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At least 685 records · Page 38
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DOE OSTI2011

THE NATURE OF THE WARM/HOT INTERGALACTIC MEDIUM. I. NUMERICAL METHODS, CONVERGENCE, AND O VI ABSORPTION

We perform a series of cosmological simulations using Enzo, an Eulerian adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the warm/hot intergalactic medium (WHIM). The WHIM may be an important component of the baryons missing observationally at low redshift. We investigate the dependence of the global star formation rate and mass fraction in various baryonic phases on spatial resolution and methods of incorporating stellar feedback. Although both resolution and feedback significantly affect the total mass in the WHIM, all of our simulations find that the WHIM fraction peaks at z {approx} 0.5, declining to 35%-40% at z = 0. We construct samples of synthetic O VI absorption lines from our highest-resolution simulations, using several models of oxygen ionization balance. Models that include both collisional ionization and photoionization provide excellent fits to the observed number density of absorbers per unit redshift over the full range of column densities (10{sup 13} cm{sup -2} {approx}< N{sub OVI} {approx}< 10{sup 15} cm{sup -2}). Models that include only collisional ionization provide better fits for high column density absorbers (N{sub OVI} {approx}> 10{sup 14} cm{sup -2}). The distribution of O VI in density and temperature exhibits two populations: one at T {approx} 10{sup 5.5} K (collisionally ionized, 55% of total O VI) and one at T {approx} 10{sup 4.5} K (photoionized, 37%) with the remainder located in dense gas near galaxies. While not a perfect tracer of hot gas, O VI provides an important tool for a WHIM baryon census.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

A NEW JEANS RESOLUTION CRITERION FOR (M)HD SIMULATIONS OF SELF-GRAVITATING GAS: APPLICATION TO MAGNETIC FIELD AMPLIFICATION BY GRAVITY-DRIVEN TURBULENCE

Cosmic structure formation is characterized by the complex interplay between gravity, turbulence, and magnetic fields. The processes by which gravitational energy is converted into turbulent and magnetic energies, however, remain poorly understood. Here, we show with high-resolution, adaptive-mesh simulations that MHD turbulence is efficiently driven by extracting energy from the gravitational potential during the collapse of a dense gas cloud. Compressible motions generated during the contraction are converted into solenoidal, turbulent motions, leading to a natural energy ratio of E{sub sol}/E{sub tot} {approx} 2/3. We find that the energy injection scale of gravity-driven turbulence is close to the local Jeans scale. If small seeds of the magnetic field are present, they are amplified exponentially fast via the small-scale dynamo process. The magnetic field grows most efficiently on the smallest scales, for which the stretching, twisting, and folding of field lines, and the turbulent vortices are sufficiently resolved. We find that this scale corresponds to about 30 grid cells in the simulations. We thus suggest a new minimum resolution criterion of 30 cells per Jeans length in (magneto)hydrodynamical simulations of self-gravitating gas, in order to resolve turbulence on the Jeans scale, and to capture minimum dynamo amplification of the magnetic field. Due to numerical diffusion, however, any existing simulation today can at best provide lower limits on the physical growth rates. We conclude that a small, initial magnetic field can grow to dynamically important strength on timescales significantly shorter than the free-fall time of the cloud.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

INDUCED ROTATION IN THREE-DIMENSIONAL SIMULATIONS OF CORE-COLLAPSE SUPERNOVAE: IMPLICATIONS FOR PULSAR SPINS

It has been suggested that the observed rotation periods of radio pulsars might be induced by a non-axisymmetric spiral-mode instability in the turbulent region behind the stalled supernova bounce shock, even if the progenitor core was not initially rotating. In this paper, using the three-dimensional Adaptive Mesh Refinement code CASTRO with a realistic progenitor and equation of state and a simple neutrino heating and cooling scheme, we present a numerical study of the evolution in three dimensions of the rotational profile of a supernova core from collapse, through bounce and shock stagnation, to delayed explosion. By the end of our simulation ({approx}420 ms after core bounce), we do not witness significant spin-up of the proto-neutron star core left behind. However, we do see the development before the explosion of strong differential rotation in the turbulent gain region between the core and stalled shock. Shells in this region acquire high spin rates that reach {approx}150 Hz, but this region contains too little mass and angular momentum to translate, even if left behind, into rapid rotation for the full neutron star. We also find that much of the induced angular momentum is likely to be ejected in the explosion, and moreover that even if the optimal amount of induced angular momentum is retained in the core, the resulting spin period is likely to be quite modest. Nevertheless, induced periods of seconds are possible.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

FORMATION OF COMPACT STELLAR CLUSTERS BY HIGH-REDSHIFT GALAXY OUTFLOWS. II. EFFECT OF TURBULENCE AND METAL-LINE COOLING

In the primordial universe, low-mass structures with virial temperatures less than 10{sup 4} K were unable to cool by atomic line transitions, leading to a strong suppression of star formation. On the other hand, these 'minihalos' were highly prone to triggered star formation by interactions from nearby galaxy outflows. In Gray and Scannapieco, we explored the impact of nonequilibrium chemistry on these interactions. Here we turn our attention to the role of metals, carrying out a series of high-resolution three-dimensional adaptive mesh refinement simulations that include both metal cooling and a subgrid turbulent mixing model. Despite the presence of an additional coolant, we again find that outflow-minihalo interactions produce a distribution of dense, massive stellar clusters. We also find that these clusters are evenly enriched with metals to a final abundance of Z {approx} 10{sup -2} Z{sub sun}. As in our previous simulations, all of these properties suggest that these interactions may have given rise to present-day halo globular clusters.

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DOE OSTI2011

A HYBRID SOLAR WIND MODEL OF THE CESE+HLL METHOD WITH A YIN-YANG OVERSET GRID AND AN AMR GRID

A hybrid three-dimensional (3D) MHD model for solar wind study is proposed in the present paper with combined grid systems and solvers. The computational domain from the Sun to Earth space is decomposed into the near-Sun and off-Sun domains, which are respectively constructed with a Yin-Yang overset grid system and a Cartesian adaptive mesh refinement (AMR) grid system and coupled with a domain connection interface in the overlapping region between the near-Sun and off-Sun domains. The space-time conservation element and solution element method is used in the near-Sun domain, while the Harten-Lax-Leer method is employed in the off-Sun domain. The Yin-Yang overset grid can avoid well-known singularity and polar grid convergence problems and its body-fitting property helps achieve high-quality resolution near the solar surface. The block structured AMR Cartesian grid can automatically capture far-field plasma flow features, such as heliospheric current sheets and shock waves, and at the same time, it can save significant computational resources compared to the uniformly structured Cartesian grid. A numerical study of the solar wind structure for Carrington rotation 2069 shows that the newly developed hybrid MHD solar wind model successfully produces many realistic features of the background solar wind, in both the solar corona and interplanetary space, by comparisons with multiple solar and interplanetary observations.

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DOE OSTI2011

SIMULATED VOID GALAXIES IN THE STANDARD COLD DARK MATTER MODEL

We analyze a (120 h{sup -1} Mpc){sup 3} adaptive mesh refinement hydrodynamic simulation that contains a higher resolution 31 x 31 x 35 h{sup -3} Mpc subvolume centered on a {approx}30 Mpc diameter void. Our detailed {approx}1 kpc resolution allows us to identify 1300 galaxies within this void to a limiting halo mass of {approx}10{sup 10} M{sub sun}. Nearly 1000 galaxies are found to be in underdense regions, with 300 galaxies residing in regions less than half the mean density of the simulation volume. We construct mock observations of the stellar and gas properties of these systems and reproduce the range of colors and luminosities observed in the Sloan Digital Sky Survey for nearby (z < 0.03) galaxies. We find no trends with density for the most luminous (M{sub r} < -18) galaxies, however our dwarf void galaxies (M{sub r} > -16), though they are less reliably resolved, typically appear bluer, with higher rates of star formation and specific star formation and lower mean stellar ages than galaxies in average density environments. We find a significant population of low-luminosity (M{sub r} {approx} -14) dwarf galaxies that is preferentially located in low-density regions and specifically in the void center. This population may help to reduce, but not remove, the discrepancy between the predicted and observed number of void galaxies.

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DOE OSTI2011

THE COMPLEXITY THAT THE FIRST STARS BROUGHT TO THE UNIVERSE: FRAGILITY OF METAL-ENRICHED GAS IN A RADIATION FIELD

The initial mass function (IMF) of the first (Population III) stars and Population II (Pop II) stars is poorly known due to a lack of observations of the period between recombination and reionization. In simulations of the formation of the first stars, it has been shown that, due to the limited ability of metal-free primordial gas to cool, the IMF of the first stars is a few orders of magnitude more massive than the current IMF. The transition from a high-mass IMF of the first stars to a lower-mass current IMF is thus important to understand. To study the underlying physics of this transition, we performed several simulations using the cosmological hydrodynamic adaptive mesh refinement code Enzo for metallicities of 10{sup -4}, 10{sup -3}, 10{sup -2}, and 10{sup -1} Z{sub sun}. In our simulations, we include a star formation prescription that is derived from a metallicity-dependent multi-phase interstellar medium (ISM) structure, an external UV radiation field, and a mechanical feedback algorithm. We also implement cosmic ray heating, photoelectric heating, and gas-dust heating/cooling, and follow the metal enrichment of the ISM. It is found that the interplay between metallicity and UV radiation leads to the coexistence of Pop III and Pop II star formation in non-zero-metallicity (Z/Z{sub sun} {>=} 10{sup -2}) gas. A cold (T < 100 K) and dense ({rho} > 10{sup -22} g cm{sup -3}) gas phase is fragile to ambient UV radiation. In a metal-poor (Z/Z{sub sun} {<=} 10{sup -3}) gas, the cold and dense gas phase does not form in the presence of a radiation field of F{sub 0} {approx} 10{sup -5}-10{sup -4} erg cm{sup -2} s{sup -1}. Therefore, metallicity by itself is not a good indicator of the Pop III-Pop II transition. Metal-rich (Z/Z{sub sun} {>=} 10{sup -2}) gas dynamically evolves two to three orders of magnitude faster than metal-poor gas (Z/Z{sub sun} {<=} 10{sup -3}). The simulations including supernova explosions show that pre-enrichment of the halo does not affect the mixing of metals.

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DOE OSTI2011

EVOLUTION AND DISTRIBUTION OF MAGNETIC FIELDS FROM ACTIVE GALACTIC NUCLEI IN GALAXY CLUSTERS. II. THE EFFECTS OF CLUSTER SIZE AND DYNAMICAL STATE

Theory and simulations suggest that magnetic fields from radio jets and lobes powered by their central super massive black holes can be an important source of magnetic fields in the galaxy clusters. This is Paper II in a series of studies where we present self-consistent high-resolution adaptive mesh refinement cosmological magnetohydrodynamic simulations that simultaneously follow the formation of a galaxy cluster and evolution of magnetic fields ejected by an active galactic nucleus. We studied 12 different galaxy clusters with virial masses ranging from 1 x 10{sup 14} to 2 x 10{sup 15} M{sub sun}. In this work, we examine the effects of the mass and merger history on the final magnetic properties. We find that the evolution of magnetic fields is qualitatively similar to those of previous studies. In most clusters, the injected magnetic fields can be transported throughout the cluster and be further amplified by the intracluster medium (ICM) turbulence during the cluster formation process with hierarchical mergers, while the amplification history and the magnetic field distribution depend on the cluster formation and magnetism history. This can be very different for different clusters. The total magnetic energies in these clusters are between 4 x 10{sup 57} and 10{sup 61} erg, which is mainly decided by the cluster mass, scaling approximately with the square of the total mass. Dynamically older relaxed clusters usually have more magnetic fields in their ICM. The dynamically very young clusters may be magnetized weakly since there is not enough time for magnetic fields to be amplified.

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DOE OSTI2011

MAGNETIC FLUX EXPULSION IN STAR FORMATION

Stars form in dense cores of magnetized molecular clouds. If the magnetic flux threading the cores is dragged into the stars, the stellar field would be orders of magnitude stronger than observed. This well-known 'magnetic flux problem' demands that most of the core magnetic flux be decoupled from the matter that enters the star. We carry out the first exploration of what happens to the decoupled magnetic flux in three dimensions, using a magnetohydrodynamic (MHD) version of the ENZO adaptive mesh refinement code. The field-matter decoupling is achieved through a sink particle treatment, which is needed to follow the protostellar accretion phase of star formation. We find that the accumulation of the decoupled flux near the accreting protostar leads to a magnetic pressure buildup. The high pressure is released anisotropically along the path of least resistance. It drives a low-density expanding region in which the decoupled magnetic flux is expelled. This decoupling-enabled magnetic structure has never been seen before in three-dimensional MHD simulations of star formation. It generates a strong asymmetry in the protostellar accretion flow, potentially giving a kick to the star. In the presence of an initial core rotation, the structure presents an obstacle to the formation of a rotationally supported disk, in addition to magnetic braking, by acting as a rigid magnetic wall that prevents the rotating gas from completing a full orbit around the central object. We conclude that the decoupled magnetic flux from the stellar matter can strongly affect the protostellar collapse dynamics.

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DOE OSTI2011

FORMATION OF COMPACT STELLAR CLUSTERS BY HIGH-REDSHIFT GALAXY OUTFLOWS. III. OBSERVABILITY AND CONNECTION TO HALO GLOBULAR CLUSTERS

The early universe hosted a large population of low-mass virialized 'minihalos', that were not massive enough to form stars on their own. While most minihalos were photoevaporated by ionizing photons from star-forming galaxies, these galaxies also drove large outflows, which in some cases would have reached the minihalos in advance of ionization fronts. In the previous papers in this series, we carried out high-resolution, three-dimensional adaptive mesh refinement simulations of outflow-minihalo interactions that included non-equilibrium chemistry, radiative cooling, and turbulent mixing. We found that, for a fiducial set of parameters, minihalos were transformed into dense, chemically homogenous stellar clusters. Here we conduct a suite of simulations that follow these interactions over a wide range of parameters including minihalo mass, minihalo formation redshift, outflow energy, outflow redshift, distance, concentration, and spin. In almost all cases, the shocked minihalos form molecules through non-equilibrium reactions and then cool rapidly to become compact, chemically homogenous stellar clusters. Furthermore, we show that the unique properties of these clusters make them a prime target for direct study with the next generation of telescopes, and that there are many reasons to suspect that their low-redshift counterparts are the observed population of halo globular clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

GAS CONDENSATION IN THE GALACTIC HALO

Using adaptive mesh refinement (AMR) hydrodynamic simulations of vertically stratified hot halo gas, we examine the conditions under which clouds can form and condense out of the hot halo medium to potentially fuel star formation in the gaseous disk. We find that halo clouds do not develop from linear isobaric perturbations. This is a regime where the cooling time is longer than the Brunt-Vaeisaelae time, confirming previous linear analysis. We extend the analysis into the nonlinear regime by considering mildly or strongly nonlinear perturbations with overdensities up to 100, also varying the initial height, the cloud size, and the metallicity of the gas. Here, the result depends on the ratio of cooling time to the time required to accelerate the cloud to the sound speed (similar to the dynamical time). If the ratio exceeds a critical value near unity, the cloud is accelerated without further cooling and gets disrupted by Kelvin-Helmholtz and/or Rayleigh-Taylor instabilities. If it is less than the critical value, the cloud cools and condenses before disruption. Accreting gas with overdensities of 10-20 is expected to be marginally unstable; the cooling fraction will depend on the metallicity, the size of the incoming cloud, and the distance to the galaxy. Locally enhanced overdensities within cold streams have a higher likelihood of cooling out. Our results have implications on the evolution of clouds seeded by cold accretion that are barely resolved in current cosmological hydrodynamic simulations and absorption line systems detected in galaxy halos.

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DOE OSTI2012

SIMULATING THE COOLING FLOW OF COOL-CORE CLUSTERS

We carry out high-resolution adaptive mesh refinement simulations of a cool core cluster, resolving the flow from Mpc scales down to pc scales. We do not (yet) include any active galactic nucleus (AGN) heating, focusing instead on cooling in order to understand how gas reaches the supermassive black hole at the center of the cluster. We find that, as the gas cools, the cluster develops a very flat temperature profile, undergoing a cooling catastrophe only in the central 10-100 pc of the cluster. Outside of this region, the flow is smooth, with no local cooling instabilities, and naturally produces very little low-temperature gas (below a few keV), in agreement with observations. The gas cooling in the center of the cluster rapidly forms a thin accretion disk. The amount of cold gas produced at the very center grows rapidly until a reasonable estimate of the resulting AGN heating rate (assuming even a moderate accretion efficiency) would overwhelm cooling. We argue that this naturally produces a thermostat which links the cooling of gas out to 100 kpc with the cold gas accretion in the central 100 pc, potentially closing the loop between cooling and heating. Isotropic heat conduction does not affect the result significantly, but we show that including the potential well of the brightest cluster galaxy is necessary to obtain the correct result. Also, we found that the outcome is sensitive to resolution, requiring very high mass resolution to correctly reproduce the small transition radius.

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DOE OSTI2012

THE NATURE OF DAMPED Ly{alpha} SYSTEMS AND THEIR HOSTS IN THE STANDARD COLD DARK MATTER UNIVERSE

Using adaptive mesh refinement cosmological hydrodynamic simulations with a physically motivated supernova feedback prescription, we show that the standard cold dark matter model can account for extant observed properties of damped Ly{alpha} systems (DLAs). With detailed examination of DLAs identified for each redshift snapshot through ray tracing through the simulation volumes containing thousands of galaxies, we find the following: (1) While DLA hosts roughly trace the overall population of galaxies at all redshifts, they are always gas-rich and have tendencies of being slightly smaller and bluer. (2) The history of DLA evolution is cosmological in nature and reflects primarily the evolution of the underlying cosmic density, galaxy size, and galaxy interactions. With higher density and more interactions at high redshift the size of DLAs is a larger fraction of their virial radius. (3) The variety of DLAs at high redshift is richer with a large contribution coming from galactic aqueducts, created through close galaxy interactions. The portion of gaseous disks of galaxies where most stars reside makes a relatively small contribution to DLA incidence at z = 3-4. (4) The majority of DLAs arise in halos of mass M{sub h} = 10{sup 10}-10{sup 12} M{sub Sun} at z = 1.6-4, as these galaxies dominate the overall population of galaxies then. At z = 3-4, 20%-30% of DLA hosts are Lyman break galaxies (LBGs), 10%-20% are due to galaxies more massive than LBGs, and 50%-70% are from smaller galaxies. (5) Galactic winds play an indispensable role in shaping the kinematic properties of DLAs. Specifically, the high velocity width DLAs are a mixture of those arising in high-mass, high velocity dispersion halos and those arising in smaller mass systems where cold gas clouds are entrained to high velocities by galactic winds. (6) In agreement with observations, we see a weak but noticeable evolution in DLA metallicity. The metallicity distribution centers at [Z/H] = -1.5 to -1 and spans more than three decades at z = 3-4, with the peak moving to [Z/H] = -0.75 at z = 1.6 and [Z/H] = -0.5 by z = 0. (7) The star formation rate of DLA hosts is concentrated in the range 0.3-30 M{sub Sun} yr{sup -1} at z = 3-4, gradually shifting lower to peak at {approx}0.5-1 M{sub Sun} yr{sup -1} by z = 0. (8) We predict that only 20%-30% of DLAs are within 100 kpc of L* galaxies at z = 3-4, increasing to 30%-50% at z = 1.6.

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DOE OSTI2012

DWARF GALAXY FORMATION WITH H{sub 2}-REGULATED STAR FORMATION

We describe cosmological galaxy formation simulations with the adaptive mesh refinement code Enzo that incorporate a star formation prescription regulated by the local abundance of molecular hydrogen. We show that this H{sub 2}-regulated prescription leads to a suppression of star formation in low-mass halos (M{sub h} {approx}< 10{sup 10} M{sub Sun }) at z > 4, alleviating some of the dwarf galaxy problems faced by theoretical galaxy formation models. H{sub 2} regulation modifies the efficiency of star formation of cold gas directly, rather than indirectly reducing the cold gas content with 'supernova feedback'. We determine the local H{sub 2} abundance in our most refined grid cells (76 proper parsec in size at z = 4) by applying the model of Krumholz, McKee, and Tumlinson, which is based on idealized one-dimensional radiative transfer calculations of H{sub 2} formation-dissociation balance in {approx}100 pc atomic-molecular complexes. Our H{sub 2}-regulated simulations are able to reproduce the empirical (albeit lower z) Kennicutt-Schmidt relation, including the low {Sigma}{sub gas} cutoff due to the transition from atomic to molecular phase and the metallicity dependence thereof, without the use of an explicit density threshold in our star formation prescription. We compare the evolution of the luminosity function, stellar mass density, and star formation rate density from our simulations to recent observational determinations of the same at z = 4-8 and find reasonable agreement between the two.

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DOE OSTI2012

THE ORIGIN AND DISTRIBUTION OF COLD GAS IN THE HALO OF A MILKY-WAY-MASS GALAXY

We analyze an adaptive mesh refinement hydrodynamic cosmological simulation of a Milky-Way-sized galaxy to study the cold gas in the halo. H I observations of the Milky Way and other nearby spirals have revealed the presence of such gas in the form of clouds and other extended structures, which indicates ongoing accretion. We use a high-resolution simulation (136-272 pc throughout) to study the distribution of cold gas in the halo, compare it with observations, and examine its origin. The amount ({approx}10{sup 8} M{sub Sun} in H I), covering fraction, and spatial distribution of the cold halo gas around the simulated galaxy at z = 0 are consistent with existing observations. At z = 0, the H I mass accretion rate onto the disk is 0.2 M{sub Sun} yr{sup -1}. We track the histories of the 20 satellites that are detected in H I in the redshift interval 0.5 > z > 0 and find that most of them are losing gas, with a median mass-loss rate per satellite of 3.1 Multiplication-Sign 10{sup -3} M{sub Sun} yr{sup -1}. This stripped gas is a significant component of the H I gas seen in the simulation. In addition, we see filamentary material coming into the halo from the intergalactic medium at all redshifts. Most of this gas does not make it directly to the disk, but part of the gas in these structures is able to cool and form clouds. The metallicity of the gas allows us to distinguish between filamentary flows and satellite gas. We find that the former accounts for at least 25%-75% of the cold gas in the halo seen at any redshift analyzed here. Placing constraints on cloud formation mechanisms allows us to better understand how galaxies accrete gas and fuel star formation at z = 0.

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DOE OSTI2012

THE TWO STATES OF STAR-FORMING CLOUDS

We examine the effects of self-gravity and magnetic fields on supersonic turbulence in isothermal molecular clouds with high-resolution simulations and adaptive mesh refinement. These simulations use large root grids (512{sup 3}) to capture turbulence and four levels of refinement to follow the collapse to high densities, for an effective resolution of 8192{sup 3}. Three Mach 9 simulations are performed, two super-Alfvenic and one trans-Alfvenic. We find that gravity splits the clouds into two populations, one low-density turbulent state and one high-density collapsing state. The low-density state exhibits properties similar to non-self-gravitating in this regime, and we examine the effects of varied magnetic field strength on statistical properties: the density probability distribution function is approximately lognormal, the velocity power spectral slopes decrease with decreasing mean field strength, the alignment between velocity and magnetic field increases with the field, and the magnetic field probability distribution can be fitted to a stretched exponential. The high-density state is well characterized by self-similar spheres: the density probability distribution is a power law, collapse rate decreases with increasing mean field, density power spectra have positive slopes, P({rho}, k){proportional_to}k, thermal-to-magnetic pressure ratios are roughly unity for all mean field strengths, dynamic-to-magnetic pressure ratios are larger than unity for all mean field strengths, the magnetic field distribution follows a power-law distribution. The high Alfven Mach numbers in collapsing regions explain the recent observations of magnetic influence decreasing with density. We also find that the high-density state is typically found in filaments formed by converging flows, consistent with recent Herschel observations. Possible modifications to existing star formation theories are explored. The overall trans-Alfvenic nature of star-forming clouds is discussed.

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DOE OSTI2012

IMPACT OF TYPE Ia SUPERNOVA EJECTA ON BINARY COMPANIONS IN THE SINGLE-DEGENERATE SCENARIO

Type Ia supernovae are thought to be caused by thermonuclear explosions of a carbon-oxygen white dwarf in close binary systems. In the single-degenerate scenario (SDS), the companion star is non-degenerate and can be significantly affected by the explosion. We explore this interaction by means of multi-dimensional adaptive mesh refinement simulations using the FLASH code. We consider several different companion types, including main-sequence-like stars (MS), red giants (RG), and helium stars (He). In addition, we include the symmetry-breaking effects of orbital motion, rotation of the non-degenerate star, and Roche-lobe overflow. A detailed study of a sub-grid model for Type Ia supernovae is also presented. We find that the dependence of the unbound stellar mass and kick velocity on the initial binary separation can be fitted by power-law relations. By using the tracer particles in FLASH, the process leading to the unbinding of matter is dominated by ablation, which has usually been neglected in past analytical studies. The level of Ni/Fe contamination of the companion that results from the passage of the supernova ejecta is found to be {approx}10{sup -5} M{sub Sun} for the MS star, {approx}10{sup -4} M{sub Sun} for the He star, and {approx}10{sup -8} M{sub Sun} for the RG. The spinning MS companion star loses about half of its initial angular momentum during the impact, causing the rotational velocity to drop to a quarter of the original rotational velocity, suggesting that the Tycho G star is a promising progenitor candidate in the SDS.

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DOE OSTI2012

SIMULATIONS OF ACCRETION POWERED SUPERNOVAE IN THE PROGENITORS OF GAMMA-RAY BURSTS

Observational evidence suggests a link between long-duration gamma-ray bursts (LGRBs) and Type Ic supernovae. Here, we propose a potential mechanism for Type Ic supernovae in LGRB progenitors powered solely by accretion energy. We present spherically symmetric hydrodynamic simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a 'collapsar', onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in one spatial dimension and with rotation, an explicit shear viscosity, and convection in the mixing length theory approximation. Once the accretion flow becomes rotationally supported outside of the black hole, an accretion shock forms and traverses the stellar envelope. Energy is carried from the central geometrically thick accretion disk to the stellar envelope by convection. Energy losses through neutrino emission and nuclear photodisintegration are calculated but do not seem important following the rapid early drop of the accretion rate following circularization. We find that the shock velocity, energy, and unbound mass are sensitive to convective efficiency, effective viscosity, and initial stellar angular momentum. Our simulations show that given the appropriate combinations of stellar and physical parameters, explosions with energies {approx}5 Multiplication-Sign 10{sup 50} erg, velocities {approx}3000 km s{sup -1}, and unbound material masses {approx}> 6 M{sub Sun} are possible in a rapidly rotating 16 M{sub Sun} main-sequence progenitor star. Further work is needed to constrain the values of these parameters, to identify the likely outcomes in more plausible and massive LRGB progenitors, and to explore nucleosynthetic implications.

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