arXiv subjects

Jeremiah W. Murphy

Publications and source records attributed to Jeremiah W. Murphy.

36 records · Page 2Linked to original sources

arXiv2018

Inferring Properties of the ISM from Supernova Remnant Size Distributions

We model the size distribution of supernova remnants to infer the surrounding ISM density. Using simple, yet standard SNR evolution models, we find that the distribution of ambient densities is remarkably narrow; either the standard assumptions about SNR evolution are wrong, or observable SNRs are biased to a narrow range of ambient densities. We show that the size distributions are consistent with log-normal, which severely limits the number of model parameters in any SNR population synthesis model. Simple Monte Carlo simulations demonstrate that the size distribution is indistinguishable from log-normal when the SNR sample size is less than 600. This implies that these SNR distributions provide only information on the mean and variance, yielding additional information only when the sample size grows larger than $\sim{600}$ SNRs. To infer the parameters of the ambient density, we use Bayesian statistical inference under the assumption that SNR evolution is dominated by the Sedov phase. In particular, we use the SNR sizes and explosion energies to estimate the mean and variance of the ambient medium surrounding SNR progenitors. We find that the mean ISM particle density around our sample of SNRs is $μ_{\log{n}} = -1.33$, in $\log_{10}$ of particles per cubic centimeter, with variance $σ^2_{\log{n}} = 0.49$. If interpreted at face value, this implies that most SNRs result from supernovae propagating in the warm, ionized medium. However, it is also likely that either SNR evolution is not dominated by the simple Sedov evolution or SNR samples are biased to the warm, ionized medium (WIM).

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arXiv2018

The Progenitor Age and Mass of the Black-Hole-Formation Candidate N6946-BH1

The failed supernova N6946-BH1 likely formed a black hole (BH); we age-date the surrounding population and infer an age and initial mass for the progenitor of this BH formation candidate. First, we use archival Hubble Space Telescope imaging to extract broadband photometry of the resolved stellar populations surrounding this event. Using this photometry, we fit stellar evolution models to the color-magnitude diagrams to measure the recent star formation history (SFH). Modeling the photometry requires an accurate distance; therefore, we measure the tip of the red giant branch (TRGB) and infer a distance modulus of $29.47 \pm 0.079$ to NGC~6946, or a metric distance of $7.83 \pm 0.29$ Mpc. To estimate the stellar population's age, we convert the SFH and uncertainties into a probabilistic distribution for the progenitor's age. The region in the immediate vicinity of N6946-BH1 exhibits the youngest and most vigorous star formation for several hundred pc. This suggests that the progenitor is not a runaway star. From these measurements, we infer an age for the BH progenitor of $10.6^{+14.5}_{-5.9}$ Myr. Assuming that the progenitor evolved effectively as a single star, this corresponds to an initial mass of $17.9^{+29.9}_{-7.6}$ $M_{\odot}$. Previous spectral energy distribution (SED) modeling of the progenitor suggests a mass of $\sim$27 $M_{\odot}$. Formally, the SED-derived mass falls within our narrowest 68\% confidence interval; however, $91\%$ of the probability distribtuion function we measure lies below that mass, putting some tension between the age and the direct-imaging results.

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arXiv2018

Progenitor Mass Distribution for Core-Collapse Supernova Remnants in M31 and M33

Using the star formation histories (SFHs) near 94 supernova remnants (SNRs), we infer the progenitor mass distribution for core-collapse supernovae. We use Bayesian inference and model each SFH with multiple bursts of star formation (SF), one of which is assumed to be associated with the SNR. Assuming single-star evolution, the minimum mass of CCSNe is $7.33^{+0.02}_{-0.16}$ $\text{M}_\odot$, the slope of the progenitor mass distribution is $α= -2.96^{+0.45}_{-0.25}$, and the maximum mass is greater than $\text{M}_\textrm{max} > 59$ $\text{M}_\odot$ with a 68% confidence. While these results are consistent with previous inferences, they also provide tighter constraints. The progenitor distribution is somewhat steeper than a Salpeter initial mass function ($α$ = -2.35). This suggests that either SNR catalogs are biased against the youngest SF regions, or the most massive stars do not explode as easily as lower mass stars. If SNR catalogs are biased, it will most likely affect the slope but not the minimum mass. The uncertainties are dominated by three primary sources of uncertainty, the SFH resolution, the number of SF bursts, and the uncertainty on SF rate in each age bin. We address the first two of these uncertainties, with an emphasis on multiple bursts. The third will be addressed in future work.

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arXiv2018

A Complete Census of Luminous Stellar Variability on Day to Decade Timescales

Stellar photometric variability offers a novel probe of the interior structure and evolutionary state of stars. Here we present a census of stellar variability on day to decade timescales across the color-magnitude diagram for 73,000 stars brighter than $M_I$=-5 in the Whirlpool Galaxy (M51). Our Cycle 24 HST program acquired V and I-band images over 34 epochs spanning one year with pseudo-random cadences enabling sensitivity to periods from days to months. We supplement these data with archival V and I-band HST data obtained in 1995 and 2005 providing sensitivity to variability on decade timescales. At least 50% of stars brighter than $M_I$=-7 show strong evidence for variability within our Cycle 24 data; among stars with V-I>2 the variability fraction rises to ~100%. Large amplitude variability (>0.3 mag) on decade timescales is restricted to red supergiants and very luminous blue stars. Both populations display fairly smooth variability on month-year timescales. The Cepheid instability strip is clearly visible in our data, although the variability fraction within this region never exceeds ~10%. The location of variable stars across the color magnitude diagram broadly agrees with theoretical sources of variability, including the instability strip, red supergiant pulsational instabilities, long-period fundamental mode pulsations, and radiation-dominated envelopes in massive stars. Our data can be used to place stringent constraints on the precise onset of these various instabilities and their lifetimes and growth rates.

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arXiv2018

Constraints for the Progenitor Masses of Historic Core-Collapse Supernovae

We age-date the stellar populations associated with 12 historic nearby core-collapse supernovae (CCSNe) and 2 supernova impostors, and from these ages, we infer their initial masses and associated uncertainties. To do this, we have obtained new HST imaging covering these CCSNe. Using these images, we measure resolved stellar photometry for the stars surrounding the locations of the SNe. We then fit the color-magnitude distributions of this photometry with stellar evolution models to determine the ages of any young existing populations present. From these age distributions, we infer the most likely progenitor mass for all of the SNe in our sample. We find ages between 4 and 50 Myr, corresponding to masses from 7.5 to 59 solar masses. There were no SNe that lacked a young population within 50~pc. Our sample contains 4 type Ib/c SNe; their masses have a wide range of values, suggesting that the progenitors of stripped-envelope SNe are binary systems. Both impostors have masses constrained to be $\lesssim$7.5 solar masses. In cases with precursor imaging measurements, we find that age-dating and precursor imaging give consistent progenitor masses. This consistency implies that, although the uncertainties for each technique are significantly different, the results of both are reliable to the measured uncertainties. We combine these new measurements with those from our previous work and find that the distribution of 25 core-collapse SNe progenitor masses is consistent with a standard Salpeter power-law mass function, no upper mass cutoff, and an assumed minimum mass for core-collapse of 7.5~M$_{\odot}$.

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arXiv2017

Modelling Luminous-Blue-Variable Isolation

Observations show that luminous blue variables (LBVs) are far more dispersed than massive O-type stars, and Smith & Tombleson suggested that these large separations are inconsistent with a single-star evolution model of LBVs. Instead, they suggested that the large distances are most consistent with binary evolution scenarios. To test these suggestions, we modelled young stellar clusters and their passive dissolution, and we find that, indeed, the standard single-star evolution model is mostly inconsistent with the observed LBV environments. If LBVs are single stars, then the lifetimes inferred from their luminosity and mass are far too short to be consistent with their extreme isolation. This implies that there is either an inconsistency in the luminosity- to-mass mapping or the mass-to-age mapping. In this paper, we explore binary solutions that modify the mass-to-age mapping and are consistent with the isolation of LBVs. For the binary scenarios, our crude models suggest that LBVs are rejuvenated stars. They are either the result of mergers or they are mass gainers and received a kick when the primary star exploded. In the merger scenario, if the primary is about 19 solar masses, then the binary has enough time to wander far afield, merge and form a rejuvenated star. In the mass-gainer and kick scenario, we find that LBV isolation is consistent with a wide range of kick velocities, anywhere from 0 to ~ 105 km/s. In either scenario, binarity seems to play a major role in the isolation of LBVs.

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arXiv2017

An Integral Condition for Core-Collapse Supernova Explosions

We derive an integral condition for core-collapse supernova (CCSN) explosions and use it to construct a new diagnostic of explodability. The fundamental challenge in CCSN theory is to explain how a stalled accretion shock revives to explode a star. In this manuscript, we assume that the shock revival is initiated by the delayed-neutrino mechanism and derive an integral condition for spherically symmetric shock expansion, $v_s > 0$. One of the most useful one-dimensional explosion conditions is the neutrino luminosity and mass-accretion rate ($L_ν-\dot{\mathcal{M}}$) critical curve. Below this curve, steady-state stalled solutions exist, but above this curve, there are no stalled solutions. Burrows & Goshy suggested that the solutions above this curve are dynamic and explosive. In this manuscript, we take one step closer to proving this supposition; we show that all steady solutions above this curve have $v_s > 0$. Assuming that these steady $v_s > 0$ solutions correspond to explosion, we present a new dimensionless integral condition for explosion, $Ψ> 0$. $Ψ$ roughly describes the balance between pressure and gravity, and we show that this parameter is equivalent to the $τ$ condition used to infer the $L_ν-\dot{\mathcal{M}}$ critical curve. The illuminating difference is that there is a direct relationship between $Ψ$ and $v_s$. Below the critical curve, $Ψ$ may be negative, positive, and zero, which corresponds to receding, expanding, and stalled-shock solutions. At the critical curve, the minimum $Ψ$ solution is zero; above the critical curve, $Ψ_{\rm min} > 0$, and all steady solutions have $v_s > 0$. Using one-dimensional simulations, we confirm our primary assumptions and verify that $Ψ_{\rm min} > 0$ is a reliable and accurate explosion diagnostic.

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arXiv2014

The Supernova Progenitor Mass Distributions of M31 and M33: Further Evidence for an Upper Mass Limit

Using Hubble Space Telescope (HST) photometry to measure star formation histories, we age-date the stellar populations surrounding supernova remnants (SNRs) in M31 and M33. We then apply stellar evolution models to the ages to infer the corresponding masses for their supernova progenitor stars. We analyze 33 M33 SNR progenitors and 29 M31 SNR progenitors in this work. We then combine these measurements with 53 previously published M31 SNR progenitor measurements to bring our total number of progenitor mass estimates to 115. To quantify the mass distributions, we fit power laws of the form $dN/dM \propto M^{-α}$. Our new, larger sample of M31 progenitors follows a distribution with $α= 4.4\pm 0.4$, and the M33 sample follows a distribution with $α= 3.8^{+0.4}_{-0.5}$. Thus both samples are consistent within the uncertainties, and the full sample across both galaxies gives $α= 4.2\pm 0.3$. Both the individual and full distributions display a paucity of massive stars when compared to a Salpeter initial mass function (IMF), which we would expect to observe if all massive stars exploded as SN that leave behind observable SNR. If we instead fix $α= 2.35$ and treat the maximum mass as a free parameter, we find $M_{max} \sim 35-45M_{sun}$, indicative of a potential maximum cutoff mass for SN production. Our results suggest that either SNR surveys are biased against finding objects in the youngest (<10 Myr old) regions, or the highest mass stars do not produce SNe.

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arXiv2013

The Dominance of Neutrino-Driven Convection in Core-Collapse Supernovae

Multi-dimensional instabilities have become an important ingredient in core-collapse supernova (CCSN) theory. Therefore, it is necessary to understand the driving mechanism of the dominant instability. We compare our parameterized three-dimensional CCSN simulations with other buoyancy-driven simulations and propose scaling relations for neutrino-driven convection. Through these comparisons, we infer that buoyancy-driven convection dominates post-shock turbulence in our simulations. In support of this inference, we present four major results. First, the convective fluxes and kinetic energies in the neutrino-heated region are consistent with expectations of buoyancy-driven convection. Second, the convective flux is positive where buoyancy actively drives convection, and the radial and tangential components of the kinetic energy are in rough equipartition (i.e. K_r ~ K_θ + K_ϕ). Both results are natural consequences of buoyancy-driven convection, and are commonly observed in simulations of convection. Third, buoyant driving is balanced by turbulent dissipation. Fourth, the convective luminosity and turbulent dissipation scale with the driving neutrino power. In all, these four results suggest that in neutrino-driven explosions, the multi-dimensional motions are consistent with neutrino-driven convection.

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arXiv2012

Supernova Remnant Progenitor Masses in M31

Using HST photometry, we age-date 59 supernova remnants (SNRs) in the spiral galaxy M31 and use these ages to estimate zero-age main sequence masses (MZAMS) for their progenitors. To accomplish this, we create color-magnitude diagrams (CMDs) and use CMD fitting to measure the recent star formation history (SFH) of the regions surrounding cataloged SNR sites. We identify any young coeval population that likely produced the progenitor star and assign an age and uncertainty to that population. Application of stellar evolution models allows us to infer the MZAMS from this age. Because our technique is not contingent on precise location of the progenitor star, it can be applied to the location of any known SNR. We identify significant young SF around 53 of the 59 SNRs and assign progenitor masses to these, representing a factor of 2 increase over currently measured progenitor masses. We consider the remaining 6 SNRs as either probable Type Ia candidates or the result of core-collapse progenitors that have escaped their birth sites. The distribution of recovered progenitor masses is bottom heavy, showing a paucity of the most massive stars. If we assume a single power law distribution, dN/dM proportional to M^alpha, we find a distribution that is steeper than a Salpeter IMF (alpha=-2.35). In particular, we find values of alpha outside the range -2.7 to -4.4 inconsistent with our measured distribution at 95% confidence. If instead we assume a distribution that follows a Salpeter IMF up to some maximum mass, we find that values of M_max greater than 26 Msun are inconsistent with the measured distribution at 95% confidence. In either scenario, the data suggest that some fraction of massive stars may not explode. The result is preliminary and requires more SNRs and further analysis. In addition, we use our distribution to estimate a minimum mass for core collapse between 7.0 and 7.8 Msun.

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arXiv2012

Dimensional Dependence of the Hydrodynamics of Core-Collapse Supernovae

The multidimensional character of the hydrodynamics in core-collapse supernova (CCSN) cores is a key facilitator of explosions. Unfortunately, much of this work has necessarily been performed assuming axisymmetry and it remains unclear whether or not this compromises those results. In this work, we present analyses of simplified two- and three-dimensional CCSN models with the goal of comparing the multidimensional hydrodynamics in setups that differ only in dimension. Not surprisingly, we find many differences between 2D and 3D models. While some differences are subtle and perhaps not crucial to understanding the explosion mechanism, others are quite dramatic and make interpreting 2D CCSN models problematic. In particular, we find that imposing axisymmetry artificially produces excess power at the largest spatial scales, power that has been deemed critical in the success of previous explosion models and has been attributed solely to the standing accretion shock instability. Nevertheless, our 3D models, which have an order of magnitude less power on large scales compared to 2D models, explode earlier. Since we see explosions earlier in 3D than in 2D, the vigorous sloshing associated with the large scale power in 2D models is either not critical in any dimension or the explosion mechanism operates differently in 2D and 3D. Possibly related to the earlier explosions in 3D, we find that about 25% of the accreted material spends more time in the gain region in 3D than in 2D, being exposed to more integrated heating and reaching higher peak entropies, an effect we associate with the differing characters of turbulence in 2D and 3D. Finally, we discuss a simple model for the runaway growth of buoyant bubbles that is able to quantitatively account for the growth of the shock radius and predicts a critical luminosity relation.

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arXiv2012

An Investigation into the Character of Pre-Explosion Core-Collapse Supernova Shock Motion

We investigate the structure of the stalled supernova shock in both 2D and 3D and explore the differences in the effects of neutrino heating and the standing accretion shock instability (SASI). We find that early on the amplitude of the dipolar mode of the shock is factors of 2 to 3 smaller in 3D than in 2D. However, later in both 3D and 2D the monopole and dipole modes start to grow until explosion. Whereas in 2D the (l,m) = (1,0) mode changes sign quasi-periodically, producing the "up-and-down" motion always seen in modern 2D simulations, in 3D this almost never happens. Rather, in 3D when the dipolar mode starts to grow, it grows in magnitude and wanders stochastically in direction until settling before explosion to a particular patch of solid angle. In 2D we find that the amplitude of the dipolar shock deformation separates into two classes. For the first, identified with the SASI and for a wide range of "low" neutrino luminosities, this amplitude remains small and roughly constant. For the other, identified with higher luminosities and neutrino-driven convection, the dipolar amplitude grows sharply. Importantly, it is only for this higher luminosity class that we see neutrino-driven explosions within ~1 second of bounce. Moreover, for the "low" luminosity runs, the power spectra of these dipolar oscillations peak in the 30-50 Hz range associated with advection timescales, while for the high-luminosity runs the power spectra at lower frequencies are significantly more prominent. We associate this enhanced power at lower frequencies with slower convective effects and the secular growth of the dipolar shock amplitude. On the basis of our study, we hypothesize that neutrino-driven buoyant convection should almost always dominate the SASI when the supernova explosion is neutrino-driven.

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arXiv2011

A Global Turbulence Model for Neutrino-Driven Convection in Core-Collapse Supernovae

Simulations of core-collapse supernovae (CCSNe) result in successful explosions once the neutrino luminosity exceeds a critical curve, and recent simulations indicate that turbulence further enables explosion by reducing this critical neutrino luminosity. We propose a theoretical framework to derive this result and take the first steps by deriving the governing mean-field equations. Using Reynolds decomposition, we decompose flow variables into background and turbulent flows and derive self-consistent averaged equations for their evolution. As basic requirements for the CCSN problem, these equations naturally incorporate steady-state accretion, neutrino heating and cooling, non-zero entropy gradients, and turbulence terms associated with buoyant driving, redistribution, and dissipation. Furthermore, analysis of two-dimensional (2D) CCSN simulations validate these Reynolds-averaged equations, and we show that the physics of turbulence entirely accounts for the differences between 1D and 2D CCSN simulations. As a prelude to deriving the reduction in the critical luminosity, we identify the turbulent terms that most influence the conditions for explosion. Generically, turbulence equations require closure models, but these closure models depend upon the macroscopic properties of the flow. To derive a closure model that is appropriate for CCSNe, we cull the literature for relevant closure models and compare each with 2D simulations. These models employ local closure approximations and fail to reproduce the global properties of neutrino-driven turbulence. Motivated by the generic failure of these local models, we propose an original model for turbulence which incorporates global properties of the flow. This global model accurately reproduces the turbulence profiles and evolution of 2D CCSN simulations.

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arXiv2011

The Progenitor Mass of SN 2011dh from Stellar Populations Analysis

Using Hubble Space Telescope (HST) photometry, we characterize the age of the stellar association in the vicinity of supernova (SN) 2011dh and use it to infer the zero-age main sequence mass (M_{ZAMS}) of the progenitor star. We find two distinct and significant star formation events with ages of <6 and 17^{+3}_{-4} Myrs, and the corresponding M_{ZAMS} are >29 and 13^{+2}_{-1} M_{Sun}, respectively. These two bursts represent 18^{+4}_{-9}% (young) and 64^{+10}_{-14}% (old) of the total star formation in the last 50 Myrs. Adopting these fractions as probabilities suggests that the most probable M_{ZAMS} is 13^{+2}_{-1} M_{Sun}. These results are most sensitive to the luminosity function along the well-understood main sequence and are less sensitive to uncertain late-stage stellar evolution. Therefore, they stand even if the progenitor suffered disruptive post-main-sequence evolution (e.g. eruptive mass loss or binary Roche-lobe overflow). Progenitor identification will help to further constrain the appropriate population. Even though pre-explosion images show a yellow supergiant (YSG) at the site of the SN, panchromatic SN light curves suggest a more compact star as the progenitor. In spite of this, our results suggest an association between the YSG and the SN. Not only was the star located at the SN site, but reinforcing an association, the star's bolometric luminosity is consistent with the final evolutionary stage of the 17 Myr old star burst. If the YSG disappears, then M_{ZAMS}=13^{+2}_{-1} M_{Sun}, but if it persists, then our results allow the possibility that the progenitor was an unseen star of >29 M_{Sun}.

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arXiv2009

A Model for Gravitational Wave Emission from Neutrino-Driven Core-Collapse Supernovae

Abridged: Using a suite of progenitor models, neutrino luminosities, and two- dimensional (2D) simulations, we investigate the matter gravitational-wave (GW) emission from postbounce phases of neutrino-driven core-collapse supernovae (CCSNe). The relevant phases are prompt and steady-state convection, the standing accretion shock instability (SASI), and asymmetric explosions. For the stages before explosion, we propose a model for the source of GW emission. Downdrafts of the postshock-convection/SASI region strike the protoneutron star "surface" with large speeds and are decelerated by buoyancy forces. We find that the GW amplitude is set by the magnitude of deceleration and, by extension, the downdraft's speed and the vigor of postshock-convective/SASI motions. However, the characteristic frequencies, which evolve from ~100 Hz after bounce to ~300-400 Hz, are primarily independent of these speeds, but are set by the deceleration timescale, which is in turn set by the buoyancy frequency at the lower boundary of postshock convection. Consequently, the GW characteristic frequencies are dependent upon a combination of core structure attributes, specifically the dense-matter equation of state (EOS) and details that determine the gradients at the boundary, including the accretion-rate history, the EOS at subnuclear densities, and neutrino transport. During explosion, the high frequency signal wanes and is replaced by a strong low frequency, ~10s of Hz, signal that reveals the general morphology of the explosion (i.e. prolate, oblate, or spherical). However, current and near-future GW detectors are sensitive to GW power at frequencies >50 Hz. Therefore, the signature of explosion will be the abrupt reduction of detectable GW emission.

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arXiv2009

The NGC 300 Transient: An Alternative Method For Measuring Progenitor Masses

We present an alternative technique for measuring the precursor masses of transient events in stars undergoing late stage stellar evolution. We use the well-established techniques of stellar population modeling to age-date the stars surrounding the site of the recent transient event in NGC 300 (NGC 300 OT2008-1). The surrounding stars must share a common turnoff mass with the transient, since almost all stars form in stellar clusters that remain physically associated for periods longer than the lifetime of the most massive stars. We find that the precursor of NGC 300 OT2008-1 is surrounded by stars that formed in a single burst between 8-13 Myr ago, with 70% confidence. The transient was therefore likely to be due to a progenitor whose mass falls between the main sequence turnoff mass (12 Msun) and the maximum stellar mass (25 Msun) found for isochrones bounding this age range. We characterize the general applicability of this technique in identifying precursor masses of historic and future transients and supernovae (SNe), noting that it requires neither precursor imaging nor sub-arcsecond accuracy in the position of the transient. It is also based on the well-understood physics of the main sequence, and thus may be a more reliable source of precursor masses than fitting evolutionary tracks to precursor magnitudes. We speculate that if the progenitor mass is close to 17 Msun, there may be a connection between optical transients such as NGC 300 OT2008-1 and the missing type II-P SNe, known as the "red supergiant problem."

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arXiv2008

Criteria for Core-Collapse Supernova Explosions by the Neutrino Mechanism

We investigate the criteria for successful core-collapse supernova explosions by the neutrino mechanism. We find that a critical-luminosity/mass-accretion-rate condition distinguishes non-exploding from exploding models in hydrodynamic one-dimensional (1D) and two-dimensional (2D) simulations. We present 95 such simulations that parametrically explore the dependence on neutrino luminosity, mass accretion rate, resolution, and dimensionality. While radial oscillations mediate the transition between 1D accretion (non-exploding) and exploding simulations, the non-radial standing accretion shock instability characterizes 2D simulations. We find that it is useful to compare the average dwell time of matter in the gain region with the corresponding heating timescale, but that tracking the residence time distribution function of tracer particles better describes the complex flows in multi-dimensional simulations. Integral quantities such as the net heating rate, heating efficiency, and mass in the gain region decrease with time in non-exploding models, but for 2D exploding models, increase before, during, and after explosion. At the onset of explosion in 2D, the heating efficiency is $\sim$2% to $\sim$5% and the mass in the gain region is $\sim$0.005 M$_{\sun}$ to $\sim$0.01 M$_{\sun}$. Importantly, we find that the critical luminosity for explosions in 2D is $\sim$70% of the critical luminosity required in 1D. This result is not sensitive to resolution or whether the 2D computational domain is a quadrant or the full 180$^{\circ}$. We suggest that the relaxation of the explosion condition in going from 1D to 2D (and to, perhaps, 3D) is of a general character and is not limited by the parametric nature of this study.

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arXiv2008

BETHE-Hydro: An Arbitrary Lagrangian-Eulerian Multi-dimensional Hydrodynamics Code for Astrophysical Simulations

In this paper, we describe a new hydrodynamics code for 1D and 2D astrophysical simulations, BETHE-hydro, that uses time-dependent, arbitrary, unstructured grids. The core of the hydrodynamics algorithm is an arbitrary Lagrangian-Eulerian (ALE) approach, in which the gradient and divergence operators are made compatible using the support-operator method. We present 1D and 2D gravity solvers that are finite differenced using the support-operator technique, and the resulting system of linear equations are solved using the tridiagonal method for 1D simulations and an iterative multigrid-preconditioned conjugate-gradient method for 2D simulations. Rotational terms are included for 2D calculations using cylindrical coordinates. We document an incompatibility between a subcell pressure algorithm to suppress hourglass motions and the subcell remapping algorithm and present a modified subcell pressure scheme that avoids this problem. Strengths of this code include a straightforward structure, enabling simple inclusion of additional physics packages, the ability to use a general equation of state, and most importantly, the ability to solve self-gravitating hydrodynamic flows on time-dependent, arbitrary grids. In what follows, we describe in detail the numerical techniques employed and, with a large suite of tests, demonstrate that BETHE-hydro finds accurate solutions with 2$^{nd}$-order convergence.

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