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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

I-process nucleosynthesis and mass retention efficiency in He-shell flash evolution of rapidly accreting white dwarfs

Based on stellar evolution simulations, we demonstrate that rapidly accreting white dwarfs (WDs) in close binary systems are an astrophysical site for the intermediate neutron-capture process. During recurrent and very strong He-shell flashes in the stable H-burning accretion regime H-rich material enters the He-shell flash convection zone. {sup 12}C(p,γ){sup 13}N reactions release enough energy to potentially impact convection, and i process is activated through the {sup 13}C(α,n){sup 16}O reaction. The H-ingestion flash may not cause a split of the convection zone as it was seen in simulations of He-shell flashes in post-AGB and low-Z asymptotic giant branch (AGB) stars. We estimate that for the production of first-peak heavy elements this site can be of similar importance for galactic chemical evolution as the s-process production by low-mass AGB stars. The He-shell flashes result in the expansion and, ultimately, ejection of the accreted and then i-process enriched material, via super-Eddington-luminosity winds or Roche-lobe overflow. The WD models do not retain any significant amount of the accreted mass, with a He retention efficiency of ≲10% depending on mass and convective boundary mixing assumptions. This makes the evolutionary path of such systems to supernova Ia explosion highly unlikely.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Radiation Transport Two-temperature GRMHD Simulations of Warped Accretion Disks

In many black hole (BH) systems, the accretion disk is expected to be misaligned with respect to the BH spin axis. If the scale height of the disk is much smaller than the misalignment angle, the spin of the BH can tear the disk into multiple, independently precessing "sub-disks." This is most likely to happen during outbursts in black hole X-Ray binaries (BHXRBs) and in active galactic nuclei (AGNs) accreting above a few percent of the Eddington limit, because the disk becomes razor-thin. Disk tearing has the potential to explain variability phenomena including quasi-periodic oscillations in BHXRBs and changing-look phenomena in AGNs. Here, we present the first radiative two-temperature general relativistic magnetohydrodynamic (GRMHD) simulation of a strongly tilted (65°) accretion disk around an M BH = 10 M ⊙ BH, which tears and precesses. This leads to luminosity swings between a few percent and 50% of the Eddington limit on sub-viscous timescales. Surprisingly, even where the disk is radiation-pressure-dominated, the accretion disk is thermally stable over t ≳ 14,000 r g /c. This suggests warps play an important role in stabilizing the disk against thermal collapse. The disk forms two nozzle shocks perpendicular to the line of nodes where the scale height of the disk decreases tenfold and the electron temperature reaches T e ~ 10 8 –10 9 K. In addition, optically thin gas crossing the tear between the inner and outer disk gets heated to T e ~ 108 K. This suggests that warped disks may emit a Comptonized spectrum that deviates substantially from idealized models.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1972

Accretion process of the moon

Recent geochemical and geophysical data suggest that the initial temperature of the moon was strongly peaked toward the lunar surface. To explain such an initial temperature distribution, a simple model of accretion process of the moon is presented. The model assumes that the moon was formed from the accumulation of the solid particles or gases in the isolated, closed cloud. Two equations are derived to calculate the accretion rate and surface temperature of the accreting moon. Numerical calculations are made for a wide range of the parameters particle concentration and particle velocity in the cloud. A limited set of the parameters gives the initial temperature profiles as required by geochemical and geophysical data. These models of the proto-moon cloud indicate that the lunar outermost shell, about 400 km thick, was partially or completely molten just after the accretion of the moon and that the moon should have been formed in a period shorter than 1000 years. If the moon formed at a position nearer to the earth than its present one, the moon might have been formed in a period of less than one year.

Mizutani, H.↗
NASA NTRS1972

Accretion process of the moon.

Development of a simple model of the accretion process of the moon to explain its initial temperature distribution. The model assumes that the moon was formed from the accumulation of the solid particles or gases in the isolated, closed cloud. Two equations are derived to calculate the accretion rate and surface temperature of the accreting moon. Numerical calculations are made for a wide range of the particle concentrations and particle velocities in the cloud. A limited set of parameters gives the initial temperature profiles as required by geochemical and geophysical data. These models of the protomoon cloud indicate that the lunar outershell, about 400 km thick, was partially or completely molten just after the accretion of the moon and that the moon should have been formed in a period shorter than 1000 yr. If the moon formed at a position nearer to the earth than its present one, the moon might have been formed in a period of less than one year.

Mizutani, H.↗
NASA NTRS1976

Steady accretion of a rotating fluid

The accretion of rotating gas on to a gravitating object is investigated by means of a perturbation to the spherically symmetrical flow. An expression is found for the correction to the accretion rate due to rotation of the gas in terms of the boundary conditions far from the object. In the case of accretion from a cloud with uniform angular velocity, the gas is accreted preferentially from a 'jet' near the axis of rotation. The angular-momentum distribution in the cloud can be altered by the propagation of inertia waves.

Cassen, P.↗
NASA NTRS1977

An investigation of the stability of the Bondi-Hoyle model of accretion flow

The Bondi-Hoyle-Lyttleton (1944) accretion model is considered which involves accretion onto a massive body moving at a high Mach number with respect to the ambient medium and the production of a high-density accretion column along the axis where particle orbits intersect. The stability of steady-state solutions with respect to short-wavelength perturbations is analyzed using the WKB approximation, and the accretion column is shown to be unstable toward such perturbations. It is noted that this instability is not affected by the position of the stagnation point in the steady-state solution.

Cowie, L. L.↗
NASA NTRS1978

Helium-burning flashes on an accreting neutron star - A model for X-ray burst sources

Detailed numerical models of X-ray bursts resulting from thermonuclear flashes near the surface of an accreting neutron star have been constructed. The models assume a nonrotating nonmagnetized spherically accreting neutron star of 1.4 solar masses, radius 6.6 km, core temperature in the range from 250 million to 570 million K, and accretion rate in the range (0.3-3) x 10 to the 17th power g/s. Under many conditions the helium-burning shell undergoes thermonuclear flashes that result in the emission of X-ray bursts, the gross properties of which are remarkably similar to those of most observed X-ray burst sources. Neutron stars with moderately low core temperatures, low accretion rates, and weak magnetic fields are most likely to produce X-ray bursts.

Joss, P. C.↗
NASA NTRS1978

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

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

Cowie, L. L.↗
NASA NTRS1980

The Eddington limit and supercritical accretion. I - Time-independent calculations

Spherically symmetric, steady state accretion of an ionized hydrogen plasma onto a neutron star is considered for accretion rates which exceed a critical rate at which the Eddington limiting luminosity is produced. The coupled hydrodynamic and frequency integrated, radiative transfer equations are solved for accretion rates up to 10 times the nominal limit. Steady state solutions are presented that imply a multiplicity of different luminosity solutions for a single accretion rate in this 'supercritical' regime.

Burger, H. L.↗
NASA NTRS1980

Supercritical time-dependent accretion onto compact objects. I - Neutron stars

The time-dependent flow resulting from spherically symmetric, supercritical accretion onto a nonmagnetic neutron star is computed for accretion rates between 3 and 30. In addition, the fully time-dependent self-consistent set of coupled radiation-hydrodynamic equations governing such flow has been solved. Effects that have not previously been treated are taken into account, including separate energy equations for ions and electrons allowing for nonequilibrium, variable Eddington factors in the radiation transfer to account for anisotropy of the radiation field due to highly spherically extended envelopes, and convection of ions, electrons and photons in the accretion envelope. It is found that supercritical flow, for material falling from a radius of less than 10 to the 10th cm, is characterized by stable accretion with no evidence of stellar wind outflow at any phase of the evolution.

Klein, R. I.↗
NASA NTRS1980

Radial accretion of H-rich material onto a He white dwarf

The nova outburst is modeled by spherically accreting H-rich material onto a 1 solar mass He white dwarf at a rate of 10 to the -8th solar mass/yr. The star accretes for 5848 years, when the nuclear reactions run away near the base of the accreted envelope. The nuclear-energy generation rate rises to 4.6(8) solar luminosities, and the envelope expands in response to it. However, nova-like mass ejection does not occur because the envelope is of insufficient mass, the base of the envelope is only mildly electron degenerate, and there is no enrichment of the CNO abundance. To overcome these limiting conditions, it is suggested that the H-rich material be accreted either more slowly than 10 to the -10th solar mass/yr or with angular momentum. Outbursts resulting in the former case should be similar to the nova models computed by Starrfield, Truran, and Sparks. Outbursts in the latter case should be strengthened, and novae might result because the H-rich material will be mixed into the surface layers of the white dwarf, as first suggested by Kippenhahn and Thomas (1978).

Kutter, G. S.↗
NASA NTRS1981

Accretion disk models for QSOs and active galactic nuclei - The role of magnetic viscosity

The inner regions of standard accretion disk models are known to be thermally unstable, and when scaled to quasar black hole masses, optically thin. Alternative accretion disk models are constructed under the assumption of a purely magnetic viscosity in the limiting cases of equipartition of gas and magnetic pressures and global flux conservation. The inner regions of these models are considerably denser than the standard model and therefore remain optically thick in all regions. The equipartition model is thermally stable throughout, while flux conservation leads to a localized thermal instability at the gas pressure/radiation pressure boundary and marginal stability as the radial distance approaches zero. The outer regions of quasar scaled accretion disks are strongly self-gravitating, leading to a vertical scale height which is smaller than that found in the inner region. Most of the outer region is gravitationally unstable, implying that the outer parts of galactic nuclei accretion disks are populated by dense self-gravitating gas clouds or possibly by stars.

Sakimoto, P. J.↗
NASA NTRS1981

On stellar wind accretion in widely separated X-ray binaries, and the nature of 4U0115+63

Widely separated, eccentric pulsating X-ray binaries are shown to be appropriate systems for studying the basic properties of stellar wind accretion. They are free from the complications encountered in closed binaries, where the optical star nearly fills its critical lobe (Avni, 1977). A framework of four observational tests was set up to study the characteristics of wind accretion, and to distinguish between different models of wind acceleration. The tests are applied to the source 4U0115+63, and the accretion process is discussed in both its quiescent states, and during its transient high luminosity outbursts. For the quiescent steady state, two wind profiles, determined from observational data, are compared (Barlow-Lohen, 1977; Castor et al., 1975). The difficulties encountered during the transient high states with respect to the possibility of stellar wind accretion are discussed.

Avni, Y.↗
NASA NTRS1982

Thermonuclear processes on accreting neutron stars - A systematic study

A series of model calculations for the evolution of the surface layers of an accreting neutron star is carried out. The neutron star mass, radius, core temperature, and surface magnetic field strength are systematically varied, as are the accretion rate onto the neutron star surface and the metallicity of the accreting matter, in order to determine the effects of these parameters on the properties of thermonuclear flashes in the surface layers and the emitted X-ray bursts that result from such flashes. The core temperatures required for thermal equilibrium are found to be approximately a factor of 2 lower than estimated in earlier work. Owing to the effects of the gravitational redshift, the emitted X-ray bursts have lower peak luminosities and longer durations than those calculated in the Newtonian approximation. The entrainment of hydrogen into helium flashes can cause the flashes to exhibit a rather wide range of observable effects and can decrease by a factor of more than 2 the ratio of persistent accretion-driven luminosity to time-averaged burst luminosity emitted by the neutron star.

Ayasli, S.↗
NASA NTRS1983

Compton heated winds and coronae above accretion disks. I Dynamics

X rays emitted in the inner part of an accretion disk system can heat the surface of the disk farther out, producing a corona and possibly driving off a strong wind. The dynamics of Compton-heated coronae and winds are analyzed using an approximate two-dimensional technique to estimate the mass loss rate as a function of distance from the source of X rays. The findings have important dynamical implications for accretion disks in quasars, active galactic nuclei, X ray binaries, and cataclysmic variables. These include: mass loss from the disk possibly comparable with or exceeding the net accretion rate onto the central compact object, which may lead to unstable accretion; sufficient angular momentum loss in some cases to truncate the disk in a semidetached binary at a smaller radius than that predicted by tidal truncation theories; and combined static plus ram pressure in the wind adequate to confine line-emitting clouds in quasars and Seyfert galaxies.

Begelman, M. C.↗
NASA NTRS1983

Gamma-burst emission from neutron-star accretion

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

Colgate, S. A.↗
NASA NTRS1984

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

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

Ahrens, T. J.↗
NASA NTRS1984

Models of the hard X-ray spectrum of AM Herculis and implications for the accretion rate

Phenomenological fits to the hard X-ray spectrum of AM Hercules have failed to explain the high equivalent width (0.8 + or - 0.1 keV) of Fe K-alpha emission. The scattering and fluorescent line production in a cold or partially ionized accretion column of hard X-rays emitted at the base are investigated using the Monte Carlo method. It is shown that the Fe emission equivalent width observed in the spectrum of AM Her can be explained without resorting to an overabundance of Fe in the accreting material. For sufficient optical depth across the accretion column, equivalent widths of K-alpha emission even larger than the observed values can be obtained. The accretion rate and gravitational power can be deduced from the optical depth across the column, if the column size is known, and, together with the observed hard X-ray and polarized light luminosities, imply a lower limit for the luminosity in the UV to soft X-ray range, for which the observations give model-dependent values.

Swank, J. H.↗