arXiv subjects

F. X. Timmes

Publications and source records attributed to F. X. Timmes.

At least 73 records · Page 4Linked to original sources

arXiv2011

Remnants of Binary White Dwarf Mergers

We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

astro-ph.HE↗

arXiv2011

Freeze-out yields of radioactivities in core-collapse supernovae

We explore the nucleosynthesis trends from two mechanisms during freeze-out expansions in core-collapse supernovae. The first mechanism is related to the convection and instabilities within homogeneous stellar progenitor matter that is accreted through the supernova shock. The second mechanism is related to the impact of the supersonic wind termination shock (reverse shock) within the tumultuous inner regions of the ejecta above the proto-neutron star. Our results suggest that isotopes in the mass range 12<=A<=122 that are produced during the freeze-out expansions may be classified in two families. The isotopes of the first family manifest a common mass fraction evolutionary profile, whose specific shape per isotope depends on the characteristic transition between two equilibrium states (equilibrium state transition) during each type of freeze-out expansion. The first family includes the majority of isotopes in this mass range. The second family is limited to magic nuclei and isotopes in their locality, which do not sustain any transition, become nuclear flow hubs, and dominate the final composition. We use exponential and power-law adiabatic profiles to identify dynamic large-scale and small-scale equilibrium patterns among nuclear reactions. (truncated abstract) In addition, we introduce non-monotonic parameterized profiles to probe the impact of the reverse shock and multi-dimensional explosion asymmetries on nucleosynthesis. Cases are shown in which the non-monotonic profiles favor the production of radioactivities. Non-monotonic freeze-out profiles involve longer non-equilibrium nucleosynthesis intervals compared with the exponential and power-law profiles, resulting in mass fraction trends and yield distributions that may not be achieved by the monotonic freeze-out profiles.

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arXiv2010

56Ni Production in Double Degenerate White Dwarf Collisions

We present a comprehensive study of white dwarf collisions as an avenue for creating type Ia supernovae. Using a smooth particle hydrodynamics code with a 13-isotope, α-chain nuclear network, we examine the resulting 56Ni yield as a function of total mass, mass ratio, and impact parameter. We show that several combinations of white dwarf masses and impact parameters are able to produce sufficient quantities of 56Ni to be observable at cosmological distances. We find the 56Ni production in double-degenerate white dwarf collisions ranges from sub-luminous to the super-luminous, depending on the parameters of the collision. For all mass pairs, collisions with small impact parameters have the highest likelihood of detonating, but 56Ni production is insensitive to this parameter in high-mass combinations, which significantly increases their likelihood of detection. We also find that the 56Ni dependence on total mass and mass ratio is not linear, with larger mass primaries producing disproportionately more 56Ni than their lower mass secondary counterparts, and symmetric pairs of masses producing more 56Ni than asymmetric pairs.

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arXiv2010

Evaluating Systematic Dependencies of Type Ia Supernovae: The Influence of Deflagration to Detonation Density

We explore the effects of the deflagration to detonation transition (DDT) density on the production of Ni-56 in thermonuclear supernova explosions (type Ia supernovae). Within the DDT paradigm, the transition density sets the amount of expansion during the deflagration phase of the explosion and therefore the amount of nuclear statistical equilibrium (NSE) material produced. We employ a theoretical framework for a well-controlled statistical study of two-dimensional simulations of thermonuclear supernovae with randomized initial conditions that can, with a particular choice of transition density, produce a similar average and range of Ni-56 masses to those inferred from observations. Within this framework, we utilize a more realistic "simmered" white dwarf progenitor model with a flame model and energetics scheme to calculate the amount of Ni-56 and NSE material synthesized for a suite of simulated explosions in which the transition density is varied in the range 1-3x10^7 g/cc. We find a quadratic dependence of the NSE yield on the log of the transition density, which is determined by the competition between plume rise and stellar expansion. By considering the effect of metallicity on the transition density, we find the NSE yield decreases by 0.055 +/- 0.004 solar masses for a 1 solar metallicity increase evaluated about solar metallicity. For the same change in metallicity, this result translates to a 0.067 +/- 0.004 solar mass decrease in the Ni-56 yield, slightly stronger than that due to the variation in electron fraction from the initial composition. Observations testing the dependence of the yield on metallicity remain somewhat ambiguous, but the dependence we find is comparable to that inferred from some studies.

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arXiv2010

On variations of the brightness of type Ia supernovae with the age of the host stellar population

Recent observational studies of type Ia supernovae (SNeIa) suggest correlations between the peak brightness of an event and the age of the progenitor stellar population. This trend likely follows from properties of the progenitor white dwarf (WD), such as central density, that follow from properties of the host stellar population. We present a statistically well-controlled, systematic study utilizing a suite of multi-dimensional SNeIa simulations investigating the influence of central density of the progenitor WD on the production of Fe-group material, particularly radioactive Ni-56, which powers the light curve. We find that on average, as the progenitor's central density increases, production of Fe-group material does not change but production of Ni-56 decreases. We attribute this result to a higher rate of neutronization at higher density. The central density of the progenitor is determined by the mass of the WD and the cooling time prior to the onset of mass transfer from the companion, as well as the subsequent accretion heating and neutrino losses. The dependence of this density on cooling time, combined with the result of our central density study, offers an explanation for the observed age-luminosity correlation: a longer cooling time raises the central density at ignition thereby producing less Ni-56 and thus a dimmer event. While our ensemble of results demonstrates a significant trend, we find considerable variation between realizations, indicating the necessity for averaging over an ensemble of simulations to demonstrate a statistically significant result.

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arXiv2009

On Type Ia Supernovae From The Collisions of Two White Dwarfs

We explore collisions between two white dwarfs as a pathway for making Type Ia Supernovae (SNIa). White dwarf number densities in globular clusters allow 10-100 redshift <1 collisions per year, and observations by (Chomiuk et al. 2008) of globular clusters in the nearby S0 galaxy NGC 7457 have detected what is likely to be a SNIa remnant. We carry out simulations of the collision between two 0.6 solar mass white dwarfs at various impact parameters and mass resolutions. For impact parameters less than half the radius of the white dwarf, we find such collisions produce approximately 0.4 solar masses of Ni56, making such events potential candidates for underluminous SNIa or a new class of transients between Novae and SNIa.

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arXiv2009

Evaluating Systematic Dependencies of Type Ia Supernovae: The Influence of Progenitor Ne22 Content on Dynamics

We present a theoretical framework for formal study of systematic effects in Supernovae Type Ia (SN Ia) that utilizes 2-d simulations to implement a form of the deflagration-detonation transition (DDT) explosion scenario. The framework is developed from a randomized initial condition that leads to a sample of simulated SN Ia whose Ni56 masses have a similar average and range to those observed, and have many other modestly realistic features such as the velocity extent of intermediate mass elements. The intended purpose is to enable statistically well-defined studies of both physical and theoretical parameters of the SN Ia explosion simulation. We present here a thorough description of the outcome of the SN Ia explosions produced by our current simulations. A first application of this framework is utilized to study the dependence of the SN Ia on the Ne22 content, which is known to be directly influenced by the progenitor stellar population's metallicity. Our study is very specifically tailored to measure how the Ne22 content influences the competition between the rise of plumes of burned material and the expansion of the star before these plumes reach DDT conditions. This competition controls the amount of material in nuclear statistical equilibrium (NSE) and therefore Ni56 produced by setting the density at which nucleosynthesis takes place during the detonation phase of the explosion. Although the outcome following from any particular ignition condition can change dramatically with Ne22 content, with a sample of 20 ignition conditions we find that the systematic change in the expansion of the star prior to detonation is not large enough to compete with the dependence on initial neutron excess discussed by Timmes, Brown & Truran (2003). (Abridged)

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arXiv2008

The Effects of the pep Nuclear Reaction and Other Improvements in the Nuclear Reaction Rate Library on Simulations of the Classical Nova Outburst

We have continued our studies of the Classical Nova outburst by evolving TNRs on 1.25Msun and 1.35Msun WDs (ONeMg composition) under conditions which produce mass ejection and a rapid increase in the emitted light, by examining the effects of changes in the nuclear reaction rates on both the observable features and the nucleosynthesis during the outburst. In order to improve our calculations over previous work, we have incorporated a modern nuclear reaction network into our hydrodynamic computer code. We find that the updates in the nuclear reaction rate libraries change the amount of ejected mass, peak luminosity, and the resulting nucleosynthesis. In addition, as a result of our improvements, we discovered that the pep reaction was not included in our previous studies of CN explosions. Although the energy production from this reaction is not important in the Sun, the densities in WD envelopes can exceed $10^4$ gm cm$^{-3}$ and the presence of this reaction increases the energy generation during the time that the p-p chain is operating. The effect of the increased energy generation is to reduce the evolution time to the peak of the TNR and, thereby, the accreted mass as compared to the evolutionary sequences done without this reaction included. As expected from our previous work, the reduction in accreted mass has important consequences on the characteristics of the resulting TNR and is discussed in this paper.

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arXiv2008

Proton-Rich Nuclear Statistical Equilibrium

Proton-rich material in a state of nuclear statistical equilibrium (NSE) is one of the least studied regimes of nucleosynthesis. One reason for this is that after hydrogen burning, stellar evolution proceeds at conditions of equal number of neutrons and protons or at a slight degree of neutron-richness. Proton-rich nucleosynthesis in stars tends to occur only when hydrogen-rich material that accretes onto a white dwarf or neutron star explodes, or when neutrino interactions in the winds from a nascent proto-neutron star or collapsar-disk drive the matter proton-rich prior to or during the nucleosynthesis. In this paper we solve the NSE equations for a range of proton-rich thermodynamic conditions. We show that cold proton-rich NSE is qualitatively different from neutron-rich NSE. Instead of being dominated by the Fe-peak nuclei with the largest binding energy per nucleon that have a proton to nucleon ratio close to the prescribed electron fraction, NSE for proton-rich material near freeze-out temperature is mainly composed of Ni56 and free protons. Previous results of nuclear reaction network calculations rely on this non-intuitive high proton abundance, which this paper will explain. We show how the differences and especially the large fraction of free protons arises from the minimization of the free energy as a result of a delicate competition between the entropy and the nuclear binding energy.

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arXiv2008

The Reduction of the Electron Abundance during the Pre-explosion Simmering in White Dwarf Supernovae

Prior to the explosion of a carbon-oxygen white dwarf in a Type Ia supernova there is a long "simmering," during which the 12C + 12C reaction gradually heats the white dwarf on a long (~ 1000 yr) timescale. Piro & Bildsten showed that weak reactions during this simmering set a maximum electron abundance Ye at the time of the explosion. We investigate the nuclear reactions during this simmering with a series of self-heating, at constant pressure, reaction network calculations. Unlike in AGB stars, proton captures onto 22Ne and heavier trace nuclei do not play a significant role. The 12C abundance is sufficiently high that the neutrons preferentially capture onto 12C, rather than iron group nuclei. As an aid to hydrodynamical simulations of the simmering phase, we present fits to the rates of heating, electron capture, change in mean atomic mass, and consumption of 12C in terms of the screened thermally averaged cross section for 12C + 12C. Our evaluation of the net heating rate includes contributions from electron captures into the 3.68 MeV excited state of 13C. This results in a slightly larger energy release, per 12C consumed, than that found by Piro & Bildsten, but less than that released for a burn to only 20Ne and 23Na. We compare our one-zone results to more accurate integrations over the white dwarf structure to estimate the amount of 12C that must be consumed to raise the white dwarf temperature, and hence to determine the net reduction of Ye during simmering.

astro-ph↗

arXiv2007

The Effects of Changes in Reaction Rates on Simulations of Nova Explosions

Classical novae participate in the cycle of Galactic chemical evolution in which grains and metal enriched gas in their ejecta, supplementing those of supernovae, AGB stars, and Wolf-Rayet stars, are a source of heavy elements for the ISM. Once in the diffuse gas, this material is mixed with the existing gases and then incorporated into young stars and planetary systems during star formation. Infrared observations have confirmed the presence of carbon, SiC, hydrocarbons, and oxygen-rich silicate grains in nova ejecta, suggesting that some fraction of the pre-solar grains identified in meteoritic material come from novae. The mean mass returned by a nova outburst to the ISM probably exceeds ~2 x 10^{-4} Solar Masses. Using the observed nova rate of 35 per year in our Galaxy, it follows that novae introduce more than ~7 x 10^{-3} Solar Masses per year of processed matter into the ISM. Novae are expected to be the major source of 15N and 17O in the Galaxy and to contribute to the abundances of other isotopes in this atomic mass range. Here, we report on how changes in the nuclear reaction rates affect the properties of the outburst and alter the predictions of the contributions of novae to Galactic chemical evolution. We also discuss the necessity of including the pep reaction in studies of thermonuclear runaways in material accreted onto white dwarfs.

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arXiv2006

Local Ignition in Carbon/Oxygen White Dwarfs -- I: One-zone Ignition and Spherical Shock Ignition of Detonations

The details of ignition of Type Ia supernovae remain fuzzy, despite the importance of this input for any large-scale model of the final explosion. Here, we begin a process of understanding the ignition of these hotspots by examining the burning of one zone of material, and then investigate the ignition of a detonation due to rapid heating at single point. We numerically measure the ignition delay time for onset of burning in mixtures of degenerate material and provide fitting formula for conditions of relevance in the Type Ia problem. Using the neon abundance as a proxy for the white dwarf metallicity, we then find that ignition times can decrease by ~20% with addition of even 5% of neon by mass. When temperature fluctuations that successfully kindle a region are very rare, such a reduction in ignition time can increase the probability of ignition by orders of magnitude. If the neon comes largely at the expense of carbon, a similar increase in the ignition time can occur. We then consider the ignition of a detonation by an explosive energy input in one localized zone, eg a Sedov blast wave leading to a shock-ignited detonation. Building on previous work on curved detonations, we find that surprisingly large inputs of energy are required to successfully launch a detonation, leading to required matchheads of ~4500 detonation thicknesses - tens of centimeters to hundreds of meters - which is orders of magnitude larger than naive considerations might suggest. This is a very difficult constraint to meet for some pictures of a deflagration-to-detonation transition, such as a Zel'dovich gradient mechanism ignition in the distributed burning regime.

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arXiv2005

Constraints on the Progenitor of Cassiopeia A

We compare a suite of 3D explosion calculations and stellar models incorporating advanced physics with observational constraints on the progenitor of Cassiopeia A. We consider binary and single stars from 16 to 40 solar masses with a range of explosion energies and geometries. The parameter space allowed by observations of nitrogen rich high velocity ejecta, ejecta mass, compact remnant mass, and 44Ti and 56Ni abundances individually and as an ensemble is considered. A progenitor of 15-25 solar masses which loses its hydrogen envelope to a binary interaction and undergoes an energetic explosion can match all the observational constraints.

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arXiv2005

Understanding Compact Object Formation and Natal Kicks I. Calculation Methods and the case of GRO J1655-40

In recent years proper motion measurements have been added to the set of observational constraints on the current properties of Galactic X-ray binaries. We develop an analysis that allows us to consider all this available information and reconstruct the full evolutionary history of X-ray binaries back to the time of compact object formation. This analysis accounts for mass transfer through the ongoing X-ray phase, tidal circularization before the onset of Roche-lobe overflow, motion through the Galactic potential after the formation of the compact object, and binary orbital dynamics and hydrodynamic modeling of the core collapse. We apply the analysis to the soft X-ray transient GRO J1655-40 and, for the first time, use its full 3D peculiar velocity constraints right after core collapse instead of lower limits on the current space velocity given by the present-day radial velocity. We find that the system has remained within 200pc from the Galactic plane throughout its entire life time and that the mass loss and a kick possibly associated with the black hole (BH) formation imparted a kick velocity of 45-115 km/s to the binary's center of mass. Right after BH formation, the system consists of a 3.5-6.3 Msun BH and a 2.3-4 Msun main-sequence star. At the onset of the X-ray phase the donor is still on the main sequence. We find that a symmetric BH formation event cannot be formally excluded, but that the associated system parameters are only marginally consistent with the currently observed binary properties. BH formation mechanisms involving an asymmetric supernova explosion with associated BH kick velocities of a few tens of km/s, on the other hand, satisfy the constraints much more comfortably. We also derive an upper limit on the BH kick magnitude of 210 km/s. (abridged)

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arXiv2004

Surface Hydrogen Modeling of Super Soft X-ray Sources: Are They Supernova Ia Progenitors?

Nova explosions occur on the white dwarf (WD) component of a Cataclysmic Variable stellar system which is accreting matter lost by a companion. A Type Ia supernova explosion is thought to result when a WD, in a similar binary configuration, grows in mass to the Chandrasekhar Limit. Here, we present calculations of accretion of Solar matter, at a variety of mass accretion rates, onto hot ($2.3 \times 10^{5}$K), luminous (30L$_\odot$), massive (1.25M$_\odot$, 1.35M$_\odot$) Carbon-Oxygen WDs. In contrast to our nova simulations where the WD has a low initial luminosity and a thermonuclear runaway (TNR) occurs and ejects material, these simulations do not eject material (or only a small fraction of the accreted material) and the WD grows in mass. A hydrogen TNR does not occur because hydrogen fuses to helium in the surface layers, and we call this process Surface Hydrogen Burning (SHB). As the helium layer grows in mass, it gradually fuses either to carbon and oxygen or to more massive nuclei depending on the WD mass and mass accretion rate. If such a WD were to explode in a SN Ia event, therefore, it would show neither hydrogen nor helium in its spectrum as is observed. Moreover, the luminosities and effective temperatures of our simulations agree with the observations of some of the Super Soft X-ray Binary Sources and, therefore, our results strengthen previous speculation that some of them (CAL 83 and CAL 87 for example) are probably progenitors of SN Ia explosions. Finally, we have achieved SHB for values of the mass accretion rate that almost span the observed values of the Cataclysmic Variables.

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arXiv2003

On Heavy Element Enrichment in Classical Novae

Many classical nova ejecta are enriched in CNO and Ne. Rosner et al. recently suggested that the enrichment might originate in the resonant interaction between large-scale shear flows in the accreted H/He envelope and gravity waves at the interface between the envelope and the underlying C/O white dwarf. The shear flow amplifies the waves, which eventually form cusps and break. This wave breaking injects a spray of C/O into the superincumbent H/He. Using two-dimensional simulations, we formulate a quantitative expression for the amount of C/O per unit area that can be entrained into the H/He at saturation. The fraction of the envelope that is enriched depends on the horizontal distribution of shear velocity and the density contrast between the C/O white dwarf and the H/He layer but is roughly independent of the vertical shape of the shear profile. Using this parameterization for the mixed mass, we then perform several one-dimensional Lagrangian calculations of an accreting white dwarf envelope and consider two scenarios: that the wave breaking and mixing is driven by the convective flows; and that the mixing occurs prior to the onset of convection. In the absence of enrichment prior to ignition, the base of the convective zone, as calculated from mixing-length theory with the Ledoux instability criterion, does not reach the C/O interface. As a result, there is no additional mixing, and the runaway is slow. In contrast, the formation of a mixed layer during the accretion of H/He, prior to ignition, causes a more violent runaway. The envelope can be enriched by <25% of C/O by mass (consistent with that observed in some ejecta) for shear velocities, over the surface, with Mach numbers <0.4.

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arXiv2003

The Response of Model and Astrophysical Thermonuclear Flames to Curvature and Stretch

Critically understanding the `standard candle'-like behavior of Type Ia supernovae requires understanding their explosion mechanism. One family of models for Type Ia Supernovae begins with a deflagration in a Carbon-Oxygen white dwarf which greatly accelerates through wrinkling and flame instabilities. While the planar speed and behavior of astrophysically-relevant flames is increasingly well understood, more complex behavior, such as the flame's response to stretch and curvature, has not been extensively explored in the astrophysical literature; this behavior can greatly enhance or suppress instabilities and local flame-wrinkling, which in turn can increase or decrease the bulk burning rate. In this paper, we explore the effects of curvature on both nuclear flames and simpler model flames to understand the effect of curvature on the flame structure and speed.

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arXiv2003

On Variations in the Peak Luminosity of Type Ia Supernovae

We explore the idea that the observed variations in the peak luminosities of Type Ia supernovae originate in part from a scatter in metallicity of the main-sequence stars that become white dwarfs. Previous, numerical, studies have not self-consistently explored metallicities greater than solar. One-dimensional Chandrasekhar mass models of SNe Ia produce most of their 56Ni in a burn to nuclear statistical equilibrium between the mass shells 0.2 and 0.8 solar masses, for which the electron to nucleon ratio is constant during the burn. We show analytically that, under these conditions, charge and mass conservation constrain the mass of 56Ni produced to depend linearly on the original metallicity of the white dwarf progenitor. Detailed post-processing of W7-like models confirms this linear dependence. The effect that we identify is most evident at metallicities larger than solar, and is in agreement with previous self-consistent calculations over the metallicity range common to both calculations. The observed scatter in the metallicity (1/3--3 times solar) of the solar neighborhood is enough to induce a 25% variation in the mass of 56Ni ejected by Type Ia supernovae. This is sufficient to vary the peak V-band brightness by approximately 0.2. This scatter in metallicity is present out to the limiting redshifts of current observations (z < 1). Sedimentation of 22Ne can possibly amplify the variation in 56Ni mass up to 50%. Further numerical studies can determine if other metallicity-induced effects, such as a change in the mass of the 56Ni-producing region, offset or enhance this variation.

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