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At least 1,027 records · Page 57
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DOE OSTI2012

FULLY GENERAL RELATIVISTIC SIMULATIONS OF CORE-COLLAPSE SUPERNOVAE WITH AN APPROXIMATE NEUTRINO TRANSPORT

We present results from the first generation of multi-dimensional hydrodynamic core-collapse simulations in full general relativity (GR) that include an approximate treatment of neutrino transport. Using an M1 closure scheme with an analytic variable Eddington factor, we solve the energy-independent set of radiation energy and momentum based on the Thorne's momentum formalism. Our newly developed code is designed to evolve the Einstein field equation together with the GR radiation hydrodynamic equations. We follow the dynamics starting from the onset of gravitational core collapse of a 15 M{sub Sun} star, through bounce, up to about 100 ms postbounce in this study. By computing four models that differ according to 1D to 3D and by switching from special relativistic (SR) to GR hydrodynamics, we study how the spacial multi-dimensionality and GR would affect the dynamics in the early postbounce phase. Our 3D results support the anticipation in previous 1D results that the neutrino luminosity and average neutrino energy of any neutrino flavor in the postbounce phase increase when switching from SR to GR hydrodynamics. This is because the deeper gravitational well of GR produces more compact core structures, and thus hotter neutrino spheres at smaller radii. By analyzing the residency timescale to the neutrino-heating timescale in the gain region, we show that the criterion to initiate neutrino-driven explosions can be most easily satisfied in 3D models, irrespective of SR or GR hydrodynamics. Our results suggest that the combination of GR and 3D hydrodynamics provides the most favorable condition to drive a robust neutrino-driven explosion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EARLY-TIME VELOCITY AUTOCORRELATION FOR CHARGED PARTICLES DIFFUSION AND DRIFT IN STATIC MAGNETIC TURBULENCE

Using test-particle simulations, we investigate the temporal dependence of the two-point velocity correlation function for charged particles scattering in a time-independent spatially fluctuating magnetic field derived from a three-dimensional isotropic turbulence power spectrum. Such a correlation function allowed us to compute the spatial coefficients of diffusion both parallel and perpendicular to the average magnetic field. Our simulations confirm the dependence of the perpendicular diffusion coefficient on turbulence energy density and particle energy predicted previously by a model for early-time charged particle transport. Using the computed diffusion coefficients, we exploit the particle velocity autocorrelation to investigate the timescale over which the particles 'decorrelate' from the solution to the unperturbed equation of motion. Decorrelation timescales are evaluated for parallel and perpendicular motions, including the drift of the particles from the local magnetic field line. The regimes of strong and weak magnetic turbulence are compared for various values of the ratio of the particle gyroradius to the correlation length of the magnetic turbulence. Our simulation parameters can be applied to energetic particles in the interplanetary space, cosmic rays at the supernova shocks, and cosmic-rays transport in the intergalactic medium.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ON THE EVOLUTION AND SURVIVAL OF PROTOPLANETS EMBEDDED IN A PROTOPLANETARY DISK

We model the evolution of a Jupiter-mass protoplanet formed by the disk instability mechanism at various radial distances accounting for the presence of the disk. Using three different disk models, it is found that a newly formed Jupiter-mass protoplanet at a radial distance of {approx}<5-10 AU cannot undergo a dynamical collapse and evolve further to become a gravitational bound planet. We therefore conclude that giant planets, if formed by the gravitational instability mechanism, must form and remain at large radial distances during the first {approx}10{sup 5}-10{sup 6} years of their evolution. The minimum radial distances in which protoplanets of 1 Saturn-mass, 3 and 5 Jupiter-mass protoplanets can evolve using a disk model with M-dot = 10{sup -6} M{sub Sun} yr{sup -1} and {alpha} = 10{sup -2} are found to be 12, 9, and 7 AU, respectively. The effect of gas accretion on the planetary evolution of a Jupiter-mass protoplanet is also investigated. It is shown that gas accretion can shorten the pre-collapse timescale substantially. Our study suggests that the timescale of the pre-collapse stage does not only depend on the planetary mass, but is greatly affected by the presence of the disk and efficient gas accretion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ANOMALOUS LOW STATES AND LONG-TERM VARIABILITY IN THE BLACK HOLE BINARY LMC X-3

Rossi X-ray Timing Explorer observations of the black hole binary LMC X-3 reveal an extended very low X-ray state lasting from 2003 December 13 until 2004 March 18, unprecedented both in terms of its low luminosity (>15 times fainter than ever before seen in this source) and long duration ({approx}3 times longer than a typical low/hard state excursion). During this event little to no source variability is observed on timescales of {approx}hours-weeks, and the X-ray spectrum implies an upper limit of 1.2 Multiplication-Sign 10{sup 35} erg s{sup -1}. Five years later another extended low state occurs, lasting from 2008 December 11 until 2009 June 17. This event lasts nearly twice as long as the first, and while significant variability is observed, the source remains reliably in the low/hard spectral state for the {approx}188 day duration. These episodes share some characteristics with the 'anomalous low states' in the neutron star binary Her X-1. The average period and amplitude of the variability of LMC X-3 have different values between these episodes. We characterize the long-term variability of LMC X-3 before and after the two events using conventional and nonlinear time series analysis methods, and show that, as is the case in Her X-1, the characteristic amplitude of the variability is related to its characteristic timescale. Furthermore, the relation is in the same direction in both systems. This suggests that a similar mechanism gives rise to the long-term variability, which in the case of Her X-1 is reliably modeled with a tilted, warped precessing accretion disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

OBLIQUITIES OF HOT JUPITER HOST STARS: EVIDENCE FOR TIDAL INTERACTIONS AND PRIMORDIAL MISALIGNMENTS

We provide evidence that the obliquities of stars with close-in giant planets were initially nearly random, and that the low obliquities that are often observed are a consequence of star-planet tidal interactions. The evidence is based on 14 new measurements of the Rossiter-McLaughlin effect (for the systems HAT-P-6, HAT-P-7, HAT-P-16, HAT-P-24, HAT-P-32, HAT-P-34, WASP-12, WASP-16, WASP-18, WASP-19, WASP-26, WASP-31, Gl 436, and Kepler-8), as well as a critical review of previous observations. The low-obliquity (well-aligned) systems are those for which the expected tidal timescale is short, and likewise the high-obliquity (misaligned and retrograde) systems are those for which the expected timescale is long. At face value, this finding indicates that the origin of hot Jupiters involves dynamical interactions like planet-planet interactions or the Kozai effect that tilt their orbits rather than inspiraling due to interaction with a protoplanetary disk. We discuss the status of this hypothesis and the observations that are needed for a more definitive conclusion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

BIPOLAR JETS LAUNCHED FROM MAGNETICALLY DIFFUSIVE ACCRETION DISKS. I. EJECTION EFFICIENCY VERSUS FIELD STRENGTH AND DIFFUSIVITY

We investigate the launching of jets and outflows from magnetically diffusive accretion disks. Using the PLUTO code, we solve the time-dependent resistive magnetohydrodynamic equations taking into account the disk and jet evolution simultaneously. The main question we address is which kind of disks launch jets and which kind of disks do not? In particular, we study how the magnitude and distribution of the (turbulent) magnetic diffusivity affect mass loading and jet acceleration. We apply a turbulent magnetic diffusivity based on {alpha}-prescription, but also investigate examples where the scale height of diffusivity is larger than that of the disk gas pressure. We further investigate how the ejection efficiency is governed by the magnetic field strength. Our simulations last for up to 5000 dynamical timescales corresponding to 900 orbital periods of the inner disk. As a general result, we observe a continuous and robust outflow launched from the inner part of the disk, expanding into a collimated jet of superfast-magnetosonic speed. For long timescales, the disk's internal dynamics change, as due to outflow ejection and disk accretion the disk mass decreases. For magnetocentrifugally driven jets, we find that for (1) less diffusive disks, (2) a stronger magnetic field, (3) a low poloidal diffusivity, or (4) a lower numerical diffusivity (resolution), the mass loading of the outflow is increased-resulting in more powerful jets with high-mass flux. For weak magnetization, the (weak) outflow is driven by the magnetic pressure gradient. We consider in detail the advection and diffusion of magnetic flux within the disk and we find that the disk and outflow magnetization may substantially change in time. This may have severe impact on the launching and formation process-an initially highly magnetized disk may evolve into a disk of weak magnetization which cannot drive strong outflows. We further investigate the jet asymptotic velocity and the jet rotational velocity in respect of the different launching scenarios. We find a lower degree of jet collimation than previous studies, most probably due to our revised outflow boundary condition.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

OCCURRENCE OF EXTREME SOLAR PARTICLE EVENTS: ASSESSMENT FROM HISTORICAL PROXY DATA

The probability of occurrence of extreme solar particle events (SPEs) with proton fluence (>30 MeV) F{sub 30} {>=} 10{sup 10} cm{sup -2} is evaluated based on data on the cosmogenic isotopes {sup 14}C and {sup 10}Be in terrestrial archives covering centennial-millennial timescales. Four potential candidates with F{sub 30} = (1-1.5) Multiplication-Sign 10{sup 10} cm{sup -2} and no events with F{sub 30} > 2 Multiplication-Sign 10{sup 10} cm{sup -2} are identified since 1400 AD in the annually resolved {sup 10}Be data. A strong SPE related to the Carrington flare of 1859 AD is not supported by the data. For the last 11,400 years, 19 SPE candidates with F{sub 30} = (1-3) Multiplication-Sign 10{sup 10} cm{sup -2} are found and clearly no event with F{sub 30} > 5 Multiplication-Sign 10{sup 10} cm{sup -2} (50 times the SPE of 1956 February 23) has occurred. These values serve as observational upper limits on the strength of SPEs on the timescale of tens of millennia. Two events, ca. 780 and 1460 AD, appear in different data series making them strong candidates for extreme SPEs. We build a distribution of the occurrence probability of extreme SPEs, providing a new strict observational constraint. Practical limits can be set as F{sub 30} Almost-Equal-To 1, 2-3, and 5 Multiplication-Sign 10{sup 10} cm{sup -2} for occurrence probabilities Almost-Equal-To 10{sup -2}, 10{sup -3}, and 10{sup -4} yr{sup -1}, respectively. Because of the uncertainties, our results should be interpreted as a conservative upper limit on the SPE occurrence near Earth. The mean solar energetic particle (SEP) flux is evaluated as Almost-Equal-To 40 (cm{sup 2} s){sup -1}, in agreement with estimates from lunar rocks. On average, extreme SPEs contribute about 10% to the total SEP fluence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

PLASMA EFFECTS ON FAST PAIR BEAMS IN COSMIC VOIDS

The interaction of TeV gamma rays from distant blazars with the extragalactic background light produces relativistic electron-positron pair beams by the photon- photon annihilation process. The created pair beam distribution is unstable to linear two-stream instabilities of both electrostatic and electromagnetic nature in the unmagnetized intergalactic medium (IGM). The maximum electrostatic growth rate occurs at angles of 39.{sup 0}2 with respect to the pair beam direction, and is more than three orders of magnitude greater than the maximum Weibel growth rate, indicating that the linear oblique electrostatic instability operates much faster than the Weibel instability. The dissipation of the generated electrostatic turbulence is different for intense and weak gamma-ray blazars. For intense blazars, the normalized number of generated pairs n {sub 22} = n{sub b} /[10{sup -22} cm{sup -3}] exceeds the critical density n{sub c} (T) = 4.8 Multiplication-Sign 10{sup -3} T {sub 4} for given normalized IGM temperature T {sub 4} = T/[10{sup 4} K] necessary for the onset of the modulation instability, so that all free kinetic pair energy is dissipated in heating the IGM in cosmic voids. For weak blazars, half of the initial energy density of the beam particles is transferred to the electrostatic and electromagnetic fluctuations on timescales smaller than the inverse Compton energy loss timescale of the pairs. In both cases, this prevents the development of a full electromagnetic pair cascade as in vacuum. For weak blazars, the superluminal electrostatic fluctuations are dissipated by the inverse Compton scattering into transverse electromagnetic waves by the relaxed relativistic pair particles to optical frequencies, implying the occurrence of optical electrostatic bremsstrahlung pair halos from weak blazars with spectral flux densities below 50 {mu}Jy.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE NATURE OF STARBURSTS. III. THE SPATIAL DISTRIBUTION OF STAR FORMATION

We map the spatial distribution of recent star formation over a few Multiplication-Sign 100 Myr timescales in 15 starburst dwarf galaxies using the location of young blue helium burning stars identified from optically resolved stellar populations in archival Hubble Space Telescope observations. By comparing the star formation histories from both the high surface brightness central regions and the diffuse outer regions, we measure the degree to which the star formation has been centrally concentrated during the galaxies' starbursts, using three different metrics for the spatial concentration. We find that the galaxies span a full range in spatial concentration, from highly centralized to broadly distributed star formation. Since most starbursts have historically been identified by relatively short timescale star formation tracers (e.g., H{alpha} emission), there could be a strong bias toward classifying only those galaxies with recent, centralized star formation as starbursts, while missing starbursts that are spatially distributed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

OPTICAL TO X-RAY SUPERNOVA LIGHT CURVES FOLLOWING SHOCK BREAKOUT THROUGH A THICK WIND

Recent supernova (SN) observations have motivated renewed interest in SN shock breakouts from stars surrounded by thick winds. In such events the interaction with the wind powers the observed luminosity, and predictions include observable hard X-rays. Wind breakouts on timescales of a day or longer are currently the most probable for detection. Here, we study the signal that follows such events. We start from the breakout of the radiation-mediated shock, finding that the breakout temperature can vary significantly from one event to another (10{sup 4} to 5 Multiplication-Sign 10{sup 6} K) due to possible deviation from thermal equilibrium. In general, events with longer breakout pulse duration, t {sub bo}, are softer. We follow the observed radiation through the evolution of the collisionless shock that forms after the breakout of the radiation-mediated shock. We restrict the study of the collisionless shock evolution to cases where the breakout itself is in thermal equilibrium, peaking in optical/UV. In these cases the post-breakout emission contains two spectral components-soft (optical/UV) and hard (X-rays and possibly soft {gamma}-rays). Right after the breakout pulse X-rays are strongly suppressed, and they carry only a small fraction of the total luminosity. The hard component becomes harder, and its luminosity rises quickly afterward, gaining dominance at {approx}10-50 t {sub bo}. The ratio of the peak optical/UV to the peak X-ray luminosity depends mostly on the breakout time. In early breakouts (t {sub bo} {approx}< 20 days for typical parameters) they are comparable, while in late breakouts (t {sub bo} {approx}> 80 days for typical parameters) the X-rays become dominant only after the total luminosity has dropped significantly. In terms of prospects for X-ray and soft gamma-ray detections, it is best to observe 100-500 days after explosions with breakout timescales between a week and a month.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

RADIAL ANGULAR MOMENTUM TRANSFER AND MAGNETIC BARRIER FOR SHORT-TYPE GAMMA-RAY-BURST CENTRAL ENGINE ACTIVITY

Soft extended emission (EE) following initial hard spikes up to 100 s was observed with Swift/BAT for about half of known short-type gamma-ray bursts (SGRBs). This challenges the conversional central engine models of SGRBs, i.e., compact star merger models. In the framework of black-hole-neutron-star merger models, we study the roles of radial angular momentum transfer in the disk and the magnetic barrier around the black hole in the activity of SGRB central engines. We show that radial angular momentum transfer may significantly prolong the lifetime of the accretion process, which may be divided into multiple episodes by the magnetic barrier. Our numerical calculations based on models of neutrino-dominated accretion flows suggest that disk mass is critical for producing the observed EE. In the case of the mass being {approx}0.8 M {sub Sun }, our model can reproduce the observed timescale and luminosity of both the main and the EE episodes in a reasonable parameter set. The predicted luminosity of the EE component is lower than the observed EE within about one order of magnitude and the timescale is shorter than 20 s if the disk mass is {approx}0.2 M {sub Sun }. Swift/BAT-like instruments may be not sensitive enough to detect the EE component in this case. We argue that the EE component could be a probe for the merger process and disk formation for compact star mergers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

LUNAR ACCRETION FROM A ROCHE-INTERIOR FLUID DISK

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

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

SEARCH FOR A CORRELATION BETWEEN VERY-HIGH-ENERGY GAMMA RAYS AND GIANT RADIO PULSES IN THE CRAB PULSAR

We present the results of a joint observational campaign between the Green Bank radio telescope and the VERITAS gamma-ray telescope, which searched for a correlation between the emission of very-high-energy (VHE) gamma rays (E {sub {gamma}} > 150 GeV) and giant radio pulses (GRPs) from the Crab pulsar at 8.9 GHz. A total of 15,366 GRPs were recorded during 11.6 hr of simultaneous observations, which were made across four nights in 2008 December and in 2009 November and December. We searched for an enhancement of the pulsed gamma-ray emission within time windows placed around the arrival time of the GRP events. In total, eight different time windows with durations ranging from 0.033 ms to 72 s were positioned at three different locations relative to the GRP to search for enhanced gamma-ray emission which lagged, led, or was concurrent with, the GRP event. Furthermore, we performed separate searches on main pulse GRPs and interpulse GRPs and on the most energetic GRPs in our data sample. No significant enhancement of pulsed VHE emission was found in any of the preformed searches. We set upper limits of 5-10 times the average VHE flux of the Crab pulsar on the flux simultaneous with interpulse GRPs on single-rotation-period timescales. On {approx}8 s timescales around interpulse GRPs, we set an upper limit of 2-3 times the average VHE flux. Within the framework of recent models for pulsed VHE emission from the Crab pulsar, the expected VHE-GRP emission correlations are below the derived limits.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

FAST FOSSIL ROTATION OF NEUTRON STAR CORES

It is argued that the superfluid core of a neutron star super-rotates relative to the crust, because stratification prevents the core from responding to the electromagnetic braking torque, until the relevant dissipative (viscous or Eddington-Sweet) timescale, which can exceed {approx}10{sup 3} yr and is much longer than the Ekman timescale, has elapsed. Hence, in some young pulsars, the rotation of the core today is a fossil record of its rotation at birth, provided that magnetic crust-core coupling is inhibited, e.g., by buoyancy, field-line topology, or the presence of uncondensed neutral components in the superfluid. Persistent core super-rotation alters our picture of neutron stars in several ways, allowing for magnetic field generation by ongoing dynamo action and enhanced gravitational wave emission from hydrodynamic instabilities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE FORMATION OF COSMIC FULLERENES FROM AROPHATIC CLUSTERS

Fullerenes have recently been identified in space and they may play a significant role in the gas and dust budget of various astrophysical objects including planetary nebulae (PNe), reflection nebulae, and H II regions. The tenuous nature of the gas in these environments precludes the formation of fullerene materials following known vaporization or combustion synthesis routes even on astronomical timescales. We have studied the processing of hydrogenated amorphous carbon (a-C:H or HAC) nanoparticles and their specific derivative structures, which we name ''arophatics'', in the circumstellar environments of young, carbon-rich PNe. We find that UV-irradiation of such particles can result in the formation of fullerenes, consistent with the known physical conditions in PNe and with available timescales.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

DENSITY FLUCTUATIONS AND THE ACCELERATION OF ELECTRONS BY BEAM-GENERATED LANGMUIR WAVES IN THE SOLAR CORONA

Non-thermal electron populations are observed throughout the heliosphere. The relaxation of an electron beam is known to produce Langmuir waves which, in turn, may substantially modify the electron distribution function. As the Langmuir waves are refracted by background density gradients and as the solar and heliospheric plasma density is naturally perturbed with various levels of inhomogeneity, the interaction of Langmuir waves with non-thermal electrons in inhomogeneous plasmas is an important topic. We investigate the role played by ambient density fluctuations on the beam-plasma relaxation, focusing on the effect of acceleration of beam electrons. The scattering of Langmuir waves off turbulent density fluctuations is modeled as a wavenumber diffusion process which is implemented in numerical simulations of the one-dimensional quasilinear kinetic equations describing the beam relaxation. The results show that a substantial number of beam electrons are accelerated when the diffusive timescale in wavenumber space {tau}{sub D} is of the order of the quasilinear timescale {tau}{sub ql}, while when {tau}{sub D} << {tau}{sub ql}, the beam relaxation is suppressed. Plasma inhomogeneities are therefore an important means of energy redistribution for waves and hence electrons, and so must be taken into account when interpreting, for example, hard X-ray or Type III emission from flare-accelerated electrons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

COLD FRONTS AND GAS SLOSHING IN GALAXY CLUSTERS WITH ANISOTROPIC THERMAL CONDUCTION

Cold fronts in cluster cool cores should be erased on short timescales by thermal conduction, unless protected by magnetic fields that are 'draped' parallel to the front surfaces, suppressing conduction perpendicular to the sloshing fronts. We present a series of MHD simulations of cold front formation in the core of a galaxy cluster with anisotropic thermal conduction, exploring a parameter space of conduction strengths parallel and perpendicular to the field lines. Including conduction has a strong effect on the temperature distribution of the core and the appearance of the cold fronts. Though magnetic field lines are draping parallel to the front surfaces, preventing conduction directly across them, the temperature jumps across the fronts are nevertheless reduced. The geometry of the field is such that the cold gas below the front surfaces can be connected to hotter regions outside via field lines along directions perpendicular to the plane of the sloshing motions and along sections of the front that are not perfectly draped. This results in the heating of this gas below the front on a timescale of a Gyr, but the sharpness of the density and temperature jumps may nevertheless be preserved. By modifying the gas density distribution below the front, conduction may indirectly aid in suppressing Kelvin-Helmholtz instabilities. If conduction along the field lines is unsuppressed, we find that the characteristic sharp jumps seen in Chandra observations of cold front clusters do not form. Therefore, the presence of cold fronts in hot clusters is in contradiction with our simulations with full Spitzer conduction. This suggests that the presence of cold fronts in hot clusters could be used to place upper limits on conduction in the bulk of the intracluster medium. Finally, the combination of sloshing and anisotropic thermal conduction can result in a larger flux of heat to the core than either process in isolation. While still not sufficient to prevent a cooling catastrophe in the very central (r {approx} 5 kpc) regions of the cool core (where something else is required, such as active galactic nucleus feedback), it reduces significantly the mass of gas that experiences a cooling catastrophe outside those small radii.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

NONLINEAR EVOLUTION OF RAYLEIGH-TAYLOR INSTABILITY IN A RADIATION-SUPPORTED ATMOSPHERE

The nonlinear regime of Rayleigh-Taylor instability (RTI) in a radiation supported atmosphere, consisting of two uniform fluids with different densities, is studied numerically. We perform simulations using our recently developed numerical algorithm for multi-dimensional radiation hydrodynamics based on a variable Eddington tensor (VET) as implemented in Athena, focusing on the regime where scattering opacity greatly exceeds absorption opacity. We find that the radiation field can reduce the growth and mixing rate of RTI, but this reduction is only significant when radiation pressure significantly exceeds gas pressure. Small-scale structures are also suppressed in this case. In the nonlinear regime, dense fingers sink faster than rarefied bubbles can rise, leading to asymmetric structures about the interface. By comparing the calculations that use a VET versus the Eddington approximation, we demonstrate that anisotropy in the radiation field can affect the nonlinear development of RTI significantly. We also examine the disruption of a shell of cold gas being accelerated by strong radiation pressure, motivated by models of radiation driven outflows in ultraluminous infrared galaxies. We find that when the growth timescale of RTI is smaller than acceleration timescale, the amount of gas that would be pushed away by the radiation field is reduced due to RTI.

79 ASTRONOMY AND ASTROPHYSICS↗