=4-7 with CANDELS and S-CANDELS

NASA Astrophysics Data System (ADS)

Song, Mimi; Finkelstein, Steven L.; Ashby, Matthew; Merlin, Emiliano

2015-01-01

Over the last few years the advent of the Hubble Space Telescope (HST) Wide Field Camera 3 has enabled us to build statistically significant samples of galaxies out to z=8. We have subsequently witnessed remarkable progress in our understanding of galaxy evolution in the early universe. However, our understanding of the galaxy stellar mass growth in this era has been limited due to the lack of rest-frame optical data at a comparable depth as the HST data. Here we present results on the galaxy stellar mass function at z=4-7 from a sample of ~7500 galaxies over an area of ~280 square arcmin in the CANDELS GOODS-South and North fields, as well as the Hubble Ultra Deep Field. Utilizing deep IRAC data from the S-CANDELS and IUDF10 programs to robustly constrain the stellar masses of galaxies in our sample, we measure the stellar-mass to rest-frame ultraviolet (UV) luminosity trends in each of our redshift bins. We convolve these trends with recent measurements of the rest-frame ultraviolet luminosity function to derive the stellar mass functions. Contrary to initial studies at these redshifts, we find steeper low-mass-end slopes (-1.6 at z=4, and -2.0 at z=7), similar to recent simulations. Our results provide the most accurate estimates to date of the cosmic stellar mass density over the first two billion years after the Big Bang.

Planetary nebulae: 20 years of Hubble inquiry

NASA Astrophysics Data System (ADS)

Balick, Bruce

2012-08-01

The Hubble Space Telescope has served the critical roles of microscope and movie camera in the past 20 years of research on planetary nebulae (``PNe''). We have glimpsed the details of the evolving structures of neutral and ionized post-AGB objects, built ingenious heuristic models that mimic these structures, and constrained most of the relevant physical processes with careful observations and interpretation. We have searched for close physical binary stars with spatial resolution ~50 AU at 1 AU, located jets emerging from the nucleus at speeds up to 2000 km s-1 and matched newly discovered molecular and X-ray emission regions to physical substructures in order to better understand how stellar winds and ionizing radiation interact to form the lovely symmetries that are observed. Ultraviolet spectra of CNO in PNe help to uncover how stars process deep inside AGB stars with unstable nuclear burning zones. HST broadband imaging has been at the forefront of uncovering surprisingly complex wind morphologies produced at the tip of the AGB, and has led to an increasing realization of the potentially vital roles of close binary stars and emerging magnetic fields in shaping stellar winds.

ALMA reveals sunburn: CO dissociation around AGB stars in the globular cluster 47 Tucanae

NASA Astrophysics Data System (ADS)

McDonald, I.; Zijlstra, A. A.; Lagadec, E.; Sloan, G. C.; Boyer, M. L.; Matsuura, M.; Smith, R. J.; Smith, C. L.; Yates, J. A.; van Loon, J. Th.; Jones, O. C.; Ramstedt, S.; Avison, A.; Justtanont, K.; Olofsson, H.; Blommaert, J. A. D. L.; Goldman, S. R.; Groenewegen, M. A. T.

2015-11-01

Atacama Large Millimetre Array observations show a non-detection of carbon monoxide around the four most luminous asymptotic giant branch (AGB) stars in the globular cluster 47 Tucanae. Stellar evolution models and star counts show that the mass-loss rates from these stars should be ˜1.2-3.5 × 10-7 M⊙ yr-1. We would naïvely expect such stars to be detectable at this distance (4.5 kpc). By modelling the ultraviolet radiation field from post-AGB stars and white dwarfs in 47 Tuc, we conclude that CO should be dissociated abnormally close to the stars. We estimate that the CO envelopes will be truncated at a few hundred stellar radii from their host stars and that the line intensities are about two orders of magnitude below our current detection limits. The truncation of CO envelopes should be important for AGB stars in dense clusters. Observing the CO (3-2) and higher transitions and targeting stars far from the centres of clusters should result in the detections needed to measure the outflow velocities from these stars.

Global properties of M31's stellar halo from the splash survey. II. Metallicity profile

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

Gilbert, Karoline M.; Kalirai, Jason S.; Guhathakurta, Puragra

2014-12-01

We present the metallicity distribution of red giant branch (RGB) stars in M31's stellar halo, derived from photometric metallicity estimates for over 1500 spectroscopically confirmed RGB halo stars. The stellar sample comes from 38 halo fields observed with the Keck/DEIMOS spectrograph, ranging from 9 to 175 kpc in projected distance from M31's center, and includes 52 confirmed M31 halo stars beyond 100 kpc. While a wide range of metallicities is seen throughout the halo, the metal-rich peak of the metallicity distribution function becomes significantly less prominent with increasing radius. The metallicity profile of M31's stellar halo shows a continuous gradientmore » from 9 to ∼100 kpc, with a magnitude of ∼ – 0.01 dex kpc{sup –1}. The stellar velocity distributions in each field are used to identify stars that are likely associated with tidal debris features. The removal of tidal debris features does not significantly alter the metallicity gradient in M31's halo: a gradient is maintained in fields spanning 10-90 kpc. We analyze the halo metallicity profile, as well as the relative metallicities of stars associated with tidal debris features and the underlying halo population, in the context of current simulations of stellar halo formation. We argue that the large-scale gradient in M31's halo implies M31 accreted at least one relatively massive progenitor in the past, while the field to field variation seen in the metallicity profile indicates that multiple smaller progenitors are likely to have contributed substantially to M31's outer halo.« less

Constructing Integrable Full-pressure Full-current Free-boundary Stellarator Magnetohydrodynamic Equilibria

NASA Astrophysics Data System (ADS)

Hudson, S. R.; Monticello, D. A.; Reiman, A. H.; Strickler, D. J.; Hirshman, S. P.

2003-06-01

For the (non-axisymmetric) stellarator class of plasma confinement devices to be feasible candidates for fusion power stations it is essential that, to a good approximation, the magnetic field lines lie on nested flux surfaces; however, the inherent lack of a continuous symmetry implies that magnetic islands are guaranteed to exist. Magnetic islands break the smooth topology of nested flux surfaces and chaotic field lines result when magnetic islands overlap. An analogous case occurs with 11/2-dimension Hamiltonian systems where resonant perturbations cause singularities in the transformation to action-angle coordinates and destroy integrability. The suppression of magnetic islands is a critical issue for stellarator design, particularly for small aspect ratio devices. Techniques for `healing' vacuum fields and fixed-boundary plasma equilibria have been developed, but what is ultimately required is a procedure for designing stellarators such that the self-consistent plasma equilibrium currents and the coil currents combine to produce an integrable magnetic field, and such a procedure is presented here for the first time. Magnetic islands in free-boundary full-pressure full-current stellarator magnetohydrodynamic equilibria are suppressed using a procedure based on the Princeton Iterative Equilibrium Solver [A.H.Reiman & H.S.Greenside, Comp. Phys. Comm., 43:157, 1986.] which iterates the equilibrium equations to obtain the plasma equilibrium. At each iteration, changes to a Fourier representation of the coil geometry are made to cancel resonant fields produced by the plasma. As the iterations continue, the coil geometry and the plasma simultaneously converge to an equilibrium in which the island content is negligible. The method is applied to a candidate plasma and coil design for the National Compact Stellarator eXperiment [G.H.Neilson et.al., Phys. Plas., 7:1911, 2000.].

2D and 3D Models of Convective Turbulence and Oscillations in Intermediate-Mass Main-Sequence Stars

NASA Astrophysics Data System (ADS)

Guzik, Joyce Ann; Morgan, Taylor H.; Nelson, Nicholas J.; Lovekin, Catherine; Kitiashvili, Irina N.; Mansour, Nagi N.; Kosovichev, Alexander

2015-08-01

We present multidimensional modeling of convection and oscillations in main-sequence stars somewhat more massive than the sun, using three separate approaches: 1) Applying the spherical 3D MHD ASH (Anelastic Spherical Harmonics) code to simulate the core convection and radiative zone. Our goal is to determine whether core convection can excite low-frequency gravity modes, and thereby explain the presence of low frequencies for some hybrid gamma Dor/delta Sct variables for which the envelope convection zone is too shallow for the convective blocking mechanism to drive g modes; 2) Using the 3D planar ‘StellarBox’ radiation hydrodynamics code to model the envelope convection zone and part of the radiative zone. Our goals are to examine the interaction of stellar pulsations with turbulent convection in the envelope, excitation of acoustic modes, and the role of convective overshooting; 3) Applying the ROTORC 2D stellar evolution and dynamics code to calculate evolution with a variety of initial rotation rates and extents of core convective overshooting. The nonradial adiabatic pulsation frequencies of these nonspherical models will be calculated using the 2D pulsation code NRO of Clement. We will present new insights into gamma Dor and delta Sct pulsations gained by multidimensional modeling compared to 1D model expectations.

Collisionless high energy particle losses in optimized stellarators calculated in real-space coordinates

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

Nemov, V. V.; Kasilov, S. V.; Institut für Theoretische Physik—Computational Physics, Technische Universität Graz, Fusion@ÖAW, Petersgasse 16, A-8010 Graz

An approach for the direct computation of collisionless losses of high energy charged particles is developed for stellarator magnetic fields given in real space coordinates. With this approach, the corresponding computations can be performed for magnetic fields with three-dimensional inhomogeneities in the presence of stochastic regions as well as magnetic islands. A code, which is based on this approach, is applied to various stellarator configurations. It is found that the life time of fast particles obtained in real-space coordinates can be smaller than that obtained in magnetic coordinates.

A new method to unveil embedded stellar clusters

NASA Astrophysics Data System (ADS)

Lombardi, Marco; Lada, Charles J.; Alves, João

2017-11-01

In this paper we present a novel method to identify and characterize stellar clusters deeply embedded in a dark molecular cloud. The method is based on measuring stellar surface density in wide-field infrared images using star counting techniques. It takes advantage of the differing H-band luminosity functions (HLFs) of field stars and young stellar populations and is able to statistically associate each star in an image as a member of either the background stellar population or a young stellar population projected on or near the cloud. Moreover, the technique corrects for the effects of differential extinction toward each individual star. We have tested this method against simulations as well as observations. In particular, we have applied the method to 2MASS point sources observed in the Orion A and B complexes, and the results obtained compare very well with those obtained from deep Spitzer and Chandra observations where presence of infrared excess or X-ray emission directly determines membership status for every star. Additionally, our method also identifies unobscured clusters and a low resolution version of the Orion stellar surface density map shows clearly the relatively unobscured and diffuse OB 1a and 1b sub-groups and provides useful insights on their spatial distribution.

Cosmic-Ray Propagation in Turbulent Spiral Magnetic Fields Associated with Young Stellar Objects

NASA Astrophysics Data System (ADS)

Fatuzzo, Marco; Adams, Fred C.

2018-04-01

External cosmic rays impinging upon circumstellar disks associated with young stellar objects provide an important source of ionization, and, as such, play an important role in disk evolution and planet formation. However, these incoming cosmic rays are affected by a variety of physical processes internal to stellar/disk systems, including modulation by turbulent magnetic fields. Globally, these fields naturally provide both a funneling effect, where cosmic rays from larger volumes are focused into the disk region, and a magnetic mirroring effect, where cosmic rays are repelled due to the increasing field strength. This paper considers cosmic-ray propagation in the presence of a turbulent spiral magnetic field, analogous to that produced by the solar wind. The interaction of this wind with the interstellar medium defines a transition radius, analogous to the heliopause, which provides the outer boundary to this problem. We construct a new coordinate system where one coordinate follows the spiral magnetic field lines and consider magnetic perturbations to the field in the perpendicular directions. The presence of magnetic turbulence replaces the mirroring points with a distribution of values and moves the mean location outward. Our results thus help quantify the degree to which cosmic-ray fluxes are reduced in circumstellar disks by the presence of magnetic field structures that are shaped by stellar winds. The new coordinate system constructed herein should also be useful in other astronomical applications.

Wide-Field Structure of Local Group Dwarf Irregular Galaxy IC1613

NASA Astrophysics Data System (ADS)

Pucha, Ragadeepika; Carlin, Jeffrey; Willman, Beth; Sand, David J.; Bechtol, Keith

2018-01-01

IC1613 is a typical dwarf irregular galaxy in the Local Group. Being an isolated dwarf, as opposed to the dwarfs around the Milky Way, it is likely to be subjected to fewer strong environmental effects. As a result, it serves as a good prototype for the study of the structure and evolution of dwarf galaxies. We present g- and i- band photometry from deep imaging of four fields around IC1613, that resolved stars up to ~ 4 magnitudes fainter than the tip of the RGB. This photometry was obtained using Hyper-Suprime Cam (HSC) on the Subaru Telescope. The large (1.5o) field-of-view of HSC provides us with a unique opportunity to study the wide-field structure of this dwarf galaxy. This project explores the structure of IC1613 to radii of about ~ 25 kpc using different types of stellar tracers. The aim is to search for evidence of a stellar halo or stellar over-densities around IC1613. The relative contributions of the different stellar populations as a function of position in IC1613 are also shown.

Surface temperature of a magnetized neutron star and interpretation of the ROSAT data. 1: Dipole fields

NASA Technical Reports Server (NTRS)

Page, Dany

1995-01-01

We model the temperature distribution at the surface of a magnetized neutron star and study the effects on the observed X-ray spectra and light curves. Generalrelativistic effects, i.e., redshift and lensing, are fully taken into account. Atmospheric effects on the emitted spectral flux are not included: we consider only blackbody emission at the local effective temperature. In this first paper we restrict ourselves to dipole fields. General features are studied and compared with the ROSAT data from the pulsars 0833 - 45 (Vela), 0656 + 14, 0630 + 178 (Geminga), and 1055 - 52, the four cases for which there is strong evidence that thermal radiation from the stellar surface is detected. The composite spectra we obtain are not very different from a blackbody spectrum at the star's effective temperature. We conclude that, as far as blackbody spectra are considered, temperature estimates using single-temperature models give results practically identical to our composite models. The change of the (composite blackbody) spectrum with the star's rotational phase is also not very large and may be unobservable inmost cases. Gravitational lensing strongly suppresses the light curve pulsations. If a dipole field is assumed, pulsed fractions comparable to the observed ones can be obtained only with stellar radii larger than those which are predicted by current models of neutron star struture, or with low stellar masses. Moreover, the shapes of the theoretical light curves with dipole fields do not correspond to the observations. The use of magnetic spectra may raise the pulsed fraction sufficiently but will certainly make the discrepancy with the light curve shapes worse: dipole fields are not sufficient to interpret the data. Many neutron star models with a meson condensate or hypersons predict very small radii, and hence very strong lensing, which will require highly nondipolar fields to be able to reproduce the observed pulsed fractions, if possible at all: this may be a new tool to constrain the size of neutron stars. The pulsed fractions obtained in all our models increase with photon energy: the strong decrease observed in Geminga at energies 0.3-0.5 keV is definitely a genuine effect of the magnetic field on the spectrum in contradistinction to the magnetic effects on the surface temperature considered her. Thus, a detailed analysis of thermal emission from the four pulsars we consider will require both complex surface field configurations and the inclusion of magnetic effects in the atmosphere (i.e., on the emitted spectrum).

Formation and Assembly of Massive Star Clusters

NASA Astrophysics Data System (ADS)

McMillan, Stephen

The formation of stars and star clusters is a major unresolved problem in astrophysics. It is central to modeling stellar populations and understanding galaxy luminosity distributions in cosmological models. Young massive clusters are major components of starburst galaxies, while globular clusters are cornerstones of the cosmic distance scale and represent vital laboratories for studies of stellar dynamics and stellar evolution. Yet how these clusters form and how rapidly and efficiently they expel their natal gas remain unclear, as do the consequences of this gas expulsion for cluster structure and survival. Also unclear is how the properties of low-mass clusters, which form from small-scale instabilities in galactic disks and inform much of our understanding of cluster formation and star-formation efficiency, differ from those of more massive clusters, which probably formed in starburst events driven by fast accretion at high redshift, or colliding gas flows in merging galaxies. Modeling cluster formation requires simulating many simultaneous physical processes, placing stringent demands on both software and hardware. Simulations of galaxies evolving in cosmological contexts usually lack the numerical resolution to simulate star formation in detail. They do not include detailed treatments of important physical effects such as magnetic fields, radiation pressure, ionization, and supernova feedback. Simulations of smaller clusters include these effects, but fall far short of the mass of even single young globular clusters. With major advances in computing power and software, we can now directly address this problem. We propose to model the formation of massive star clusters by integrating the FLASH adaptive mesh refinement magnetohydrodynamics (MHD) code into the Astrophysical Multi-purpose Software Environment (AMUSE) framework, to work with existing stellar-dynamical and stellar evolution modules in AMUSE. All software will be freely distributed on-line, allowing open access to state-of- the-art simulation techniques within a modern, modular software environment. We will follow the gravitational collapse of 0.1-10 million-solar mass gas clouds through star formation and coalescence into a star cluster, modeling in detail the coupling of the gas and the newborn stars. We will study the effects of star formation by detecting accreting regions of gas in self-gravitating, turbulent, MHD, FLASH models that we will translate into collisional dynamical systems of stars modeled with an N-body code, coupled together in the AMUSE framework. Our FLASH models will include treatments of radiative transfer from the newly formed stars, including heating and radiative acceleration of the surrounding gas. Specific questions to be addressed are: (1) How efficiently does the gas in a star forming region form stars, how does this depend on mass, metallicity, and other parameters, and what terminates star formation? What observational predictions can be made to constrain our models? (2) How important are different mechanisms for driving turbulence and removing gas from a cluster: accretion, radiative feedback, and mechanical feedback? (3) How does the infant mortality rate of young clusters depend on the initial properties of the parent cloud? (4) What are the characteristic formation timescales of massive star clusters, and what observable imprints does the assembly process leave on their structure at an age of 10-20 Myr, when formation is essentially complete and many clusters can be observed? These studies are directly relevant to NASA missions at many electromagnetic wavelengths, including Chandra, GALEX, Hubble, and Spitzer. Each traces different aspects of cluster formation and evolution: X-rays trace supernovae, ultraviolet traces young stars, visible colors can distinguish between young blue stars and older red stars, and the infrared directly shows young embedded star clusters.

A Framework for Finding and Interpreting Stellar CMEs

NASA Astrophysics Data System (ADS)

Osten, Rachel A.; Wolk, Scott J.

2017-10-01

The astrophysical study of mass loss, both steady-state and transient, on the cool half of the HR diagram has implications both for the star itself and the conditions created around the star that can be hospitable or inimical to supporting life. Stellar coronal mass ejections (CMEs) have not been conclusively detected, despite the ubiquity with which their radiative counterparts in an eruptive event (flares) have been. I will review some of the different observational methods which have been used and possibly could be used in the future in the stellar case, emphasizing some of the difficulties inherent in such attempts. I will provide a framework for interpreting potential transient stellar mass loss in light of the properties of flares known to occur on magnetically active stars. This uses a physically motivated way to connect the properties of flares and coronal mass ejections and provides a testable hypothesis for observing or constraining transient stellar mass loss. Finally I will describe recent results using observations at low radio frequencies to detect stellar coronal mass ejections, and give updates on prospects using future facilities to make headway in this important area.

The Planck Dusty Gravitationally Enhanced subMillimeter Sources (GEMS)

NASA Astrophysics Data System (ADS)

Frye, Brenda

2015-10-01

The brightest, strongly lensed high-redshift galaxies are veritable gems to study intense star formation in the early Universe. How do the high and irregular clumpy stellar and gas mass surface densities, strong radiation fields, and high turbulence regulate the rapid growth of these galaxies? We will use HST/WFC3 to investigate the stellar component of 6 of the brightest high-redshift sub-millimeter galaxies on the sky, which were recently discovered with the Planck all-sky survey. All are giant arcs or partial Einstein rings with angular sizes up to 17 in shallow CFHT K-band or Spitzer/IRAC imaging. FIR luminosities are 10^13-14 L_sun, with dust SEDs strongly dominated by intense star formation near the maximum possible rates (maximal starbursts). All have spectroscopic redshifts z=2.2-3.6 and magnification factors >/=20. We already have multiwavelength data sets to characterize their gas and dust column densities and kinematics, and propose here to acquire deep, high-resolution rest-frame optical imaging to study the stellar populations and morphologies. With WFC3 imaging in F110W & F160W we will: (1) constrain the stellar morphologies, ages, and mass-to-light ratios (2) Identify clumps and measure their properties to test several clump formation scenarios(3) Enhance our on-going lens modeling through the most accurate positions, morphologies and colorsOnly the brightest of the arc clumps are visible from the ground in the NIR. To register the flux along the full extent of the arcs, and importantly to probe individual star forming regions of the size of 30 Dor at z 2-3 in the brightest high-z sub-mm galaxies requires HST.

Synthesizing Planetary Nebulae for Large Scale Surveys: Predictions for LSST

NASA Astrophysics Data System (ADS)

Vejar, George; Montez, Rodolfo; Morris, Margaret; Stassun, Keivan G.

2017-01-01

The short-lived planetary nebula (PN) phase of stellar evolution is characterized by a hot central star and a bright, ionized, nebula. The PN phase forms after a low- to intermediate-mass star stops burning hydrogen in its core, ascends the asymptotic giant branch, and expels its outer layers of material into space. The exposed hot core produces ionizing UV photons and a fast stellar wind that sweeps up the surrounding material into a dense shell of ionized gas known as the PN. This fleeting stage of stellar evolution provides insight into rare atomic processes and the nucleosynthesis of elements in stars. The inherent brightness of the PNe allow them to be used to obtain distances to nearby stellar systems via the PN luminosity function and as kinematic tracers in other galaxies. However, the prevalence of non-spherical morphologies of PNe challenge the current paradigm of PN formation. The role of binarity in the shaping of the PN has recently gained traction ultimately suggesting single stars might not form PN. Searches for binary central stars have increased the binary fraction but the current PN sample is incomplete. Future wide-field, multi-epoch surveys like the Large Synoptic Survey Telescope (LSST) can impact studies of PNe and improve our understanding of their origin and formation. Using a suite of Cloudy radiative transfer calculations, we study the detectability of PNe in the proposed LSST multiband observations. We compare our synthetic PNe to common sources (stars, galaxies, quasars) and establish discrimination techniques. Finally, we discuss follow-up strategies to verify new LSST-discovered PNe and use limiting distances to estimate the potential sample of PNe enabled by LSST.

The Constant Average Relationship Between Dust-obscured Star Formation and Stellar Mass from z=0 to z=2.5

NASA Astrophysics Data System (ADS)

Whitaker, Katherine E.; Pope, Alexandra; Cybulski, Ryan; Casey, Caitlin M.; Popping, Gergo; Yun, Min; 3D-HST Collaboration

2018-01-01

The total star formation budget of galaxies consists of the sum of the unobscured star formation, as observed in the rest-frame ultraviolet (UV), together with the obscured component that is absorbed and re-radiated by dust grains in the infrared. We explore how the fraction of obscured star formation depends (SFR) and stellar mass for mass-complete samples of galaxies at 0 < z < 2.5. We combine GALEX and WISE photometry for SDSS-selected galaxies with the 3D-HST treasury program and Spitzer/MIPS 24μm photometry in the well-studied 5 extragalactic CANDELS fields. We find a strong dependence of the fraction of obscured star formation (f_obscured=SFR_IR/SFR_UV+IR) on stellar mass, with remarkably little evolution in this fraction with redshift out to z=2.5. 50% of star formation is obscured for galaxies with log(M/M⊙)=9.4 although unobscured star formation dominates the budget at lower masses, there exists a tail of low mass extremely obscured star-forming galaxies at z > 1. For log(M/M⊙)>10.5, >90% of star formation is obscured at all redshifts. We also show that at fixed total SFR, f_obscured is lower at higher redshift. At fixed mass, high-redshift galaxies are observed to have more compact sizes and much higher star formation rates, gas fractions and hence surface densities (implying higher dust obscuration), yet we observe no redshift evolution in f_obscured with stellar mass. This poses a challenge to theoretical models to reproduce, where the observed compact sizes at high redshift seem in tension with lower dust obscuration.

The Constant Average Relationship between Dust-obscured Star Formation and Stellar Mass from z = 0 to z = 2.5

NASA Astrophysics Data System (ADS)

Whitaker, Katherine E.; Pope, Alexandra; Cybulski, Ryan; Casey, Caitlin M.; Popping, Gergö; Yun, Min S.

2017-12-01

The total star formation budget of galaxies consists of the sum of the unobscured star formation, as observed in the rest-frame ultraviolet (UV), together with the obscured component that is absorbed and re-radiated by dust grains in the infrared. We explore how the fraction of obscured star formation depends on stellar mass for mass-complete samples of galaxies at 0< z< 2.5. We combine GALEX and WISE photometry for SDSS-selected galaxies with the 3D-HST treasury program and Spitzer/MIPS 24 μm photometry in the well-studied five extragalactic Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) fields. We find a strong dependence of the fraction of obscured star formation (f obscured = SFRIR/SFRUV+IR) on stellar mass, with remarkably little evolution in this fraction with redshift out to z = 2.5. 50% of star formation is obscured for galaxies with log(M/M ⊙) = 9.4 although unobscured star formation dominates the budget at lower masses, there exists a tail of low-mass, extremely obscured star-forming galaxies at z> 1. For log(M/M ⊙) > 10.5, >90% of star formation is obscured at all redshifts. We also show that at fixed total SFR, {f}{obscured} is lower at higher redshift. At fixed mass, high-redshift galaxies are observed to have more compact sizes and much higher star formation rates, gas fractions, and hence surface densities (implying higher dust obscuration), yet we observe no redshift evolution in {f}{obscured} with stellar mass. This poses a challenge to theoretical models, where the observed compact sizes at high redshift seem in tension with lower dust obscuration.

Optical, UV, and X-ray evidence for a 7-yr stellar cycle in Proxima Centauri

NASA Astrophysics Data System (ADS)

Wargelin, B. J.; Saar, S. H.; Pojmański, G.; Drake, J. J.; Kashyap, V. L.

2017-01-01

Stars of stellar type later than about M3.5 are believed to be fully convective and therefore unable to support magnetic dynamos like the one that produces the 11-yr solar cycle. Because of their intrinsic faintness, very few late M stars have undergone long-term monitoring to test this prediction, which is critical to our understanding of magnetic field generation in such stars. Magnetic activity is also of interest as the driver of UV and X-ray radiation, as well as energetic particles and stellar winds, that affects the atmospheres of close-in planets that lie within habitable zones, such as the recently discovered Proxima b. We report here on several years of optical, UV, and X-ray observations of Proxima Centauri (GJ 551; dM5.5e): 15 yr of All Sky Automated Survey photometry in the V band (1085 nights) and 3 yr in the I band (196 nights), 4 yr of Swift X-Ray Telescope and UV/Optical Telescope observations (more than 120 exposures), and nine sets of X-ray observations from other X-ray missions (ASCA, XMM-Newton, and three Chandra instruments) spanning 22 yr. We confirm previous reports of an 83-d rotational period and find strong evidence for a 7-yr stellar cycle, along with indications of differential rotation at about the solar level. X-ray/UV intensity is anticorrelated with optical V-band brightness for both rotational and cyclical variations. From comparison with other stars observed to have X-ray cycles, we deduce a simple empirical relationship between X-ray cyclic modulation and Rossby number, and we also present Swift UV grism spectra covering 2300-6000 Å.

The Spatial Distribution of Complex Organic Molecules in the L1544 Pre-stellar Core.

PubMed

Jiménez-Serra, Izaskun; Vasyunin, Anton I; Caselli, Paola; Marcelino, Nuria; Billot, Nicolas; Viti, Serena; Testi, Leonardo; Vastel, Charlotte; Lefloch, Bertrand; Bachiller, Rafael

2016-10-10

The detection of complex organic molecules (COMs) toward cold sources such as pre-stellar cores (with T<10 K), has challenged our understanding of the formation processes of COMs in the interstellar medium. Recent modelling on COM chemistry at low temperatures has provided new insight into these processes predicting that COM formation depends strongly on parameters such as visual extinction and the level of CO freeze out. We report deep observations of COMs toward two positions in the L1544 pre-stellar core: the dense, highly-extinguished continuum peak with A V ≥30 mag within the inner 2700 au; and a low-density shell with average A V ~7.5-8 mag located at 4000 au from the core's center and bright in CH 3 OH. Our observations show that CH 3 O, CH 3 OCH 3 and CH 3 CHO are more abundant (by factors ~2-10) toward the low-density shell than toward the continuum peak. Other COMs such as CH 3 OCHO, c-C 3 H 2 O, HCCCHO, CH 2 CHCN and HCCNC show slight enhancements (by factors ≤3) but the associated uncertainties are large. This suggests that COMs are actively formed and already present in the low-density shells of pre-stellar cores. The modelling of the chemistry of O-bearing COMs in L1544 indicates that these species are enhanced in this shell because i) CO starts freezing out onto dust grains driving an active surface chemistry; ii) the visual extinction is sufficiently high to prevent the UV photo-dissociation of COMs by the external interstellar radiation field; and iii) the density is still moderate to prevent severe depletion of COMs onto grains.

The Spatial Distribution of Complex Organic Molecules in the L1544 Pre-stellar Core

PubMed Central

Jiménez-Serra, Izaskun; Vasyunin, Anton I.; Caselli, Paola; Marcelino, Nuria; Billot, Nicolas; Viti, Serena; Testi, Leonardo; Vastel, Charlotte; Lefloch, Bertrand; Bachiller, Rafael

2016-01-01

The detection of complex organic molecules (COMs) toward cold sources such as pre-stellar cores (with T<10 K), has challenged our understanding of the formation processes of COMs in the interstellar medium. Recent modelling on COM chemistry at low temperatures has provided new insight into these processes predicting that COM formation depends strongly on parameters such as visual extinction and the level of CO freeze out. We report deep observations of COMs toward two positions in the L1544 pre-stellar core: the dense, highly-extinguished continuum peak with AV ≥30 mag within the inner 2700 au; and a low-density shell with average AV ~7.5-8 mag located at 4000 au from the core’s center and bright in CH3OH. Our observations show that CH3O, CH3OCH3 and CH3CHO are more abundant (by factors ~2-10) toward the low-density shell than toward the continuum peak. Other COMs such as CH3OCHO, c-C3H2O, HCCCHO, CH2CHCN and HCCNC show slight enhancements (by factors ≤3) but the associated uncertainties are large. This suggests that COMs are actively formed and already present in the low-density shells of pre-stellar cores. The modelling of the chemistry of O-bearing COMs in L1544 indicates that these species are enhanced in this shell because i) CO starts freezing out onto dust grains driving an active surface chemistry; ii) the visual extinction is sufficiently high to prevent the UV photo-dissociation of COMs by the external interstellar radiation field; and iii) the density is still moderate to prevent severe depletion of COMs onto grains. PMID:27733899

TRUST. I. A 3D externally illuminated slab benchmark for dust radiative transfer

NASA Astrophysics Data System (ADS)

Gordon, K. D.; Baes, M.; Bianchi, S.; Camps, P.; Juvela, M.; Kuiper, R.; Lunttila, T.; Misselt, K. A.; Natale, G.; Robitaille, T.; Steinacker, J.

2017-07-01

Context. The radiative transport of photons through arbitrary three-dimensional (3D) structures of dust is a challenging problem due to the anisotropic scattering of dust grains and strong coupling between different spatial regions. The radiative transfer problem in 3D is solved using Monte Carlo or Ray Tracing techniques as no full analytic solution exists for the true 3D structures. Aims: We provide the first 3D dust radiative transfer benchmark composed of a slab of dust with uniform density externally illuminated by a star. This simple 3D benchmark is explicitly formulated to provide tests of the different components of the radiative transfer problem including dust absorption, scattering, and emission. Methods: The details of the external star, the slab itself, and the dust properties are provided. This benchmark includes models with a range of dust optical depths fully probing cases that are optically thin at all wavelengths to optically thick at most wavelengths. The dust properties adopted are characteristic of the diffuse Milky Way interstellar medium. This benchmark includes solutions for the full dust emission including single photon (stochastic) heating as well as two simplifying approximations: One where all grains are considered in equilibrium with the radiation field and one where the emission is from a single effective grain with size-distribution-averaged properties. A total of six Monte Carlo codes and one Ray Tracing code provide solutions to this benchmark. Results: The solution to this benchmark is given as global spectral energy distributions (SEDs) and images at select diagnostic wavelengths from the ultraviolet through the infrared. Comparison of the results revealed that the global SEDs are consistent on average to a few percent for all but the scattered stellar flux at very high optical depths. The image results are consistent within 10%, again except for the stellar scattered flux at very high optical depths. The lack of agreement between different codes of the scattered flux at high optical depths is quantified for the first time. Convergence tests using one of the Monte Carlo codes illustrate the sensitivity of the solutions to various model parameters. Conclusions: We provide the first 3D dust radiative transfer benchmark and validate the accuracy of this benchmark through comparisons between multiple independent codes and detailed convergence tests.

Stellar activity and coronal heating: an overview of recent results

PubMed Central

Testa, Paola; Saar, Steven H.; Drake, Jeremy J.

2015-01-01

Observations of the coronae of the Sun and of solar-like stars provide complementary information to advance our understanding of stellar magnetic activity, and of the processes leading to the heating of their outer atmospheres. While solar observations allow us to study the corona at high spatial and temporal resolution, the study of stellar coronae allows us to probe stellar activity over a wide range of ages and stellar parameters. Stellar studies therefore provide us with additional tools for understanding coronal heating processes, as well as the long-term evolution of solar X-ray activity. We discuss how recent studies of stellar magnetic fields and coronae contribute to our understanding of the phenomenon of activity and coronal heating in late-type stars. PMID:25897087

X-rays from radio pulsars - The detection of PSR 1055-52

NASA Technical Reports Server (NTRS)

Cheng, A. F.; Helfand, D. J.

1983-01-01

The short-period pulsar PSR 1055-52 has been detected as a soft X-ray source in the course of an Einstein Observatory survey of radio pulsars. Its X-ray to radio luminosity ratio is about 10,000, although the X-rays are not modulated at the neutron star's rotation frequency. High spatial resolution observations suggest that a significant fraction of the emission comes from an extended region surrounding the pulsar. Several possible scenarios for the origin of both point and extended X-ray emission from isolated neutron stars are investigated: radiation from the hot stellar surface, from hot polar caps, and from an optically thick atmosphere, as well as from a circumstellar nebula emitting thermal bremsstrahlung or synchrotron radiation. It is concluded that the spatial, spectral, and temporal characteristics of this source are most consistent with a model in which relativistic particles generated by the pulsar are radiating synchrotron X-rays in the surrounding magnetic field; i.e., that PSR 1055 is embedded in a mini-Crab nebula. Observational tests of this hypothesis are suggested, and the implications of this result for pulsar evolution are briefly discussed.

Helical axis stellarator with noninterlocking planar coils

DOEpatents

Reiman, Allan; Boozer, Allen H.

1987-01-01

A helical axis stellarator using only noninterlocking planar, non-circular coils, generates magnetic fields having a magnetic well and large rotational transform with resultant large equilibrium beta.

A tunable integrated system to simulate colder stellar radiation

NASA Astrophysics Data System (ADS)

Erculiani, Marco S.; Claudi, Riccardo; Barbisan, Diego; Giro, Enrico; Bonato, Matteo; Cocola, Lorenzo; Farisato, Giancarlo; Meneghini, Metteo; Poletto, Luca; Salasnich, Bernardo; Trivellin, Nicola

2015-09-01

In the last years, a lot of extrasolar planets have been discovered in any direction of the Galaxy. More interesting, some of them have been found in the habitable zone of their host stars. A large diversity of spectral type, from early types (A) to colder ones (M), is covered by the planetary system host stars. A lot of efforts are done in order to find habitable planets around M stars and indeed some habitable super earths were found. In this framework, "Atmosphere in a Test Tube", a project started at Astronomical observatory of Padua, simulates planetary environmental condition in order to understand how and how much the behavior of photosynthetic bacteria in different planetary/star scenarios can modify the planet atmosphere. The particular case of an habitable planet orbiting a M dwarf star is under study for the time being. The irradiation of an M star, due to its lower surface temperature is very different in quality and quantity by the irradiation of a star like our Sun. We would like to describe the study of feasibility of a new kind of tunable led stellarlight simulator capable to recreate the radiation spectrum of M type stars (but with the potential to be expanded even to F, G, K star spectra types) incident on the planet. The radiation source is a multiple LED matrix cooled by means of air fan technology. In order to endow it with modularity this device will be composed by a mosaic of circuit boards arranged in a pie-chart shape, on the surface of which will be welded the LEDs. This concept is a smart way in order to replace blown out pieces instead of changing the entire platform as well as implement the device with new modules suitable to reproduce other type of stars. The device can be driven by a PC to raise or lower the intensity of both each LED and the lamp, in order to simulate as close as possible a portion of the star spectrum. The wavelength intervals overlap the limits of photosynthetic pigment absorption range (280-850 nm), while the range of the radiation source will be between 365 nm and 940 nm. The reason why we chose a higher outer limit is that M stars have the emission peak at about 1000 nm and we want to study the effects of low-light radiation on bacterial vitality. The innovative concept behind this radiative source is the use of the LED components to simulate the main stellar absorption lines and to make this a dynamic-light. Last but not least the use of LED is crucial to keep the device compact and handy. This device could help us to better understand the link between radiation and NIR-photosynthesis and could find applications in the field of photobioreactors as a test bench for the choice of the wavelength to be used in order to maximize the production rate. Other fields of application are the microscopy light sources field and the yeasts growth sector.

The distribution of interstellar dust in CALIFA edge-on galaxies via oligochromatic radiative transfer fitting

NASA Astrophysics Data System (ADS)

De Geyter, Gert; Baes, Maarten; Camps, Peter; Fritz, Jacopo; De Looze, Ilse; Hughes, Thomas M.; Viaene, Sébastien; Gentile, Gianfranco

2014-06-01

We investigate the amount and spatial distribution of interstellar dust in edge-on spiral galaxies, using detailed radiative transfer modelling of a homogeneous sample of 12 galaxies selected from the Calar Alto Legacy Integral Field Area survey. Our automated fitting routine, FITSKIRT, was first validated against artificial data. This is done by simultaneously reproducing the Sloan Digital Sky Survey g-, r-, i- and z-band observations of a toy model in order to combine the information present in the different bands. We show that this combined, oligochromatic fitting has clear advantages over standard monochromatic fitting especially regarding constraints on the dust properties. We model all galaxies in our sample using a three-component model, consisting of a double-exponential disc to describe the stellar and dust discs and using a Sérsic profile to describe the central bulge. The full model contains 19 free parameters, and we are able to constrain all these parameters to a satisfactory level of accuracy without human intervention or strong boundary conditions. Apart from two galaxies, the entire sample can be accurately reproduced by our model. We find that the dust disc is about 75 per cent more extended but only half as high as the stellar disc. The average face-on optical depth in the V band is 0.76 and the spread of 0.60 within our sample is quite substantial, which indicates that some spiral galaxies are relatively opaque even when seen face-on.

Observations by Juno's Radiation Monitoring Investigation During the First Year at Jupiter

NASA Astrophysics Data System (ADS)

Becker, H. N.; Adumitroaie, V.; Alexander, J. W.; Daubar, I.; Joergensen, J. L.; Denver, T.; Benn, M.; Adriani, A.; Mura, A.; Cicchetti, A.; Noschese, R.; Connerney, J. E. P.; Gladstone, R.; Hue, V.; Versteeg, M.; Santos-Costa, D.; Bolton, S. J.; Levin, S.; Thorne, R. M.

2017-12-01

Juno's Radiation Monitoring (RM) Investigation measures MeV electron fluxes at Jupiter by utilizing the noise signatures of penetrating high-energy particles which are visible in images collected by Juno's heavily shielded star cameras and science instruments. Image processing is used to identify and extract the characteristic signatures of penetrating high-energy electrons and ions and derive count rates which are used to infer external integral electron flux levels [Becker, H.N., et al. (2017), Space Sci Rev, doi: 10.1007/s11214-017-0345-9; Becker H.N. et al. (2017), Geophys. Res. Lett., 44, doi:10.1002/2017GL073091]. The count rate data from each RM instrument represents detection of electrons from within a broad energy channel (e.g. > 5 MeV or > 10 MeV electron sensitivity, determined using Geant4 shielding analysis). Simultaneous observations by the instruments therefore allow study of the external spectra where coordinated measurements are achieved. The spacecraft Stellar Reference Unit (SRU), the Magnetic Field Investigation's Advanced Stellar Compass (ASC) camera head D, and the Jovian Infrared Auroral Mapper (JIRAM) infrared imager are the primary instruments used in RM's collaborative observation campaigns. Penetrating particle signatures and trends across a broader range of Juno instruments and spacecraft housekeeping data also contribute to the analysis. This paper presents an overview of RM measurements of the Jovian high energy particle environment observed during the first eight science orbits of Juno's prime mission.

On the Magnetic Protection of the Atmosphere of Proxima Centauri b

NASA Astrophysics Data System (ADS)

Garcia-Sage, K.; Glocer, A.; Drake, J. J.; Gronoff, G.; Cohen, O.

2017-07-01

The discovery of exoplanets orbiting red dwarfs, such as Proxima Centauri b, has led to questions of their habitability and capacity to retain liquid surface water. While Proxima b is in a “temperate orbit,” I.e., an Earth at that location would not freeze or boil its oceans, its proximity to a parent star with quite high magnetic activity is likely to influence its atmospheric evolution and habitability. Planetary magnetic fields can prevent direct stripping away of the planetary atmosphere by the stellar wind, but ion escape can still occur at the magnetic poles. This process, the polar wind, is well known to occur at Earth and may have contributed to the habitability of Earth’s early atmosphere. The polar wind is highly variable and sensitive to both ionizing radiation and geomagnetic activity. The higher ionizing radiation levels of M dwarfs at habitable zone distances are expected to increase the polar wind by orders of magnitude and, instead of helping create a habitable atmosphere, may strip away enough volatiles to render the planet inhospitable. Here, we compute the ionospheric outflow of an Earth-twin subject to the enhanced stellar EUV flux of Proxima b, and the effect on atmospheric escape timescales. We show that an Earth-like planet would not survive the escape of its atmosphere at that location, and therefore the pathway to habitability for Proxima b requires a very different atmospheric history than that of Earth.

On the Magnetic Protection of the Atmosphere of Proxima Centauri b

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

Garcia-Sage, K.; Glocer, A.; Drake, J. J.

The discovery of exoplanets orbiting red dwarfs, such as Proxima Centauri b, has led to questions of their habitability and capacity to retain liquid surface water. While Proxima b is in a “temperate orbit,” i.e., an Earth at that location would not freeze or boil its oceans, its proximity to a parent star with quite high magnetic activity is likely to influence its atmospheric evolution and habitability. Planetary magnetic fields can prevent direct stripping away of the planetary atmosphere by the stellar wind, but ion escape can still occur at the magnetic poles. This process, the polar wind, is wellmore » known to occur at Earth and may have contributed to the habitability of Earth’s early atmosphere. The polar wind is highly variable and sensitive to both ionizing radiation and geomagnetic activity. The higher ionizing radiation levels of M dwarfs at habitable zone distances are expected to increase the polar wind by orders of magnitude and, instead of helping create a habitable atmosphere, may strip away enough volatiles to render the planet inhospitable. Here, we compute the ionospheric outflow of an Earth-twin subject to the enhanced stellar EUV flux of Proxima b, and the effect on atmospheric escape timescales. We show that an Earth-like planet would not survive the escape of its atmosphere at that location, and therefore the pathway to habitability for Proxima b requires a very different atmospheric history than that of Earth.« less

The X-Ray Luminosity Functions of Field Low-Mass X-Ray Binaries in Early-Type Galaxies: Evidence for a Stellar Age Dependence

NASA Technical Reports Server (NTRS)

Lehmer, B. D.; Berkeley, M.; Zezas, A.; Alexander, D. M.; Basu-Zych, A.; Bauer, F. E.; Brandt, W. N.; Fragos, T.; Hornschemeier, A. E.; Kalogera, V.;

A STELLAR MASS THRESHOLD FOR QUENCHING OF FIELD GALAXIES

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

Geha, M.; Blanton, M. R.; Yan, R.

2012-09-20

We demonstrate that dwarf galaxies (10{sup 7} < M{sub stellar} < 10{sup 9} M{sub Sun }, -12 > M{sub r} > -18) with no active star formation are extremely rare (<0.06%) in the field. Our sample is based on the NASA-Sloan Atlas which is a reanalysis of the Sloan Digital Sky Survey Data Release 8. We examine the relative number of quenched versus star-forming dwarf galaxies, defining quenched galaxies as having no H{alpha} emission (EW{sub H{alpha}} < 2 A) and a strong 4000 A break. The fraction of quenched dwarf galaxies decreases rapidly with increasing distance from a massive host,more » leveling off for distances beyond 1.5 Mpc. We define galaxies beyond 1.5 Mpc of a massive host galaxy to be in the field. We demonstrate that there is a stellar mass threshold of M{sub stellar} < 1.0 Multiplication-Sign 10{sup 9} M{sub Sun} below which quenched galaxies do not exist in the field. Below this threshold, we find that none of the 2951 field dwarf galaxies are quenched; all field dwarf galaxies show evidence for recent star formation. Correcting for volume effects, this corresponds to a 1{sigma} upper limit on the quenched fraction of 0.06%. In more dense environments, quenched galaxies account for 23% of the dwarf population over the same stellar mass range. The majority of quenched dwarf galaxies (often classified as dwarf elliptical galaxies) are within 2 virial radii of a massive galaxy, and only a few percent of quenched dwarf galaxies exist beyond 4 virial radii. Thus, for galaxies with stellar mass less than 1.0 Multiplication-Sign 10{sup 9} M{sub Sun }, ending star formation requires the presence of a more massive neighbor, providing a stringent constraint on models of star formation feedback.« less

Dust spectral energy distributions of nearby galaxies: an insight from the Herschel Reference Survey

NASA Astrophysics Data System (ADS)

Ciesla, L.; Boquien, M.; Boselli, A.; Buat, V.; Cortese, L.; Bendo, G. J.; Heinis, S.; Galametz, M.; Eales, S.; Smith, M. W. L.; Baes, M.; Bianchi, S.; De Looze, I.; di Serego Alighieri, S.; Galliano, F.; Hughes, T. M.; Madden, S. C.; Pierini, D.; Rémy-Ruyer, A.; Spinoglio, L.; Vaccari, M.; Viaene, S.; Vlahakis, C.

2014-05-01

Although it accounts only for a small fraction of the baryonic mass, dust has a profound impact on the physical processes at play in galaxies. Thus, to understand the evolution of galaxies, it is essential not only to characterize dust properties per se, but also in relation to global galaxy properties. To do so, we derive the dust properties of galaxies in a volume limited, K-band selected sample, the Herschel Reference Survey (HRS). We gather infrared photometric data from 8 μm to 500 μm from Spitzer, WISE, IRAS, and Herschel for all of the HRS galaxies. Draine & Li (2007, ApJ, 663, 866) models are fit to the data from which the stellar contribution has been carefully removed. We find that our photometric coverage is sufficient to constrain all of the parameters of the Draine & Li models and that a strong constraint on the 20-60 μm range is mandatory to estimate the relative contribution of the photo-dissociation regions to the infrared spectral energy distribution (SED). The SED models tend to systematically underestimate the observed 500 μm flux densities, especially for low-mass systems. We provide the output parameters for all of the galaxies, i.e., the minimum intensity of the interstellar radiation field, the fraction of polycyclic aromatic hydrocarbon (PAH), the relative contribution of PDR and evolved stellar population to the dust heating, the dust mass, and the infrared luminosity. For a subsample of gas-rich galaxies, we analyze the relations between these parameters and the main integrated properties of galaxies, such as stellar mass, star formation rate, infraredluminosity, metallicity, Hα and H-band surface brightness, and the far-ultraviolet attenuation. A good correlation between the fraction of PAH and the metallicity is found, implying a weakening of the PAH emission in galaxies with low metallicities and, thus, low stellar masses. The intensity of the diffuse interstellar radiation field and the H-band and Hα surface brightnesses are correlated, suggesting that the diffuse dust component is heated by both the young stars in star-forming regions and the diffuse evolved population. We use these results to provide a new set of infrared templates calibrated with Herschel observations on nearby galaxies and a mean SED template to provide the z = 0 reference for cosmological studies. For the same purpose, we place our sample on the SFR - M∗ diagram. The templates are compared to the most popular infrared SED libraries, enlightening a large discrepancy between all of them in the 20-100 μm range. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Table 4 and appendices are available in electronic form at http://www.aanda.org

Super-Eddington stellar winds: unifying radiative-enthalpy versus flux-driven models

NASA Astrophysics Data System (ADS)

Owocki, Stanley P.; Townsend, Richard H. D.; Quataert, Eliot

2017-12-01

We derive semi-analytic solutions for optically thick, super-Eddington stellar winds, induced by an assumed steady energy addition Δ {\\dot{E}} concentrated around a near-surface heating radius R in a massive star of central luminosity L*. We show that obtaining steady wind solutions requires both that the resulting total luminosity L_o = L_\\ast + Δ {\\dot{E}} exceed the Eddington luminosity, Γo ≡ Lo/LEdd > 1, and that the induced mass-loss rate be such that the 'photon-tiring' parameter, m ≡ {\\dot{M}} GM/R L_o ≤ 1-1/Γ _o, ensuring the luminosity is sufficient to overcome the gravitational potential GM/R. Our analysis unifies previous super-Eddington wind models that either: (1) assumed a direct radiative flux-driving without accounting for the advection of radiative enthalpy that can become important in such an optically thick flow; or (2) assumed that such super-Eddington outflows are adiabatic, neglecting the effects of the diffusive radiative flux. We show that these distinct models become applicable in the asymptotic limits of small versus large values of mΓo, respectively. By solving the coupled differential equations for radiative diffusion and wind momentum, we obtain general solutions that effectively bridge the behaviours of these limiting models. Two key scaling results are for the terminal wind speed to escape speed, which is found to vary as v_∞^2/v_esc^2 = Γ _o/(1+m Γ _o) -1, and for the final observed luminosity Lobs, which for all allowed steady-solutions with m < 1 - 1/Γo exceeds the Eddington luminosity, Lobs > LEdd. Our super-Eddington wind solutions have potential applicability for modelling phases of eruptive mass-loss from massive stars, classical novae, and the remnants of stellar mergers.

H2O from R Cas: ISO LWS-SWS observations and detailed modelling

NASA Astrophysics Data System (ADS)

Truong-Bach; Sylvester, R. J.; Barlow, M. J.; Nguyen-Q-Rieu; Lim, T.; Liu, X. W.; Baluteau, J. P.; Deguchi, S.; Justtanont, K.; Tielens, A. G. G. M.

1999-05-01

We present 29-197 mu m spectra of the oxygen-rich Mira variable star, R Cas, obtained with the Long- and Short- Wavelength Spectrometers (LWS and SWS) on board the Infrared Space Observatory (ISO). The LWS grating observations were made during two pulsational stellar phases, phi { ~ } 0.5 and 0.2 in August 1996 and June 1997 when the stellar luminosity was near its minimum and mean values, respectively. The infrared flux at the latter epoch was { ~ } 30-40% stronger than at the former. SWS grating observations were also made in June 1997. The spectrum presents a strong far-infrared (FIR) continuum and is rich in water lines suitable for use as circumstellar diagnostics. We have constructed a circumstellar model which consistently treats radiative transfer, chemical exchanges, photodissociation, and heating and cooling effects. The overall FIR excitation field was scaled by a factor which varied with the stellar phase. By fitting the model to the observed FIR water line fluxes and continuum while adopting the stellar parameters based on the Hipparcos distance we have found a mass-loss rate of dot {M} { ~ } 3.4*E(-7) Msun yr(-1) and a total ortho and para water vapour abundance (relative to {H_2} ) of f { ~ } 1.1x\\ex{-5}. The kinetic temperature and the relative abundances of {H2O} , OH, and O in chemical equilibrium have been derived as functions of radial distance r. {H2O} excitation is mainly dominated by FIR emitted by dust grains. The deduced model continuum flux at 29-197 mu m for the phi ~ 0.5 phase was 61% of the flux at phi ~ 0.2. Photodissociation by the FUV interstellar field and CO cooling effects operate farther out than the {H2O} excitation region. Our derived mass-loss rate of R Cas is similar to the value 6x\\ex{-7} Msun yr(-1) previously published for WHya, another oxygen-rich AGB star. Based on observations with ISO, an ESA project with instruments funded by ESA Members States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

Gradient of the stellar magnetic field in measurements of hydrogen line cores

NASA Astrophysics Data System (ADS)

Kudryavtsev, Dimitry O.; Romanyuk, Iosif I.

2009-04-01

We report the observed systematic differences in longitudinal magnetic field values, obtained from measurements of metal lines and the core of the Hβ line for a number of Ap stars, having strong global magnetic fields. In overwhelming majority of cases the magnetic field values, obtained from measurements of hydrogen lines cores, is smaller then the ones obtained from metal lines. We discuss some possible explanations of this effect, the most probable of which is the existence of the gradient of the magnetic field in stellar atmospheres.

Comparing Dark Energy Survey and HST –CLASH observations of the galaxy cluster RXC J2248.7-4431: implications for stellar mass versus dark matter

DOE PAGES

Palmese, A.; Lahav, O.; Banerji, M.; ...

2016-08-20

We derive the stellar mass fraction in the galaxy cluster RXC J2248.7-4431 observed with the Dark Energy Survey (DES) during the Science Verification period. We compare the stellar mass results from DES (5 filters) with those from the Hubble Space Telescope CLASH (17 filters). When the cluster spectroscopic redshift is assumed, we show that stellar masses from DES can be estimated within 25% of CLASH values. We compute the stellar mass contribution coming from red and blue galaxies, and study the relation between stellar mass and the underlying dark matter using weak lensing studies with DES and CLASH. An analysismore » of the radial profiles of the DES total and stellar mass yields a stellar-to-total fraction of f*=7.0+-2.2x10^-3 within a radius of r_200c~3 Mpc. Our analysis also includes a comparison of photometric redshifts and star/galaxy separation efficiency for both datasets. We conclude that space-based small field imaging can be used to calibrate the galaxy properties in DES for the much wider field of view. The technique developed to derive the stellar mass fraction in galaxy clusters can be applied to the ~100 000 clusters that will be observed within this survey. The stacking of all the DES clusters would reduce the errors on f* estimates and deduce important information about galaxy evolution.« less

The PyCASSO database: spatially resolved stellar population properties for CALIFA galaxies

NASA Astrophysics Data System (ADS)

de Amorim, A. L.; García-Benito, R.; Cid Fernandes, R.; Cortijo-Ferrero, C.; González Delgado, R. M.; Lacerda, E. A. D.; López Fernández, R.; Pérez, E.; Vale Asari, N.

2017-11-01

The Calar Alto Legacy Integral Field Area (CALIFA) survey, a pioneer in integral field spectroscopy legacy projects, has fostered many studies exploring the information encoded on the spatially resolved data on gaseous and stellar features in the optical range of galaxies. We describe a value-added catalogue of stellar population properties for CALIFA galaxies analysed with the spectral synthesis code starlight and processed with the pycasso platform. Our public database (http://pycasso.ufsc.br/, mirror at http://pycasso.iaa.es/) comprises 445 galaxies from the CALIFA Data Release 3 with COMBO data. The catalogue provides maps for the stellar mass surface density, mean stellar ages and metallicities, stellar dust attenuation, star formation rates, and kinematics. Example applications both for individual galaxies and for statistical studies are presented to illustrate the power of this data set. We revisit and update a few of our own results on mass density radial profiles and on the local mass-metallicity relation. We also show how to employ the catalogue for new investigations, and show a pseudo Schmidt-Kennicutt relation entirely made with information extracted from the stellar continuum. Combinations to other databases are also illustrated. Among other results, we find a very good agreement between star formation rate surface densities derived from the stellar continuum and the H α emission. This public catalogue joins the scientific community's effort towards transparency and reproducibility, and will be useful for researchers focusing on (or complementing their studies with) stellar properties of CALIFA galaxies.

Comparing Dark Energy Survey and HST-CLASH observations of the galaxy cluster RXC J2248.7-4431: implications for stellar mass versus dark matter

NASA Astrophysics Data System (ADS)

Palmese, A.; Lahav, O.; Banerji, M.; Gruen, D.; Jouvel, S.; Melchior, P.; Aleksić, J.; Annis, J.; Diehl, H. T.; Hartley, W. G.; Jeltema, T.; Romer, A. K.; Rozo, E.; Rykoff, E. S.; Seitz, S.; Suchyta, E.; Zhang, Y.; Abbott, T. M. C.; Abdalla, F. B.; Allam, S.; Benoit-Lévy, A.; Bertin, E.; Brooks, D.; Buckley-Geer, E.; Burke, D. L.; Capozzi, D.; Carnero Rosell, A.; Carrasco Kind, M.; Carretero, J.; Crocce, M.; Cunha, C. E.; D'Andrea, C. B.; da Costa, L. N.; Desai, S.; Dietrich, J. P.; Doel, P.; Estrada, J.; Evrard, A. E.; Flaugher, B.; Frieman, J.; Gerdes, D. W.; Goldstein, D. A.; Gruendl, R. A.; Gutierrez, G.; Honscheid, K.; James, D. J.; Kuehn, K.; Kuropatkin, N.; Li, T. S.; Lima, M.; Maia, M. A. G.; Marshall, J. L.; Miller, C. J.; Miquel, R.; Nord, B.; Ogando, R.; Plazas, A. A.; Roodman, A.; Sanchez, E.; Scarpine, V.; Sevilla-Noarbe, I.; Smith, R. C.; Soares-Santos, M.; Sobreira, F.; Swanson, M. E. C.; Tarle, G.; Thomas, D.; Tucker, D.; Vikram, V.

2016-12-01

We derive the stellar mass fraction in the galaxy cluster RXC J2248.7-4431 observed with the Dark Energy Survey (DES) during the Science Verification period. We compare the stellar mass results from DES (five filters) with those from the Hubble Space Telescope Cluster Lensing And Supernova Survey (CLASH; 17 filters). When the cluster spectroscopic redshift is assumed, we show that stellar masses from DES can be estimated within 25 per cent of CLASH values. We compute the stellar mass contribution coming from red and blue galaxies, and study the relation between stellar mass and the underlying dark matter using weak lensing studies with DES and CLASH. An analysis of the radial profiles of the DES total and stellar mass yields a stellar-to-total fraction of f⋆ = (6.8 ± 1.7) × 10-3 within a radius of r200c ≃ 2 Mpc. Our analysis also includes a comparison of photometric redshifts and star/galaxy separation efficiency for both data sets. We conclude that space-based small field imaging can be used to calibrate the galaxy properties in DES for the much wider field of view. The technique developed to derive the stellar mass fraction in galaxy clusters can be applied to the ˜100 000 clusters that will be observed within this survey and yield important information about galaxy evolution.

Comparing Dark Energy Survey and HST –CLASH observations of the galaxy cluster RXC J2248.7-4431: implications for stellar mass versus dark matter

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

Palmese, A.; Lahav, O.; Banerji, M.

We derive the stellar mass fraction in the galaxy cluster RXC J2248.7-4431 observed with the Dark Energy Survey (DES) during the Science Verification period. We compare the stellar mass results from DES (5 filters) with those from the Hubble Space Telescope CLASH (17 filters). When the cluster spectroscopic redshift is assumed, we show that stellar masses from DES can be estimated within 25% of CLASH values. We compute the stellar mass contribution coming from red and blue galaxies, and study the relation between stellar mass and the underlying dark matter using weak lensing studies with DES and CLASH. An analysismore » of the radial profiles of the DES total and stellar mass yields a stellar-to-total fraction of f*=7.0+-2.2x10^-3 within a radius of r_200c~3 Mpc. Our analysis also includes a comparison of photometric redshifts and star/galaxy separation efficiency for both datasets. We conclude that space-based small field imaging can be used to calibrate the galaxy properties in DES for the much wider field of view. The technique developed to derive the stellar mass fraction in galaxy clusters can be applied to the ~100 000 clusters that will be observed within this survey. The stacking of all the DES clusters would reduce the errors on f* estimates and deduce important information about galaxy evolution.« less

Comparing Dark Energy Survey and HST –CLASH observations of the galaxy cluster RXC J2248.7-4431: implications for stellar mass versus dark matter

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

Palmese, A.; Lahav, O.; Banerji, M.

We derive the stellar mass fraction in the galaxy cluster RXC J2248.7-4431 observed with the Dark Energy Survey (DES) during the Science Verification period. We compare the stellar mass results from DES (five filters) with those from the Hubble Space Telescope Cluster Lensing And Supernova Survey (CLASH; 17 filters). When the cluster spectroscopic redshift is assumed, we show that stellar masses from DES can be estimated within 25 per cent of CLASH values. We compute the stellar mass contribution coming from red and blue galaxies, and study the relation between stellar mass and the underlying dark matter using weak lensingmore » studies with DES and CLASH. An analysis of the radial profiles of the DES total and stellar mass yields a stellar-to-total fraction of f(star) = (6.8 +/- 1.7) x 10(-3) within a radius of r(200c) similar or equal to 2 Mpc. Our analysis also includes a comparison of photometric redshifts and star/galaxy separation efficiency for both data sets. We conclude that space-based small field imaging can be used to calibrate the galaxy properties in DES for the much wider field of view. The technique developed to derive the stellar mass fraction in galaxy clusters can be applied to the similar to 100 000 clusters that will be observed within this survey and yield important information about galaxy evolution.« less

Herschel Detects a Massive Dust Reservoir in Supernova 1987A

NASA Technical Reports Server (NTRS)

Matsuura, M.; Dwek, E.; Meixner, M.; Otsuka, M.; Babler, B.; Barlow, M. J.; Roman-Duval, J.; Engelbracht, C.; Sandstrom K.; Lakicevic, M.;

Extended hard-X-ray emission in the inner few parsecs of the Galaxy.

PubMed

Perez, Kerstin; Hailey, Charles J; Bauer, Franz E; Krivonos, Roman A; Mori, Kaya; Baganoff, Frederick K; Barrière, Nicolas M; Boggs, Steven E; Christensen, Finn E; Craig, William W; Grefenstette, Brian W; Grindlay, Jonathan E; Harrison, Fiona A; Hong, Jaesub; Madsen, Kristin K; Nynka, Melania; Stern, Daniel; Tomsick, John A; Wik, Daniel R; Zhang, Shuo; Zhang, William W; Zoglauer, Andreas

2015-04-30

The Galactic Centre hosts a puzzling stellar population in its inner few parsecs, with a high abundance of surprisingly young, relatively massive stars bound within the deep potential well of the central supermassive black hole, Sagittarius A* (ref. 1). Previous studies suggest that the population of objects emitting soft X-rays (less than 10 kiloelectronvolts) within the surrounding hundreds of parsecs, as well as the population responsible for unresolved X-ray emission extending along the Galactic plane, is dominated by accreting white dwarf systems. Observations of diffuse hard-X-ray (more than 10 kiloelectronvolts) emission in the inner 10 parsecs, however, have been hampered by the limited spatial resolution of previous instruments. Here we report the presence of a distinct hard-X-ray component within the central 4 × 8 parsecs, as revealed by subarcminute-resolution images in the 20-40 kiloelectronvolt range. This emission is more sharply peaked towards the Galactic Centre than is the surface brightness of the soft-X-ray population. This could indicate a significantly more massive population of accreting white dwarfs, large populations of low-mass X-ray binaries or millisecond pulsars, or particle outflows interacting with the surrounding radiation field, dense molecular material or magnetic fields. However, all these interpretations pose significant challenges to our understanding of stellar evolution, binary formation, and cosmic-ray production in the Galactic Centre.

Ongoing Massive Star Formation in NGC 604

NASA Astrophysics Data System (ADS)

Martínez-Galarza, J. R.; Hunter, D.; Groves, B.; Brandl, B.

2012-12-01

NGC 604 is the second most massive H II region in the Local Group, thus an important laboratory for massive star formation. Using a combination of observational and analytical tools that include Spitzer spectroscopy, Herschel photometry, Chandra imaging, and Bayesian spectral energy distribution fitting, we investigate the physical conditions in NGC 604 and quantify the amount of massive star formation currently taking place. We derive an average age of 4 ± 1 Myr and a total stellar mass of 1.6+1.6 - 1.0 × 105 M ⊙ for the entire region, in agreement with previous optical studies. Across the region, we find an effect of the X-ray field on both the abundance of aromatic molecules and the [Si II] emission. Within NGC 604, we identify several individual bright infrared sources with diameters of about 15 pc and luminosity-weighted masses between 103 M ⊙ and 104 M ⊙. Their spectral properties indicate that some of these sources are embedded clusters in process of formation, which together account for ~8% of the total stellar mass in the NGC 604 system. The variations of the radiation field strength across NGC 604 are consistent with a sequential star formation scenario, with at least two bursts in the last few million years. Our results indicate that massive star formation in NGC 604 is still ongoing, likely triggered by the earlier bursts.

HABITABLE ZONES OF POST-MAIN SEQUENCE STARS

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

Ramirez, Ramses M.; Kaltenegger, Lisa

Once a star leaves the main sequence and becomes a red giant, its Habitable Zone (HZ) moves outward, promoting detectable habitable conditions at larger orbital distances. We use a one-dimensional radiative-convective climate and stellar evolutionary models to calculate post-MS HZ distances for a grid of stars from 3700 to 10,000 K (∼M1 to A5 stellar types) for different stellar metallicities. The post-MS HZ limits are comparable to the distances of known directly imaged planets. We model the stellar as well as planetary atmospheric mass loss during the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) phases for super-Moons tomore » super-Earths. A planet can stay between 200 million years up to 9 Gyr in the post-MS HZ for our hottest and coldest grid stars, respectively, assuming solar metallicity. These numbers increase for increased stellar metallicity. Total atmospheric erosion only occurs for planets in close-in orbits. The post-MS HZ orbital distances are within detection capabilities of direct imaging techniques.« less

Non-radial pulsations and large-scale structure in stellar winds

NASA Astrophysics Data System (ADS)

Blomme, R.

2009-07-01

Almost all early-type stars show Discrete Absorption Components (DACs) in their ultraviolet spectral lines. These can be attributed to Co-rotating Interaction Regions (CIRs): large-scale spiral-shaped structures that sweep through the stellar wind. We used the Zeus hydrodynamical code to model the CIRs. In the model, the CIRs are caused by ``spots" on the stellar surface. Through the radiative acceleration these spots create fast streams in the stellar wind material. Where the fast and slow streams collide, a CIR is formed. By varying the parameters of the spots, we quantitatively fit the observed DACs in HD~64760. An important result from our work is that the spots do not rotate with the same velocity as the stellar surface. The fact that the cause of the CIRs is not fixed on the surface eliminates many potential explanations. The only remaining explanation is that the CIRs are due to the interference pattern of a number of non-radial pulsations.

Scale covariant gravitation. V - Kinetic theory. VI - Stellar structure and evolution

NASA Technical Reports Server (NTRS)

Hsieh, S.-H.; Canuto, V. M.

1981-01-01

A scale covariant kinetic theory for particles and photons is developed. The mathematical framework of the theory is given by the tangent bundle of a Weyl manifold. The Liouville equation is derived, and solutions to corresponding equilibrium distributions are presented and shown to yield thermodynamic results identical to the ones obtained previously. The scale covariant theory is then used to derive results of interest to stellar structure and evolution. A radiative transfer equation is derived that can be used to study stellar evolution with a variable gravitational constant. In addition, it is shown that the sun's absolute luminosity scales as L approximately equal to GM/kappa, where kappa is the stellar opacity. Finally, a formula is derived for the age of globular clusters as a function of the gravitational constant using a previously derived expression for the absolute luminosity.

Earth's Radiation Belts: The View from Juno's Cameras

NASA Astrophysics Data System (ADS)

Becker, H. N.; Joergensen, J. L.; Hansen, C. J.; Caplinger, M. A.; Ravine, M. A.; Gladstone, R.; Versteeg, M. H.; Mauk, B.; Paranicas, C.; Haggerty, D. K.; Thorne, R. M.; Connerney, J. E.; Kang, S. S.

2013-12-01

Juno's cameras, particle instruments, and ultraviolet imaging spectrograph have been heavily shielded for operation within Jupiter's high radiation environment. However, varying quantities of >1-MeV electrons and >10-MeV protons will be energetic enough to penetrate instrument shielding and be detected as transient background signatures by the instruments. The differing shielding profiles of Juno's instruments lead to differing spectral sensitivities to penetrating electrons and protons within these regimes. This presentation will discuss radiation data collected by Juno in the Earth's magnetosphere during Juno's October 9, 2013 Earth flyby (559 km altitude at closest approach). The focus will be data from Juno's Stellar Reference Unit, Advanced Stellar Compass star cameras, and JunoCam imager acquired during coordinated proton measurements within the inner zone and during the spacecraft's inbound and outbound passages through the outer zone (L ~3-5). The background radiation signatures from these cameras will be correlated with dark count background data collected at these geometries by Juno's Ultraviolet Spectrograph (UVS) and Jupiter Energetic Particle Detector Instrument (JEDI). Further comparison will be made to Van Allen Probe data to calibrate Juno's camera results and contribute an additional view of the Earth's radiation environment during this unique event.

Subsonic structure and optically thick winds from Wolf-Rayet stars

NASA Astrophysics Data System (ADS)

Grassitelli, L.; Langer, N.; Grin, N. J.; Mackey, J.; Bestenlehner, J. M.; Gräfener, G.

2018-06-01

Mass loss by stellar wind is a key agent in the evolution and spectroscopic appearance of massive main sequence and post-main sequence stars. In Wolf-Rayet stars the winds can be so dense and so optically thick that the photosphere appears in the highly supersonic part of the outflow, veiling the underlying subsonic part of the star, and leaving the initial acceleration of the wind inaccessible to observations. Here we investigate the conditions and the structure of the subsonic part of the outflow of Galactic Wolf-Rayet stars, in particular of the WNE subclass; our focus is on the conditions at the sonic point of their winds. We compute 1D hydrodynamic stellar structure models for massive helium stars adopting outer boundaries at the sonic point. We find that the outflows of our models are accelerated to supersonic velocities by the radiative force from opacity bumps either at temperatures of the order of 200 kK by the iron opacity bump or of the order of 50 kK by the helium-II opacity bump. For a given mass-loss rate, the diffusion approximation for radiative energy transport allows us to define the temperature gradient based purely on the local thermodynamic conditions. For a given mass-loss rate, this implies that the conditions in the subsonic part of the outflow are independent from the detailed physical conditions in the supersonic part. Stellar atmosphere calculations can therefore adopt our hydrodynamic models as ab initio input for the subsonic structure. The close proximity to the Eddington limit at the sonic point allows us to construct a sonic HR diagram, relating the sonic point temperature to the luminosity-to-mass ratio and the stellar mass-loss rate, thereby constraining the sonic point conditions, the subsonic structure, and the stellar wind mass-loss rates of WNE stars from observations. The minimum stellar wind mass-loss rate necessary to have the flow accelerated to supersonic velocities by the iron opacity bump is derived. A comparison of the observed parameters of Galactic WNE stars to this minimum mass-loss rate indicates that these stars have their winds launched to supersonic velocities by the radiation pressure arising from the iron opacity bump. Conversely, stellar models which do not show transonic flows from the iron opacity bump form low-density extended envelopes. We derive an analytic criterion for the appearance of envelope inflation and of a density inversion in the outer sub-photospheric layers.

Solar and stellar coronal plasmas

NASA Technical Reports Server (NTRS)

Golub, L.

1985-01-01

Progress made in describing and interpreting coronal plasma processes and the relationship between the solar corona and its stellar counterparts is reported. Topics covered include: stellar X-ray emission, HEAO 2 X-ray survey of the Pleiades, closed coronal structures, X-ray survey of main-sequence stars with shallow convection zones, implications of the 1400 MHz flare emission, and magnetic field stochasticity.

Effects of Combined Stellar Feedback on Star Formation in Stellar Clusters

NASA Astrophysics Data System (ADS)

Wall, Joshua Edward; McMillan, Stephen; Pellegrino, Andrew; Mac Low, Mordecai; Klessen, Ralf; Portegies Zwart, Simon

2018-01-01

We present results of hybrid MHD+N-body simulations of star cluster formation and evolution including self consistent feedback from the stars in the form of radiation, winds, and supernovae from all stars more massive than 7 solar masses. The MHD is modeled with the adaptive mesh refinement code FLASH, while the N-body computations are done with a direct algorithm. Radiation is modeled using ray tracing along long characteristics in directions distributed using the HEALPIX algorithm, and causes ionization and momentum deposition, while winds and supernova conserve momentum and energy during injection. Stellar evolution is followed using power-law fits to evolution models in SeBa. We use a gravity bridge within the AMUSE framework to couple the N-body dynamics of the stars to the gas dynamics in FLASH. Feedback from the massive stars alters the structure of young clusters as gas ejection occurs. We diagnose this behavior by distinguishing between fractal distribution and central clustering using a Q parameter computed from the minimum spanning tree of each model cluster. Global effects of feedback in our simulations will also be discussed.

The STAGGER-grid: A grid of 3D stellar atmosphere models. V. Synthetic stellar spectra and broad-band photometry

NASA Astrophysics Data System (ADS)

Chiavassa, A.; Casagrande, L.; Collet, R.; Magic, Z.; Bigot, L.; Thévenin, F.; Asplund, M.

2018-03-01

Context. The surface structures and dynamics of cool stars are characterised by the presence of convective motions and turbulent flows which shape the emergent spectrum. Aims: We used realistic three-dimensional (3D) radiative hydrodynamical simulations from the STAGGER-grid to calculate synthetic spectra with the radiative transfer code OPTIM3D for stars with different stellar parameters to predict photometric colours and convective velocity shifts. Methods: We calculated spectra from 1000 to 200 000 Å with a constant resolving power of λ/Δλ = 20 000 and from 8470 and 8710 Å (Gaia Radial Velocity Spectrometer - RVS - spectral range) with a constant resolving power of λ/Δλ = 300 000. Results: We used synthetic spectra to compute theoretical colours in the Johnson-Cousins UBV (RI)C, SDSS, 2MASS, Gaia, SkyMapper, Strömgren systems, and HST-WFC3. Our synthetic magnitudes are compared with those obtained using 1D hydrostatic models. We showed that 1D versus 3D differences are limited to a small percent except for the narrow filters that span the optical and UV region of the spectrum. In addition, we derived the effect of the convective velocity fields on selected Fe I lines. We found the overall convective shift for 3D simulations with respect to the reference 1D hydrostatic models, revealing line shifts of between -0.235 and +0.361 km s-1. We showed a net correlation of the convective shifts with the effective temperature: lower effective temperatures denote redshifts and higher effective temperatures denote blueshifts. We conclude that the extraction of accurate radial velocities from RVS spectra need an appropriate wavelength correction from convection shifts. Conclusions: The use of realistic 3D hydrodynamical stellar atmosphere simulations has a small but significant impact on the predicted photometry compared with classical 1D hydrostatic models for late-type stars. We make all the spectra publicly available for the community through the POLLUX database. Tables 5-8 are only available at the CDS and Table B.1 is also available at the CDS and via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A11

Habitable zones exposed: astrosphere collapse frequency as a function of stellar mass.

PubMed

Smith, David S; Scalo, John M

2009-09-01

Stellar astrospheres--the plasma cocoons carved out of the interstellar medium by stellar winds--are one of several buffers that partially screen planetary atmospheres and surfaces from high-energy radiation. Screening by astrospheres is continually influenced by the passage of stars through the fluctuating density field of the interstellar medium (ISM). The most extreme events occur inside dense interstellar clouds, where the increased pressure may compress an astrosphere to a size smaller than the liquid-water habitable-zone distance. Habitable planets then enjoy no astrospheric buffering from exposure to the full flux of galactic cosmic rays and interstellar dust and gas, a situation we call "descreening" or "astrospheric collapse." Under such conditions the ionization fraction in the atmosphere and contribution to radiation damage of putative coding organisms at the surface would increase significantly, and a series of papers have suggested a variety of global responses to descreening. These possibilities motivate a more careful calculation of the frequency of descreening events. Using a ram-pressure balance model, we compute the size of the astrosphere in the apex direction as a function of parent-star mass and velocity and ambient interstellar density, emphasizing the importance of gravitational focusing of the interstellar flow. The interstellar densities required to descreen planets in the habitable zone of solar- and subsolar-mass stars are found to be about 600(M/M[middle dot in circle])(-2) cm(-3) for the Sun's velocity relative to the local ISM. Such clouds are rare and small, indicating that descreening encounters are rare. We use statistics from two independent catalogues of dense interstellar clouds to derive a dependence of descreening frequency on the parent-star mass that decreases strongly with decreasing stellar mass, due to the weaker gravitational focusing and smaller habitable-zone distances for lower-mass stars. We estimate an uncertain upper limit to the absolute frequency of descreening encounters as 1-10 Gyr(-1) for solar-type stars and 10(2) to 10(9) times smaller for stars between 0.5 and 0.1 M[middle dot in circle]. Habitable-zone planets orbiting late-K to M stars are virtually never exposed to the severe consequences that have been proposed for astrospheric descreening events, but descreening events at a moderate rate may occur for stars with the Sun's mass or larger.

Laboratory Astrophysics Prize: Laboratory Astrophysics with Nuclei

NASA Astrophysics Data System (ADS)

Wiescher, Michael

2018-06-01

Nuclear astrophysics is concerned with nuclear reaction and decay processes from the Big Bang to the present star generation controlling the chemical evolution of our universe. Such nuclear reactions maintain stellar life, determine stellar evolution, and finally drive stellar explosion in the circle of stellar life. Laboratory nuclear astrophysics seeks to simulate and understand the underlying processes using a broad portfolio of nuclear instrumentation, from reactor to accelerator from stable to radioactive beams to map the broad spectrum of nucleosynthesis processes. This talk focuses on only two aspects of the broad field, the need of deep underground accelerator facilities in cosmic ray free environments in order to understand the nucleosynthesis in stars, and the need for high intensity radioactive beam facilities to recreate the conditions found in stellar explosions. Both concepts represent the two main frontiers of the field, which are being pursued in the US with the CASPAR accelerator at the Sanford Underground Research Facility in South Dakota and the FRIB facility at Michigan State University.

KSC-08pd2465

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, technicians observe as the Wide Field Camera 3, or WFC3, is rotated. The WFC3 will be transferred to the Super Lightweight Interchangeable Carrier. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. The curved edge shown at the back is the radiator, the "outside" of WFC3 that will be exposed to space and will expel heat out of Hubble and into space through black body radiation. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

KSC-08pd2466

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, technicians observe as the Wide Field Camera 3, or WFC3, is rotated to vertical. The WFC3 will be transferred to the Super Lightweight Interchangeable Carrier. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. The curved edge shown at top is the radiator, the "outside" of WFC3 that will be exposed to space and will expel heat out of Hubble and into space through black body radiation. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

KSC-08pd2467

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – After rotation of the Wide Field Camera 3 (background left), or WFC3, in the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, technicians check the data. The WFC3 will be transferred to the Super Lightweight Interchangeable Carrier. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. The curved edge shown at top is the radiator, the "outside" of WFC3 that will be exposed to space and will expel heat out of Hubble and into space through black body radiation. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

Resource Letter GrW-1: Gravitational Waves

NASA Technical Reports Server (NTRS)

White, Nicholas E. (Technical Monitor); Centrella, Joan M.

2003-01-01

The phenomenon of gravitational radiation was one of the first predictions of Einstein's general theory of relativity. Progress in understanding this radiation theoretically was slow at first, owing to the difficulty of the nonlinear field equations and the subtleties of their physical effects. The experimental side of this subject also has taken a long time to develop, with efforts at detection severely challenged by the extreme weakness of the waves impinging on the Earth. However, as the 21st century begins, observations of the gravitational waves from astrophysical sources such as black holes, neutron stars, and stellar collapse are expected to open a new window on the universe. Vigorous experimental programs centered on ground-based detectors are being carried out worldwide, and a space-based detector is in the planning stages. On the theoretical side, much effort is being expended to produce robust models of the astrophysical sources and accurate calculations of the waveforms they produce. In this Resource Letter, a set of basic references will be presented first, to provide a general introduction to and overview of the literature in this field. The focus then will shift to highlighting key resources in more specialized areas at the forefront of current research.

KSC-08pd2464

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, the Wide Field Camera 3, or WFC3, has been rotated. The WFC3 will be transferred to the Super Lightweight Interchangeable Carrier. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. The curved edge shown at top is the radiator, the "outside" of WFC3 that will be exposed to space and will expel heat out of Hubble and into space through black body radiation. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

KSC-08pd2461

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, technicians wait for the rotation of the Wide Field Camera 3, or WFC3, in order to attach a crane. The WFC3 will be transferred to the Super Lightweight Interchangeable Carrier. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. The part shown here is the radiator, the "outside" of WFC3 that will be exposed to space and will expel heat out of Hubble and into space through black body radiation. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

KSC-08pd2462

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, technicians wait for the rotation of the Wide Field Camera 3, or WFC3, in order to attach a crane. The WFC3 will be transferred to the Super Lightweight Interchangeable Carrier. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. The curved edge shown at left is the radiator, the "outside" of WFC3 that will be exposed to space and will expel heat out of Hubble and into space through black body radiation. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

KSC-08pd2463

NASA Image and Video Library

2008-08-18

CAPE CANAVERAL, Fla. – In the Payload Hazardous Servicing Facility at NASA's Kennedy Space Center, technicians stand by as the Wide Field Camera 3, or WFC3, is rotated. The WFC3 will be transferred to the Super Lightweight Interchangeable Carrier. WFC3 is part of the payload on space shuttle Atlantis' STS-125 mission for the fifth and final Hubble servicing flight to NASA's Hubble Space Telescope. The curved edge shown at left is the radiator, the "outside" of WFC3 that will be exposed to space and will expel heat out of Hubble and into space through black body radiation. As Hubble enters the last stage of its life, WFC3 will be Hubble's next evolutionary step, allowing Hubble to peer ever further into the mysteries of the cosmos. WFC3 will study a diverse range of objects and phenomena, from young and extremely distant galaxies, to much more nearby stellar systems, to objects within our very own solar system. WFC3 will take the place of Wide Field Planetary Camera 2, which astronauts will bring back to Earth aboard the shuttle. Launch of Atlantis is targeted at 1:34 a.m. EDT Oct. 8. Photo credit: NASA/Amanda Diller

Evaporating Atmospheres Around Close-in Exoplanets.

NASA Astrophysics Data System (ADS)

Owen, J.; Jackson, A.; Wu, Y.; Adams, F.

2014-12-01

The majority of currently observed exoplanets appear exceeding close to the central star (<0.1 AU) and as such are subject to intense high energy radiation from UV & X-ray photons. We will discuss that in such environments the atmospheres these planets are heated sufficiently that they can escape the planet's gravitational field in a hydrodynamic trans-sonic wind. We will show that this hydrodynamic mass-loss occurs for the majority of exoplanets at short periods, and for low-mass planets (<50 Mearth) is vigorous enough to significantly alter the planet's evolution. In some cases we will argue that an originally gas rich exoplanet can be completely evaporated leaving behind a bare rock core. In addition, we will present new multi-dimensional simulations of evaporation that include realistic treatment of the radiative transfer. These new simulations show that evaporation from 'hot' Jupiters is likely to be magnetically controlled, where mass-loss can only occur along open filed lines, where the interaction between the stellar and planetary magnetic field strongly controls the geometry of the evaporative flow. We will indicate how these new multi-dimensional radiation-magneto-hydrodynamic calculations can be used to study the time-dependence of the outflow and link the small but growing number of observations of exoplanet evaporation to the theoretical models. Finally, we will indicate that asymmetric evaporative flows can lead to orbital evolution of planets at close separations. Figure Caption: "Flow structure from an evaporating Hot Jupiter with a magnetic field strength of 0.3 Gauss. Top panels show density and magnetic field configuration and bottom panel shows plasma beta and velocity structure; left panels show simulation domain, right panels show a zoom in on the planet."

THE DEPENDENCE OF STELLAR MASS AND ANGULAR MOMENTUM LOSSES ON LATITUDE AND THE INTERACTION OF ACTIVE REGION AND DIPOLAR MAGNETIC FIELDS

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

Garraffo, Cecilia; Drake, Jeremy J.; Cohen, Ofer

Rotation evolution of late-type stars is dominated by magnetic braking and the underlying factors that control this angular momentum loss are important for the study of stellar spin-down. In this work, we study angular momentum loss as a function of two different aspects of magnetic activity using a calibrated Alfvén wave-driven magnetohydrodynamic wind model: the strengths of magnetic spots and their distribution in latitude. By driving the model using solar and modified solar surface magnetograms, we show that the topology of the field arising from the net interaction of both small-scale and large-scale field is important for spin-down rates andmore » that angular momentum loss is not a simple function of large scale magnetic field strength. We find that changing the latitude of magnetic spots can modify mass and angular momentum loss rates by a factor of two. The general effect that causes these differences is the closing down of large-scale open field at mid- and high-latitudes by the addition of the small-scale field. These effects might give rise to modulation of mass and angular momentum loss through stellar cycles, and present a problem for ab initio attempts to predict stellar spin-down based on wind models. For all the magnetogram cases considered here, from dipoles to various spotted distributions, we find that angular momentum loss is dominated by the mass loss at mid-latitudes. The spin-down torque applied by magnetized winds therefore acts at specific latitudes and is not evenly distributed over the stellar surface, though this aspect is unlikely to be important for understanding spin-down and surface flows on stars.« less

M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets

NASA Astrophysics Data System (ADS)

Vidotto, A. A.; Jardine, M.; Morin, J.; Donati, J. F.; Opher, M.; Gombosi, T. I.

2014-02-01

We perform three-dimensional numerical simulations of stellar winds of early-M-dwarf stars. Our simulations incorporate observationally reconstructed large-scale surface magnetic maps, suggesting that the complexity of the magnetic field can play an important role in the angular momentum evolution of the star, possibly explaining the large distribution of periods in field dM stars, as reported in recent works. In spite of the diversity of the magnetic field topologies among the stars in our sample, we find that stellar wind flowing near the (rotational) equatorial plane carries most of the stellar angular momentum, but there is no preferred colatitude contributing to mass-loss, as the mass flux is maximum at different colatitudes for different stars. We find that more non-axisymmetric magnetic fields result in more asymmetric mass fluxes and wind total pressures ptot (defined as the sum of thermal, magnetic and ram pressures). Because planetary magnetospheric sizes are set by pressure equilibrium between the planet's magnetic field and ptot, variations of up to a factor of 3 in ptot (as found in the case of a planet orbiting at several stellar radii away from the star) lead to variations in magnetospheric radii of about 20 per cent along the planetary orbital path. In analogy to the flux of cosmic rays that impact the Earth, which is inversely modulated with the non-axisymmetric component of the total open solar magnetic flux, we conclude that planets orbiting M-dwarf stars like DT Vir, DS Leo and GJ 182, which have significant non-axisymmetric field components, should be the more efficiently shielded from galactic cosmic rays, even if the planets lack a protective thick atmosphere/large magnetosphere of their own.

Weaving the history of the solar wind with magnetic field lines

NASA Astrophysics Data System (ADS)

Alvarado Gomez, Julian

2017-08-01

Despite its fundamental role for the evolution of the solar system, our observational knowledge of the wind properties of the young Sun comes from a single stellar observation. This unexpected fact for a field such as astrophysics arises from the difficulty of detecting Sun-like stellar winds. Their detection relies on the appearance of an astrospheric signature (from the stellar wind-ISM interaction region), visible only with the aid of high-resolution HST Lyman-alpha spectra. However, observations and modelling of the present day Sun have revealed that magnetic fields constitute the main driver of the solar wind, providing guidance on how such winds would look like back in time. In this context we propose observations of four young Sun-like stars in order to detect their astrospheres and characterise their stellar winds. For all these objects we have recovered surface magnetic field maps using the technique of Zeeman Doppler Imaging, and developed detailed wind models based on these observed field distributions. Even a single detection would represent a major step forward for our understanding of the history of the solar wind, and the outflows in more active stars. Mass loss rate estimates from HST will be confronted with predictions from realistic models of the corona/stellar wind. In one of our objects the comparison would allow us to quantify the wind variability induced by the magnetic cycle of a star, other than the Sun, for the first time. Three of our targets are planet hosts, thus the HST spectra would also provide key information on the high-energy environment of these systems, guaranteeing their legacy value for the growing field of exoplanet characterisation.

Does Stellar Feedback Create HI Holes? An HST/VLA Study of Holmberg II

NASA Astrophysics Data System (ADS)

Weisz, Daniel R.; Skillman, E. D.; Cannon, J. M.; Dolphin, A. E.; Kennicutt, R. C., Jr.; Lee, J.; Walter, F.

2010-01-01

We use deep HST/ACS F555W and F814W photometry of resolved stars in the M81 Group dwarf irregular galaxy Holmberg II to study the hypothesis that the holes identified in the neutral ISM (HI) are created by stellar feedback. From the deep photometry, we construct color-magnitude diagrams (CMDs) and measure the star formation histories (SFHs) for stars contained in HI holes from two independent holes catalogs, as well as select control fields, i.e., similar sized regions that span a range of HI column densities. The recent SFHs confirm multiple episodes of star formation within most holes. Converting the recent SFHs into stellar feedback energies, we find that enough energy has been generated to have created all holes. However, the required energy is not always produced over a time scale that is less than the estimated kinematic age of the hole. A similar analysis of stars in the control fields finds that the stellar populations of the control fields and HI holes are statistically indistinguishable. However, because we are only sensitive to holes ˜ 100 pc in diameter, we cannot tell if there are smaller holes inside the control fields. The combination of the CMDs, recent SFHs, and locations of young stars shows that the stellar populations inside HI holes are not coherent, single-aged, stellar clusters, as previously suggested, but rather multi-age populations distributed across each hole. From a comparison of the modeled and observed integrated magnitudes, and the locations and energetics of stars inside of HI holes, we propose a potential new model: a viable mechanism for creating the observed HI holes in Holmberg II is stellar feedback from multiple generations of SF spread out over tens or hundreds of Myr, and thus, the concept of an age for an HI hole is intrinsically ambiguous.

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

NASA Technical Reports Server (NTRS)

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

1994-01-01

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

The Pan-STARRS1 medium-deep survey: The role of galaxy group environment in the star formation rate versus stellar mass relation and quiescent fraction out to z ∼ 0.8

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

Lin, Lihwai; Chen, Chin-Wei; Coupon, Jean

2014-02-10

Using a large optically selected sample of field and group galaxies drawn from the Pan-STARRS1 Medium-Deep Survey (PS1/MDS), we present a detailed analysis of the specific star formation rate (SSFR)—stellar mass (M {sub *}) relation, as well as the quiescent fraction versus M {sub *} relation in different environments. While both the SSFR and the quiescent fraction depend strongly on stellar mass, the environment also plays an important role. Using this large galaxy sample, we confirm that the fraction of quiescent galaxies is strongly dependent on environment at a fixed stellar mass, but that the amplitude and the slope ofmore » the star-forming sequence is similar between the field and groups: in other words, the SSFR-density relation at a fixed stellar mass is primarily driven by the change in the star-forming and quiescent fractions between different environments rather than a global suppression in the star formation rate for the star-forming population. However, when we restrict our sample to the cluster-scale environments (M > 10{sup 14} M {sub ☉}), we find a global reduction in the SSFR of the star-forming sequence of 17% at 4σ confidence as opposed to its field counterpart. After removing the stellar mass dependence of the quiescent fraction seen in field galaxies, the excess in the quiescent fraction due to the environment quenching in groups and clusters is found to increase with stellar mass, although deeper and larger data from the full PS1/MDS will be required to draw firm conclusions. We argue that these results are in favor of galaxy mergers to be the primary environment quenching mechanism operating in galaxy groups whereas strangulation is able to reproduce the observed trend in the environment quenching efficiency and stellar mass relation seen in clusters. Our results also suggest that the relative importance between mass quenching and environment quenching depends on stellar mass—the mass quenching plays a dominant role in producing quiescent galaxies for more massive galaxies, while less massive galaxies are quenched mostly through the environmental effect, with the transition mass around 1-2 × 10{sup 10} M {sub ☉} in the group/cluster environment.« less

Systematic variation of the stellar initial mass function in early-type galaxies.

PubMed

Cappellari, Michele; McDermid, Richard M; Alatalo, Katherine; Blitz, Leo; Bois, Maxime; Bournaud, Frédéric; Bureau, M; Crocker, Alison F; Davies, Roger L; Davis, Timothy A; de Zeeuw, P T; Duc, Pierre-Alain; Emsellem, Eric; Khochfar, Sadegh; Krajnović, Davor; Kuntschner, Harald; Lablanche, Pierre-Yves; Morganti, Raffaella; Naab, Thorsten; Oosterloo, Tom; Sarzi, Marc; Scott, Nicholas; Serra, Paolo; Weijmans, Anne-Marie; Young, Lisa M

2012-04-25

Much of our knowledge of galaxies comes from analysing the radiation emitted by their stars, which depends on the present number of each type of star in the galaxy. The present number depends on the stellar initial mass function (IMF), which describes the distribution of stellar masses when the population formed, and knowledge of it is critical to almost every aspect of galaxy evolution. More than 50 years after the first IMF determination, no consensus has emerged on whether it is universal among different types of galaxies. Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot both be universal, but they could not convincingly discriminate between the two possibilities. Only recently were indications found that massive elliptical galaxies may not have the same IMF as the Milky Way. Here we report a study of the two-dimensional stellar kinematics for the large representative ATLAS(3D) sample of nearby early-type galaxies spanning two orders of magnitude in stellar mass, using detailed dynamical models. We find a strong systematic variation in IMF in early-type galaxies as a function of their stellar mass-to-light ratios, producing differences of a factor of up to three in galactic stellar mass. This implies that a galaxy's IMF depends intimately on the galaxy's formation history.

A hot compact dust disk around a massive young stellar object.

PubMed

Kraus, Stefan; Hofmann, Karl-Heinz; Menten, Karl M; Schertl, Dieter; Weigelt, Gerd; Wyrowski, Friedrich; Meilland, Anthony; Perraut, Karine; Petrov, Romain; Robbe-Dubois, Sylvie; Schilke, Peter; Testi, Leonardo

2010-07-15

Circumstellar disks are an essential ingredient of the formation of low-mass stars. It is unclear, however, whether the accretion-disk paradigm can also account for the formation of stars more massive than about 10 solar masses, in which strong radiation pressure might halt mass infall. Massive stars may form by stellar merging, although more recent theoretical investigations suggest that the radiative-pressure limit may be overcome by considering more complex, non-spherical infall geometries. Clear observational evidence, such as the detection of compact dusty disks around massive young stellar objects, is needed to identify unambiguously the formation mode of the most massive stars. Here we report near-infrared interferometric observations that spatially resolve the astronomical-unit-scale distribution of hot material around a high-mass ( approximately 20 solar masses) young stellar object. The image shows an elongated structure with a size of approximately 13 x 19 astronomical units, consistent with a disk seen at an inclination angle of approximately 45 degrees . Using geometric and detailed physical models, we found a radial temperature gradient in the disk, with a dust-free region less than 9.5 astronomical units from the star, qualitatively and quantitatively similar to the disks observed in low-mass star formation. Perpendicular to the disk plane we observed a molecular outflow and two bow shocks, indicating that a bipolar outflow emanates from the inner regions of the system.

Massive Stars and the Energy Balance of the Interstellar Medium. 1; The Impact of an Isolated 60 M. Star

NASA Technical Reports Server (NTRS)

Freyer, Tim; Hensler, Gerhard; Yorke, Harold W.

2003-01-01

We present results of numerical simulations carried out with a two-dimensional radiation hydrodynamics code in order to study the impact of massive stars on their surrounding interstellar medium. This first paper deals with the evolution of the circumstellar gas around an isolated 60 M. star. The interaction of the photo- ionized H II region with the stellar wind bubble forms a variety of interesting structures like shells, clouds, fingers, and spokes. These results demonstrate that complex structures found in H II regions are not necessarily relics from the time before the gas became ionized but may result from dynamical processes during the course of the H II region evolution. We have also analyzed the transfer and deposit of the stellar wind and radiation energy into the circumstellar medium until the star explodes as a supernova. Although the total mechanical wind energy supplied by the star is negligible compared to the accumulated energy of the Lyman continuum photons, the kinetic energy imparted to the circumstellar gas over the star s lifetime is 4 times higher than for a comparable windless simulation. Furthermore, the thermal energy of warm photoionized gas is lower by some 55%). Our results document the necessity to consider both ionizing radiation and stellar winds for an appropriate description of the interaction of OB stars with their circumstellar environment.

A higher-than-predicted measurement of iron opacity at solar interior temperatures.

PubMed

Bailey, J E; Nagayama, T; Loisel, G P; Rochau, G A; Blancard, C; Colgan, J; Cosse, Ph; Faussurier, G; Fontes, C J; Gilleron, F; Golovkin, I; Hansen, S B; Iglesias, C A; Kilcrease, D P; MacFarlane, J J; Mancini, R C; Nahar, S N; Orban, C; Pain, J-C; Pradhan, A K; Sherrill, M; Wilson, B G

2015-01-01

Nearly a century ago it was recognized that radiation absorption by stellar matter controls the internal temperature profiles within stars. Laboratory opacity measurements, however, have never been performed at stellar interior conditions, introducing uncertainties in stellar models. A particular problem arose when refined photosphere spectral analysis led to reductions of 30-50 per cent in the inferred amounts of carbon, nitrogen and oxygen in the Sun. Standard solar models using the revised element abundances disagree with helioseismic observations that determine the internal solar structure using acoustic oscillations. This could be resolved if the true mean opacity for the solar interior matter were roughly 15 per cent higher than predicted, because increased opacity compensates for the decreased element abundances. Iron accounts for a quarter of the total opacity at the solar radiation/convection zone boundary. Here we report measurements of wavelength-resolved iron opacity at electron temperatures of 1.9-2.3 million kelvin and electron densities of (0.7-4.0) × 10(22) per cubic centimetre, conditions very similar to those in the solar region that affects the discrepancy the most: the radiation/convection zone boundary. The measured wavelength-dependent opacity is 30-400 per cent higher than predicted. This represents roughly half the change in the mean opacity needed to resolve the solar discrepancy, even though iron is only one of many elements that contribute to opacity.

Dynamical Model for Spindown of Solar-type Stars

NASA Astrophysics Data System (ADS)

Sood, Aditi; Kim, Eun-jin; Hollerbach, Rainer

2016-12-01

After their formation, stars slow down their rotation rates by the removal of angular momentum from their surfaces, e.g., via stellar winds. Explaining how this rotation of solar-type stars evolves in time is currently an interesting but difficult problem in astrophysics. Despite the complexity of the processes involved, a traditional model, where the removal of angular momentum by magnetic fields is prescribed, has provided a useful framework to understand observational relations between stellar rotation, age, and magnetic field strength. Here, for the first time, a spindown model is proposed where loss of angular momentum by magnetic fields evolves dynamically, instead of being prescibed kinematically. To this end, we evolve the stellar rotation and magnetic field simultaneously over stellar evolution time by extending our previous work on a dynamo model which incorporates nonlinear feedback mechanisms on rotation and magnetic fields. We show that our extended model reproduces key observations and is capable of explaining the presence of the two branches of (fast and slow rotating) stars which have different relations between rotation rate Ω versus time (age), magnetic field strength | B| versus rotation rate, and frequency of magnetic field {ω }{cyc} versus rotation rate. For fast rotating stars we find that: (I) there is an exponential spindown {{Ω }}\\propto {e}-1.35t, with t measured in Gyr; (II) magnetic activity saturates for higher rotation rate; (III) {ω }{cyc}\\propto {{{Ω }}}0.83. For slow rotating stars we find: (I) a power-law spindown {{Ω }}\\propto {t}-0.52; (II) that magnetic activity scales roughly linearly with rotation rate; (III) {ω }{cyc}\\propto {{{Ω }}}1.16. The results obtained from our investigations are in good agreement with observations. The Vaughan-Preston gap is consistently explained in our model by the shortest spindown timescale in this transition from fast to slow rotators. Our results highlight the importance of self-regulation of magnetic fields and rotation by direct and indirect interactions involving nonlinear feedback in stellar evolution.

Classification of stellar populations in globular clusters

NASA Astrophysics Data System (ADS)

Wang, Yue; Zhao, Gang; Li, Hai-Ning

2017-04-01

Possessing multiple stellar populations has been accepted as a common feature of globular clusters (GCs). Different stellar populations manifest themselves with different chemical features, e.g. the well-known O-Na anti-correlation. Generally, the first (primordial) population has O and Na abundances consistent with those of field stars with similar metallicity; while the second (polluted) population is identified by their Na overabundance and O deficiency. The fraction of the populations is an important constraint on the GC formation scenario. Several methods have been proposed for the classification of GC populations. Here we examine a criterion derived based on the distribution of Galactic field stars, which relies on Na abundance as a function of [Fe/H], to distinguish first and second stellar populations in GCs. By comparing the first population fractions of 17 GCs estimated by the field star criterion with those in the literature derived by methods related to individual GCs, we find that the field star criterion tends to overestimate the first population fractions. The population separation methods, which are related to an individual GC sample, are recommended because the diversity of GCs can be taken into consideration. Currently, more caution should be exercised if one wants to regard field stars as a reference for the identification of a GC population. However, further study on the connection between field stars and GCs populations is still needed.

Tomography of the Red Supergiant Star MU Cep

NASA Astrophysics Data System (ADS)

Kravchenko, Kateryna

2018-04-01

We present a tomographic method allowing to recover the velocity field at different optical depths in a stellar atmosphere. It is based on the computation of the contribution function to identify the depth of formation of spectral lines in order to construct numerical masks probing different optical depths. These masks are cross-correlated with observed spectra to extract information about the average shape of lines forming at a given optical depth and to derive the velocity field projected on the line of sight. We applied this method to series of spectra of the red supergiant star mu Cep and derived velocities in different atmospheric layers. The resulting velocity variations reveal complex atmospheric dynamics and indicate that convective cells are present in the atmosphere of the mu Cep. The mu Cep velocities were compared with those obtained by applying the tomographic masks to series of snapshot spectra from 3D radiative-hydrodynamics CO5BOLD simulations.

Core Radial Electric Field and Transport in Wendelstein 7-X Plasmas

NASA Astrophysics Data System (ADS)

Pablant, Novimir

2016-10-01

Results from the investigation of core transport and the role of the radial electric field profile (Er) in the first operational phase of the Wendelstein 7-X (W7-X) stellarator are presented. In stellarator plasmas, the details of the Er profile are expected to have a strong effect on both the particle and heat fluxes. Neoclassical particle fluxes are not intrinsically ambipolar, which leads to the formation of a radial electric field that enforces ambipolarity. The radial electric field is closely related to the perpendicular plasma flow (u⊥) through the force balance equation. This allows the radial electric field to be inferred from measurements of the perpendicular flow velocity from the x-ray imaging crystal spectrometer (XICS) and correlation reflectometry diagnostics. Large changes in the perpendicular rotation, on the order of Δu⊥ 5km /s (ΔEr 12kV / m), have been observed within a set of experiments where the heating power was stepped down from 2 MW to 0.6 MW . These experiments are examined in detail to explore the relationship between, heating power, response of the temperature and density profiles and the response of the radial electric field. Estimations of the core transport are based on power balance and utilize electron temperature (Te) profiles from the ECE and Thomson scattering, electron density profiles (ne) from interferometry and Thomson scattering, ion temperature (Ti) profiles from XICS, along with measurements of the total stored energy and radiated power. Also described are a set core impurity confinement experiments and results. Impurity confinement has been investigated through the injection of trace amount of argon impurity gas at the plasma edge in conjunction with measurements of the density of various ionization states of argon from the XICS and High Efficiency eXtreme-UV Overview Spectrometer (HEXOS) diagnostics. Finally the inferred Er and heat flux profiles are compared to initial neoclassical calculations using measured plasma profiles. On behalf of the W7-X Team.

High-energy Gamma Rays from the Milky Way: Three-dimensional Spatial Models for the Cosmic-Ray and Radiation Field Densities in the Interstellar Medium

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

Porter, T. A.; Moskalenko, I. V.; Jóhannesson, G., E-mail: tporter@stanford.edu

High-energy γ -rays of interstellar origin are produced by the interaction of cosmic-ray (CR) particles with the diffuse gas and radiation fields in the Galaxy. The main features of this emission are well understood and are reproduced by existing CR propagation models employing 2D galactocentric cylindrically symmetrical geometry. However, the high-quality data from instruments like the Fermi Large Area Telescope reveal significant deviations from the model predictions on few to tens of degrees scales, indicating the need to include the details of the Galactic spiral structure and thus requiring 3D spatial modeling. In this paper, the high-energy interstellar emissions frommore » the Galaxy are calculated using the new release of the GALPROP code employing 3D spatial models for the CR source and interstellar radiation field (ISRF) densities. Three models for the spatial distribution of CR sources are used that are differentiated by their relative proportion of input luminosity attributed to the smooth disk or spiral arms. Two ISRF models are developed based on stellar and dust spatial density distributions taken from the literature that reproduce local near- to far-infrared observations. The interstellar emission models that include arms and bulges for the CR source and ISRF densities provide plausible physical interpretations for features found in the residual maps from high-energy γ -ray data analysis. The 3D models for CR and ISRF densities provide a more realistic basis that can be used for the interpretation of the nonthermal interstellar emissions from the Galaxy.« less

Magnetic field in IRC+10216 and other C-rich evolved stars

NASA Astrophysics Data System (ADS)

Duthu, A.; Herpin, F.; Wiesemeyer, H.; Baudry, A.; Lèbre, A.; Paubert, G.

2017-07-01

Context. During the transition from the asymptotic giant branch (AGB) to planetary nebulae (PN), the circumstellar geometry and morphology change dramatically. Another characteristic of this transition is the high mass-loss rate, that can be partially explained by radiation pressure and a combination of various factors, such as the stellar pulsation, the dust grain condensation, and opacity in the upper atmosphere. The magnetic field can also be one of the main ingredients that shapes the stellar upper atmosphere and envelope. Aims: Our main goal is to investigate for the first time the spatial distribution of the magnetic field in the envelope of IRC+10216. More generally we intend to determine the magnetic field strength in the circumstellar envelope (CSE) of C-rich evolved stars, compare this field with previous studies for O-rich stars, and constrain the variation of the magnetic field with r the distance to the star's centre. Methods: We use spectropolarimetric observations of the Stokes V parameter, collected with Xpol on the IRAM-30 m radiotelescope, observing the Zeeman effect in seven hyperfine components of the CN J = 1-0 line. We use the Crutcher et al. (1996, ApJ, 456, 217) method to estimate the magnetic field. For the first time, the instrumental contamination is investigated, through dedicated studies of the power patterns in Stokes V and I in detail. Results: For C-rich evolved stars, we derive a magnetic field strength (B) between 1.6 and 14.2 mG while B is estimated to be 6 mG for the proto-PN (PPN) AFGL618, and an upper value of 8 mG is found for the PN NGC 7027. These results are consistent with a decrease of B as 1/r in the environment of AGB objects, that is, with the presence of a toroidal field. But this is not the case for PPN and PN stars. Our map of IRC+10216 suggests that the magnetic field is not homogeneously strong throughout or aligned with the envelope and that the morphology of the CN emission might have changed with time.

The Stellar Populations of Two Ultra-diffuse Galaxies from Optical and Near-infrared Photometry

NASA Astrophysics Data System (ADS)

Pandya, Viraj; Romanowsky, Aaron J.; Laine, Seppo; Brodie, Jean P.; Johnson, Benjamin D.; Glaccum, William; Villaume, Alexa; Cuillandre, Jean-Charles; Gwyn, Stephen; Krick, Jessica; Lasker, Ronald; Martín-Navarro, Ignacio; Martinez-Delgado, David; van Dokkum, Pieter

2018-05-01

We present observational constraints on the stellar populations of two ultra-diffuse galaxies (UDGs) using optical through near-infrared (NIR) spectral energy distribution (SED) fitting. Our analysis is enabled by new Spitzer-IRAC 3.6 and 4.5 μm imaging, archival optical imaging, and the prospector fully Bayesian SED fitting framework. Our sample contains one field UDG (DGSAT I), one Virgo cluster UDG (VCC 1287), and one Virgo cluster dwarf elliptical for comparison (VCC 1122). We find that the optical–NIR colors of the three galaxies are significantly different from each other. We infer that VCC 1287 has an old (≳7.7 Gyr) and surprisingly metal-poor ([Z/Z ⊙] ≲ ‑1.0) stellar population, even after marginalizing over uncertainties on diffuse interstellar dust. In contrast, the field UDG DGSAT I shows evidence of being younger than the Virgo UDG, with an extended star formation history and an age posterior extending down to ∼3 Gyr. The stellar metallicity of DGSAT I is sub-solar but higher than that of the Virgo UDG, with [Z/{Z}ȯ ]=-{0.63}-0.62+0.35; in the case of exactly zero diffuse interstellar dust, DGSAT I may even have solar metallicity. With VCC 1287 and several Coma UDGs, a general picture is emerging where cluster UDGs may be “failed” galaxies, but the field UDG DGSAT I seems more consistent with a stellar feedback-induced expansion scenario. In the future, our approach can be applied to a large and diverse sample of UDGs down to faint surface brightness limits, with the goal of constraining their stellar ages, stellar metallicities, and circumstellar and diffuse interstellar dust content.

Simulated stellar kinematics studies of high-redshift galaxies with the HARMONI Integral Field Spectrograph

NASA Astrophysics Data System (ADS)

Kendrew, S.; Zieleniewski, S.; Houghton, R. C. W.; Thatte, N.; Devriendt, J.; Tecza, M.; Clarke, F.; O'Brien, K.; Häußler, B.

2016-05-01

We present a study into the capabilities of integrated and spatially resolved integral field spectroscopy of galaxies at z = 2-4 with the future HARMONI spectrograph for the European Extremely Large Telescope (E-ELT) using the simulation pipeline, HSIM. We focus particularly on the instrument's capabilities in stellar absorption line integral field spectroscopy, which will allow us to study the stellar kinematics and stellar population characteristics. Such measurements for star-forming and passive galaxies around the peak star formation era will provide a critical insight into the star formation, quenching and mass assembly history of high-z, and thus present-day galaxies. First, we perform a signal-to-noise study for passive galaxies at a range of stellar masses for z = 2-4, assuming different light profiles; for this population, we estimate that integrated stellar absorption line spectroscopy with HARMONI will be limited to galaxies with M* ≳ 1010.7 M⊙. Secondly, we use HSIM to perform a mock observation of a typical star-forming 1010 M⊙ galaxy at z = 3 generated from the high-resolution cosmological simulation NUTFB. We demonstrate that the input stellar kinematics of the simulated galaxy can be accurately recovered from the integrated spectrum in a 15-h observation, using common analysis tools. Whilst spatially resolved spectroscopy is likely to remain out of reach for this particular galaxy, we estimate HARMONI's performance limits in this regime from our findings. This study demonstrates how instrument simulators such as HSIM can be used to quantify instrument performance and study observational biases on kinematics retrieval; and shows the potential of making observational predictions from cosmological simulation output data.

Quenching of Star-formation Activity of High-redshift Galaxies in Clusters and Field

NASA Astrophysics Data System (ADS)

Lee, Seong-Kook; Im, Myungshin; Kim, Jae-Woo; Lotz, Jennifer; McPartland, Conor; Peth, Michael; Koekemoer, Anton

At local, galaxy properties are well known to be clearly different in different environments. However, it is still an open question how this environment-dependent trend has been shaped. We present the results of our investigation about the evolution of star-formation properties of galaxies over a wide redshift range, from z ~ 2 to z ~ 0.5, focusing its dependence on their stellar mass and environment (Lee et al. 2015). In the UKIDSS/UDS region, covering ~2800 square arcmin, we estimated photometric redshifts and stellar population properties, such as stellar masses and star-formation rates, using the deep optical and near-infrared data available in this field. Then, we identified galaxy cluster candidates within the given redshift range. Through the analysis and comparison of star-formation (SF) properties of galaxies in clusters and in field, we found interesting results regarding the evolution of SF properties of galaxies: (1) regardless of redshifts, stellar mass is a key parameter controlling quenching of star formation in galaxies; (2) At z < 1, environmental effects become important at quenching star formation regardless of stellar mass of galaxies; and (3) However, the result of the environmental quenching is prominent only for low mass galaxies (M* < 1010 M⊙) since the star formation in most of high mass galaxies are already quenched at z > 1.

RVxK2: Simultaneous PRV Program with Kepler/K2 Campaign 16

NASA Astrophysics Data System (ADS)

Xuesong Wang, Sharon; RVxK2 team

2018-01-01

Introducing the RVxK2 program: simultaneous precise radial velocity (PRV) observations with Kepler/K2 Campaign 16, from Dec 7 2017 through Feb 26 2018 (website: rvxk2.com). K2 will provide short cadence photometric data on five carefully selected stars, including the brightest M dwarf in C16 field, a solar analog, and three bright G/K subgiants. K2 will also provide long cadence data on the next three brightest K or M dwarfs in the C16 field. Our campaign includes several precise RV instruments around the globe, including Keck/HIRES, APF, IRTF/iSHELL, SONG, MINERVA, PARAS, representing the first organized, extensive, simultaneous RV campaign with space photometry. Our primary science goal is to characterize and understand stellar jitter and stellar activity. This RV+K2 campaign will provide the community with an unprecedented dataset of RV spectra and precise photometry to study stellar jitter on a broad range of time scales, from minutes to 80 days. We will gather the first ever RV+photometry dataset aiming at characterizing stellar granulation, the most poorly understood term among all astrophysical sources of stellar jitter. This project will also enable the first asteroseismic studies with simultaneous precise RVs and photometry, while searching for planets in the brightest nearby G to M dwarfs in the field.

A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars.

PubMed

Stello, Dennis; Cantiello, Matteo; Fuller, Jim; Huber, Daniel; García, Rafael A; Bedding, Timothy R; Bildsten, Lars; Aguirre, Victor Silva

2016-01-21

Magnetic fields play a part in almost all stages of stellar evolution. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes. Intermediate-mass stars do not have deep convective envelopes, although 10 per cent exhibit strong surface fields that are presumed to be residuals from the star formation process. These stars do have convective cores that might produce internal magnetic fields, and these fields might survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface. Here we report the strength of dipolar oscillation modes for a sample of 3,600 red giant stars. About 20 per cent of our sample show mode suppression, by strong magnetic fields in the cores, but this fraction is a strong function of mass. Strong core fields occur only in red giants heavier than 1.1 solar masses, and the occurrence rate is at least 50 per cent for intermediate-mass stars (1.6-2.0 solar masses), indicating that powerful dynamos were very common in the previously convective cores of these stars.

Energy balance in solar and stellar chromospheres

NASA Technical Reports Server (NTRS)

Avrett, E. H.

1981-01-01

Net radiative cooling rates for quiet and active regions of the solar chromosphere and for two stellar chromospheres are calculated from corresponding atmospheric models. Models of chromospheric temperature and microvelocity distributions are derived from observed spectra of a dark point within a cell, the average sun and a very bright network element on the quiet sun, a solar plage and flare, and the stars Alpha Boo and Lambda And. Net radiative cooling rates due to the transitions of various atoms and ions are then calculated from the models as a function of depth. Large values of the net radiative cooling rate are found at the base of the chromosphere-corona transition region which are due primarily to Lyman alpha emission, and a temperature plateau is obtained in the transition region itself. In the chromospheric regions, the calculated cooling rate is equal to the mechanical energy input as a function of height and thus provides a direct constraint on theories of chromospheric heating.

Atomic Physics of Shocked Plasma in Winds of Massive Stars

NASA Technical Reports Server (NTRS)

Leutenegger, Maurice A.; Cohen, David H.; Owocki, Stanley P.

2012-01-01

High resolution diffraction grating spectra of X-ray emission from massive stars obtained with Chandra and XMM-Newton have revolutionized our understanding of their powerful, radiation-driven winds. Emission line shapes and line ratios provide diagnostics on a number of key wind parameters. Modeling of resolved emission line velocity profiles allows us to derive independent constraints on stellar mass-loss rates, leading to downward revisions of a factor of a few from previous measurements. Line ratios in He-like ions strongly constrain the spatial distribution of Xray emitting plasma, confirming the expectations of radiation hydrodynamic simulations that X-ray emission begins moderately close to the stellar surface and extends throughout the wind. Some outstanding questions remain, including the possibility of large optical depths in resonance lines, which is hinted at by differences in line shapes of resonance and intercombination lines from the same ion. Resonance scattering leads to nontrivial radiative transfer effects, and modeling it allows us to place constraints on shock size, density, and velocity structure

Calculation of the Neoclassical Radial Electric Field using a Gyrokinetic δ f Code

NASA Astrophysics Data System (ADS)

Lewandowski, J. L. V.; Boozer, A.; Williams, J.; Lin, Z.; Zarnstorff, M.

2000-10-01

The calculation of the radial electric field in stellarator devices is an important issue in neoclassical transport. The radial electric field, which is also related to the formation of transport barriers, can affect the anomalous transport. In stellarator configurations which depart only weakly from axi-symmetry, a direct Monte Carlo calculations of the radial electric is difficult due to the large statistical fluctuations. We present a novel method based on the evaluation of the perpendicular ( p_⊥ ) and parallel ( p_|| ) pressures. The variation of widehatp ≡ ( p_|| + p_⊥ ) /2 on the magnetic surface provides a low-noise calculation of the radial electric field. The low-noise method has been implemented in a three-dimensional gyro-kinetic particle code [1]. The calculation of the radial electric field for the National Compact Stellarator Experiment [2] will be presented. [ 1 ] Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang, and R. White Science 281, 1835 (1998). [ 2 ] A. Reiman et al, invited talk (this conference).

Role of Massive Stars in the Evolution of Primitive Galaxies

NASA Technical Reports Server (NTRS)

Heap, Sara

2012-01-01

An important factor controlling galaxy evolution is feedback from massive stars. It is believed that the nature and intensity of stellar feedback changes as a function of galaxy mass and metallicity. At low mass and metallicity, feedback from massive stars is mainly in the form of photoionizing radiation. At higher mass and metallicity, it is in stellar winds. IZw 18 is a local blue, compact dwarf galaxy that meets the requirements for a primitive galaxy: low halo mass greater than 10(exp 9)Msun, strong photoionizing radiation, no galactic outflow, and very low metallicity,log(O/H)+12=7.2. We will describe the properties of massive stars and their role in the evolution of IZw 18, based on analysis of ultraviolet images and spectra obtained with HST.

On Star-Planet Interaction: Magnetospheric Dynamics and Atmospheric Evolution

NASA Astrophysics Data System (ADS)

Tilley, Matthew Tilley

With the explosion of exoplanetary discoveries, the question of planetary habitability is at the forefront, and generates many interesting and complex questions. One of those questions: Are planetary global magnetic fields necessary for the development of complex surface organics and the development of life? Does a global field protect planetary atmospheres? What detection signatures can be gleaned from a planet or moon with a global field as opposed to one without? We have a wealth of in situ magnetospheric data from Earth, as well as solar system planets and their moons from several vital satellite missions, such as the Voyager missions, the Pioneer missions, Galileo, Cassini, Messenger, MAVEN, and New Horizons. Due to the distances involved, it is not tenable to send satellites to obtain data at exoplanetary bodies, so we rely on simulations and using solar system data as analog environments to help set ground truth validation for the numerical work. In this dissertation, I use a multifluid plasma model for gas giant magnetospheres to predict the potential dynamical consequences and detection signatures for giant exoplanets in a warm orbit (˜0.2 AU). I discuss the dynamics of plasma loss from an exomoon injected torus, and how the total mass flux out of the system is altered by increased stellar wind forcing as a function of orbital semi-major axis. Detection signatures for such a planet, including transit depth modifications due to plasma densities and radio emissions, show promise for further detecting and characterizing future systems. I also improve the multifluid model by implementing a full treatment of pressure anisotropy at Saturn, with a focus on the dynamics and structure of the magnetosphere. The improvements to the physics of the model generate more accurate system when compared to Cassini data; the anisotropic simulations show stronger current confinement of the Enceladus torus, consistent and well-structure flux interchange events, and global corotational convection that match more closely with the Cassini data than the isotropic model. Turning from giant planets to terrestrial, I use a coupled one-dimensional photochemical and radiative-convective climate model to investigate the effects of M dwarf stellar flare activity on an Earth-like atmosphere for an unmagnetized planet in the nominal habitable zone. I find that EM-only activity - even to the level of some of the most active stars yet observed - is insufficient at the age of the universe to reduce the ozone column to the point that UV-C radiation can reach the surface. However, repeated proton events from frequent daily flare activity, which has been observed on several M dwarfs, can erode the ozone column by several orders of magnitude, allowing the surface of the planet to be bathed in UV-C, which is sterilizing and detrimental to the development of complex organic structures. The ability of a strong planetary magnetic field to deflect incoming stellar wind and flare-energized protons seems to be of import to maintain surface habitability. I also use scaling laws to predict a potential atomic oxygen auroral signal from Proxima Centauri b, the detection of which would constrain the presence of an atmosphere and point to the presence of a magnetic field. The increased forcing from Proxima Centauri's stellar wind is expected to drive powerful emissions, orders of magnitude stronger than at Earth, and within easy reach of the next generation of observational telescopic instruments. Magnetic fields do seem to be important for both detection and potential shielding of the atmosphere of exoplanets, but much work remains to be performed. Future observations combined with simulations validated against solar system star-planet interactions will likely provide answers to these questions, and perhaps lead to a focus on specific planetary targets for extensive investigation of astrobiological interest.

Star formation history and chemical enrichment in the early Universe: clues from the rest-optical and rest-UV spectra of z~2-3 star-forming galaxies in the Keck Baryonic Structure Survey

NASA Astrophysics Data System (ADS)

Strom, Allison L.

2017-01-01

Galaxies at the peak of cosmic star formation (z~2-3) exhibit significantly higher star formation rates and gas fractions at fixed stellar mass than nearby galaxies. These z~2-3 galaxies are also distinct in terms of their nebular spectra, reflecting important differences not only in the physical conditions of their interstellar medium (e.g., electron density and gas-phase metallicity), but also in the details of their massive stellar populations, especially their ionizing radiation fields. Jointly observing galaxies' HII regions, at rest-UV and rest-optical wavelengths, and massive stars, at rest-UV wavelengths, is central to constructing a framework for understanding the differences between z~2-3 and z~0 star-forming galaxies and for self-consistently explaining the trends observed in the high-redshift population. My thesis is based on data from the Keck Baryonic Structure Survey (KBSS), which uniquely combines observations of individual galaxies in these two bandpasses. In total, the near-infrared component of the KBSS includes spectra of >700 z~2-3 galaxies obtained with Keck/MOSFIRE. I will present these results along with a detailed analysis of the full rest-optical (3600-7000 Ang) nebular spectra of ~400 galaxies, showing that high-redshift galaxies exhibit uniformly high degrees of ionization and excitation with respect to most z~0 galaxies. Combined with observations of the same galaxies' rest-UV spectra (obtained with Keck/LRIS) and photoionization model predictions, these results suggest that the disparity arises from differences in the shape of the ionizing radiation field at fixed gas-phase oxygen abundance, most likely due to the effects of Fe-poor massive binary stars. My comprehensive spectroscopic study of an unprecedentedly large sample of z~2-3 galaxies offers compelling evidence that the distinct chemical abundance patterns observed in these galaxies are the result of systematic differences in their star formation histories.

Gamma rays from clumpy wind-jet interactions in high-mass microquasars

NASA Astrophysics Data System (ADS)

de la Cita, V. M.; del Palacio, S.; Bosch-Ramon, V.; Paredes-Fortuny, X.; Romero, G. E.; Khangulyan, D.

2017-07-01

Context. The stellar winds of the massive stars in high-mass microquasars are thought to be inhomogeneous. The interaction of these inhomogeneities, or clumps, with the jets of these objects may be a major factor in gamma-ray production. Aims: Our goal is to characterize a typical scenario of clump-jet interaction, and calculate the contribution of these interactions to the gamma-ray emission from these systems. Methods: We use axisymmetric, relativistic hydrodynamical simulations to model the emitting flow in a typical clump-jet interaction. Using the simulation results we perform a numerical calculation of the high-energy emission from one of these interactions. The radiative calculations are performed for relativistic electrons locally accelerated at the jet shock, and the synchrotron and inverse Compton radiation spectra are computed for different stages of the shocked clump evolution. We also explore different parameter values, such as viewing angle and magnetic field strength. The results derived from one clump-jet interaction are generalized phenomenologically to multiple interactions under different wind models, estimating the clump-jet interaction rates, and the resulting luminosities in the GeV range. Results: If particles are efficiently accelerated in clump-jet interactions, the apparent gamma-ray luminosity through inverse Compton scattering with the stellar photons can be significant even for rather strong magnetic fields and thus efficient synchrotron cooling. Moreover, despite the standing nature or slow motion of the jet shocks for most of the interaction stage, Doppler boosting in the postshock flow is relevant even for mildly relativistic jets. Conclusions: For clump-to-average wind density contrasts greater than or equal to ten, clump-jet interactions could be bright enough to match the observed GeV luminosity in Cyg X-1 and Cyg X-3 when a jet is present in these sources, with required non-thermal-to-total available power fractions greater than 0.01 and 0.1, respectively.

Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions.

PubMed

Driscoll, P E; Barnes, R

2015-09-01

The internal thermal and magnetic evolution of rocky exoplanets is critical to their habitability. We focus on the thermal-orbital evolution of Earth-mass planets around low-mass M stars whose radiative habitable zone overlaps with the "tidal zone," where tidal dissipation is expected to be a significant heat source in the interior. We develop a thermal-orbital evolution model calibrated to Earth that couples tidal dissipation, with a temperature-dependent Maxwell rheology, to orbital circularization and migration. We illustrate thermal-orbital steady states where surface heat flow is balanced by tidal dissipation and cooling can be stalled for billions of years until circularization occurs. Orbital energy dissipated as tidal heat in the interior drives both inward migration and circularization, with a circularization time that is inversely proportional to the dissipation rate. We identify a peak in the internal dissipation rate as the mantle passes through a viscoelastic state at mantle temperatures near 1800 K. Planets orbiting a 0.1 solar-mass star within 0.07 AU circularize before 10 Gyr, independent of initial eccentricity. Once circular, these planets cool monotonically and maintain dynamos similar to that of Earth. Planets forced into eccentric orbits can experience a super-cooling of the core and rapid core solidification, inhibiting dynamo action for planets in the habitable zone. We find that tidal heating is insignificant in the habitable zone around 0.45 (or larger) solar-mass stars because tidal dissipation is a stronger function of orbital distance than stellar mass, and the habitable zone is farther from larger stars. Suppression of the planetary magnetic field exposes the atmosphere to stellar wind erosion and the surface to harmful radiation. In addition to weak magnetic fields, massive melt eruption rates and prolonged magma oceans may render eccentric planets in the habitable zone of low-mass stars inhospitable for life.

Molecular Dynamics of Hot Dense Plasmas: New Horizons

NASA Astrophysics Data System (ADS)

Graziani, Frank

2011-10-01

We describe the status of a new time-dependent simulation capability for hot dense plasmas. The backbone of this multi-institutional computational and experimental effort--the Cimarron Project--is the massively parallel molecular dynamics (MD) code ``ddcMD''. The project's focus is material conditions such as exist in inertial confinement fusion experiments, and in many stellar interiors: high temperatures, high densities, significant electromagnetic fields, mixtures of high- and low- Zelements, and non-Maxwellian particle distributions. Of particular importance is our ability to incorporate into this classical MD code key atomic, radiative, and nuclear processes, so that their interacting effects under non-ideal plasma conditions can be investigated. This talk summarizes progress in computational methodology, discusses strengths and weaknesses of quantum statistical potentials as effective interactions for MD, explains the model used for quantum events possibly occurring in a collision and highlights some significant results obtained to date. We describe the status of a new time-dependent simulation capability for hot dense plasmas. The backbone of this multi-institutional computational and experimental effort--the Cimarron Project--is the massively parallel molecular dynamics (MD) code ``ddcMD''. The project's focus is material conditions such as exist in inertial confinement fusion experiments, and in many stellar interiors: high temperatures, high densities, significant electromagnetic fields, mixtures of high- and low- Zelements, and non-Maxwellian particle distributions. Of particular importance is our ability to incorporate into this classical MD code key atomic, radiative, and nuclear processes, so that their interacting effects under non-ideal plasma conditions can be investigated. This talk summarizes progress in computational methodology, discusses strengths and weaknesses of quantum statistical potentials as effective interactions for MD, explains the model used for quantum events possibly occurring in a collision and highlights some significant results obtained to date. This work is performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Close Binaries in the Orion Nebula Cluster: On the Universality of Stellar Multiplicity and the Origin of Field Stars

NASA Astrophysics Data System (ADS)

Duchene, Gaspard; Lacour, Sylvestre; Moraux, Estelle; Bouvier, Jerome; Goodwin, Simon

2018-01-01

While stellar multiplicity is an ubiquitous outcome of star formation, there is a clear dichotomy between the multiplicity properties of young (~1 Myr-old) stellar clusters, like the ONC, which host a mostly field-like population of visual binaries, and those of equally young sparse populations, like the Taurus-Auriga region, which host twice as many stellar companions. Two distinct scenarios can account for this observation: one in which different star-forming regions form different number of stars, and one in which multiplicity properties are universal at birth but where internal cluster dynamics destroy many wide binaries. To solve this ambiguity, one must probe binaries that are sufficiently close so as not to be destroyed through interactions with other cluster members. To this end, we have conducted a survey for 10-100 au binaries in the ONC using the aperture masking technique with the VLT adaptive optics system. Among our sample of the 42 ONC members, we discovered 13 companions in this range of projected separations. This is consistent with the companion frequency observed in the Taurus population and twice as high as that observed among field stars. This survey thus strongly supports the idea that stellar multiplicity is characterized by near-universal initial properties that can later be dynamically altered. On the other hand, this exacerbates the question of the origin of field stars, since only clusters much denser than the ONC can effectively destroyed binaries closer than 100 au.

Infrared spectroscopy of interstellar shocks

NASA Technical Reports Server (NTRS)

Mckee, C. F.; Chernoff, D. F.; Hollenbach, D. J.

1984-01-01

Infrared emission lines from interstellar shocks provide valuable diagnostics for violent events in the interstellar medium, such as supernova remnants and mass outflow from young stellar objects. There are two types of interstellar shocks: in J shocks, gas properties 'jump' from their preshock to their postshock values in a shock front with a thickness equal to or less than one mean free path; radiation is emitted behind the shock front, primarily in the visible and ultraviolet, but with a few strong infrared lines, such as OI(63 microns). Such shocks occur in ionized or neutral atomic gas, or at high velocities (equal to or greater than 50 km/s) in molecular gas. In C shocks, gas is accelerated and heated by collisions between charged particles, which have a low concentration and are coupled to the magnetic field, and neutral particles; radiation is generated throughout the shock and is emitted almost entirely in infrared emission lines. Such shocks occur in weakly ionized molecular gas for shock velocities below about 50 km/s.

The Statistical Sobolev-Rosseland Mean and the Effects of Frequency Redistribution on Wolf-Rayet Wind Driving

NASA Astrophysics Data System (ADS)

Onifer, A. J.; Gayley, K. G.

2003-06-01

The optically thick character of Wolf-Rayet winds implies that stellar continuum photons are multiply scattered, as a result of both free electron opacity and overlapping wind-broadened spectral lines. This allows the wind to accumulate a substantial excess in momentum flux relative to the driving radiation field, as is observationally required. Nevertheless, sustaining such a high degree of multiple scattering requires not only a large optical depth spatially but also substantial spectral blanketing. The latter is difficult to maintain when redistribution during scattering allows radiative flux to shift preferentially into spectral regions with fewer lines, since then the channels carrying much of the flux are also the least well blanketed. This paper parameterizes the potential severity of this effect in simple terms, using a generalization of the Rosseland mean treated in the Sobolev approximation. We show that our approach provides an informative starting point for characterizing and conceptualizing nongray effects in optically thick supersonic flows.

Probing Black Holes With Gravitational Radiation

NASA Astrophysics Data System (ADS)

Cornish, Neil J.

2006-09-01

Gravitational radiation can provide unique insights into the dynamics and evolution of black holes. Gravitational waveforms encode detailed information about the spacetime geometry, much as the sounds made by a musical instrument reflect the geometry of the instrument. The LISA gravitational wave observatory will be able to record black holes colliding out to the edge of the visible Universe, with an expected event rate of tens to thousands per year. LISA has unmatched capabilities for studying the role of black holes in galactic evolution, in particular, by studying the mergers of seed black holes at very high redshift, z > 5. Merger events at lower redshift will be detected at extremely high signal-to-noise, allowing for precision tests of the black hole paradigm. Below z=1 LISA will be able to record stellar remnants falling into supermassive black holes. These extreme mass ratio inspiral events will yield insights into the dynamics of galactic cusps, and the brighter events will provide incredibly precise tests of strong field, dynamical gravity.

THE TRIFID NEBULA: STELLAR SIBLING RIVALRY

NASA Technical Reports Server (NTRS)

2002-01-01

This NASA Hubble Space Telescope image of the Trifid Nebula reveals a stellar nursery being torn apart by radiation from a nearby, massive star. The picture also provides a peek at embryonic stars forming within an ill-fated cloud of dust and gas, which is destined to be eaten away by the glare from the massive neighbor. This stellar activity is a beautiful example of how the life cycles of stars like our Sun is intimately connected with their more powerful siblings. The Hubble image shows a small part of a dense cloud of dust and gas, a stellar nursery full of embryonic stars. This cloud is about 8 light-years away from the nebula's central star, which is beyond the top of this picture. Located about 9,000 light-years from Earth, the Trifid resides in the constellation Sagittarius. A stellar jet [the thin, wispy object pointing to the upper left] protrudes from the head of a dense cloud and extends three-quarters of a light-year into the nebula. The jet's source is a very young stellar object that lies buried within the cloud. Jets such as this are the exhaust gases of star formation. Radiation from the massive star at the center of the nebula is making the gas in the jet glow, just as it causes the rest of the nebula to glow. The jet in the Trifid is a 'ticker tape,' telling the history of one particular young stellar object that is continuing to grow as its gravity draws in gas from its surroundings. But this particular ticker tape will not run for much longer. Within the next 10,000 years the glare from the central, massive star will continue to erode the nebula, overrunning the forming star, and bringing its growth to an abrupt and possibly premature end. Another nearby star may have already faced this fate. The Hubble picture shows a 'stalk' [the finger-like object] pointing from the head of the dense cloud directly toward the star that powers the Trifid. This stalk is a prominent example of the evaporating gaseous globules, or 'EGGs,' that were seen previously in the Eagle Nebula, another star-forming region photographed by Hubble. The stalk has survived because at its tip there is a knot of gas that is dense enough to resist being eaten away by the powerful radiation. Reflected starlight at the tip of the EGG may be due to light from the Trifid's central star, or from a young stellar object buried within the EGG. Similarly, a tiny spike of emission pointing outward from the EGG looks like a small stellar jet. Hubble astronomers are tentatively interpreting this jet as the last gasp from a star that was cut off from its supply lines 100,000 years ago. The images were taken Sept. 8, 1997 through filters that isolate emission from hydrogen atoms, ionized sulfur atoms, and doubly ionized oxygen atoms. The images were combined in a single color composite picture. While the resulting picture is not true color, it is suggestive of what a human eye might see. Credits: NASA and Jeff Hester (Arizona State University)

Quasiperiodicity and chaos in post-AGB stars

NASA Astrophysics Data System (ADS)

Icke, V.

2003-03-01

This is a mini-presentation of three subjects, which are all related to the atmospheric motion in post-AGB stars. First, a summary of my 1990 equation of a driven stellar oscillator that exhibits chaotic solutions. Second, an advertisement for the subtle interplay of hydrodynamics, gas/dust drift, gas chemistry, dust formation, and radiation pressure, as presented in the thesis by Simis. Third, a new model equation for nonspherical stellar oscillations that resembles the FPU-equation which shows permanent non-equilibrium, with possibly intermittent solutions.

Computational Design of Short Pulse Laser Driven Iron Opacity Measurements at Stellar-Relevant Conditions

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

Martin, Madison E.

Opacity is a critical parameter in the simulation of radiation transport in systems such as inertial con nement fusion capsules and stars. The resolution of current disagreements between solar models and helioseismological observations would bene t from experimental validation of theoretical opacity models. Overall, short pulse laser heated iron experiments reaching stellar-relevant conditions have been designed with consideration of minimizing tamper emission and optical depth effects while meeting plasma condition and x-ray emission goals.

Resolving Star Formation, Multiphase ISM Structure, and Wind Driving with MHD and RHD Models of Galactic Disks

NASA Astrophysics Data System (ADS)

Ostriker, Eve

Current studies of star and galaxy formation have concluded that energetic feedback from young stars and supernovae (SNe) is crucial, both for controlling observed interstellar medium (ISM) properties and star formation rates in the Milky Way and other galaxies, and for driving galactic winds that govern the baryon abundance in dark matter halos. However, in many numerical studies of the ISM, energy inputs have not been implemented self-consistently with the evolving rate of gravitational collapse to make stars, or have considered only isolated star-forming clouds without a realistic galactic environment (including sheared rotation and externally-originating SNe), or have not directly incorporated radiation, magnetic, and chemical effects that are important or even dominant. In models of galaxy formation and evolution in the cosmic context, galactic winds are indispensable but highly uncertain as the physics of superbubble evolution and radiation-gas interactions cannot be resolved. Our central objectives are (1) to address the above limitations of current models, developing self-consistent simulations of the multiphase ISM in disk galaxies that resolve both star formation and stellar feedback, covering the range of scales needed to connect star cluster formation to galactic superwind ejection, and the range of environments from dwarfs to ULIRGs; and (2) to analyze the detailed properties of the gas, magnetic field, radiation field, and star formation/SNe in our simulations, including dependencies on local galactic disk environment, and to connect intrinsic properties with observable diagnostics. The proposed project will employ the Athena code for numerical magneto-hydrodynamic (MHD) and radiation-hydrodynamic (RHD) simulations, using comprehensive physics modules that have been developed, tested, and demonstrated in sample simulations. We will consider local ``shearing box'' disk models with gas surface density Sigma = 2 - 10,000 Msun/pc^2, and a range of stellar potentials and galactic rotation rates. Our simulations follow all thermal phases of the gas, the driving of turbulence, and the expulsion of material in high-velocity galactic winds as well as the circulation of lowervelocity material in galactic ``fountains.'' We resolve gravitational collapse and apply stellar population modeling to determine radiation emitted by star cluster particles, and both in situ and runaway O-star SN events. With time-dependent chemistry, we will be able to follow C+/C/CO transitions and assess the relationship between the observed molecular component and self-gravitating or diffuse clouds in varying galactic environments, also determining how cloud properties (e.g. distributions of mass, size, virial parameter, internal/external pressure, magnetization) and lifetimes depend on environment. We will also investigate the dependence on local galactic environment of: * mass and volume fractions, and turbulent and magnetic state, of each thermal and chemical ISM phase * star formation rate, and galactic wind mass loss rate in each ISM phase * metrics of ISM energy gain/loss, large-scale force balance, wind acceleration * roles of SN and radiation feedback in setting cloud SFEs, overall SFRs, and wind massloss rates Our models will be valuable for interpreting a wide range of observations with Chandra, Hubble, Spitzer, Herschel, Planck, and ground-based telescopes. Obtaining self-consistent solutions for the dynamical, thermal, magnetic, chemical, and radiative state of the star-forming ISM is a long-sought goal of galactic theory. Understanding why ISM and star formation properties vary among and within galaxies is essential for interpreting new multiwavelength extragalactic surveys. Connecting galactic winds to star formation via resolved physical mechanisms will provide a missing link in contemporary galaxy formation models. With our planned research program, we are in a position to achieve all of these advances.

Ly α Absorption at Transits of HD 209458b: A Comparative Study of Various Mechanisms Under Different Conditions

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

Khodachenko, M. L.; Lammer, H.; Kislyakova, K. G.

To shed more light on the nature of the observed Ly α absorption during transits of HD 209458b and to quantify the major mechanisms responsible for the production of fast hydrogen atoms (the so-called energetic neutral atoms, ENAs) around the planet, 2D hydrodynamic multifluid modeling of the expanding planetary upper atmosphere, which is driven by stellar XUV, and its interaction with the stellar wind has been performed. The model self-consistently describes the escaping planetary wind, taking into account the generation of ENAs due to particle acceleration by the radiation pressure and by the charge exchange between the stellar wind protonsmore » and planetary atoms. The calculations in a wide range of stellar wind parameters and XUV flux values showed that under typical Sun-like star conditions, the amount of generated ENAs is too small, and the observed absorption at the level of 6%–8% can be attributed only to the non-resonant natural line broadening. For lower XUV fluxes, e.g., during the activity minima, the number of planetary atoms that survive photoionization and give rise to ENAs increases, resulting in up to 10%–15% absorption at the blue wing of the Ly α line, caused by resonant thermal line broadening. A similar asymmetric absorption can be seen under the conditions realized during coronal mass ejections, when sufficiently high stellar wind pressure confines the escaping planetary material within a kind of bowshock around the planet. It was found that the radiation pressure in all considered cases has a negligible contribution to the production of ENAs and the corresponding absorption.« less

Stellar feedback as the origin of an extended molecular outflow in a starburst galaxy.

PubMed

Geach, J E; Hickox, R C; Diamond-Stanic, A M; Krips, M; Rudnick, G H; Tremonti, C A; Sell, P H; Coil, A L; Moustakas, J

2014-12-04

Recent observations have revealed that starburst galaxies can drive molecular gas outflows through stellar radiation pressure. Molecular gas is the phase of the interstellar medium from which stars form, so these outflows curtail stellar mass growth in galaxies. Previously known outflows, however, involve small fractions of the total molecular gas content and have typical scales of less than a kiloparsec. In at least some cases, input from active galactic nuclei is dynamically important, so pure stellar feedback (the momentum return into the interstellar medium) has been considered incapable of rapidly terminating star formation on galactic scales. Molecular gas has been detected outside the galactic plane of the archetypal starburst galaxy M82 (refs 4 and 5), but so far there has been no evidence that starbursts can propel substantial quantities of cold molecular gas to the same galactocentric radius (about 10 kiloparsecs) as the warmer gas that has been traced by metal ion absorbers in the circumgalactic medium. Here we report observations of molecular gas in a compact (effective radius 100 parsecs) massive starburst galaxy at redshift 0.7, which is known to drive a fast outflow of ionized gas. We find that 35 per cent of the total molecular gas extends approximately 10 kiloparsecs, and one-third of this extended gas has a velocity of up to 1,000 kilometres per second. The kinetic energy associated with this high-velocity component is consistent with the momentum flux available from stellar radiation pressure. This demonstrates that nuclear bursts of star formation are capable of ejecting large amounts of cold gas from the central regions of galaxies, thereby strongly affecting their evolution by truncating star formation and redistributing matter.

Super-Eddington stellar winds driven by near-surface energy deposition

NASA Astrophysics Data System (ADS)

Quataert, Eliot; Fernández, Rodrigo; Kasen, Daniel; Klion, Hannah; Paxton, Bill

2016-05-01

We develop analytic and numerical models of the properties of super-Eddington stellar winds, motivated by phases in stellar evolution when super-Eddington energy deposition (via, e.g. unstable fusion, wave heating, or a binary companion) heats a region near the stellar surface. This appears to occur in the giant eruptions of luminous blue variables (LBVs), Type IIn supernovae progenitors, classical novae, and X-ray bursts. We show that when the wind kinetic power exceeds Eddington, the photons are trapped and behave like a fluid. Convection does not play a significant role in the wind energy transport. The wind properties depend on the ratio of a characteristic speed in the problem v_crit˜ (dot{E} G)^{1/5} (where dot{E} is the heating rate) to the stellar escape speed near the heating region vesc(rh). For vcrit ≳ vesc(rh), the wind kinetic power at large radii dot{E}_w ˜ dot{E}. For vcrit ≲ vesc(rh), most of the energy is used to unbind the wind material and thus dot{E}_w ≲ dot{E}. Multidimensional hydrodynamic simulations without radiation diffusion using FLASH and one-dimensional hydrodynamic simulations with radiation diffusion using MESA are in good agreement with the analytic predictions. The photon luminosity from the wind is itself super-Eddington but in many cases the photon luminosity is likely dominated by `internal shocks' in the wind. We discuss the application of our models to eruptive mass-loss from massive stars and argue that the wind models described here can account for the broad properties of LBV outflows and the enhanced mass-loss in the years prior to Type IIn core-collapse supernovae.

Novel design methods for magnetic flux loops in the National Compact Stellarator Experiment

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

Pomphrey, N.; Lazarus, E.; Zarnstorff, M.

2007-05-15

Magnetic pickup loops on the vacuum vessel (VV) can provide an abundance of equilibrium information for stellarators. A substantial effort has gone into designing flux loops for the National Compact Stellarator Experiment (NCSX) [Zarnstorff et al., Plasma Phys. Controlled Fusion 43, A237 (2001)], a three-field period quasi-axisymmetric stellarator under construction at the Princeton Plasma Physics Laboratory. The design philosophy, to measure all of the magnetic field distributions normal to the VV that can be measured, has necessitated the development of singular value decomposition algorithms for identifying efficient loop locations. Fields are expected to be predominantly stellarator symmetric (SS)--the symmetry ofmore » the machine design--with toroidal mode numbers per torus (n) equal to a multiple of 3 and possessing reflection symmetry in a period. However, plasma instabilities and coil imperfections will generate non-SS fields that must also be diagnosed. The measured symmetric fields will yield important information on the plasma current and pressure profile as well as on the plasma shape. All fields that obey the design symmetries could be measured by placing flux loops in a single half-period of the VV, but accurate resolution of nonsymmetric modes, quantified by the condition number of a matrix, requires repositioning loops to equivalent locations on the full torus. A subarray of loops located along the inside wall of the vertically elongated cross section was designed to detect n=3, m=5 or 6 resonant field perturbations that can cause important islands. Additional subarrays included are continuous in the toroidal and poloidal directions. Loops are also placed at symmetry points of the VV to obtain maximal sensitivity to asymmetric perturbations. Combining results from various calculations which have made extensive use of a database of 2500 free-boundary VMEC equilibria, has led to the choice of 225 flux loops for NCSX, of which 151 have distinct shapes.« less

EXor OUTBURSTS FROM DISK AMPLIFICATION OF STELLAR MAGNETIC CYCLES

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

Armitage, Philip J., E-mail: pja@jilau1.colorado.edu

EXor outbursts—moderate-amplitude disk accretion events observed in Class I and Class II protostellar sources—have timescales and amplitudes that are consistent with the viscous accumulation and release of gas in the inner disk near the dead zone boundary. We suggest that outbursts are indirectly triggered by stellar dynamo cycles, via poloidal magnetic flux that diffuses radially outward through the disk. Interior to the dead zone the strength of the net field modulates the efficiency of angular momentum transport by the magnetorotational instability. In the dead zone changes in the polarity of the net field may lead to stronger outbursts because ofmore » the dominant role of the Hall effect in this region of the disk. At the level of simple estimates we show that changes to kG-strength stellar fields could stimulate disk outbursts on 0.1 au scales, though this optimistic conclusion depends upon the uncertain efficiency of net flux transport through the inner disk. The model predicts a close association between observational tracers of stellar magnetic activity and EXor events.« less

Chromospheric Activity in Cool Luminous Stars

NASA Astrophysics Data System (ADS)

Dupree, Andrea

2018-04-01

Spatially unresolved spectra of giant and supergiant stars demonstrate ubiquitous signatures of chromospheric activity, variable outflows, and winds. The advent of imaging techniques and spatially resolved spectra reveal complex structures in these extended stellar atmospheres that we do not understand. The presence and behavior of these atmospheres is wide ranging and impacts stellar activity, magnetic fields, angular momentum loss, abundance determinations, and the understanding of stellar cluster populations.

Magnetohydrodynamic simulations of hot jupiter upper atmospheres

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

Trammell, George B.; Li, Zhi-Yun; Arras, Phil, E-mail: gbt8f@virginia.edu, E-mail: zl4h@virginia.edu, E-mail: arras@virginia.edu

Two-dimensional simulations of hot Jupiter upper atmospheres including the planet's magnetic field are presented. The goal is to explore magnetic effects on the layer of the atmosphere that is ionized and heated by stellar EUV radiation, and the imprint of these effects on the Lyα transmission spectrum. The simulations are axisymmetric, isothermal, and include both rotation and azimuth-averaged stellar tides. Mass density is converted to atomic hydrogen density through the assumption of ionization equilibrium. The three-zone structure—polar dead zone (DZ), mid-latitude wind zone (WZ), and equatorial DZ—found in previous analytic calculations is confirmed. For a magnetic field comparable to thatmore » of Jupiter, the equatorial DZ, which is confined by the magnetic field and corotates with the planet, contributes at least half of the transit signal. For even stronger fields, the gas escaping in the mid-latitude WZ is found to have a smaller contribution to the transit depth than the equatorial DZ. Transmission spectra computed from the simulations are compared to Hubble Space Telescope Space Telescope Imaging Spectrograph and Advanced Camera for Surveys data for HD 209458b and HD 189733b, and the range of model parameters consistent with the data is found. The central result of this paper is that the transit depth increases strongly with magnetic field strength when the hydrogen ionization layer is magnetically dominated, for dipole magnetic field B {sub 0} ≳ 10 G. Hence transit depth is sensitive to magnetic field strength, in addition to standard quantities such as the ratio of thermal to gravitational binding energies. Another effect of the magnetic field is that the planet loses angular momentum orders of magnitude faster than in the non-magnetic case, because the magnetic field greatly increases the lever arm for wind braking of the planet's rotation. Spin-down timescales for magnetized models of HD 209458b that agree with the observed transit depth can be as short as ≅ 30 Myr, which is much shorter than the age of the system.« less

Ensemble asteroseismology of solar-type stars with the NASA Kepler mission.

PubMed

Chaplin, W J; Kjeldsen, H; Christensen-Dalsgaard, J; Basu, S; Miglio, A; Appourchaux, T; Bedding, T R; Elsworth, Y; García, R A; Gilliland, R L; Girardi, L; Houdek, G; Karoff, C; Kawaler, S D; Metcalfe, T S; Molenda-Żakowicz, J; Monteiro, M J P F G; Thompson, M J; Verner, G A; Ballot, J; Bonanno, A; Brandão, I M; Broomhall, A-M; Bruntt, H; Campante, T L; Corsaro, E; Creevey, O L; Doğan, G; Esch, L; Gai, N; Gaulme, P; Hale, S J; Handberg, R; Hekker, S; Huber, D; Jiménez, A; Mathur, S; Mazumdar, A; Mosser, B; New, R; Pinsonneault, M H; Pricopi, D; Quirion, P-O; Régulo, C; Salabert, D; Serenelli, A M; Silva Aguirre, V; Sousa, S G; Stello, D; Stevens, I R; Suran, M D; Uytterhoeven, K; White, T R; Borucki, W J; Brown, T M; Jenkins, J M; Kinemuchi, K; Van Cleve, J; Klaus, T C

2011-04-08

In addition to its search for extrasolar planets, the NASA Kepler mission provides exquisite data on stellar oscillations. We report the detections of oscillations in 500 solar-type stars in the Kepler field of view, an ensemble that is large enough to allow statistical studies of intrinsic stellar properties (such as mass, radius, and age) and to test theories of stellar evolution. We find that the distribution of observed masses of these stars shows intriguing differences to predictions from models of synthetic stellar populations in the Galaxy.

Stellar Parameter Determination With J-Plus Using Artificial Neural Networks

NASA Astrophysics Data System (ADS)

Whitten, Devin D.

2017-10-01

The J-PLUS narrow-band filter system provides a unique opportunity for the determination of stellar parameters and chemical abundances from photometry alone. Mapping stellar magnitudes to estimates of surface temperature, [Fe/H], and [C/Fe] is an excellent application of machine learning and in particular, artificial neural networks (ANN). The logistics and performance of this ANN methodology is explored with the J-PLUS Early Data Release, as well as the potential impact of stellar parameters from J-PLUS on the field of Galactic chemical evolution.

Radiation-hydrodynamical simulations of massive star formation using Monte Carlo radiative transfer - II. The formation of a 25 solar-mass star

NASA Astrophysics Data System (ADS)

Harries, Tim J.; Douglas, Tom A.; Ali, Ahmad

2017-11-01

We present a numerical simulation of the formation of a massive star using Monte Carlo-based radiation hydrodynamics (RHD). The star forms via stochastic disc accretion and produces fast, radiation-driven bipolar cavities. We find that the evolution of the infall rate (considered to be the mass flux across a 1500 au spherical boundary) and the accretion rate on to the protostar, are broadly consistent with observational constraints. After 35 kyr the star has a mass of 25 M⊙ and is surrounded by a disc of mass 7 M⊙ and 1500 au radius, and we find that the velocity field of the disc is close to Keplerian. Once again these results are consistent with those from recent high-resolution studies of discs around forming massive stars. Synthetic imaging of the RHD model shows good agreement with observations in the near- and far-IR, but may be in conflict with observations that suggest that massive young stellar objects are typically circularly symmetric in the sky at 24.5 μm. Molecular line simulations of a CH3CN transition compare well with observations in terms of surface brightness and line width, and indicate that it should be possible to reliably extract the protostellar mass from such observations.

An unusually brilliant transient in the galaxy M85.

PubMed

Kulkarni, S R; Ofek, E O; Rau, A; Cenko, S B; Soderberg, A M; Fox, D B; Gal-Yam, A; Capak, P L; Moon, D S; Li, W; Filippenko, A V; Egami, E; Kartaltepe, J; Sanders, D B

2007-05-24

Historically, variable and transient sources have both surprised astronomers and provided new views of the heavens. Here we report the discovery of an optical transient in the outskirts of the lenticular galaxy Messier 85 in the Virgo cluster. With a peak absolute R magnitude of -12, this event is distinctly brighter than novae, but fainter than type Ia supernovae (which are expected in a population of old stars in lenticular galaxies). Archival images of the field do not show a luminous star at that position with an upper limit in the g filter of about -4.1 mag, so it is unlikely to be a giant eruption from a luminous blue variable star. Over a two-month period, the transient source emitted radiation energy of almost 10(47) erg and subsequently faded in the optical sky. It is similar to, but six times more luminous at peak than, an enigmatic transient in the galaxy M31 (ref. 1). A possible origin of M85 OT2006-1 is a stellar merger. If so, searches for similar events in nearby galaxies will not only allow study of the physics of hyper-Eddington sources, but also probe an important phase in the evolution of stellar binary systems.

Failures no More: The Radical Consequences of Realistic Stellar Feedback for Dwarf Galaxies, the Milky Way, and Reionization

NASA Astrophysics Data System (ADS)

Hopkins, Philip F.

2016-06-01

Many of the most fundamental unsolved questions in star and galaxy formation revolve around star formation and "feedback" from massive stars, in-extricably linking galaxy formation and stellar evolution. I'll present simulations with un-precedented resolution of Milky-Way (MW) mass galaxies, followed cosmologically to redshift zero. For the first time, these simulations resolve the internal structure of small dwarf satellites around a MW-like host, with detailed models for stellar evolution including radiation pressure, supernovae, stellar winds, and photo-heating. I'll show that, without fine-tuning, these feedback processes naturally resolve the "missing satellites," "too big to fail," and "cusp-core" problems, and produce realistic galaxy populations. At high redshifts however, the realistic ISM structure predicted, coupled to standard stellar population models, naively leads to the prediction that only ~1-2% of ionizing photons can ever escape galaxies, insufficient to ionize the Universe. But these models assume all stars are single: if we account for binary evolution, the escape fraction increases dramatically to ~20% for the small, low-metallicity galaxies believed to ionize the Universe.

Gravitational Waves from Stellar Black Hole Binaries and the Impact on Nearby Sun-like Stars

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

Lopes, Ilídio; Silk, Joseph, E-mail: ilidio.lopes@tecnico.ulisboa.pt, E-mail: silk@astro.ox.ac.uk

We investigate the impact of resonant gravitational waves on quadrupole acoustic modes of Sun-like stars located nearby stellar black hole binary systems (such as GW150914 and GW151226). We find that the stimulation of the low-overtone modes by gravitational radiation can lead to sizeable photometric amplitude variations, much larger than the predictions for amplitudes driven by turbulent convection, which in turn are consistent with the photometric amplitudes observed in most Sun-like stars. For accurate stellar evolution models, using up-to-date stellar physics, we predict photometric amplitude variations of 1–10{sup 3} ppm for a solar mass star located at a distance between 1more » au and 10 au from the black hole binary and belonging to the same multi-star system. The observation of such a phenomenon will be within the reach of the Plato mission because the telescope will observe several portions of the Milky Way, many of which are regions of high stellar density with a substantial mixed population of Sun-like stars and black hole binaries.« less

Metallicity gradients in local field star-forming galaxies: insights on inflows, outflows, and the coevolution of gas, stars and metals

NASA Astrophysics Data System (ADS)

Ho, I.-Ting; Kudritzki, Rolf-Peter; Kewley, Lisa J.; Zahid, H. Jabran; Dopita, Michael A.; Bresolin, Fabio; Rupke, David S. N.

2015-04-01

We present metallicity gradients in 49 local field star-forming galaxies. We derive gas-phase oxygen abundances using two widely adopted metallicity calibrations based on the [O III]/Hβ, [N II]/Hα, and [N II]/[O II] line ratios. The two derived metallicity gradients are usually in good agreement within ± 0.14 dex R_{25}^{-1} (R25 is the B-band iso-photoal radius), but the metallicity gradients can differ significantly when the ionization parameters change systematically with radius. We investigate the metallicity gradients as a function of stellar mass (8 < log (M*/M⊙) < 11) and absolute B-band luminosity (-16 > MB > -22). When the metallicity gradients are expressed in dex kpc-1, we show that galaxies with lower mass and luminosity, on average, have steeper metallicity gradients. When the metallicity gradients are expressed in dex R_{25}^{-1}, we find no correlation between the metallicity gradients, and stellar mass and luminosity. We provide a local benchmark metallicity gradient of field star-forming galaxies useful for comparison with studies at high redshifts. We investigate the origin of the local benchmark gradient using simple chemical evolution models and observed gas and stellar surface density profiles in nearby field spiral galaxies. Our models suggest that the local benchmark gradient is a direct result of the coevolution of gas and stellar disc under virtually closed-box chemical evolution when the stellar-to-gas mass ratio becomes high (≫0.3). These models imply low current mass accretion rates ( ≲ 0.3 × SFR), and low-mass outflow rates ( ≲ 3 × SFR) in local field star-forming galaxies.

Influence of stellar multiplicity on planet formation. I. Evidence of suppressed planet formation due to stellar companions within 20 au and validation of four planets from the Kepler multiple planet candidates

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

Wang, Ji; Fischer, Debra A.; Xie, Ji-Wei

2014-03-01

The planet occurrence rate for multiple stars is important in two aspects. First, almost half of stellar systems in the solar neighborhood are multiple systems. Second, the comparison of the planet occurrence rate for multiple stars to that for single stars sheds light on the influence of stellar multiplicity on planet formation and evolution. We developed a method of distinguishing planet occurrence rates for single and multiple stars. From a sample of 138 bright (K{sub P} < 13.5) Kepler multi-planet candidate systems, we compared the stellar multiplicity rate of these planet host stars to that of field stars. Using dynamicalmore » stability analyses and archival Doppler measurements, we find that the stellar multiplicity rate of planet host stars is significantly lower than field stars for semimajor axes less than 20 AU, suggesting that planet formation and evolution are suppressed by the presence of a close-in companion star at these separations. The influence of stellar multiplicity at larger separations is uncertain because of search incompleteness due to a limited Doppler observation time baseline and a lack of high-resolution imaging observation. We calculated the planet confidence for the sample of multi-planet candidates and find that the planet confidences for KOI 82.01, KOI 115.01, KOI 282.01, and KOI 1781.02 are higher than 99.7% and thus validate the planetary nature of these four planet candidates. This sample of bright Kepler multi-planet candidates with refined stellar and orbital parameters, planet confidence estimation, and nearby stellar companion identification offers a well-characterized sample for future theoretical and observational study.« less

Exozodiacal clouds: hot and warm dust around main sequence stars

NASA Astrophysics Data System (ADS)

Kral, Quentin; Krivov, Alexander V.; Defrère, Denis; van Lieshout, Rik; Bonsor, Amy; Augereau, Jean-Charles; Thébault, Philippe; Ertel, Steve; Lebreton, Jérémy; Absil, Olivier

2017-04-01

A warm/hot dust component (at temperature $>$ 300K) has been detected around $\\sim$ 20% of stars. This component is called "exozodiacal dust" as it presents similarities with the zodiacal dust detected in our Solar System, even though its physical properties and spatial distribution can be significantly different. Understanding the origin and evolution of this dust is of crucial importance, not only because its presence could hamper future detections of Earth-like planets in their habitable zones, but also because it can provide invaluable information about the inner regions of planetary systems. In this review, we present a detailed overview of the observational techniques used in the detection and characterisation of exozodiacal dust clouds ("exozodis") and the results they have yielded so far, in particular regarding the incidence rate of exozodis as a function of crucial parameters such as stellar type and age, or the presence of an outer cold debris disc. We also present the important constraints that have been obtained, on dust size distribution and spatial location, by using state-of-the-art radiation transfer models on some of these systems. Finally, we investigate the crucial issue of how to explain the presence of exozodiacal dust around so many stars (regardless of their ages) despite the fact that such dust so close to its host star should disappear rapidly due to the coupled effect of collisions and stellar radiation pressure. Several potential mechanisms have been proposed to solve this paradox and are reviewed in detail in this paper. The review finishes by presenting the future of this growing field.

A model for the wind of the M supergiant VX Sagittarii

NASA Astrophysics Data System (ADS)

Pijpers, F. P.

1990-11-01

The velocity distribution of the stellar wind from the M supergiant VX Sgr deduced from interferometric measurements of maser lines by Chapman and Cohen (1986) has been modeled using the linearized theory of stellar winds driven by short period sound waves proposed by Pijpers and Hearn (1989) and the theory of stellar winds driven by short period shocks proposed by Pijpers and Habing (1989). The effect of the radiative forces on the dust formed in the wind is included in a simple way. Good agreement with the observations is obtained by a range of parameters in the theory. A series of observations of the maser lines at invervals of one or a few days may provide additional constraints on the interpretation.

The Spectroscopic Ages of Passive Galaxies in a z=1.62 Protocluster

NASA Astrophysics Data System (ADS)

Lee-Brown, Donald

2017-07-01

IRC 0218 is a protocluster at z = 1.62 with a wealth of observations that make it an ideal target for resolving the interplay between galaxy properties and environment at high redshift. We have used extremely deep HST spectroscopic data to derive unambiguous membership and stellar ages via the 4000 angstrom break for 14 members with stellar masses log(M) > 10.2. We find that at high stellar masses, log(M) > 10.85, the fraction of quiescent galaxies in the cluster is 2× higher than the field value. At lower stellar masses, the protocluster and field have consistent quiescent fractions. Despite this mass trend, we see no comparable relation between galaxy stellar age and mass for the quiescent members. Taken together, these results may reflect the impact of dry mergers on the protocluster galaxies. Alternately, the results may imply that the mass trend we observe in the IRC 0218 quiescent fraction was imprinted over a short timescale. This talk will place our results in the context of studies of other high redshift clusters and likely descendent environments at z = 1.

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.

Ultimate Spectrum of Solar/Stellar Cosmic Rays

NASA Astrophysics Data System (ADS)

Struminsky, Alexei

2015-08-01

We reconstruct an ultimate spectrum of solar/stellar cosmic rays (SCR) in a given point in the heliosphere (stellar sphere) basing on maximal value of magnetic field strenght in active region and its characteristic linear dimension. An accelerator of given dimensions and magnetic field strengh may accelarate to a finite energy for a given time (a maximal energy of SCR). We will use spectrum of SCR proposed by Syrovatsky (1961) for relativistic and non-relativistic energies normaliszing it to galactic cosmic ray (GCR) intensity at maximal SCR energy. Maximal values of SCR flux propagating in the heliosphere are determined by equilibrium between pressure of interplanetary magnrtic field and dynamic pressure of SCR (Frier&Webber, 1963). The obtained spectra would be applied to explain the extreme solar particle event occurred in about 775 AD basing on the tree-ring chronology (Miyake et al., 2012).

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 white dwarfs through the formation of strong radiating shock waves. A comparative study of the IPS and Polars can elucidate the primary effects of the magnetic fields on the dynamics and thermodynamics in accreting white dwarf systems.

The effects of magnetic fields and protostellar feedback on low-mass cluster formation

NASA Astrophysics Data System (ADS)

Cunningham, Andrew J.; Krumholz, Mark R.; McKee, Christopher F.; Klein, Richard I.

2018-05-01

We present a large suite of simulations of the formation of low-mass star clusters. Our simulations include an extensive set of physical processes - magnetohydrodynamics, radiative transfer, and protostellar outflows - and span a wide range of virial parameters and magnetic field strengths. Comparing the outcomes of our simulations to observations, we find that simulations remaining close to virial balance throughout their history produce star formation efficiencies and initial mass function (IMF) peaks that are stable in time and in reasonable agreement with observations. Our results indicate that small-scale dissipation effects near the protostellar surface provide a feedback loop for stabilizing the star formation efficiency. This is true regardless of whether the balance is maintained by input of energy from large-scale forcing or by strong magnetic fields that inhibit collapse. In contrast, simulations that leave virial balance and undergo runaway collapse form stars too efficiently and produce an IMF that becomes increasingly top heavy with time. In all cases, we find that the competition between magnetic flux advection towards the protostar and outward advection due to magnetic interchange instabilities, and the competition between turbulent amplification and reconnection close to newly formed protostars renders the local magnetic field structure insensitive to the strength of the large-scale field, ensuring that radiation is always more important than magnetic support in setting the fragmentation scale and thus the IMF peak mass. The statistics of multiple stellar systems are similarly insensitive to variations in the initial conditions and generally agree with observations within the range of statistical uncertainty.

Upper stellar mass limit by radiative feedback at low-metallicities: metallicity and accretion rate dependence

NASA Astrophysics Data System (ADS)

Fukushima, Hajime; Omukai, Kazuyuki; Hosokawa, Takashi

2018-02-01

We investigate the upper stellar mass limit set by radiative feedback for a forming star with various accretion rates and metallicities. Thus, we numerically solve the structures of both a protostar and its surrounding accretion envelope assuming a spherical symmetric and steady flow. The optical depth of the dust cocoon, a dusty part of the accretion envelope, differs for direct light from the stellar photosphere and diffuse light re-emitted as dust thermal emission. As a result, varying the metallicity qualitatively changes the way that the radiative feedback suppresses the accretion flow. With a fixed accretion rate of 10-3 M⊙ yr-1, both direct and diffuse light jointly operate to prevent mass accretion at Z ≳ 10-1 Z⊙. At Z ≲ 10-1 Z⊙, the diffuse light is no longer effective and the direct light solely limits the mass accretion. At Z ≲ 10-3 Z⊙, formation of the H II region plays an important role in terminating the accretion. The resultant upper mass limit increases with decreasing metallicity, from a few × 10 M⊙ to ∼103 M⊙ over Z = 1 Z⊙-10-4 Z⊙. We also illustrate how the radiation spectrum of massive star-forming cores changes with decreasing metallicity. First, the peak wavelength of the spectrum, which is located around 30 μm at 1 Z⊙, shifts to < 3 μm at Z ≲ 0.1 Z⊙. Secondly, a characteristic feature at 10 μm due to the amorphous silicate band appears as a dip at 1 Z⊙, but changes to a bump at Z ≲ 0.1 Z⊙. Using these spectral signatures, we can search massive accreting protostars in nearby low-metallicity environments with upcoming observations.

Cosmic Metal Production and the Contribution of QSO Absorption Systems to the Ionizing Background

NASA Technical Reports Server (NTRS)

Madau, Piero; Shull, J. Michael

1996-01-01

The recent discovery by Cowie et al. (1995) and Tytler et al. (1995) of metals in the Ly alpha clouds shows that the intergalactic medium (IGM) at high redshift is contaminated by the products of stars and suggests that ionizing photons from massive star formation may be a significant contributor to the UV background radiation at early epochs. We assess the validity of the stellar photoionization hypothesis. Based on recent computations of metal yields and 0-star Lyman continuum (Lyc) fluxes, we find that 0.2 percent of the rest-mass energy of the metals produced is radiated as Lyc. By modeling the transfer of ionizing radiation through the IGM and the rate of chemical enrichment, we demonstrate that the background intensity of photons at 1 ryd that accompanies the production of metals in the Ly alpha forest clouds may be significant, approaching 0.5 x 10(exp -21) ergs cm squared s(-1) Hz(-1) sr(-1) at z approximately equals 3 if the Lyc escape fraction is greater than of equal to 0.25. Together with quasars, massive stars could then, in principle, provide the hydrogen and helium Lyc photons required to ionize the universe at high redshifts. We propose that observations of the He2 Gunn-Peterson effect and of the metal ionization states of the Ly alpha forest and Lyman-limit absorbers should show the signature of a stellar spectrum. We also note that the stellar photoionization model fails if a large fraction of the UV radiation emitted from stars cannot escape into the IGM, as suggested by the recent Hopkins Ultraviolet Telescope observations by Leitherer et al. (1995) of low-redshift starburst galaxies, or if most of the metals observed at z is approximately 3 were produced at much earlier epochs.

A time-dependent radiative model of HD 209458b

NASA Astrophysics Data System (ADS)

Iro, N.; Bézard, B.; Guillot, T.

2005-06-01

We present a time-dependent radiative model of the atmosphere of HD 209458b and investigate its thermal structure and chemical composition. In a first step, the stellar heating profile and radiative timescales were calculated under planet-averaged insolation conditions. We find that 99.99% of the incoming stellar flux has been absorbed before reaching the 7 bar level. Stellar photons cannot therefore penetrate deeply enough to explain the large radius of the planet. We derive a radiative time constant which increases with depth and reaches about 8 h at 0.1 bar and 2.3 days at 1 bar. Time-dependent temperature profiles were also calculated, in the limit of a zonal wind that is independent of height (i.e. solid-body rotation) and constant absorption coefficients. We predict day-night variations of the effective temperature of ~600 K, for an equatorial rotation rate of 1 km s-1, in good agreement with the predictions by Showmann & Guillot (2002). This rotation rate yields day-to-night temperature variations in excess of 600 K above the 0.1-bar level. These variations rapidly decrease with depth below the 1-bar level and become negligible below the ~5-bar level for rotation rates of at least 0.5 km s-1. At high altitudes (mbar pressures or less), the night temperatures are low enough to allow sodium to condense into Na2S. Synthetic transit spectra of the visible Na doublet show a much weaker sodium absorption on the morning limb than on the evening limb. The calculated dimming of the sodium feature during planetary transites agrees with the value reported by Charbonneau et al. (2002).

Photoionized Plasma and Opacity Experiments on the Z Machine

NASA Astrophysics Data System (ADS)

Bailey, James

2008-04-01

Laboratory experiments at Z use high energy density to create plasma conditions similar to extreme astrophysical environments, including stellar interiors and accretion powered objects. The importance of radiation unifies these topics, even though the plasmas involved are very different. Understanding stellar interiors requires knowledge of radiation transport in dense, hot, collision-dominated plasma. A Z x-ray source was used to measure iron plasma transmission at 156 eV electron temperature, 2x higher than in prior work. The data provide the first experimental tests of absorption features critical for stellar interior opacity models and may provide insight into whether the present discrepancy between solar models and helioseismology originates in opacity model deficiencies or in some other aspect of the solar model. In contrast, accretion physics requires interpretation of x-ray spectra from lower density photoionization-dominated plasma. Exploiting astrophysical spectra requires a spectral model that connects the observations with a model that describes the overall picture of the astrophysical object. However, photoionized plasma spectral models are largely untested. Z-pinch radiation was used to create photoionized iron and neon plasmas with photoionization parameter 5-25 erg cm /s. Comparisons with the data improve x-ray photoionization models and promote more accurate interpretation of spectra acquired with astrophysical observatories. The prospects for new experiments at the higher radiation powers provided by the recently upgraded Z facility will be described.* In collaboration with scientists from CEA, LANL, LLNL, Oxford, Prism, Queens University, Swarthmore College, U. Nevada Reno, and Sandia ++Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.

Black holes on FIRE: stellar feedback limits early feeding of galactic nuclei

NASA Astrophysics Data System (ADS)

Anglés-Alcázar, Daniel; Faucher-Giguère, Claude-André; Quataert, Eliot; Hopkins, Philip F.; Feldmann, Robert; Torrey, Paul; Wetzel, Andrew; Kereš, Dušan

2017-11-01

We introduce massive black holes (BHs) in the Feedback In Realistic Environments (FIRE) project and perform high-resolution cosmological hydrodynamic simulations of quasar-mass haloes [Mhalo(z = 2) ≈ 1012.5 M⊙] down to z = 1. These simulations model stellar feedback by supernovae, stellar winds and radiation, and BH growth using a gravitational torque-based prescription tied to the resolved properties of galactic nuclei. We do not include BH feedback. We show that early BH growth occurs through short (≲1 Myr) accretion episodes that can reach or even exceed the Eddington rate. In this regime, BH growth is limited by bursty stellar feedback continuously evacuating gas from galactic nuclei, and BHs remain undermassive in low-mass galaxies relative to the local MBH-Mbulgerelation. BH growth is more efficient at later times, when the nuclear stellar potential retains a significant gas reservoir, star formation becomes less bursty and galaxies settle into a more ordered state. BHs rapidly converge on to the observed scaling relations when the host reaches Mbulge ∼ 1010 M⊙. We show that resolving the effects of stellar feedback on the gas supply in the inner ∼100 pc of galaxies is necessary to accurately capture the growth of central BHs. Our simulations imply that bursty stellar feedback has important implications for BH-galaxy relations, AGN demographics and time variability, the formation of early quasars and massive BH mergers.

Stellar winds in binary X-ray systems

NASA Technical Reports Server (NTRS)

Macgregor, K. B.; Vitello, P. A. J.

1982-01-01

It is thought that accretion from a strong stellar wind by a compact object may be responsible for the X-ray emission from binary systems containing a massive early-type primary. To investigate the effect of X-ray heating and ionization on the mass transfer process in systems of this type, an idealized model is constructed for the flow of a radiation-driven wind in the presence of an X-ray source of specified luminosity, L sub x. It is noted that for low values of L sub x, X-ray photoionization gives rise to additional ions having spectral lines with wavelengths situated near the peak of the primary continuum flux distribution. As a consequence, the radiation force acting on the gas increases in relation to its value in the absence of X-rays, and the wind is accelerated to higher velocities. As L sub x is increased, the degree of ionization of the wind increases, and the magnitude of the radiation force is diminished in comparison with the case in which L sub x = 0. This reduction leads at first to a decrease in the wind velocity and ultimately (for L sub x sufficiently large) to the termination of radiatively driven mass loss.

Massive stars: privileged sources of cosmic-rays for interstellar astrochemistry

NASA Astrophysics Data System (ADS)

De Becker, M.

2015-01-01

Massive stars can be considered as crucial engines for interstellar physics. They are indeed the main providers of UV radiation field, and constitute a substantial source of chemical enrichment. On their evolution time-scale (at most about 10 Myr), they typically stay close to their formation site, i.e. close to molecular clouds very rich in interstellar molecules. These stellar objects have also the property to be involved in particle acceleration processes leading to the production of high energy charged particles (cosmic-rays). After rejection in the interstellar medium, these particles will play a substantial role in processes such as those simulated in various facilities dedicated to experimental astrochemistry. This short contribution intends to put these particles, crucial for astrochemistry, in their adequate astrophysical context.

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

Not Available

This journal contains 7 articles pertaining to astrophysics. The first article is an overview of the other 6 articles and also a tribute to Jim Wilson and his work in the fields of general relativity and numerical astrophysics. The six articles are on the following subjects: (1) computer simulations of black hole accretion; (2) calculations on the collapse of the iron core of a massive star; (3) stellar-collapse models which reveal a possible site for nucleosynthesis of elements heavier than iron; (4) modeling sources for gravitational radiation; (5) the development of a computer program for finite-difference mesh calculations and itsmore » applications to astrophysics; (6) the existence of neutrinos with nonzero rest mass are used to explain the universe. Abstracts of each of the articles were prepared separately. (SC)« less

Orbital fingerprints of ultralight scalar fields around black holes

NASA Astrophysics Data System (ADS)

Ferreira, Miguel C.; Macedo, Caio F. B.; Cardoso, Vitor

2017-10-01

Ultralight scalars have been predicted in a variety of scenarios and advocated as a possible component of dark matter. These fields can form compact regular structures known as boson stars, or—in the presence of horizons—give rise to nontrivial time-dependent scalar hair and a stationary geometry. Because these fields can be coherent over large spatial extents, their interaction with "regular" matter can lead to very peculiar effects, most notably resonances. Here we study the motion of stars in a background describing black holes surrounded by nonaxially symmetric scalar field profiles. By analyzing the system in a weak-field approach, we find that the presence of a scalar field gives rise to secular effects akin to ones existing in planetary and accretion disks. Particularly, the existence of resonances between the orbiting stars and the scalar field may enable angular momentum exchange between them, providing mechanisms similar to planetary migration. Additionally, these mechanisms may allow floating orbits, which are stable radiating orbits. We also show, in the full relativistic case, that these effects also appear when there is a direct coupling between the scalar field and the stellar matter, which can arise due to the presence of a scalar core in the star or in alternative theories of gravity.

Stellarator Coil Design and Plasma Sensitivity

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

Long-Poe Ku and Allen H. Boozer

2010-11-03

The rich information contained in the plasma response to external magnetic perturbations can be used to help design stellarator coils more effectively. We demonstrate the feasibility by first devel- oping a simple, direct method to study perturbations in stellarators that do not break stellarator symmetry and periodicity. The method applies a small perturbation to the plasma boundary and evaluates the resulting perturbed free-boundary equilibrium to build up a sensitivity matrix for the important physics attributes of the underlying configuration. Using this sensitivity information, design methods for better stellarator coils are then developed. The procedure and a proof-of-principle application are givenmore » that (1) determine the spatial distributions of external normal magnetic field at the location of the unperturbed plasma boundary to which the plasma properties are most sen- sitive, (2) determine the distributions of external normal magnetic field that can be produced most efficiently by distant coils, (3) choose the ratios of the magnitudes of the the efficiently produced magnetic distributions so the sensitive plasma properties can be controlled. Using these methods, sets of modular coils are found for the National Compact Stellarator Experiment (NCSX) that are either smoother or can be located much farther from the plasma boundary than those of the present design.« less

Doradus Nebula

NASA Image and Video Library

1999-12-01

A panoramic view of a vast, sculpted area of gas and dust where thousands of stars are being born has been captured by NASA's Hubble Space Telescope. The image, taken by Hubble's Wide Field and Planetary Camera 2, is online at http://hubblesite.org/newscenter/archive/releases/2001/21/image/a/. The camera was designed and built by NASA's Jet Propulsion Laboratory, Pasadena, Calif. The photo offers an unprecedented, detailed view of the entire inner region of the fertile, star-forming 30 Doradus Nebula. The mosaic picture shows that ultraviolet radiation and high-speed material unleashed by the stars in the cluster, called R136 (the large blue blob left of center), are weaving a tapestry of creation and destruction, triggering the collapse of looming gas and dust clouds and forming pillar-like structures that incubate newborn stars. The 30 Doradus Nebula is in the Large Magellanic Cloud, a satellite galaxy of the Milky Way located 170,000 light-years from Earth. Nebulas like 30 Doradus are signposts of recent star birth. High-energy ultraviolet radiation from young, hot, massive stars in R136 causes surrounding gaseous material to glow. Previous Hubble telescope observations showed that R136 contains several dozen of the most massive stars known, each about 100 times the mass of the Sun and about 10 times as hot. These stellar behemoths formed about 2 million years ago. The stars in R136 produce intense "stellar winds," streams of material traveling at several million miles an hour. These winds push the gas away from the cluster and compress the inner regions of the surrounding gas and dust clouds (seen in the image as the pinkish material). The intense pressure triggers the collapse of parts of the clouds, producing a new star formation around the central cluster. Most stars in the nursery are not visible because they are still encased in cocoons of gas and dust. This mosaic image of 30 Doradus consists of five overlapping pictures taken between January 1994 and September 2000 by the Wide Field and Planetary Camera 2. Several color filters enhance important details in the stars and the nebula. Blue corresponds to the hot stars. The greenish color denotes hot gas energized by the central cluster of stars. Pink depicts the glowing edges of the gas and dust clouds facing the cluster, which are being bombarded by winds and radiation. Reddish-brown represents the cooler surfaces of the clouds, which are not receiving direct radiation from the central cluster. http://photojournal.jpl.nasa.gov/catalog/PIA04200

Time dependent radiation spectra from jets of microquasars

NASA Astrophysics Data System (ADS)

Gupta, Swati

X-ray binary systems in our galaxy exhibiting relativistic jets (microquasars) present one of the most recent additions to the field of high energy astrophysics. Jet models of high energy emission from these sources have created significant interest lately with detailed spectral and timing studies of the X-ray emission from microquasars, and their recent establishment as a new distinctive class of g-ray emitting sources after the detection of very - high-energy (VHE) g-rays from the microquasars LS 5039 and LS I +61° 303. This dissertation presents a study of radiation signatures from a leptonic jet model, based on time-dependent electron injection and acceleration, followed by their subsequent adiabatic and radiative cooling. The radiation mechanisms included are synchrotron, synchrotron self Compton and external Compton with soft photons provided by the companion star and the accretion disk. Compton scattering is treated both in the Thomson and the Klein-Nishina regimes, thus making the model applicable to microquasars that are candidates for VHE g-ray emission as well. An analytical solution to the electron kinetic equation is introduced for the Thomson regime treatment, while a numerical approach is adopted for the Klein-Nishina regime. Predictions regarding rapid flux and spectral variability signatures in the form of spectral hysteresis in the X-ray hardness intensity diagrams are made, which should be testable with monitoring observations using Chandra and/or XMM - Newton . Detections of such variability would help in distinguishing between various competing models for the high energy emission from these sources. Our results show that the shape and orientation of the hysteresis loops would allow identification of the dominant emission components as well as quantify physical parameters like the magnetic field, spectral index, Doppler boosting factor etc. The model is applied to available broadband observations of the two microquasars that have been very recently detected in VHE g-rays, namely LSI +61° 303 and LS 5039. In the case of LS I +61° 303, we explain the observed orbital modulation of the VHE g-ray emission solely by the geometrical effect of changes in the relative orientation of the stellar companion with respect to the compact object affecting the position and depth of the gg absorption trough. For LS 5039, our results imply that an orbital modulation of the velocity of the stellar wind in addition to gg absorption effects may be necessary to explain the orbital variability of the VHE g-ray emission.

X-rays from radio pulsars: the detection of PSR 1055-52

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

Cheng, A.F.; Helfand, D.J.

The short-period pulsar PSR 1055-52 has been detected as a soft X-ray source in the course of an Einstein Observatory survey of radio pulsars. Its X-ray to radio luminosity ratio is approx.10/sup 4/, although the X-rays are not modulated at the neutron star's rotation frequency. We present high spatial resolution observations which suggest that a significant fraction of the emission comes from an extended region surrounding the pulsar. We investigate several possible scenarios for the origin of both point and extended X-ray emission from isolated neutron stars: radiation from the hot stellar surface, from hot polar caps, and from anmore » optically thick atmosphere, as well as from a circumstellar nebula emitting thermal bremsstrahlung or synchrotron radiation. We conclude that the spatial, spectral, and temporal characteristic of this source are most consistent with a model in which relativistic particles generated by the pulsar are radiating synchrotron X-rays in the surrounding magnetic field; i.e., that PSR 1055 is embedded in a mini-Crab nebula. Observational tests of this hypothesis are suggested, and the implications of this result for pulsar evolution are briefly discussed.« less

Reconciling solar and stellar magnetic cycles with nonlinear dynamo simulations.

PubMed

Strugarek, A; Beaudoin, P; Charbonneau, P; Brun, A S; do Nascimento, J-D

2017-07-14

The magnetic fields of solar-type stars are observed to cycle over decadal periods-11 years in the case of the Sun. The fields originate in the turbulent convective layers of stars and have a complex dependency upon stellar rotation rate. We have performed a set of turbulent global simulations that exhibit magnetic cycles varying systematically with stellar rotation and luminosity. We find that the magnetic cycle period is inversely proportional to the Rossby number, which quantifies the influence of rotation on turbulent convection. The trend relies on a fundamentally nonlinear dynamo process and is compatible with the Sun's cycle and those of other solar-type stars. Copyright © 2017, American Association for the Advancement of Science.

Magnifying the Early Episodes of Star Formation: Super Star Clusters at Cosmological Distances

NASA Astrophysics Data System (ADS)

Vanzella, E.; Castellano, M.; Meneghetti, M.; Mercurio, A.; Caminha, G. B.; Cupani, G.; Calura, F.; Christensen, L.; Merlin, E.; Rosati, P.; Gronke, M.; Dijkstra, M.; Mignoli, M.; Gilli, R.; De Barros, S.; Caputi, K.; Grillo, C.; Balestra, I.; Cristiani, S.; Nonino, M.; Giallongo, E.; Grazian, A.; Pentericci, L.; Fontana, A.; Comastri, A.; Vignali, C.; Zamorani, G.; Brusa, M.; Bergamini, P.; Tozzi, P.

2017-06-01

We study the spectrophotometric properties of a highly magnified (μ ≃ 40{--}70) pair of stellar systems identified at z = 3.2222 behind the Hubble Frontier Field galaxy cluster MACS J0416. Five multiple images (out of six) have been spectroscopically confirmed by means of VLT/MUSE and VLT/X-Shooter observations. Each image includes two faint ({m}{UV}≃ 30.6), young (≲ 100 Myr), low-mass (< {10}7 {M}⊙ ), low-metallicity (12 + Log(O/H) ≃ 7.7, or 1/10 solar), and compact (30 pc effective radius) stellar systems separated by ≃ 300 pc after correcting for lensing amplification. We measured several rest-frame ultraviolet and optical narrow ({σ }v≲ 25 km s-1) high-ionization lines. These features may be the signature of very hot (T> {{50,000}} K) stars within dense stellar clusters, whose dynamical mass is likely dominated by the stellar component. Remarkably, the ultraviolet metal lines are not accompanied by Lyα emission (e.g., C IV/Lyα > 15), despite the fact that the Lyα line flux is expected to be 150 times brighter (inferred from the Hβ flux). A spatially offset, strongly magnified (μ > 50) Lyα emission with a spatial extent ≲ 7.6 kpc2 is instead identified 2 kpc away from the system. The origin of such a faint emission could be the result of fluorescent Lyα induced by a transverse leakage of ionizing radiation emerging from the stellar systems and/or may be associated with an underlying and barely detected object (with {m}{UV}> 34 de-lensed). This is the first confirmed metal-line emitter at such low-luminosity and redshift without Lyα emission—suggesting that, at least in some cases, a non-uniform covering factor of the neutral gas might hamper the Lyα detection. Based on observations collected at the European Southern Observatory for Astronomical research in the southern hemisphere, under ESO programmes P095.A-0840, P095.A-0653, and P186.A-0798.

Fluctuations of the intergalactic ionization field at redshift z ~ 2

NASA Astrophysics Data System (ADS)

Agafonova, I. I.; Levshakov, S. A.; Reimers, D.; Hagen, H.-J.; Tytler, D.

2013-04-01

Aims: To probe the spectral energy distribution (SED) of the ionizing background radiation at z ≲ 2 and to specify the sources contributing to the intergalactic radiation field. Methods: The spectrum of a bright quasar HS 1103+6416 (zem = 2.19) contains five successive metal-line absorption systems at zabs = 1.1923, 1.7193, 1.8873, 1.8916, and 1.9410. The systems are optically thin and reveal multiple lines of different metal ions with the ionization potentials lying in the extreme ultraviolet (EUV) range (~1 Ryd to ~0.2 keV). For each system, the EUV SED of the underlying ionization field is reconstructed by means of a special technique developed for solving the inverse problem in spectroscopy. For the zabs = 1.8916 system, the analysis also involves the He I resonance lines of the Lyman series and the He iλ504 Å continuum, which are seen for the first time in any cosmic object except the Sun. Results: From one system to another, the SED of the ionizing continuum changes significantly, indicating that the intergalactic ionization field at z ≲ 2 fluctuates at the scale of at least Δz ~ 0.004. This is consistent with Δz ≲ 0.01 estimated from He II and H I Lyman-α forest measurements between the redshifts 2 and 3. A radiation intensity break by approximately an order of magnitude at E = 4 Ryd in SEDs restored for the zabs = 1.1923, 1.8873, 1.8916, and 1.9410 systems points to quasars as the main sources of the ionizing radiation. The SED variability is mostly caused by a small number of objects contributing at any given redshift to the ionizing background; at scales Δz ≳ 0.05, the influence of local radiation sources becomes significant. A remarkable SED restored for the zabs = 1.7193 system, with a sharp break shifted to E ~ 3.5 Ryd and a subsequent intensity decrease by ~1.5 dex, indicates a source with comparable inputs of both hard (active galactic nuclei, AGN) and soft (stellar) radiation components. Such a continuum can be emitted by (ultra) luminous infrared galaxies, many of which reveal both a strong AGN activity and intense star formation in the circumnuclear regions.

On the extended stellar structure around NGC 288

NASA Astrophysics Data System (ADS)

Piatti, Andrés E.

2018-01-01

We report on observational evidence of an extra-tidal clumpy structure around NGC 288 from homogeneous coverage of a large area with the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) PS1 data base. The extra-tidal star population has been disentangled from that of the Milky Way (MW) field by using a cleaning technique that successfully reproduces the stellar density, luminosity function and colour distributions of MW field stars. We have produced the cluster stellar density radial profile and a stellar density map from independent approaches, and we found the results to be in excellent agreement - the feature extends up to 3.5 times further than the cluster tidal radius. Previous works based on shallower photometric data sets have speculated on the existence of several long tidal tails, similar to that found in Pal 5. The present outcome shows that NGC 288 could hardly have such tails, but it favours the notion that the use of interactions with the MW tidal field has been a relatively inefficient process for stripping stars off the cluster. These results point to the need for a renewed overall study of the external regions of Galactic globular clusters (GGCs) in order to reliably characterize them. It will then be possible to investigate whether there is any connection between detected tidal tails, extra-tidal stellar populations and extended diffuse halo-like structures, and the dynamical histories of GGCs in the Galaxy.

Effect of Stellar Wind and Poynting-Robertson Drag on Photogravitational Elliptic Restricted Three Body Problem

NASA Astrophysics Data System (ADS)

Chakraborty, A.; Narayan, A.

2018-03-01

The existence and linear stability of the planar equilibrium points for photogravitational elliptical restricted three body problem is investigated in this paper. Assuming that the primaries, one of which is radiating are rotating in an elliptical orbit around their common center of mass. The effect of the radiation pressure, forces due to stellar wind and Poynting-Robertson drag on the dust particles are considered. The location of the five equilibrium points are found using analytical methods. It is observed that the collinear equilibrium points L 1, L 2 and L 3 do not lie on the line joining the primaries but are shifted along the y-coordinate. The instability of the libration points due to the presence of the drag forces is demonstrated by Lyapunov's first method of stability.

Inclination Angles of Black Hole X-Ray Binaries Manifest Strong Gravity around Black Holes

NASA Technical Reports Server (NTRS)

Zhang, S. N.; Zhang, Xiao-Ling; Yao, Yangsen

2002-01-01

System inclination angles have been determined for about 15 X-ray binaries, in which stellar mass black holes are considered to exist. These inclination angles range between 25 degrees and 80 degrees, but peaked between 60-70 degrees. This peak is not explained in the frame work of Newtonian gravity. However, this peak is reproduced naturally if we model the observed X-ray radiations as being produced in the accretion disks very close to the black hole horizons, where the extremely strong general and special relativistic effects, caused by the extremely strong gravity near the black hole horizons, modify the local radiation significantly as the X-rays propagate to the remote observer. Therefore the peak of the inclination angle distribution provides evidence or strong gravity around stellar mass black holes.

The signatures of the parental cluster on field planetary systems

NASA Astrophysics Data System (ADS)

Cai, Maxwell Xu; Portegies Zwart, Simon; van Elteren, Arjen

2018-03-01

Due to the high stellar densities in young clusters, planetary systems formed in these environments are likely to have experienced perturbations from encounters with other stars. We carry out direct N-body simulations of multiplanet systems in star clusters to study the combined effects of stellar encounters and internal planetary dynamics. These planetary systems eventually become part of the Galactic field population as the parental cluster dissolves, which is where most presently known exoplanets are observed. We show that perturbations induced by stellar encounters lead to distinct signatures in the field planetary systems, most prominently, the excited orbital inclinations and eccentricities. Planetary systems that form within the cluster's half-mass radius are more prone to such perturbations. The orbital elements are most strongly excited in the outermost orbit, but the effect propagates to the entire planetary system through secular evolution. Planet ejections may occur long after a stellar encounter. The surviving planets in these reduced systems tend to have, on average, higher inclinations and larger eccentricities compared to systems that were perturbed less strongly. As soon as the parental star cluster dissolves, external perturbations stop affecting the escaped planetary systems, and further evolution proceeds on a relaxation time-scale. The outer regions of these ejected planetary systems tend to relax so slowly that their state carries the memory of their last strong encounter in the star cluster. Regardless of the stellar density, we observe a robust anticorrelation between multiplicity and mean inclination/eccentricity. We speculate that the `Kepler dichotomy' observed in field planetary systems is a natural consequence of their early evolution in the parental cluster.

Flexible helical-axis stellarator

DOEpatents

Harris, Jeffrey H.; Hender, Timothy C.; Carreras, Benjamin A.; Cantrell, Jack L.; Morris, Robert N.

1988-01-01

An 1=1 helical winding which spirals about a conventional planar, circular central conductor of a helical-axis stellarator adds a significant degree of flexibility by making it possible to control the rotational transform profile and shear of the magnetic fields confining the plasma in a helical-axis stellarator. The toroidal central conductor links a plurality of toroidal field coils which are separately disposed to follow a helical path around the central conductor in phase with the helical path of the 1=1 winding. This coil configuration produces bean-shaped magnetic flux surfaces which rotate around the central circular conductor in the same manner as the toroidal field generating coils. The additional 1=1 winding provides flexible control of the magnetic field generated by the central conductor to prevent the formation of low-order resonances in the rotational transform profile which can produce break-up of the equilibrium magnetic surfaces. Further, this additional winding can deepen the magnetic well which together with the flexible control provides increased stability.

Experimental design to understand the interaction of stellar radiation with molecular clouds

NASA Astrophysics Data System (ADS)

Vandervort, Robert; Davis, Josh; Trantham, Matt; Klein, Sallee; Frank, Yechiel; Raicher, Erez; Fraenkel, Moshe; Shvarts, Dov; Keiter, Paul; Drake, R. Paul

2016-10-01

Enhanced star formation triggered by local O and B type stars is an astrophysical problem of interest. O and B type stars are massive, hot stars that emit an enormous amount of radiation. This radiation acts to either compress or blow apart clumps of gas in the interstellar media. For example, in the optically thick limit, when the x-ray radiation in the gas clump has a short mean free path length the x-ray radiation is absorbed near the clump edge and compresses the clump. In the optically thin limit, when the mean free path is long, the radiation is absorbed throughout acting to heat the clump. This heating explodes the gas clump. Careful selection of parameters, such as foam density or source temperature, allow the experimental platform to access different hydrodynamic regimes. The stellar radiation source is mimicked by a laser irradiated thin gold foil. This will provide a source of thermal x-rays (around 100 eV). The gas clump is mimicked by a low-density foam around 0.12 g/cc. Simulations were done using radiation hydrodynamics codes to tune the experimental parameters. The experiment will be carried out at the Omega laser facility on OMEGA 60. Funding acknowledgements: This work is funded by the U.S. DOE, through the NNSA-DS and SC-OFES Joint Program in HEDPLP, Grant No. DE-NA0001840, and the NLUF Program, Grant No. DE-NA0000850, and through LLE, University of Rochester by the NNSA/OICF under Agreement No. DE-FC52-08NA28302.

A SIMPLE EVOLUTIONAL MODEL OF THE UV HABITABLE ZONE AND THE POSSIBILITY OF PERSISTENT LIFE EXISTENCE: THE EFFECTS OF MASS AND METALLICITY

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

Oishi, Midori; Kamaya, Hideyuki

In addition to the habitable zone (HZ), the UV habitable zone (UV-HZ) is important when considering the existence of persistent life in the universe. The UV-HZ is defined as the area where the UV radiation field from a host star is moderate for persistent life existence. This is because UV is necessary for the synthesis of biochemical compounds. The UV-HZ must overlap the HZ when life appears and evolves. In this paper, following our previous study of the HZ, we examine the UV-HZ in cases with a stellar mass range from 0.08 to 4.00 M {sub ☉} with various metallicities during the main sequencemore » phase. This mass range was chosen because we are interested in an environment similar to that of Earth. The effect of metallicity is reflected in the spectrum of the host stars, and we reexamine it in the context of the UV-HZ. The present work shows the effect of metallicity when that in the UV-HZ is less than that in the HZ. Furthermore, we find that the chance of persistent life existence declines as the metallicity decreases, as long as the UV radiation is not protected and/or boosted by any mechanisms. This is because the overlapped region of a persistent HZ and UV-HZ decreases. We find that the most appropriate stellar mass for the persistence of life existence is from 1.0 to 1.5 M {sub ☉} with metallicity Z  = 0.02, and only about 1.2 M {sub ☉} with Z  = 0.002. When Z  = 0.0002, the chance of persistent life existence is very low, assuming that the ocean does not protect the life from UV radiation.« less

Coupling hydrodynamics with comoving frame radiative transfer. II. Stellar wind stratification in the high-mass X-ray binary Vela X-1

NASA Astrophysics Data System (ADS)

Sander, A. A. C.; Fürst, F.; Kretschmar, P.; Oskinova, L. M.; Todt, H.; Hainich, R.; Shenar, T.; Hamann, W.-R.

2018-02-01

Context. Vela X-1, a prototypical high-mass X-ray binary (HMXB), hosts a neutron star (NS) in a close orbit around an early-B supergiant donor star. Accretion of the donor star's wind onto the NS powers its strong X-ray luminosity. To understand the physics of HMXBs, detailed knowledge about the donor star winds is required. Aims: To gain a realistic picture of the donor star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model describing the wind stratification while properly reproducing the observed donor spectrum. To investigate how X-ray illumination affects the stellar wind, we calculated additional models for different X-ray luminosity regimes. Methods: We used the recently updated version of the Potsdam Wolf-Rayet code to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer. Results: The wind flow in Vela X-1 is driven by ions from various elements, with Fe III and S III leading in the outer wind. The model-predicted mass-loss rate is in line with earlier empirical studies. The mass-loss rate is almost unaffected by the presence of the accreting NS in the wind. The terminal wind velocity is confirmed at v∞≈ 600 km s-1. On the other hand, the wind velocity in the inner region where the NS is located is only ≈100 km s-1, which is not expected on the basis of a standard β-velocity law. In models with an enhanced level of X-rays, the velocity field in the outer wind can be altered. If the X-ray flux is too high, the acceleration breaks down because the ionization increases. Conclusions: Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB.

Advancing the understanding of plasma transport in mid-size stellarators

NASA Astrophysics Data System (ADS)

Hidalgo, Carlos; Talmadge, Joseph; Ramisch, Mirko; TJ-II, the; HXS; TJ-K Teams

2017-01-01

The tokamak and the stellarator are the two main candidate concepts for magnetically confining fusion plasmas. The flexibility of the mid-size stellarator devices together with their unique diagnostic capabilities make them ideally suited to study the relation between magnetic topology, electric fields and transport. This paper addresses advances in the understanding of plasma transport in mid-size stellarators with an emphasis on the physics of flows, transport control, impurity and particle transport and fast particles. The results described here emphasize an improved physics understanding of phenomena in stellarators that complements the empirical approach. Experiments in mid-size stellarators support the development of advanced plasma scenarios in Wendelstein 7-X (W7-X) and, in concert with better physics understanding in tokamaks, may ultimately lead to an advance in the prediction of burning plasma behaviour.

Dynamical effects of stellar companions

NASA Astrophysics Data System (ADS)

Naoz, Smadar

2015-08-01

The fraction of stellar binaries in the field is extremely high (about 40% - 70% for > 1 Msun stars), and thus, given this frequency, a large fraction of all exoplanetary systems may reside in binaries. While close-in giant planets tend to be found preferentially in binary stellar systems it seems that the frequency of giant planets in close binaries (<100 AU) is significantly lower than in the overall population. Stellar companions’ gravitational perturbations may significantly alter the planetary orbits around their partner on secular timescales. They can drive planets to large eccentric orbits which can either result in plunging these planets into the star or shrinking their orbits and forming short period planets. I will review the dynamical effects stellar binaries have on a planetary systems. I will also present new results on the influence that stellar evolution has on the dynamical processes in these systems.

The pulsar force-free magnetosphere linked to its striped wind: time-dependent pseudo-spectral simulations

NASA Astrophysics Data System (ADS)

Pétri, J.

2012-07-01

Pulsar activity and its related radiation mechanism are usually explained by invoking some plasma processes occurring inside the magnetosphere, be it polar caps, outer/slot gaps or the transition region between the quasi-static magnetic dipole regime and the wave zone, like the striped wind. Despite many detailed local investigations, the global electrodynamics around those neutron stars remains poorly described with only little quantitative studies on the largest scales, i.e. of several light-cylinder radii rL. A better understanding of these compact objects requires a deep and accurate knowledge of their immediate electromagnetic surrounding within the magnetosphere and its link to the relativistic pulsar wind. This is compulsory to make any reliable predictions about the whole electric circuit, energy losses, sites of particle acceleration and the possibly associated emission mechanisms. The aim of this work is to present accurate solutions to the nearly stationary force-free pulsar magnetosphere and its link to the striped wind, for various spin periods and arbitrary inclination. To this end, the time-dependent Maxwell equations are solved in spherical geometry in the force-free approximation using a vector spherical harmonic expansion of the electromagnetic field. An exact analytical enforcement of the divergencelessness of the magnetic part is obtained by a projection method. Special care has been given to designing an algorithm able to look deeply into the magnetosphere with physically realistic ratios of stellar R* to light-cylinder rL radius. However, currently available computational resources allow us only to set R*/rL= 10-1 corresponding to pulsars with a period of 2 ms. The spherical geometry permits a proper and mathematically well-posed imposition of self-consistent physical boundary conditions on the stellar crust. We checked our code against several analytical solutions, like the Deutsch vacuum rotator solution and the Michel monopole field. We also retrieve energy losses comparable to the magnetodipole radiation formula and consistent with previous similar works. Finally, for arbitrary obliquity, we give an expression for the total electric charge of the system. It does not vanish except for the perpendicular rotator. This is due to the often ignored point charge located at the centre of the neutron star. It is questionable if such solutions with huge electric charges could exist in reality except for configurations close to an orthogonal rotator. The charge spread over the stellar crust is not a tunable parameter as often hypothesized.

Numerical investigation of non-perturbative kinetic effects of energetic particles on toroidicity-induced Alfvén eigenmodes in tokamaks and stellarators

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

Slaby, Christoph; Könies, Axel; Kleiber, Ralf

2016-09-15

The resonant interaction of shear Alfvén waves with energetic particles is investigated numerically in tokamak and stellarator geometry using a non-perturbative MHD-kinetic hybrid approach. The focus lies on toroidicity-induced Alfvén eigenmodes (TAEs), which are most easily destabilized by a fast-particle population in fusion plasmas. While the background plasma is treated within the framework of an ideal-MHD theory, the drive of the fast particles, as well as Landau damping of the background plasma, is modelled using the drift-kinetic Vlasov equation without collisions. Building on analytical theory, a fast numerical tool, STAE-K, has been developed to solve the resulting eigenvalue problem usingmore » a Riccati shooting method. The code, which can be used for parameter scans, is applied to tokamaks and the stellarator Wendelstein 7-X. High energetic-ion pressure leads to large growth rates of the TAEs and to their conversion into kinetically modified TAEs and kinetic Alfvén waves via continuum interaction. To better understand the physics of this conversion mechanism, the connections between TAEs and the shear Alfvén wave continuum are examined. It is shown that, when energetic particles are present, the continuum deforms substantially and the TAE frequency can leave the continuum gap. The interaction of the TAE with the continuum leads to singularities in the eigenfunctions. To further advance the physical model and also to eliminate the MHD continuum together with the singularities in the eigenfunctions, a fourth-order term connected to radiative damping has been included. The radiative damping term is connected to non-ideal effects of the bulk plasma and introduces higher-order derivatives to the model. Thus, it has the potential to substantially change the nature of the solution. For the first time, the fast-particle drive, Landau damping, continuum damping, and radiative damping have been modelled together in tokamak- as well as in stellarator geometry.« less

The vertical structure of the boundary layer around compact objects

NASA Astrophysics Data System (ADS)

Hertfelder, Marius; Kley, Wilhelm

2017-09-01

Context. Mass transfer due to Roche lobe overflow leads to the formation of an accretion disk around a weakly magnetized white dwarf (WD) in cataclysmic variables. At the inner edge of the disk, the gas comes upon the surface of the WD and has to get rid of its excess kinetic energy in order to settle down on the more slowly rotating outer stellar layers. This region is known as the boundary layer (BL). Aims: In this work we investigate the vertical structure of the BL, which is still poorly understood. We shall provide details of the basic structure of the two-dimensional (2D) BL and how it depends on parameters such as stellar mass and rotation rate, as well as the mass-accretion rate. We further investigate the destination of the disk material and compare our results with previous one-dimensional (1D) simulations. Methods: We solve the 2D equations of radiation hydrodynamics in a spherical (r-ϑ) geometry using a parallel grid-based code that employs a Riemann solver. The radiation energy is considered in the two-temperature approach with a radiative flux given by the flux-limited diffusion approximation. Results: The BL around a non-rotating WD is characterized by a steep drop in angular velocity over a width of only 1% of the stellar radius, a heavy depletion of mass, and a high temperature ( 500 000 K) as a consequence of the strong shear. Variations in Ω∗,M∗, and Ṁ influence the extent of the changes of the variables in the BL but not the general structure. Depending on Ω∗, the disk material travels up to the poles or is halted at a certain latitude. The extent of mixing with the stellar material also depends on Ω∗. We find that the 1D approximation matches the 2D data well, apart from an underestimated temperature.

Ground based THz Spectroscopy of Obscured Starbursts in the Early Universe enabled by the 2nd generation Redshift (z) & Early Universe Spectrometer (ZEUS-2)

NASA Astrophysics Data System (ADS)

Vishwas, Amit; Stacey, Gordon; Nikola, Thomas; Ferkinhoff, Carl; Parshley, Stephen; Schoenwald, Justin; Lamarche, Cody James; Higdon, James; Higdon, Sarah; Brisbin, Drew; Güesten, Rolf; Weiss, Axel; Menten, Karl; Irwin, Kent; Cho, Hsiao-Mei; Niemack, Michael; Hilton, Gene; Hubmayr, Johannes; Amiri, Mandana; Halpern, Mark; Wiebe, Donald; Hasselfield, Matthew; Ade, Peter; Tucker, Carole

2018-01-01

Galaxies were surprisingly dusty in the early Universe, with more than half of the light emitted from stars being absorbed by dust within the system and re-radiated into far infrared (FIR, ~50-150μm) wavelengths. Dusty star forming galaxies (DSFGs) dominate the co-moving star formation rate density of the Universe that peaks around redshift, z~2, making it compelling to study them in rest frame FIR bands. From galaxies at z > 1, the FIR line emission from abundant ions like [O III], [C II] and [N II], are redshifted into the short sub-mm telluric windows. My thesis work is based on building and deploying the 2nd Generation Redshift (z) and Early Universe Spectrometer (ZEUS-2), a long-slit, echelle grating spectrometer optimized to study broad (Δv = 300km/s) spectral lines from galaxies in the 200-650µm telluric windows using TES bolometers. These far-IR lines being extinction free and major coolants of the gas heated by (young) massive stars, are powerful probes of the physical conditions of the gas and the stellar radiation field. I present results from our survey of the [O III] 88µm line in galaxies at redshift, z ~ 2.8 to 4.6, with ZEUS-2 at the Atacama Pathfinder Experiment (APEX) Telescope. To interpret our observations along with ancillary data from optical to radio facilities, we apply photoionization models for HII regions and Photo Dissociation Region (PDR) models and confirm that the galaxies host substantial ongoing obscured star formation. The presence of doubly ionized oxygen suggests hard radiation fields and hence, elevated ionization parameters that can only be accounted for by a large population of massive stars formed during the ongoing starburst, that contribute a large fraction of the infrared luminosity. This study highlights the use of FIR line emission to trace the assembly of current day massive galaxies, conditions of star formation and details of their stellar populations. The construction and operation of ZEUS-2 were funded by NSF ATI and AAG grants including AAG 1109476 and has served as a training ground for students interested in astronomical instrumentation.

Could the stellar magnetic field explain the structures in the AU Mic debris disk?

NASA Astrophysics Data System (ADS)

Sezestre, Élie; Augereau, Jean-Charles

2016-05-01

Recent SPHERE and reprocessed HST images of the edge-on AU Mic debris disk have revealed arch-like structures moving away from the star on unbound trajectories. No model in the literature can readily explain these features. Here, we explore the effect of the large-scale, stellar magnetic field on the dust dynamics. We show that our study may place constraints on the dust production location.

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

Redi, M.H.; Mynick, H.E.; Suewattana, M.

Hamiltonian coordinate, guiding-center code calculations of the confinement of suprathermal ions in quasi-axisymmetric stellarator (QAS) designs have been carried out to evaluate the attractiveness of compact configurations which are optimized for ballooning stability. A new stellarator particle-following code is used to predict ion loss rates and particle confinement for thermal and neutral beam ions in a small experiment with R = 145 cm, B = 1-2 T and for alpha particles in a reactor-size device. In contrast to tokamaks, it is found that high edge poloidal flux has limited value in improving ion confinement in QAS, since collisional pitch-angle scatteringmore » drives ions into ripple wells and stochastic field regions, where they are quickly lost. The necessity for reduced stellarator ripple fields is emphasized. The high neutral beam ion loss predicted for these configurations suggests that more interesting physics could be explored with an experiment of less constrained size and magnetic field geometry.« less

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

Batiste, Merida; Bentz, Misty C.; Manne-Nicholas, Emily R.

We present new bulge stellar velocity dispersion measurements for 10 active galaxies with secure M {sub BH} determinations from reverberation mapping. These new velocity dispersion measurements are based on spatially resolved kinematics from integral-field (IFU) spectroscopy. In all but one case, the field of view of the IFU extends beyond the effective radius of the galaxy, and in the case of Mrk 79 it extends to almost one half the effective radius. This combination of spatial resolution and field of view allows for secure determinations of stellar velocity dispersion within the effective radius for all 10 target galaxies. Spatially resolvedmore » maps of the first ( V ) and second ( σ {sub ⋆}) moments of the line of sight velocity distribution indicate the presence of kinematic substructure in most cases. In future projects we plan to explore methods of correcting for the effects of kinematic substructure in the derived bulge stellar velocity dispersion measurements.« less

The evolution of magnetic hot massive stars: Implementation of the quantitative influence of surface magnetic fields in modern models of stellar evolution

NASA Astrophysics Data System (ADS)

Keszthelyi, Zsolt; Wade, Gregg A.; Petit, Veronique

2017-11-01

Large-scale dipolar surface magnetic fields have been detected in a fraction of OB stars, however only few stellar evolution models of massive stars have considered the impact of these fossil fields. We are performing 1D hydrodynamical model calculations taking into account evolutionary consequences of the magnetospheric-wind interactions in a simplified parametric way. Two effects are considered: i) the global mass-loss rates are reduced due to mass-loss quenching, and ii) the surface angular momentum loss is enhanced due to magnetic braking. As a result of the magnetic mass-loss quenching, the mass of magnetic massive stars remains close to their initial masses. Thus magnetic massive stars - even at Galactic metallicity - have the potential to be progenitors of "heavy" stellar mass black holes. Similarly, at Galactic metallicity, the formation of pair instability supernovae is plausible with a magnetic progenitor.

Gas expulsion vs gas retention in young stellar clusters II: effects of cooling and mass segregation

NASA Astrophysics Data System (ADS)

Silich, Sergiy; Tenorio-Tagle, Guillermo

2018-05-01

Gas expulsion or gas retention is a central issue in most of the models for multiple stellar populations and light element anti-correlations in globular clusters. The success of the residual matter expulsion or its retention within young stellar clusters has also a fundamental importance in order to understand how star formation proceeds in present-day and ancient star-forming galaxies and if proto-globular clusters with multiple stellar populations are formed in the present epoch. It is usually suggested that either the residual gas is rapidly ejected from star-forming clouds by stellar winds and supernova explosions, or that the enrichment of the residual gas and the formation of the second stellar generation occur so rapidly, that the negative stellar feedback is not significant. Here we continue our study of the early development of star clusters in the extreme environments and discuss the restrictions that strong radiative cooling and stellar mass segregation provide on the gas expulsion from dense star-forming clouds. A large range of physical initial conditions in star-forming clouds which include the star-forming cloud mass, compactness, gas metallicity, star formation efficiency and effects of massive stars segregation are discussed. It is shown that in sufficiently massive and compact clusters hot shocked winds around individual massive stars may cool before merging with their neighbors. This dramatically reduces the negative stellar feedback, prevents the development of the global star cluster wind and expulsion of the residual and the processed matter into the ambient interstellar medium. The critical lines which separate the gas expulsion and the gas retention regimes are obtained.

The Prospect for Detecting Stellar Coronal Mass Ejections

NASA Astrophysics Data System (ADS)

Osten, Rachel A.; Crosley, Michael Kevin

2018-06-01

The astrophysical study of mass loss, both steady-state and transient, on the cool half of the HR diagram has implications bothfor the star itself and the conditions created around the star that can be hospitable or inimical to supporting life. Recent results from exoplanet studies show that planets around M dwarfs are exceedingly common, which together with the commonality of M dwarfs in our galaxy make this the dominant mode of star and planet configurations. The closeness of the exoplanets to the parent M star motivate a comprehensive understanding of habitability for these systems. Radio observations provide the most clear signature of accelerated particles and shocks in stars arising as the result of MHD processes in the stellar outer atmosphere. Stellar coronal mass ejections have not been conclusively detected, despite the ubiquity with which their radiative counterparts in an eruptive event (stellar flares) have. I will review some of the different observational methods which have been used and possibly could be used in the future in the stellar case, emphasizing some of the difficulties inherent in such attempts. I will provide a framework for interpreting potential transient stellar mass loss in light of the properties of flares known to occur on magnetically active stars. This uses a physically motivated way to connect the properties of flares and coronal mass ejections and provides a testable hypothesis for observing or constraining transient stellar mass loss. I will describe recent results using radio observations to detect stellar coronal mass ejections, and what those results imply about transient stellar mass loss. I will provide some motivation for what could be learned in this topic from space-based low frequency radio experiments.

IBIS: An Interferometer-Based Imaging System for Detecting Extrasolar Planets with a Next Generation Space Telescope

NASA Technical Reports Server (NTRS)

Diner, David J.

1989-01-01

The direct detection of extrasolar planetary systems is a challenging observational objective. The observing system must be able to detect faint planetary signals against the background of diffracted and scattered starlight, zodiacal light, and in the IR, mirror thermal radiation. As part of a JPL study, we concluded that the best long-term approach is a 10-20 m filled-aperture telescope operating in the thermal IR (10-15 microns). At these wavelengths, the star/planet flux ratio is on the order of 10(exp 6)-10(exp 8). Our study supports the work of Angel et al., who proposed a cooled 16-m IR telescope and a special apodization mask to suppress the stellar light within a limited angular region around the star. Our scheme differs in that it is capable of stellar suppression over a much broader field-of- view, enabling more efficient planet searches. To do this, certain key optical signal-processing components are needed, including a coronagraph to apodize the stellar diffraction pattern, an infrared interferometer to provide further starlight suppression, a complementary visible-wavelength interferometer to sense figure errors in the telescope optics, and a deformable mirror to adaptively compensate for these errors. Because of the central role of interferometry we have designated this concept the Interferometer-Based Imaging System (IBIS). IBIS incorporates techniques originally suggested by Ken Knight for extrasolar planet detection at visible wavelengths. The type of telescope discussed at this workshop is well suited to implementation of the IBIS concept.

Packing extra mass in compact stellar structures: an interplay between Kalb-Ramond field and extra dimensions

NASA Astrophysics Data System (ADS)

Chakraborty, Sumanta; SenGupta, Soumitra

2018-05-01

We have derived the Buchdahl's limit for a relativistic star in presence of the Kalb-Ramond field in four as well as in higher dimensions. It turns out that the Buchdahl's limit gets severely affected by the inclusion of the Kalb-Ramond field. In particular, the Kalb-Ramond field in four spacetime dimensions enables one to pack extra mass in any compact stellar structure of a given radius. On the other hand, a completely opposite picture emerges if the Kalb-Ramond field exists in higher dimensions, where the mass content of a compact star is smaller compared to that in general relativity. Implications are discussed.

Spin Evolution of Stellar Progenitors in Compact Binaries

NASA Astrophysics Data System (ADS)

Steinle, Nathan; Kesden, Michael

2018-01-01

Understanding the effects of various processes on the spins of stellar progenitors in compact binary systems is important for modeling the binary’s evolution and thus for interpreting the gravitational radiation emitted during inspiral and merger. Tides, winds, and natal kicks can drastically modify the binary parameters: tidal interactions increase the spin magnitudes, align the spins with the orbital angular momentum, and circularize the orbit; stellar winds decrease the spin magnitudes and cause mass loss; and natal kicks can misalign the spins and orbital angular momentum or even disrupt the binary. Also, during Roche lobe overflow, the binary may experience either stable mass transfer or common envelope evolution. The former can lead to a mass ratio reversal and alter the component spins, while the latter can dramatically shrink the binary separation. For a wide range of physically reasonable stellar-evolution scenarios, we compare the timescales of these processes to assess their relative contributions in determining the initial spins of compact binary systems.

Dynamical Scaling Relations and the Angular Momentum Problem in the FIRE Simulations

NASA Astrophysics Data System (ADS)

Schmitz, Denise; Hopkins, Philip F.; Quataert, Eliot; Keres, Dusan; Faucher-Giguere, Claude-Andre

2015-01-01

Simulations are an extremely important tool with which to study galaxy formation and evolution. However, even state-of-the-art simulations still fail to accurately predict important galaxy properties such as star formation rates and dynamical scaling relations. One possible explanation is the inadequacy of sub-grid models to capture the range of stellar feedback mechanisms which operate below the resolution limit of simulations. FIRE (Feedback in Realistic Environments) is a set of high-resolution cosmological galaxy simulations run using the code GIZMO. It includes more realistic models for various types of feedback including radiation pressure, supernovae, stellar winds, and photoionization and photoelectric heating. Recent FIRE results have demonstrated good agreement with the observed stellar mass-halo mass relation as well as more realistic star formation histories than previous simulations. We investigate the effects of FIRE's improved feedback prescriptions on the simulation "angular momentum problem," i.e., whether FIRE can reproduce observed scaling relations between galaxy stellar mass and rotational/dispersion velocities.

A Period-Activity Relation for Active RS CVN Stars

NASA Astrophysics Data System (ADS)

Simon, Theodore

Soft X ray observations of RS CVn binaries point to a correlation between L x /Lbol (the X ray to bolometric luminosity ratio that measures the coronal heating rate) and Omega (the stellar angular velocity). This correlation is almost certainly caused by a stellar dynamo, operating in rapidly-rotating late-type stars with deep convection zones. We are proposing to extend the X ray "rotation-activity relation" to the uv transition region and chromospheric emission lines observable with IUE. If the non-radiative heating rates of stellar transition regions and chromospheres are determined largely by magnetic processes associated with a stellar dynamo, then a similar correlation may be found. We have selected a group of recently discovered active long-period systems, which we believe will be very bright at uv wavelengths. One important goal of this program is to determine whether past studies of the "rotation-activity connection" have been compromised by the omission of active long-period RS CVn systems.

Dust formation and wind acceleration around the aluminum oxide-rich AGB star W Hydrae

NASA Astrophysics Data System (ADS)

Takigawa, Aki; Kamizuka, Takafumi; Tachibana, Shogo; Yamamura, Issei

2017-11-01

Dust grains, formed around asymptotic giant branch (AGB) stars, are accelerated by stellar radiation to drive stellar winds, which supply freshly synthesized nuclides to the Galaxy. Silicate is the dominant dust species in space, but 40% of oxygen-rich AGB stars are thought to have comparable amounts of aluminum oxide dust. Dust formation and the wind-driving mechanism around these oxygen-rich stars, however, are poorly understood. We report on the spatial distributions of AlO and 29SiO molecules around an aluminum oxide-rich M-type AGB star, W Hydrae, based on observations obtained with the Atacama Large Millimeter/submillimeter Array. AlO molecules were only observed within three stellar radii (Rstar), whereas 29SiO was distributed in the accelerated wind beyond 5 Rstar without significant depletion. This strongly suggests that condensed aluminum oxide dust plays a key role in accelerating the stellar wind and in preventing the efficient formation of silicate dust around W Hydrae.

Dust formation and wind acceleration around the aluminum oxide–rich AGB star W Hydrae

PubMed Central

Takigawa, Aki; Kamizuka, Takafumi; Tachibana, Shogo; Yamamura, Issei

2017-01-01

Dust grains, formed around asymptotic giant branch (AGB) stars, are accelerated by stellar radiation to drive stellar winds, which supply freshly synthesized nuclides to the Galaxy. Silicate is the dominant dust species in space, but ~40% of oxygen-rich AGB stars are thought to have comparable amounts of aluminum oxide dust. Dust formation and the wind-driving mechanism around these oxygen-rich stars, however, are poorly understood. We report on the spatial distributions of AlO and 29SiO molecules around an aluminum oxide–rich M-type AGB star, W Hydrae, based on observations obtained with the Atacama Large Millimeter/submillimeter Array. AlO molecules were only observed within three stellar radii (Rstar), whereas 29SiO was distributed in the accelerated wind beyond 5 Rstar without significant depletion. This strongly suggests that condensed aluminum oxide dust plays a key role in accelerating the stellar wind and in preventing the efficient formation of silicate dust around W Hydrae. PMID:29109978

Variable magnification glancing incidence x ray telescope

NASA Technical Reports Server (NTRS)

Hoover, Richard (Inventor)

1990-01-01

A multispectral glancing incidence x ray telescope is disclosed, which capable of broadband, high resolution imaging of solar and stellar x ray and extreme ultraviolet radiation sources includes a primary optical system which focuses the incoming radiation to a primary focus. Two or more ellipsoidal mirrors are positioned behind the primary focus at an inclination to the optical axis, each mirror having a concave surface coated with a multilayer synthetic microstructure coating to reflect a desired wavelength. The ellipsoidal mirrors are segments of respective ellipsoids having a common first focus coincident with the primary focus. A detector such as an x ray sensitive photographic film is positioned at the second focus of each of the ellipsoids so that each of the ellipsoidal mirrors may reflect the image at the first focus to the detector. In one embodiment the mirrors are inclined at different angles and has its respective second focus at a different location, separate detectors being located at the respective second focus. The mirrors are arranged so that the magnification and field of view differ, and a solenoid activated arm may withdraw at least one mirror from the beam to select the mirror upon which the beam is to impinge so that selected magnifications and fields of view may be detected.

The numerical frontier of the high-redshift Universe

NASA Astrophysics Data System (ADS)

Greif, Thomas H.

2015-03-01

The first stars are believed to have formed a few hundred million years after the big bang in so-called dark matter minihalos with masses . Their radiation lit up the Universe for the first time, and the supernova explosions that ended their brief lives enriched the intergalactic medium with the first heavy elements. Influenced by their feedback, the first galaxies assembled in halos with masses , and hosted the first metal-enriched stellar populations. In this review, I summarize the theoretical progress made in the field of high-redshift star and galaxy formation since the turn of the millennium, with an emphasis on numerical simulations. These have become the method of choice to understand the multi-scale, multi-physics problem posed by structure formation in the early Universe. In the first part of the review, I focus on the formation of the first stars in minihalos - in particular the post-collapse phase, where disk fragmentation, protostellar evolution, and radiative feedback become important. I also discuss the influence of additional physical processes, such as magnetic fields and streaming velocities. In the second part of the review, I summarize the various feedback mechanisms exerted by the first stars, followed by a discussion of the first galaxies and the various physical processes that operate in them.

STELLAR DYNAMO MODELS WITH PROMINENT SURFACE TOROIDAL FIELDS

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

Bonanno, Alfio

2016-12-20

Recent spectro-polarimetric observations of solar-type stars have shown the presence of photospheric magnetic fields with a predominant toroidal component. If the external field is assumed to be current-free it is impossible to explain these observations within the framework of standard mean-field dynamo theory. In this work, it will be shown that if the coronal field of these stars is assumed to be harmonic, the underlying stellar dynamo mechanism can support photospheric magnetic fields with a prominent toroidal component even in the presence of axisymmetric magnetic topologies. In particular, it is argued that the observed increase in the toroidal energy inmore » low-mass fast-rotating stars can be naturally explained with an underlying α Ω mechanism.« less

THE SPATIAL DISTRIBUTION OF COMPLEX ORGANIC MOLECULES IN THE L1544 PRE-STELLAR CORE

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

Jiménez-Serra, Izaskun; Vasyunin, Anton I.; Caselli, Paola

The detection of complex organic molecules (COMs) toward cold sources such as pre-stellar cores (with T < 10 K) has challenged our understanding of the formation processes of COMs in the interstellar medium. Recent modeling on COM chemistry at low temperatures has provided new insight into these processes predicting that COM formation depends strongly on parameters such as visual extinction and the level of CO freeze out. We report deep observations of COMs toward two positions in the L1544 pre-stellar core: the dense, highly extinguished continuum peak with A{sub V}≥ 30 mag within the inner 2700 au; and a low-densitymore » shell with average A{sub V}∼ 7.5–8 mag located at 4000 au from the core’s center and bright in CH{sub 3}OH. Our observations show that CH{sub 3}O, CH{sub 3}OCH{sub 3}, and CH{sub 3}CHO are more abundant (by factors of ∼2–10) toward the low-density shell than toward the continuum peak. Other COMs such as CH{sub 3}OCHO, c-C{sub 3}H{sub 2}O, HCCCHO, CH{sub 2}CHCN, and HCCNC show slight enhancements (by factors ≤3), but the associated uncertainties are large. This suggests that COMs are actively formed and already present in the low-density shells of pre-stellar cores. The modeling of the chemistry of O-bearing COMs in L1544 indicates that these species are enhanced in this shell because (i) CO starts freezing out onto dust grains driving an active surface chemistry; (ii) the visual extinction is sufficiently high to prevent the UV photo-dissociation of COMs by the external interstellar radiation field; and (iii) the density is still moderate to prevent severe depletion of COMs onto grains.« less

REVIEW ARTICLE: Major results from the stellarator Wendelstein 7-AS

NASA Astrophysics Data System (ADS)

Hirsch, M.; Baldzuhn, J.; Beidler, C.; Brakel, R.; Burhenn, R.; Dinklage, A.; Ehmler, H.; Endler, M.; Erckmann, V.; Feng, Y.; Geiger, J.; Giannone, L.; Grieger, G.; Grigull, P.; Hartfuß, H.-J.; Hartmann, D.; Jaenicke, R.; König, R.; Laqua, H. P.; Maaßberg, H.; McCormick, K.; Sardei, F.; Speth, E.; Stroth, U.; Wagner, F.; Weller, A.; Werner, A.; Wobig, H.; Zoletnik, S.; W7-AS Team

2008-05-01

Wendelstein 7-AS was the first modular stellarator device to test some basic elements of stellarator optimization: a reduced Shafranov shift and improved stability properties resulted in β-values up to 3.4% (at 0.9 T). This operational limit was determined by power balance and impurity radiation without noticeable degradation of stability or a violent collapse. The partial reduction of neoclassical transport could be verified in agreement with calculations indicating the feasibility of the concept of drift optimization. A full neoclassical optimization, in particular a minimization of the bootstrap current was beyond the scope of this project. A variety of non-ohmic heating and current drive scenarios by ICRH, NBI and in particular, ECRH were tested and compared successfully with their theoretical predictions. Besides, new heating schemes of overdense plasmas were developed such as RF mode conversion heating—Ordinary mode, Extraordinary mode, Bernstein-wave (OXB) heating—or 2nd harmonic O-mode (O2) heating. The energy confinement was about a factor of 2 above ISS95 without degradation near operational boundaries. A number of improved confinement regimes such as core electron-root confinement with central Te <= 7 keV and regimes with strongly sheared radial electric field at the plasma edge resulting in Ti <= 1.7 keV were obtained. As the first non-tokamak device, W7-AS achieved the H-mode and moreover developed a high density H-mode regime (HDH) with strongly reduced impurity confinement that allowed quasi-steady-state operation (τ ≈ 65 · τE) at densities \\bar {n}_{\\rme} \\cong 4 \\times 10^{20}\\,\\mbox{m}^{-3} (at 2.5 T). The first island divertor was tested successfully and operated with stable partial detachment in agreement with numerical simulations. With these results W7-AS laid the physics background for operation of an optimized low-shear steady-state stellarator.

Abundance measurements in stellar environments

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

Leone, F.

Most of what we know about stars, and systems of stars, is derived from the analysis of their electromagnetic radiation. This lesson is an attempt to describe to Physicists, without any Astrophysical background, the framework to understand the present status of abundance determination in stellar environments and its limit. These notes are dedicated to the recently passed, November 21, 2013, Prof. Dimitri Mihalas who spent his life confuting the 19th century positivist philosopher Auguste Comte who stated that we shall not at all be able to determine the chemical composition of stars.

A Possible Mechanism for Driving Oscillations in Hot Giant Planets

NASA Astrophysics Data System (ADS)

Dederick, Ethan; Jackiewicz, Jason

2017-03-01

The κ-mechanism has been successful in explaining the origin of observed oscillations of many types of “classical” pulsating variable stars. Here we examine quantitatively if that same process is prominent enough to excite the potential global oscillations within Jupiter, whose energy flux is powered by gravitational collapse rather than nuclear fusion. Additionally, we examine whether external radiative forcing, I.e., starlight, could be a driver for global oscillations in hot Jupiters orbiting various main-sequence stars at defined orbital semimajor axes. Using planetary models generated by the Modules for Experiments in Stellar Astrophysics and nonadiabatic oscillation calculations, we confirm that Jovian oscillations cannot be driven via the κ-mechanism. However, we do show that, in hot Jupiters, oscillations can likely be excited via the suppression of radiative cooling due to external radiation given a large enough stellar flux and the absence of a significant oscillatory damping zone within the planet. This trend does not seem to be dependent on the planetary mass. In future observations, we can thus expect that such planets may be pulsating, thereby giving greater insight into the internal structure of these bodies.

ECRH and its effects on neoclassical transport in a stellarator

NASA Astrophysics Data System (ADS)

Seol, Jaechun

The banana center orbit deviates significantly from the magnetic surface due to the symmetry-breaking term in the magnetic field configuration. Energetic electrons can escape the plasma without collision, since the drift speed is proportional to the perpendicular energy of electron and the collision frequency is reduced as the electron energy goes up. A direct loss flux can be generated from energetic electron population in a stellarator. Thus energetic electron populations can substantially modify the neoclassical transport properties in stellarators. A model accounting for this change in transport is developed assuming the presence of electron cyclotron resonance heating (ECRH). The quasilinear diffusion coefficient for second harmonic X-mode ECRH is developed for a bumpy stellarator. Care is taken in accounting for the pitch-angle dependence of the quasilinear diffusion coefficient since application to experiments with narrow resonance zones is of interest. Weakly relativistic effects are considered through the mass effect on the cyclotron frequency. For trapped particles in a three dimensional configuration, collisionless loss zones exist in velocity space. Radio-frequency (rf) waves accelerate trapped electrons into the direct loss zone in bumpy stellarators and produce a direct loss flux. An analytic expression for this loss flux is derived; it is proportional to the rf field strength and the value of the zeroth order distribution function at the minimum speed for collisionless loss. The direct loss flux of electrons is another source of a non-ambipolar particle flux in bumpy stellarators. This additional non-ambipolar flux modifies the ambipolarity equation which generally has multiple roots for the radial electric field. An electron root (large positive Er) is easily obtained if the electrons are in the 1/nu regime and the ions are in the nu regime.

Using Close White Dwarf + M Dwarf Stellar Pairs to Constrain the Flare Rates in Close Stellar Binaries

NASA Astrophysics Data System (ADS)

Morgan, Dylan P.; West, Andrew A.; Becker, Andrew C.

2016-05-01

We present a study of the statistical flare rates of M dwarfs (dMs) with close white dwarf (WD) companions (WD+dM; typical separations <1 au). Our previous analysis demonstrated that dMs with close WD companions are more magnetically active than their field counterparts. One likely implication of having a close binary companion is increased stellar rotation through disk-disruption, tidal effects, and/or angular momentum exchange; increased stellar rotation has long been associated with an increase in stellar activity. Previous studies show a strong correlation between dMs that are magnetically active (showing Hα in emission) and the frequency of stellar flare rates. We examine the difference between the flare rates observed in close WD+dM binary systems and field dMs. Our sample consists of a subset of 181 close WD+dM pairs from Morgan et al. observed in the Sloan Digital Sky Survey Stripe 82, where we obtain multi-epoch observations in the Sloan ugriz-bands. We find an increase in the overall flaring fraction in the close WD+dM pairs (0.09 ± 0.03%) compared to the field dMs (0.0108 ± 0.0007%) and a lower flaring fraction for active WD+dMs (0.05 ± 0.03%) compared to active dMs (0.28 ± 0.05%). We discuss how our results constrain both the single and binary dM flare rates. Our results also constrain dM multiplicity, our knowledge of the Galactic transient background, and may be important for the habitability of attending planets around dMs with close companions.

Group quenching and galactic conformity at low redshift

NASA Astrophysics Data System (ADS)

Treyer, M.; Kraljic, K.; Arnouts, S.; de la Torre, S.; Pichon, C.; Dubois, Y.; Vibert, D.; Milliard, B.; Laigle, C.; Seibert, M.; Brown, M. J. I.; Grootes, M. W.; Wright, A. H.; Liske, J.; Lara-Lopez, M. A.; Bland-Hawthorn, J.

2018-06-01

We quantify the quenching impact of the group environment using the spectroscopic survey Galaxy and Mass Assembly to z ˜ 0.2. The fraction of red (quiescent) galaxies, whether in groups or isolated, increases with both stellar mass and large-scale (5 Mpc) density. At fixed stellar mass, the red fraction is on average higher for satellites of red centrals than of blue (star-forming) centrals, a galactic conformity effect that increases with density. Most of the signal originates from groups that have the highest stellar mass, reside in the densest environments, and have massive, red only centrals. Assuming a colour-dependent halo-to-stellar-mass ratio, whereby red central galaxies inhabit significantly more massive haloes than blue ones of the same stellar mass, two regimes emerge more distinctly: at log (Mhalo/M⊙) ≲ 13, central quenching is still ongoing, conformity is no longer existent, and satellites and group centrals exhibit the same quenching excess over field galaxies at all mass and density, in agreement with the concept of `group quenching'; at log (Mh/M⊙) ≳ 13, a cut-off that sets apart massive (log (M⋆/M⊙) > 11), fully quenched group centrals, conformity is meaningless, and satellites undergo significantly more quenching than their counterparts in smaller haloes. The latter effect strongly increases with density, giving rise to the density-dependent conformity signal when both regimes are mixed. The star formation of blue satellites in massive haloes is also suppressed compared to blue field galaxies, while blue group centrals and the majority of blue satellites, which reside in low-mass haloes, show no deviation from the colour-stellar mass relation of blue field galaxies.

The Effect of Combined Magnetic Geometries on Thermally Driven Winds. I. Interaction of Dipolar and Quadrupolar Fields

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

Finley, Adam J.; Matt, Sean P., E-mail: af472@exeter.ac.uk

Cool stars with outer convective envelopes are observed to have magnetic fields with a variety of geometries, which on large scales are dominated by a combination of the lowest-order fields such as the dipole, quadrupole, and octupole modes. Magnetized stellar wind outflows are primarily responsible for the loss of angular momentum from these objects during the main sequence. Previous works have shown the reduced effectiveness of the stellar wind braking mechanism with increasingly complex but singular magnetic field geometries. In this paper, we quantify the impact of mixed dipolar and quadrupolar fields on the spin-down torque using 50 MHD simulationsmore » with mixed fields, along with 10 each of the pure geometries. The simulated winds include a wide range of magnetic field strength and reside in the slow-rotator regime. We find that the stellar wind braking torque from our combined geometry cases is well described by a broken power-law behavior, where the torque scaling with field strength can be predicted by the dipole component alone or the quadrupolar scaling utilizing the total field strength. The simulation results can be scaled and apply to all main-sequence cool stars. For solar parameters, the lowest-order component of the field (dipole in this paper) is the most significant in determining the angular momentum loss.« less

The polarimeters for HARPS and X-shooter

NASA Astrophysics Data System (ADS)

Snik, F.; Harpspol Team; X-Shooter-Pol Team

2013-01-01

Spectropolarimetry enables observations of stellar magnetic fields and circumstellar asymmetries, e.g. in disks and supernova explosions. To furnish better diagnostics of such stellar physics, we designed and commissioned a polarimetric unit at the successful HARPS spectrograph at ESO's 3.6-m telescope at La Silla. We present the design and performance of HARPSpol, and show some first science results. The most striking achievement of HARPSpol is its capability to measure stellar magnetic fields as small as 0.1 G. Finally, we give a sneak preview of the polarimeter we are currently designing for X-shooter at the VLT. It contains a novel type of polarimetric modulator that is able to efficiently measure all the Stokes parameters over the huge wavelength range of 300-2500 nm.

Formation Flying and the Stellar Imager Mission Concept

NASA Technical Reports Server (NTRS)

Carpenter, Kenneth G.

2003-01-01

The Stellar Imager (SI) is envisioned as a space-based, W-optical interferometer composed of 10 or more one-meter class elements distributed with a maximum baseline of 0.5 km. image stars and binaries with sufficient resolution to enable long-term studies of stellar magnetic activity patterns, for comparison with those on the sun. It will also support asteroseismology (acoustic imaging) to probe stellar internal structure, differential rotation, and large-scale circulations. SI will enable us to understand the various effects of the magnetic fields of stars, the dynamos that generate these fields, and the internal structure and dynamics of the stars. The ultimate goal of the mission is to achieve the best-possible forecasting of solar activity as a driver of climate and space weather on time scales ranging from months up to decades, and an understanding of the impact of stellar magnetic activity on life in the Universe. In this paper we briefly describe the scientific goals of the mission, the performance requirements needed to address these goals, and the "enabling technology" development efforts required, with specific attention for this meeting to the formation-flying aspects. It is designed to

The Stellar Imager (SI) Mission Concept

NASA Technical Reports Server (NTRS)

Carpenter, Kenneth G.; Schrijver, Carolus J.; Lyon, Richard G.; Mundy, Lee G.; Allen, Ronald J.; Armstrong, Thomas; Danchi, William C.; Karovska, Margarita; Marzouk, Joe; Mazzuca, Lisa M.;

Numerical Radiative Transfer and the Hydrogen Reionization of the Universe

NASA Astrophysics Data System (ADS)

Petkova, M.

2011-03-01

One of the most interesting questions in cosmology is to understand how the Universe evolved from its nearly uniform and simple state briefly after the Big Bang to the complex state we see around us today. In particular, we would like to explain how galaxies have formed, and why they have the properties that we observe in the local Universe. Computer simulations play a highly important role in studying these questions, because they allow one to follow the dynamical equations of gravity and hydrodynamics well into the non-linear regime of the growth of cosmic structures. The current generation of simulation codes for cosmological structure formation calculates the self-gravity of dark matter and cosmic gas, and the fluid dynamics of the cosmic gas, but radiation processes are typically not taken into account, or only at the level of a spatially uniform, externally imposed background field. However, we know that the radiation field has been highly inhomogeneous during certain phases of the growth of structure, and may have in fact provided important feedback effects for galaxy formation. In particular, it is well established that the diffuse gas in the universe was nearly fully neutral after recombination at very high redshift, but today this gas is highly ionized. Sometime during the evolution, a transition to the ionized state must have occurred, a process we refer to as reionization. The UV radiation responsible for this reionization is now permeating the universe and may in part explain why small dwarf galaxies have so low luminosities. It is therefore clear that accurate and self-consistent studies of galaxy formation and of the dynamics of the reionization process should ideally be done with simulation codes that directly include a treatment of radiative transfer, and that account for all relevant source and sink terms of the radiation. We present a novel numerical implementation of radiative transfer in the cosmological smoothed particle hydrodynamics (SPH) simulation code GADGET. It is based on a fast, robust and photon-conserving integration scheme where the radiation transport problem is approximated in terms of moments of the transfer equation and by using a variable Eddington tensor as a closure relation, following the "OTVET"-suggestion of Gnedin & Abel. We derive a suitable anisotropic diffusion operator for use in the SPH discretization of the local photon transport, and we combine this with an implicit solver that guarantees robustness and photon conservation. This entails a matrix inversion problem of a huge, sparsely populated matrix that is distributed in memory in our parallel code. We solve this task iteratively with a conjugate gradient scheme. Finally, to model photon sink processes we consider ionization and recombination processes of hydrogen, which is represented with a chemical network that is evolved with an implicit time integration scheme. We present several tests of our implementation, including single and multiple sources in static uniform density fields with and without temperature evolution, shadowing by a dense clump, and multiple sources in a static cosmological density field. All tests agree quite well with analytical computations or with predictions from other radiative transfer codes, except for shadowing. However, unlike most other radiative transfer codes presently in use for studying reionization, our new method can be used on-the-fly during dynamical cosmological simulations, allowing simultaneous treatments of galaxy formation and the reionization process of the Universe. We carry out hydrodynamical simulations of galaxy formation that simultaneously follow radiative transfer of hydrogen-ionizing photons, based on the optically-thin variable Eddington tensor approximation as implemented in the GADGET code. We consider only star-forming galaxies as sources and examine to what extent they can yield a reasonable reionization history and thermal state of the intergalactic medium at redshifts around z~3. This serves as an important benchmark for our self-consistent methodology to simulate galaxy formation and reionization, and for future improvements through accounting of other sources and other wavelength ranges. We find that star formation alone is sufficient for rinsing the Universe by redshift z~6. For a suitable choice of the escape fraction and the heating efficiency, our models are approximately able to account at the same time for the one-point function and the power spectrum of the Lyman-Forest. The radiation field has an important impact on the star formation rate density in our simulations and significantly lowers the gaseous and stellar fractions in low-mass dark matter halos. Our results thus directly demonstrate the importance of radiative feedback for galaxy formation. In search for even better and more accurate methods we introduce a numerical implementation of radiative transfer based on an explicitly photon conserving advection scheme, where radiative fluxes over the cell interfaces of a structured or unstructured mesh are calculated with a second-order reconstruction of the intensity field. The approach employs a direct discretization of the radiative transfer equation in Boltzmann form with adjustable angular resolution that in principle works equally well in the optically thin and optically thick regimes. In our most general formulation of the scheme, the local radiation field is decomposed into a linear sum of directional bins of equal solid-angle, tessellating the unit sphere. Each of these "cone-fields" is transported independently, with constant intensity as a function of direction within the cone. Photons propagate at the speed of light (or optionally using a reduced speed of light approximation to allow larger timesteps), yielding a fully time-dependent solution of the radiative transfer equation that can naturally cope with an arbitrary number of sources, as well as with scattering. The method casts sharp shadows, subject to the limitations induced by the adopted angular resolution. If the number of point sources is small and scattering is unimportant, our implementation can alternatively treat each source exactly in angular space, producing shadows whose sharpness is only limited by the grid resolution. A third hybrid alternative is to treat only a small number of the locally most luminous point sources explicitly, with the rest of the radiation intensity followed in a radiative diffusion approximation. We have implemented the method in the moving-mesh code AREPO, where it is coupled to the hydrodynamics in an operator splitting approach that subcycles the radiative transfer alternatingly with the hydrodynamical evolution steps. We also discuss our treatment of basic photon sink processes relevant for cosmological reionization, with a chemical network that can accurately deal with non-equilibrium effects. We discuss several tests of the new method, including shadowing configurations in two and three dimensions, ionized sphere expansion in static and dynamic density field and the ionization of a cosmological density field. The tests agree favorably with analytic expectations and results based on other numerical radiative transfer approximations. We compare how our schemes perform in a simulation of hydrogen reionization, excluding stellar winds due to development issues. The underlying cosmological simulation codes produce different star formation rate histories, which results in a different total photon budget. As a consequence reionization in GADGET happens at a higher redshift, i.e. sooner, than in AREPO. The lower number of ionizing photons in the latter code results in a higher volume-averaged neutral fraction at redshift z = 3 and a different temperature state of the baryonic gas. We find that in both reionization scenarios the baryon fraction of low mass dark matter halos is reduced due to photoheating processes and observe that the change is bigger in the GADGET simulation than in the AREPO one, which is due to the higher ionized fractions we find the in former. Both simulations compare marginally well with the Lyman-poorest observations at redshift z = 3, but results are not expected to be in very good agr! eement due the lack of the essential feedback from stellar winds in the simulations. Finally, we can conclude that despite the differences between the two realizations, both codes perform well at the given problem and are suitable for studying the process of reionization because they produce sensible results in the limits of observations. We emphasize that the reionization history depends strongly on the star formation rate density in the simulations and which should therefore be accurately reproduced.

A radiation transfer model for the Milky Way: I. Radiation fields and application to high-energy astrophysics★

NASA Astrophysics Data System (ADS)

Popescu, C. C.; Yang, R.; Tuffs, R. J.; Natale, G.; Rushton, M.; Aharonian, F.

2017-09-01

We present a solution for the ultraviolet - submillimetre (submm) interstellar radiation fields (ISRFs) of the Milky Way (MW), derived from modelling COBE, IRAS and Planck maps of the all-sky emission in the near-, mid-, far-infrared and submm. The analysis uses the axisymmetric radiative transfer model that we have previously implemented to model the panchromatic spectral energy distributions (SEDs) of star-forming galaxies in the nearby universe, but with a new methodology allowing for optimization of the radial and vertical geometry of stellar emissivity and dust opacity, as deduced from the highly resolved emission seen from the vantage point of the Sun. As such, this is the first self-consistent model of the broad-band continuum emission from the MW. In this paper, we present model predictions for the spatially integrated SED of the MW as seen from the Sun, showing good agreement with the data, and give a detailed description of the solutions for the distribution of ISRFs, as well as their physical origin, throughout the volume of the galaxy. We explore how the spatial and spectral distributions of our new predictions for the ISRF in the MW affects the amplitude and spectral distributions of the gamma rays produced via inverse Compton scattering for cosmic ray (CR) electrons situated at different positions in the galaxy, as well as the attenuation of the gamma rays due to interactions of the gamma-ray photons with photons of the ISRF. We also compare and contrast our solutions for the ISRF with those incorporated in the galprop package used for modelling the high-energy emission from CR in the MW.

Lyman alpha radiation in external galaxies

NASA Technical Reports Server (NTRS)

Neufeld, David A.; Mckee, Christopher F.

1990-01-01

The Ly alpha line of atomic hydrogen is often a luminous component of the radiation emitted by distant galaxies. Except for those galaxies which have a substantial central source of non-stellar ionizing radiation, most of the Ly alpha radiation emitted by galaxies is generated within regions of the interstellar medium which are photoionized by starlight. Conversely, much of the energy radiated by photoionized regions is carried by the Ly alpha line. Only hot, massive stars are capable of ionizing hydrogen in the interstellar medium which surrounds them, and because such stars are necessarily short-lived, Ly alpha emission traces regions of active star formation. Researchers argue that the strength of the Ly alpha emission observed from external galaxies may be used to estimate quantitatively the dust content of the emitting region, while the Ly alpha line profile is sensitive to the presence of shock waves. Interstellar dust particles and shock waves are intimately associated with the process of star formation in two senses. First, both dust particles and shock waves owe their existence to stellar activity; second, they may both serve as agents which facilitate the formation of stars, shocks by triggering gravitational instabilities in the interstellar gas that they compress, and dust by shielding star-forming molecular clouds from the ionizing and dissociative effects of external UV radiation. By using Ly alpha observations as a probe of the dust content in diffuse gas at high redshift, we might hope to learn about the earliest epochs of star formation.

A Unified Computational Model for Solar and Stellar Flares

NASA Technical Reports Server (NTRS)

Allred, Joel C.; Kowalski, Adam F.; Carlsson, Mats

2015-01-01

We present a unified computational framework that can be used to describe impulsive flares on the Sun and on dMe stars. The models assume that the flare impulsive phase is caused by a beam of charged particles that is accelerated in the corona and propagates downward depositing energy and momentum along the way. This rapidly heats the lower stellar atmosphere causing it to explosively expand and dramatically brighten. Our models consist of flux tubes that extend from the sub-photosphere into the corona. We simulate how flare-accelerated charged particles propagate down one-dimensional flux tubes and heat the stellar atmosphere using the Fokker-Planck kinetic theory. Detailed radiative transfer is included so that model predictions can be directly compared with observations. The flux of flare-accelerated particles drives return currents which additionally heat the stellar atmosphere. These effects are also included in our models. We examine the impact of the flare-accelerated particle beams on model solar and dMe stellar atmospheres and perform parameter studies varying the injected particle energy spectra. We find the atmospheric response is strongly dependent on the accelerated particle cutoff energy and spectral index.

Thermonuclear inverse magnetic pumping power cycle for stellarator reactor

DOEpatents

Ho, Darwin D.; Kulsrud, Russell M.

1991-01-01

The plasma column in a stellarator is compressed and expanded alternatively in minor radius. First a plasma in thermal balance is compressed adiabatically. The volume of the compressed plasma is maintained until the plasma reaches a new thermal equilibrium. The plasma is then expanded to its original volume. As a result of the way a stellarator works, the plasma pressure during compression is less than the corresponding pressure during expansion. Therefore, negative work is done on the plasma over a complete cycle. This work manifests itself as a back-voltage in the toroidal field coils. Direct electrical energy is obtained from this voltage. Alternatively, after the compression step, the plasma can be expanded at constant pressure. The cycle can be made self-sustaining by operating a system of two stellarator reactors in tandem. Part of the energy derived from the expansion phase of a first stellarator reactor is used to compress the plasma in a second stellarator reactor.

Active Galactic Nuclei Feedback and the Origin and Fate of the Hot Gas in Early-type Galaxies

NASA Astrophysics Data System (ADS)

Pellegrini, Silvia; Ciotti, Luca; Negri, Andrea; Ostriker, Jeremiah P.

2018-04-01

A recent determination of the relationships between the X-ray luminosity of the ISM (L X) and the stellar and total mass for a sample of nearby early-type galaxies (ETGs) is used to investigate the origin of the hot gas, via a comparison with the results of hydrodynamical simulations of the ISM evolution for a large set of isolated ETGs. After the epoch of major galaxy formation (after z ≃ 2), the ISM is replenished by stellar mass losses and SN ejecta, at the rate predicted by stellar evolution, and is depleted by star formation; it is heated by the thermalization of stellar motions, SNe explosions, and the mechanical (from winds) and radiative AGN feedback. The models agree well with the observed relations, even for the largely different L X values at the same mass, thanks to the sensitivity of the gas flow to many galaxy properties; this holds for models including AGN feedback, and those without. Therefore, the mass input from the stellar population is able to account for a major part of the observed L X; and AGN feedback, while very important to maintain massive ETGs in a time-averaged quasi-steady state, keeping low star formation and the black hole mass, does not dramatically alter the gas content originating in stellar recycled material. These conclusions are based on theoretical predictions for the stellar population contributions in mass and energy, and on a self-consistent modeling of AGN feedback.

THERMOHALINE INSTABILITIES INSIDE STARS: A SYNTHETIC STUDY INCLUDING EXTERNAL TURBULENCE AND RADIATIVE LEVITATION

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

Vauclair, Sylvie; Theado, Sylvie, E-mail: sylvie.vauclair@irap.omp.eu

2012-07-01

We have derived a new expression for the thermohaline mixing coefficient in stars, including the effects of radiative levitation and external turbulence, by solving Boussinesq equations in a nearly incompressible stratified fluid with a linear approximation. It is well known that radiative levitation of individual elements can lead to their accumulation in specific stellar layers. In some cases, it can induce important effects on the stellar structure. Here we confirm that this accumulation is moderated by thermohaline convection due to the resulting inverse {mu}-gradient. The new coefficient that we have derived shows that the effect of radiative accelerations on themore » thermohaline instability itself is small. This effect must however be checked in all computations. We also confirm that the presence of large horizontal turbulence can reduce or even suppress the thermohaline convection. These results are important as they concern all the cases of heavy element accumulation in stars. Computations of radiative diffusion must be revisited to include thermohaline convection and its consequences. It may be one of the basic reasons for the fact that the observed abundances are always smaller than those predicted by pure atomic diffusion. In any case, these processes have to compete with rotation-induced mixing, but this competition is more complex than previously thought due to their mutual interaction.« less

Dynamical Model Calculations of AGB Star Winds Including Time Dependent Dust Formation and Non-LTE Radiative Cooling

NASA Astrophysics Data System (ADS)

Schirrmacher, V.; Woitke, P.; Sedlmayr, E.

Stars on the Asymptotic Giant Branch (AGB) are pulsating objects in a late evolutionary stage. The stellar pulsation creates sound waves which steepen up to shock waves in the upper atmosphere and lead to a time dependent levitation of the outer atmosphere. Thereby, the stellar pulsation triggers and facilitates the formation of dust close to the star. The dust is accelerated by radiation pressure and drags the gas outwards due to frictional forces which is identified to provide the basic mass loss mechanism. A longstanding problem concerning the modelling of these physical processes is the influence of the propagating shock waves on the temperature structure of the wind, which strongly influences the dust formation. We have therefore improved our numerical models of AGB-star envelopes by including (i) a detailed calculation of non-LTE radiative heating and cooling rates, predominantly arising from atomic and molecular lines and (ii) atomic and molecular exitation aswell as ionisation and dissociation in the equation of state. First results, presented here, show that the cooling time scales behind the shock waves are usually rather short, but the binding energies of molecular hydrogen provide an important energy buffer capable to delay the radiative heating or cooling. Thus considerable deviations from radiative equilibrium may occur in the important inner dust forming layers.

Probing The Stellar, Gaseous, And Dust Properties Of Galaxies Through Analysis Of Their Spectral Energy Distributions

NASA Astrophysics Data System (ADS)

Eufrasio, Rafael T.

The spectral energy distributions (SEDs) of galaxies are shaped by their physical properties and they are our primary source of information on galaxies stellar, gaseous, and dust content. Nearby galaxies (less than 100 Mpc away) are spatially resolved by current telescopes from the ultraviolet (UV) to radio wavelengths, allowing the study of the SEDs of subgalactic regions. Such studies are necessary for deriving maps and spatial trends of the physical properties across a galaxy. In principle, the complex history of the formation, growth, and evolution of a galaxy or a region of a galaxy can be inferred from its radiative output. In practice, this task is complicated by the fact that a significant fraction of the star formation activity takes place in dust obscured regions, in which a significant fraction of the stellar radiative output is absorbed, scattered, and reradiated by the gas and dust in the interstellar medium (ISM). This reprocessing of the stellar radiation takes place in ionized interstellar gas regions (H II regions) surrounding massive hot stars, in diffuse atomic gas (H I regions), and in dense molecular clouds. For this work, we have analyzed two galaxies in detail, NGC 6872 and NGC 6946, also known as Condor and Fireworks Galaxy, respectively. The Condor galaxy is the largest-known spiral galaxy. It is part a group of galaxies, the Pavo group, with 12 other galaxies. It has, however, interacted in the past ~150 Myr with a smaller companion, previously believed to have shaped the physical extent of the giant spiral. We have performed detailed SED fitting from the UV to mid-infrared (mid-IR) to obtain star formation histories of seventeen sub-galactic regions across the Condor. These regions are large enough to be galaxies themselves, with 32.3 million light-years in diameter. We find that the Condor was already very massive before this interaction and that it was much less affected by the passage of the companion than previously thought. We also found that a significant fraction of the 22 micron flux, usually considered a complementary measure of the UV-optically determined star formation rate (SFR), is not associated with the recent (last 100 Myr) star formation activity. A fraction of the 22 micron flux represents the energy reradiated by dust heated by intermediate age, long-lived stars. For the Fireworks galaxy, data coverage from the UV to radio allowed us to measure the full radiative budget from the stellar emission (bolometric luminosities) and the fraction coming from reprocessing by dust and gas in the IR. We present a self-consistent, physically-motivated model to describe SEDs of subgalactic regions across the galaxy, which simultaneously fits the stellar attenuated SED from UV to mid-infrared emission, the reradiated infrared emission from the dust, the radio continuum emission from the gas, as well as the intensity of select recombination lines from the ionized gas. We present a framework capable of determine the IR fraction not associated with the recent SFR. This work provides a novel and crucial step towards understanding the physical processes responsible for various empirical laws to determine SFR in galaxies, the correlation between the IR and stellar emission, and the physical conditions of the ISM. It provides essential inputs for more detailed modeling of the spatially-resolved photometric and chemical (dust and gas) evolution of galaxies.

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

Sil’chenko, Olga K., E-mail: olga@sai.msu.su; Isaac Newton Institute, Chile, Moscow Branch

I analyze statistics of the stellar population properties for stellar nuclei and bulges of nearby lenticular galaxies in different environments by using panoramic spectral data of the integral-field spectrograph SAURON retrieved from the open archive of the Isaac Newton Group. I also estimate the fraction of nearby lenticular galaxies having inner polar gaseous disks by exploring the volume-limited sample of early-type galaxies of the ATLAS-3D survey. By inspecting the two-dimensional velocity fields of the stellar and gaseous components with the running tilted-ring technique, I have found seven new cases of inner polar disks. Together with those, the frequency of inner polar disks in nearby S0 galaxiesmore » reaches 10%, which is much higher than the frequency of large-scale polar rings. Interestingly, the properties of the nuclear stellar populations in the inner polar ring hosts are statistically the same as those in the whole S0 sample, implying similar histories of multiple gas-accretion events from various directions.« less

Research on stellarator-mirror fission-fusion hybrid

NASA Astrophysics Data System (ADS)

Moiseenko, V. E.; Kotenko, V. G.; Chernitskiy, S. V.; Nemov, V. V.; Ågren, O.; Noack, K.; Kalyuzhnyi, V. N.; Hagnestål, A.; Källne, J.; Voitsenya, V. S.; Garkusha, I. E.

2014-09-01

The development of a stellarator-mirror fission-fusion hybrid concept is reviewed. The hybrid comprises of a fusion neutron source and a powerful sub-critical fast fission reactor core. The aim is the transmutation of spent nuclear fuel and safe fission energy production. In its fusion part, neutrons are generated in deuterium-tritium (D-T) plasma, confined magnetically in a stellarator-type system with an embedded magnetic mirror. Based on kinetic calculations, the energy balance for such a system is analyzed. Neutron calculations have been performed with the MCNPX code, and the principal design of the reactor part is developed. Neutron outflux at different outer parts of the reactor is calculated. Numerical simulations have been performed on the structure of a magnetic field in a model of the stellarator-mirror device, and that is achieved by switching off one or two coils of toroidal field in the Uragan-2M torsatron. The calculations predict the existence of closed magnetic surfaces under certain conditions. The confinement of fast particles in such a magnetic trap is analyzed.

ICRF heating in a straight, helically symmetric stellarator

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

Jaeger, E.F.; Weitzner, H.; Batchelor, D.B.

1987-07-01

Experimental observations of direct ion cyclotron resonant frequency (ICRF) heating at fundamental ion cyclotron resonance on the L-2 stellarator have stimulated interest in the theoretical basis for such heating. In this paper, global solutions for the ICRF wave fields in a helically symmetric, straight stellarator are calculated in the cold plasma limit. The component of the wave electric field parallel to B-vector is assumed zero. Helical symmetry allows Fourier decomposition in the longitudinal (z) direction. The two remaining partial differential equations in tau and phi identical to THETA - hz (h is the helical pitch) are solved by finite differencing.more » Energy absorption and antenna impedance are calculated from an ad hoc collision model. Results for parameters typical of the L-2 and Advanced Toroidal Facility (ATF) stellarators show that direct resonant absorption of the fundamental ion cyclotron resonance occurs mainly near the plasma edge. The magnitude of the absorption is about half that for minority heating at the two-ion hybrid resonance.« less

Constraining stellar binary black hole formation scenarios with eLISA eccentricity measurements

NASA Astrophysics Data System (ADS)

Nishizawa, Atsushi; Sesana, Alberto; Berti, Emanuele; Klein, Antoine

2017-03-01

A space-based interferometer such as the evolved Laser Interferometer Space Antenna (eLISA) could observe a few to a few thousands of progenitors of black hole binaries (BHBs) similar to those recently detected by Advanced LIGO. Gravitational radiation circularizes the orbit during inspiral, but some BHBs retain a measurable eccentricity at the low frequencies where eLISA is the most sensitive. The eccentricity of a BHB carries precious information about its formation channel: BHBs formed in the field, in globular clusters, or close to a massive black hole (MBH) have distinct eccentricity distributions in the eLISA band. We generate mock eLISA observations, folding in measurement errors, and using a Bayesian model selection, we study whether eLISA measurements can identify the BHB formation channel. We find that a handful of observations would suffice to tell whether BHBs were formed in the gravitational field of an MBH. Conversely, several tens of observations are needed to tell apart field formation from globular cluster formation. A 5-yr eLISA mission with the longest possible armlength is desirable to shed light on BHB formation scenarios.

Constraining stellar binary black hole formation scenarios with LISA eccentricity measurements

NASA Astrophysics Data System (ADS)

Berti, Emanuele; Nishizawa, Atsushi; Sesana, Alberto; Klein, Antoine

2017-01-01

A space-based interferometer such as LISA could observe few to few thousands progenitors of black hole binaries (BHBs) similar to those recently detected by Advanced LIGO. Gravitational radiation circularizes the orbit during inspiral, but some BHBs retain a measurable eccentricity at the low frequencies where LISA is most sensitive. The eccentricity of a BHB carries precious information about its formation channel: BHBs formed in the field, in globular clusters, or close to a massive black hole (MBH) have distinct eccentricity distributions in the LISA band. We generate mock LISA observations, folding in measurement errors, and using Bayesian model selection we study whether LISA measurements can identify the BHB formation channel. We find that a handful of observations would suffice to tell whether BHBs were formed in the gravitational field of a MBH. Conversely, several tens of observations are needed to tell apart field formation from globular cluster formation. A five-year LISA mission with the longest possible armlength is desirable to shed light on BHB formation scenarios. NSF CAREER Grant No. PHY-1055103, NSF Grant No. PHY-1607130, FCT contract IF/00797/2014/CP1214/CT0012.

Effect of telescope antenna diagram on the data acquisition in a stellar interferometer

NASA Astrophysics Data System (ADS)

Longueteau, Emmanuel; Delage, Laurent; Reynaud, François

2017-11-01

This paper deals with the effect of the telescope size on accuracy of the data acquisition in a optical fibre linked stellar interferometer. In this context we introduce the concept of antenna diagram commonly used for microwaves antennae. This concept is essential to explain the contrasts and the phaseclosure acquisitions corruption in a stellar interferometer. The telescope pointing errors induces additional effects that are superimposed with the field limitation and could become critical.

AN EXPLORATION OF THE STATISTICAL SIGNATURES OF STELLAR FEEDBACK

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

Boyden, Ryan D.; Offner, Stella S. R.; Koch, Eric W.

2016-12-20

All molecular clouds are observed to be turbulent, but the origin, means of sustenance, and evolution of the turbulence remain debated. One possibility is that stellar feedback injects enough energy into the cloud to drive observed motions on parsec scales. Recent numerical studies of molecular clouds have found that feedback from stars, such as protostellar outflows and winds, injects energy and impacts turbulence. We expand upon these studies by analyzing magnetohydrodynamic simulations of molecular clouds, including stellar winds, with a range of stellar mass-loss rates and magnetic field strengths. We generate synthetic {sup 12}CO(1–0) maps assuming that the simulations aremore » at the distance of the nearby Perseus molecular cloud. By comparing the outputs from different initial conditions and evolutionary times, we identify differences in the synthetic observations and characterize these using common astrostatistics. We quantify the different statistical responses using a variety of metrics proposed in the literature. We find that multiple astrostatistics, including the principal component analysis, the spectral correlation function, and the velocity coordinate spectrum (VCS), are sensitive to changes in stellar mass-loss rates and/or time evolution. A few statistics, including the Cramer statistic and VCS, are sensitive to the magnetic field strength. These findings demonstrate that stellar feedback influences molecular cloud turbulence and can be identified and quantified observationally using such statistics.« less

Method and apparatus for maintaining equilibrium in a helical axis stellarator

DOEpatents

Reiman, A.; Boozer, A.

1984-10-31

Apparatus for maintaining three-dimensional MHD equilibrium in a plasma contained in a helical axis stellarator includes a resonant coil system, having a configuration such that current therethrough generates a magnetic field cancelling the resonant magnetic field produced by currents driven by the plasma pressure on any given flux surface resonating with the rotational transform of another flux surface in the plasma. Current through the resonant coil system is adjusted as a function of plasma beta.

Deep HST Imaging in 47 Tucanae: A Global Dynamical Model

NASA Astrophysics Data System (ADS)

Heyl, J.; Caiazzo, I.; Richer, H.; Anderson, J.; Kalirai, J.; Parada, J.

2017-12-01

Multi-epoch observations with the Advanced Camera Survey and WFC3 on the Hubble Space Telescope provide a unique and comprehensive probe of stellar dynamics within 47 Tucanae. We confront analytic models of the globular cluster with the observed stellar proper motions that probe along the main sequence from just above 0.8-0.1M ⊙ as well as white dwarfs younger than 1 Gyr. One field lies just beyond the half-light radius where dynamical models (e.g., lowered Maxwellian distributions) make robust predictions for the stellar proper motions. The observed proper motions in this outer field show evidence for anisotropy in the velocity distribution as well as skewness; the latter is evidence of rotation. The measured velocity dispersions and surface brightness distributions agree in detail with a rotating anisotropic model of the stellar distribution function with mild dependence of the proper-motion dispersion on mass. However, the best-fitting models underpredict the rotation and skewness of the stellar velocities. In the second field, centered on the core of the cluster, the mass segregation in proper motion is much stronger. Nevertheless the model developed in the outer field can be extended inward by taking this mass segregation into account in a heuristic fashion. The proper motions of the main-sequence stars yield a mass estimate of the cluster of 1.31+/- 0.02× {10}6{M}⊙ at a distance of 4.7 kpc. By comparing the proper motions of a sample of giant and subgiant stars with the observed radial velocities we estimate the distance to the cluster kinematically to be 4.29 ± 0.47 kpc.

Starspots

NASA Astrophysics Data System (ADS)

Strassmeier, Klaus G.

2009-09-01

Starspots are created by local magnetic fields on the surfaces of stars, just as sunspots. Their fields are strong enough to suppress the overturning convective motion and thus block or redirect the flow of energy from the stellar interior outwards to the surface and consequently appear as locally cool and therefore dark regions against an otherwise bright photosphere (Biermann in Astronomische Nachrichten 264:361, 1938; Z Astrophysik 25:135, 1948). As such, starspots are observable tracers of the yet unknown internal dynamo activity and allow a glimpse into the complex internal stellar magnetic field structure. Starspots also enable the precise measurement of stellar rotation which is among the key ingredients for the expected internal magnetic topology. But whether starspots are just blown-up sunspot analogs, we do not know yet. This article is an attempt to review our current knowledge of starspots. A comparison of a white-light image of the Sun (G2V, 5 Gyr) with a Doppler image of a young solar-like star (EK Draconis; G1.5V, age 100 Myr, rotation 10 × Ω Sun) and with a mean-field dynamo simulation suggests that starspots can be of significantly different appearance and cannot be explained with a scaling of the solar model, even for a star of same mass and effective temperature. Starspots, their surface location and migration pattern, and their link with the stellar dynamo and its internal energy transport, may have far reaching impact also for our understanding of low-mass stellar evolution and formation. Emphasis is given in this review to their importance as activity tracers in particular in the light of more and more precise exoplanet detections around solar-like, and therefore likely spotted, host stars.

Cosmic Masers

ERIC Educational Resources Information Center

Dickinson, Dale F.

1978-01-01

Intense radiation at microwave frequencies is emitted by certain nebular regions and stellar atmospheres. It is generated by maser action, which does for microwaves what laser action does for light. Describes in detail the types of masers and their action. (Author/MA)

The SAMI Galaxy Survey: instrument specification and target selection

NASA Astrophysics Data System (ADS)

Bryant, J. J.; Owers, M. S.; Robotham, A. S. G.; Croom, S. M.; Driver, S. P.; Drinkwater, M. J.; Lorente, N. P. F.; Cortese, L.; Scott, N.; Colless, M.; Schaefer, A.; Taylor, E. N.; Konstantopoulos, I. S.; Allen, J. T.; Baldry, I.; Barnes, L.; Bauer, A. E.; Bland-Hawthorn, J.; Bloom, J. V.; Brooks, A. M.; Brough, S.; Cecil, G.; Couch, W.; Croton, D.; Davies, R.; Ellis, S.; Fogarty, L. M. R.; Foster, C.; Glazebrook, K.; Goodwin, M.; Green, A.; Gunawardhana, M. L.; Hampton, E.; Ho, I.-T.; Hopkins, A. M.; Kewley, L.; Lawrence, J. S.; Leon-Saval, S. G.; Leslie, S.; McElroy, R.; Lewis, G.; Liske, J.; López-Sánchez, Á. R.; Mahajan, S.; Medling, A. M.; Metcalfe, N.; Meyer, M.; Mould, J.; Obreschkow, D.; O'Toole, S.; Pracy, M.; Richards, S. N.; Shanks, T.; Sharp, R.; Sweet, S. M.; Thomas, A. D.; Tonini, C.; Walcher, C. J.

2015-03-01

The SAMI Galaxy Survey will observe 3400 galaxies with the Sydney-AAO Multi-object Integral-field spectrograph (SAMI) on the Anglo-Australian Telescope in a 3-yr survey which began in 2013. We present the throughput of the SAMI system, the science basis and specifications for the target selection, the survey observation plan and the combined properties of the selected galaxies. The survey includes four volume-limited galaxy samples based on cuts in a proxy for stellar mass, along with low-stellar-mass dwarf galaxies all selected from the Galaxy And Mass Assembly (GAMA) survey. The GAMA regions were selected because of the vast array of ancillary data available, including ultraviolet through to radio bands. These fields are on the celestial equator at 9, 12 and 14.5 h, and cover a total of 144 deg2 (in GAMA-I). Higher density environments are also included with the addition of eight clusters. The clusters have spectroscopy from 2-degree Field Galaxy Redshift Survey (2dFGRS) and Sloan Digital Sky Survey (SDSS) and photometry in regions covered by the SDSS and/or VLT Survey Telescope/ATLAS. The aim is to cover a broad range in stellar mass and environment, and therefore the primary survey targets cover redshifts 0.004 < z < 0.095, magnitudes rpet < 19.4, stellar masses 107-1012 M⊙, and environments from isolated field galaxies through groups to clusters of ˜1015 M⊙.

The steady state solutions of radiatively driven stellar winds for a non-Sobolev, pure absorption model

NASA Technical Reports Server (NTRS)

Poe, C. H.; Owocki, S. P.; Castor, J. I.

1990-01-01

The steady state solution topology for absorption line-driven flows is investigated for the condition that the Sobolev approximation is not used to compute the line force. The solution topology near the sonic point is of the nodal type with two positive slope solutions. The shallower of these slopes applies to reasonable lower boundary conditions and realistic ion thermal speed v(th) and to the Sobolev limit of zero of the usual Castor, Abbott, and Klein model. At finite v(th), this solution consists of a family of very similar solutions converging on the sonic point. It is concluded that a non-Sobolev, absorption line-driven flow with a realistic values of v(th) has no uniquely defined steady state. To the extent that a pure absorption model of the outflow of stellar winds is applicable, radiatively driven winds should be intrinsically variable.

Massive Stars and Star Clusters in the Era of JWST

NASA Astrophysics Data System (ADS)

Klein, Richard

Massive stars lie at the center of the web of physical processes that has shaped the universe as we know it, governing the evolution of the interstellar medium of galaxies, producing a majority of the heavy elements, and thereby determining the evolution of galaxies. Massive stars are also important as signposts, since they produce most of the light and almost all the ionizing radiation in regions of active star formation. A significant fraction of all stars form in massive clusters, which will be observable throughout the visible universe with JWST. Their luminosities are so high that the pressure of their light on interstellar dust grains is likely the dominant feedback mechanism regulating their formation. While this process has been studied in the local Universe, much less attention has been focused on how it behaves at high redshift, where the dust abundance is much lower due to the overall lower abundance of heavy elements. The high redshift Universe also differs from the nearby one in that observations imply that high redshift star formation occurs at significantly higher densities than are typically found locally. We propose to simulate the formation of individual massive stars from the high redshift universe to the present day universe spanning metallicities ranging from 0.001 to 1.0 and column densities from 0.1to 30.0 g/cm2 focusing on how the process depends on both the dust abundance and on the density of the star-forming gas. These simulations will be among the first to treat the formation of Population II stars, which form in regions of low metallicity. Based on these results, we shall then simulate the formation of clusters of stars across also cosmic time, both of moderate mass, such as the Orion Nebula Cluster, and of high mass, such as the super star clusters seen in starburst galaxies. These state-of-the-art simulations will be carried out using our newly developed advanced techniques in our radiation-magneto-hydrodynamic AMR code ORION, for radiative transfer with both ionizing and non-ionizing radiation that accurately handle both the direct radiation from stars and the diffuse infrared radiation field that builds up when direct radiation is reprocessed by dust grains. Our simulations include all of the relevant feedback effects such as radiative heating, radiation pressure, photodissociation and photoionization, protostellar outflows and stellar winds. The challenge in simulating the formation of massive stars and massive clusters is to include all these feedback effects self-consistently as they occur collectively. We are in an excellent position to do so. The results of these simulations will be directly relevant to the interpretation of observations with JWST, which will probe cluster formation in both the nearby and distant universe, and with SOFIA, which can observe high-mass star formation in the Galaxy. We shall make direct comparison with observations of massive protostars in the Galactic disk. We shall also compare with observations of star clusters that form in dense environments, such as the Galactic Center and in merging galaxies (e.g., the Antennae), and in low metallicity environments, such as the dwarf starburst galaxy I Zw 18. Once our simulations have been benchmarked with observations of massive protostars in the Galaxy and massive protoclusters in the local universe, they will provide the theoretical basis for interpreting observations of the formation of massive star clusters at high redshift with JWST. What determines the maximum mass of a star? How does stellar feedback affect the formation of individual stars and the formation of massive star clusters and how the answers to these questions evolve with cosmic time. The proposed research will provide high-resolution input to the study of stellar feedback on galaxy formation with a significantly more accurate treatment of the physics, particularly the radiative transfer that is so important for feedback.

Initial experiments to understand the interaction of stellar radiation with molecular clouds

NASA Astrophysics Data System (ADS)

Vandervort, Robert; Davis, Josh; Trantham, Matt; Klein, Sallee; Shvarts, Dov; Keiter, Paul; Drake, R. Paul

2017-10-01

Enhanced star formation triggered by local O and B type stars is an astrophysical problem of interest. O and B type stars are massive, hot stars that emit an enormous amount of radiation. This radiation acts to either compress or blow apart gas clumps in the interstellar media. For example, in the optically thick limit, when the radiation in the gas clump has a short mean free path, radiation is absorbed near the clump edge and compresses the clump. In the optically thin limit, when the mean free path is long, the radiation is absorbed throughout, acting to heat the clump. This heating explodes the gas clump. Careful selection of parameters, such as foam density or source temperature, allow the experimental platform to access different hydrodynamic regimes. 2D CRASH simulations guide our parameter selection. A stellar radiation source is mimicked by a laser-irradiated, thin, gold foil, providing a source of thermal x-rays around 100 eV. The gas clump is mimicked by low-density CRF foam. We plan to show the preliminary experimental results of this platform in the optically thick limit, from experiments scheduled in August. This work is funded by the U.S. DOE, through the NNSA-DS and SC-OFES Joint Program in HEDPLP, Grant No. DE-NA0002956, and the NLUF Program, Grant No. DE-NA0002719, and through LLE, University of Rochester by the NNSA/OICF under Cooperative Agreement No. DE-NA0001944. This work is funded by the Lawrence Livermore National Laboratory under subcontract B614207.

Quantitative chemical tagging, stellar ages and the chemo-dynamical evolution of the Galactic disc

NASA Astrophysics Data System (ADS)

Mitschang, A. W.; De Silva, G.; Zucker, D. B.; Anguiano, B.; Bensby, T.; Feltzing, S.

2014-03-01

The early science results from the new generation of high-resolution stellar spectroscopic surveys, such as Galactic Archaeology with HERMES (GALAH) and the Gaia European Southern Observatory survey (Gaia-ESO), will represent major milestones in the quest to chemically tag the Galaxy. Yet this technique to reconstruct dispersed coeval stellar groups has remained largely untested until recently. We build on previous work that developed an empirical chemical tagging probability function, which describes the likelihood that two field stars are conatal, that is, they were formed in the same cluster environment. In this work, we perform the first ever blind chemical tagging experiment, i.e. tagging stars with no known or otherwise discernible associations, on a sample of 714 disc field stars with a number of high-quality high-resolution homogeneous metal abundance measurements. We present evidence that chemical tagging of field stars does identify coeval groups of stars, yet these groups may not represent distinct formation sites, e.g. as in dissolved open clusters, as previously thought. Our results point to several important conclusions, among them that group finding will be limited strictly to chemical abundance space, e.g. stellar ages, kinematics, colours, temperature and surface gravity do not enhance the detectability of groups. We also demonstrate that in addition to its role in probing the chemical enrichment and kinematic history of the Galactic disc, chemical tagging represents a powerful new stellar age determination technique.

Quenching of Star-formation Activity of High-redshift Galaxies in Cluster and Field

NASA Astrophysics Data System (ADS)

Lee, Seong-Kook; Im, Myungshin; Kim, Jae-Woo; Lotz, Jennifer; McPartland, Conor; Peth, Michael; Koekemoer, Anton M.

2015-08-01

How the galaxy evolution differs at different environment is one of intriguing questions in the study of structure formation. At local, galaxy properties are well known to be clearly different in different environments. However, it is still an open question how this environment-dependent trend has been shaped.In this presentation, we will present the results of our investigation about the evolution of star-formation properties of galaxies over a wide redshift range, from z~ 2 to z~0.5, focusing its dependence on their stellar mass and environment. In the UKIDSS/UDS region, covering ~2800 arcmin2, we estimated photometric redshifts and stellar population properties, such as stellar masses and star-formation rates, using the deep optical and near-infrared data available in this field. Then, we identified galaxy cluster candidates within the given redshift range.Through the analysis and comparison of star-formation (SF) properties of galaxies in clusters and in field, we found interesting results regarding the evolution of SF properties of galaxies: (1) regardless of redshifts, stellar mass is a key parameter controlling quenching of star formation in galaxies; (2) At z<1, environmental effects become important at quenching star formation regardless of stellar mass of galaxies; and (3) However, the result of the environmental quenching is prominent only for low mass galaxies (M* < 1010 M⊙) since the star formation in most of high mass galaxies are already quenched at z > 1.

Does Stellar Feedback Create H I Holes? A Hubble Space Telescope/Very Large Array Study of Holmberg II

NASA Astrophysics Data System (ADS)

Weisz, Daniel R.; Skillman, Evan D.; Cannon, John M.; Dolphin, Andrew E.; Kennicutt, Robert C., Jr.; Lee, Janice; Walter, Fabian

2009-10-01

We use deep Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) F555W and F814W photometry of resolved stars in the M81 Group dwarf irregular galaxy Ho II to study the hypothesis that the holes identified in the neutral interstellar medium (H I) are created by stellar feedback. From the deep photometry, we construct color-magnitude diagrams (CMDs) and measure the star formation histories (SFHs) for stars contained in H I holes from two independent holes catalogs, as well as select control fields, i.e., similar sized regions that span a range of H I column densities. The CMDs reveal young (< 200 Myr) stellar populations inside all H I holes, which contain very few bright OB stars with ages less than 10 Myr, indicating they are not reliable tracers of H I hole locations while the recent SFHs confirm multiple episodes of star formation within most holes. Converting the recent SFHs into stellar feedback energies, we find that enough energy has been generated to have created all holes. However, the required energy is not always produced over a timescale that is less than the estimated kinematic age of the hole. A similar analysis of stars in the control fields finds that the stellar populations of the control fields and H I holes are statistically indistinguishable. However, because we are only sensitive to holes ~100 pc in diameter, we cannot tell if there are smaller holes inside the control fields. The combination of the CMDs, recent SFHs, and locations of young stars shows that the stellar populations inside H I holes are not coherent, single-aged, stellar clusters, as previously suggested, but rather multi-age populations distributed across each hole. From a comparison of the modeled and observed integrated magnitudes, and the locations and energetics of stars inside of H I holes, we propose a potential new model: a viable mechanism for creating the observed H I holes in Ho II is stellar feedback from multiple generations of SF spread out over tens or hundreds of Myr, and thus, the concept of an age for an H I hole is intrinsically ambiguous. For H I holes in the outer parts of Ho II, located beyond the HST/ACS coverage, we use Monte Carlo simulations of expected stellar populations to show that low level SF could provide the energy necessary to form these holes. Applying the same method to the SMC, we find that the holes that appear to be void of stars could have formed via stellar feedback from low level SF. We further find that Hα and 24 μm emission, tracers of the most recent star formation, do not correlate well with the positions of the H I holes. However, UV emission, which traces star formation over roughly the last 100 Myr, shows a much better correlation with the locations of the H I holes.

Dark Matter Halos with VIRUS-P

NASA Astrophysics Data System (ADS)

Murphy, Jeremy; Gebhardt, K.

2010-05-01

We present new, two-dimensional stellar kinematic data on several of the most massive galaxies in the local universe. These data were taken with the integral field spectrograph, VIRUS-P, and extend to unprecedented radial distances. Once robust stellar kinematics are in hand, we run orbit-based axisymmetric dynamical models in order to constrain the stellar mass-to-light ratio and dark matter halo parameters. We have run a large set of dynamical models on the second rank galaxy in the Virgo cluster, M87, and find clear evidence for a massive dark matter halo. The two-dimensional stellar kinematics for several of our other targets, all first and second rank galaxies, are also presented. Dark matter halos are known to dominate the mass profile of elliptical galaxies somewhere between one to two effective radii, yet due to the low surface brightness at these radial distances, determining stellar dynamics is technologically challenging. To overcome this, constraints on the dark matter halo are often made with planetary nebulae or globular clusters at large radii. However, as results from different groups have returned contradictory results, it remains unclear whether different dynamical tracers always follow the stellar kinematics. Due to VIRUS-P's large field of view and on-sky fiber diameter, we are able to determine stellar kinematics at radial distances that overlap with other dynamical tracers. Understanding what the dynamics of stars, planetary nebula and globular clusters tell us about both the extent of the dark matter halo profile and the formation histories of the largest elliptical galaxies is a primary science driver for this work.

The Soft X-Ray/Microwave Ratio of Solar and Stellar Flares and Coronae

NASA Technical Reports Server (NTRS)

Benz, A. O.; Guedel, M.

1994-01-01

We have carried out plasma diagnostics of solar flares using soft X-ray (SXR) and simultaneous microwave observations and have compared the ratio of X-ray to microwave luminosities of solar flares with various active late-type stars available in the published literature. Both the SXR low-level ('quiescent') emission from stellar coronae and the flaring emission from the Sun and stars are generally interpreted as thermal radiations of coronal plasmas. On the other hand, the microwave emission of stars and solar flares is generally attributed to an extremely hot or nonthermal population of electrons. Solar flare SXR are conventionally measured in a narrower and harder passband than the stellar sources. Observations of the GOES-2 satellite in two energy channels have been used to estimate the luminosity of solar flares as it would appear in the ROSAT satellite passband. The solar and stellar flare luminosities fit well at the lower end of the active stellar coronae. The flare SXR/microwave ratio is similar to the ratio for stellar coronae. The average ratio follows a power-law relation L(sub X) varies as L(sub R)(sup 0.73 +/- 0.03) over 10 orders of magnitude from solar microflares to RS CVn and FK Com-type coronae. Dwarf Me and Ke stars, and RS CVn stars are also compatible with a linear SXR/microwave relation, but the ratio is slightly different for each type of star. Considering the differences between solar flares, stellar flares and the various active stellar coronae, the similarity of the SXR/microwave ratios is surprising. It suggests that the energetic electrons in low-level stellar coronae observed in microwaves are related in a similar way to the coronal thermal plasma as flare electrons to the flare thermal plasma, and, consequently, that the heating mechanism of active stellar coronae is a flare-like process.

The Role of Shocks in the Appearance and Aftermath of Stellar Mergers and Type IIn Supernovae

NASA Astrophysics Data System (ADS)

Metzger, Brian

2017-08-01

HST has played a crucial role in elucidating the environments, progenitors, explosions, and late-time behavior of Type IIn supernovae (SNe) and binary star mergers (also known as common envelope events). Although shock interaction plays a dominant role in the dynamics and appearance of these events, the details of this process and the nature of the mass loss leading up to the core collapse or dynamical stage of the merger, remain poorly understood. Mounting evidence suggests that the pre-explosion mass loss geometry is a disk or equatorially-concentrated outflow. We will perform the first multi-dimensional radiation hydrodynamical simulations of the shock interaction between the fast ejecta from the SN explosion/dynamical merger and a slower equatorially-focused outflow representing the earlier phase of mass loss. Our calculations will quantify the geometry of the ejecta and make detailed predictions for the shock-powered emission. In combination with an analytic model to be developed in parallel, we will translate the light curves and spectral information on a large sample of IIn SNe and stellar mergers into probes of their mass loss history. We will address whether the combination of hydrogen recombination and shock-powered emission can explain the common double-peaked nature of the light curves of stellar mergers. By accounting self-consistently for the role of radiative shock compression on the ejecta density structure, and thus on the global geometry and microphysical properties of dust grains formed, we will also address the late-time appearance of IIn SNe and stellar mergers observed by HST and JWST.

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

Hendler, Nathanial P.; Mulders, Gijs D.; Pascucci, Ilaria

The properties of disks around brown dwarfs and very low mass stars (hereafter VLMOs) provide important boundary conditions on the process of planet formation and inform us about the numbers and masses of planets than can form in this regime. We use the Herschel Space Observatory PACS spectrometer to measure the continuum and [O i] 63 μ m line emission toward 11 VLMOs with known disks in the Taurus and Chamaeleon I star-forming regions. We fit radiative transfer models to the spectral energy distributions of these sources. Additionally, we carry out a grid of radiative transfer models run in amore » regime that connects the luminosity of our sources with brighter T Tauri stars. We find that VLMO disks with sizes 1.3–78 au, smaller than typical T Tauri disks, fit well the spectral energy distributions assuming that disk geometry and dust properties are stellar mass independent. Reducing the disk size increases the disk temperature, and we show that VLMOs do not follow previously derived disk temperature–stellar luminosity relationships if the disk outer radius scales with stellar mass. Only 2 out of 11 sources are detected in [O i] despite a better sensitivity than was achieved for T Tauri stars, suggesting that VLMO disks are underluminous. Using thermochemical models, we show that smaller disks can lead to the unexpected [O i] 63 μ m nondetections in our sample. The disk outer radius is an important factor in determining the gas and dust observables. Hence, spatially resolved observations with ALMA—to establish if and how disk radii scale with stellar mass—should be pursued further.« less