arXiv subjects

S. E. Woosley

Publications and source records attributed to S. E. Woosley.

At least 145 records · Page 8Linked to original sources

arXiv2000

On the stability of very massive primordial stars

The stability of metal-free very massive stars ($Z$ = 0; $M = 120 - 500 \msol$) is analyzed and compared with metal-enriched stars. Such zero-metal stars are unstable to nuclear-powered radial pulsations on the main sequence, but the growth time scale for these instabilities is much longer than for their metal-rich counterparts. Since they stabilize quickly after evolving off the ZAMS, the pulsation may not have sufficient time to drive appreciable mass loss in Z = 0 stars. For reasonable assumptions regarding the efficiency of converting pulsational energy into mass loss, we find that, even for the larger masses considered, the star may die without losing a large fraction of its mass. We find a transition between the $ε$- and $κ$-mechanisms for pulsational instability at $Z\sim 2\E{-4} - 2\E{-3}$. For the most metal-rich stars, the $κ$-mechanism yields much shorter $e$-folding times, indicating the presence of a strong instability. We thus stress the fundamental difference of the stability and late stages of evolution between very massive stars born in the early universe and those that might be born today.

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arXiv2000

Presupernova Evolution with Improved Rates for Weak Interactions

Recent shell-model calculations of weak-interaction rates for nuclei in the mass range A = 45 - 65 have resulted in substantial revisions to the hitherto standard set of Fuller, Fowler, & Newman (FFN). In particular, key electron-capture rates, such as that for Co60 are much smaller. We consider here the effects of these revised rates on the presupernova (post-oxygen burning) evolution of massive stars in the mass range 11 to 40 M_sun. Moreover, we include, for the first time in models by our group, the effects of modern rates for beta-decay in addition to electron capture and positron emission. Values for the central electron mole number at the time of iron core collapse in the new models are typically larger, by delta Y_e = 0.005 to 0.015, than those of Woosley & Weaver 1995, with a tendency for the more massive models to display larger differences. About half of this change is a consequence of including beta-decay; the other half, result of the smaller rates for electron capture. Unlike what might be expected solely on basis of the larger Y_e's, the new iron core masses are systematically smaller owing to a decrease in the entropy in the outer iron core. The changes in iron core mass range from zero to 0.1 M_sun. We also observe, as predicted by Aufderheide et al. (1994), a tendency towards beta-equilibrium just prior to the collapse of the core, and the subsequent loss of that equilibrium as core collapse proceeds. We discuss the key weak reaction rates, both beta-decay and electron-capture, responsible for the evolution of Y_e and make suggestions for future measurements.

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arXiv2000

Evolution and Nucleosynthesis of Very Massive Primordial Stars

We investigate the evolution, final fate, and nucleosynthetic yields of rotating and non-rotating very massive stars (VMS) of zero metallicity. First we address the issue of mass loss during hydrogen burning due to vibrational instabilities. We find that these objects are much more stable than what was found in previous studies of VMS of solar composition, and expect only negligible mass loss driven by the pulsations. As these stars thus reach the end of their evolution with massive helium cores, they encounter the pair-creation instability. We find that for helium core masses of ~64...133 solar masses these stars are completely disrupted with explosion energies of up to ~1E53 erg and eject up to ~60 solar masses of Ni56 Stars with more massive helium cores collapse into black holes. We present the first calculations that follow the collapse of such a massive rotating star and predict that X-ray burst and significant gravitational wave emission could result.

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arXiv2000

Nucleosynthesis in massive stars revisited

We have performed the first calculations to follow the evolution of all stable nuclei and their radioactive progenitors in a finely-zoned stellar model computed from the onset of central hydrogen burning through explosion as a Type II supernova. Calculations were done for 15, 20, and 25 solar masses Pop I stars using the most recently available set of experimental and theoretical nuclear data, revised opacity tables, and taking into account mass loss due to stellar winds. Here results are presented for one 15 solar masses model.

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arXiv2000

Nuclear Aspects of Nucleosynthesis in Massive Stars

Preliminary results of a new set of stellar evolution and nucleosynthesis calculations for massive stars are presented. These results were obtained with an extended reaction network up to Bi. The discussion focuses on the importance of nuclear rates in pre- and post-explosive nucleosynthesis. The need for further experiments to study specific reactions and nuclear properties (optical alpha+nucleus potentials) is emphasized.

nucl-th↗

arXiv2000

Prediction of Astrophysical Reaction Rates: Methods, Data Needs, and Consequences for Nucleosynthesis Studies

The majority of nuclear reactions in astrophysics involve unstable nuclei which are not fully accessible by experiments yet. Therefore, there is high demand for reliable predictions of cross sections and reaction rates by theoretical means. The majority of reactions can be treated in the framework of the statistical model (Hauser-Feshbach). The global parametrizations of the nuclear properties needed for predictions far off stability probe our understanding of the strong force and take it to its limit. The sensitivity of astrophysical scenarios to nuclear inputs is illustrated in the framework of a detailed nucleosynthesis study in type II supernovae. Abundances resulting from calculations in the same explosion model with two different sets of reaction rates are compared. Key reactions and required nuclear information are identified.

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arXiv2000

Presupernova collapse models with improved weak-interaction rates

Improved values for stellar weak interaction rates have been recently calculated based upon a large shell model diagonalization. Using these new rates (for both beta decay and electron capture), we have examined the presupernova evolution of massive stars in the range 15-40 Msun. Comparing our new models with a standard set of presupernova models by Woosley and Weaver, we find significantly larger values for the electron-to-baryon ratio Ye at the onset of collapse and iron core masses reduced by approximately 0.1 Msun. The inclusion of beta-decay accounts for roughly half of the revisions, while the other half is a consequence of the improved nuclear physics. These changes will have important consequences for nucleosynthesis and the supernova explosion mechanism.

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arXiv2000

Pair-Instability Supernovae, Gravity Waves, and Gamma-Ray Transients

Growing theoretical evidence suggests that the first generation of stars may have been quite massive (~100-300 solar masses). If they retain their high mass until death, such stars will, after about 3Myr, make pair-instability supernovae. We consider the complete evolution of two zero-metallicity stars of 250 and 300 solar masses. Explosive oxygen and silicon burning cause the 130 solar mass helium core to explode, but explosive burning is unable to drive an explosion in the 300 solar mass star and it collapses to a black hole. For this star, the calculated angular momentum in the presupernova model is sufficient to delay black hole formation and the star initially forms a 50 solar mass, 1000km core within which neutrinos are trapped. Although the star does not become dynamically unstable, the calculated growth time of secular rotational instabilities is shorter than the black hole formation time, and such instabilities may develop. We estimate the energy and amplitude of the gravitational waves emitted during this collapse. After the black hole forms, accretion continues through a disk. Although the disk is far too large and cool to transport energy efficiently to the rotational axis by neutrino annihilation, it has ample potential energy to produce a 1e54erg jet driven by magnetic fields. The interaction of this jet with surrounding circumstellar gas may produce an energetic gamma-ray transient, but given the redshift and time scale, this is probably not a model for typical gamma-ray bursts.

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arXiv2000

Nucleosynthesis in Massive Stars - Including All Stable Isotopes

We present the first calculations to follow the evolution of all stable isotopes (and their abundant radioactive progenitors) in a finely zoned stellar model computed from the onset of central hydrogen burning through explosion as a Type II supernova. The calculations were performed for a 15 solar mass Pop I star using the most recently available set of experimental and theoretical nuclear data, revised opacity tables, and taking into account mass loss due to stellar winds. We find the approximately solar production of proton-rich isotopes above a mass number of A=120 due to the gamma-process. We also find a weak s-process, which along with the gamma-process and explosive helium and carbon burning, produces nearly solar abundances of almost all nuclei from A=60 to 85. A few modifications of the abundances of heavy nuclei above mass 90 by the s-process are also noted and discussed. New weak rates lead to significant alteration of the properties of the presupernova core.

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arXiv2000

Distributed Burning in Type Ia Supernovae: A Statistical Approach

We present a statistical model which shows the influence of turbulence on a thermonuclear flame propagating in C+O white dwarf matter. Based on a Monte Carlo description of turbulence, it provides a method for investigating the physics in the so-called distributed burning regime. Using this method we perform numerical simulations of turbulent flames and show that in this particular regime the flamelet model for the turbulent flame velocity loses its validity. In fact, at high turbulent intensities burning in the distributed regime can lead to a deceleration of the turbulent flame and thus induces a competing process to turbulent effects that cause a higher flame speed. It is also shown that in dense C+O matter turbulent heat transport is described adequately by the Peclet number, Pe, rather than by the Reynolds number, which means that flame propagation is decoupled from small-scale turbulence. Finally, at the onset of our results we argue that the available turbulent energy in an exploding C+O white dwarf is probably too low in order to make a deflagration to detonation transition possible.

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arXiv2000

Constraints On The Delayed Transition to Detonation in Type Ia Supernovae

We investigate the possibility of a delayed detonation in a type Ia supernova under the assumption that the transition to detonation is triggered by turbulence only. Our discussion is based on the Zeldovich mechanism and suggests that typical turbulent velocities present during the explosion are not strong enough to allow this transition to occur. Although we are able to show that in carbon-rich matter (e.g., $X(^{12}$C$) = 0.75$) the possibility of a deflagration to detonation transition (DDT) is enhanced, even in this case the turbulent velocities needed are larger than the expected value of $u'(L) \approx 10^7 {cm s}^{-1}$ on a length-scale of $L \approx 10^6$ cm. Thus we conclude that a DDT may not be a common event during a thermonuclear explosion of a Chandrasekhar-mass white dwarf.

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arXiv1999

Gamma-Ray Bursts: The Central Engine

A variety of arguments suggest that the most common form of gamma-ray bursts (GRBs), those longer than a few seconds, involve the formation of black holes in supernova-like events. Two kinds of ``collapsar'' models are discussed, those in which the black hole forms promptly - a second or so after iron core collapse - and those in which formation occurs later, following ``fallback'' over a period of minutes to hours. In most cases, extraction of energy from a rapidly accreting disk (and a rapidly rotating black hole) is achieved by magnetohydrodynamical processes, although neutrino-powered models remain viable in cases where the accretion rate is >0.05 solar masses per second. GRBs are but one observable phenomenon accompanying black hole birth and other possibilities are discussed, some of which (long, faint GRBs and soft x-ray transients) may await discovery. Since they all involve black holes of similar mass accreting one to several M\sun, collapsars have a nearly standard total energy, around 10**52 erg, but both the fraction of that energy ejected as highly relativistic matter and the distribution of that energy with angle can be highly variable. An explanation is presented why inferred GRB luminosity might correlate inversely with time scales and arguments are given against the production of ordinary GRBs by supergiant stars.

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arXiv1999

Supernovae, Jets, and Collapsars

We consider the explosion of supernovae and the possible production of a variety of high energy transients by delayed black hole formation in massive stars endowed with rotation. Following the launch of a ``successful'' shock by the usual neutrino powered mechanism, the inner layers of the star move outwards, but lack adequate energy to eject all the matter exterior to the neutron star. Over a period of minutes to hours a variable amount of mass, about 0.1 to 5 solar masses falls back into the collapsed remnant, often turning it into a black hole and establishing an accretion disk. The accretion rate, about 0.001 to 0.01 solar masses per second, is inadequate to produce a jet mediated by neutrino annihilation, but similar to that invoked in magnetohydrodynamic (MHD) models for gamma-ray bursts (GRBs). We thus consider the effect of jets formed by ``fallback'' in stars that are already in the process of exploding. We justify a parameterization of the jet power as a constant times the mass accretion rate, $ε\dot {\rm M} c^2$, and explore the consequences of $ε$ = 0.001 and 0.01. In supergiants, shock breakout produces bright x-ray transients that might be a diagnostic of the model. Jets produced by fallback should be more frequent than those made by the prompt formation of a black hole and may power the most common form of gamma-ray transient in the universe, although not the most common form seen so far by BATSE. Those are still attributed to prompt black hole formation, but it may be that the diverse energies observed for GRBs so far reflect chiefly the variable collimation of the jet inside the star and a consequently highly variable fraction of relativistic ejecta. Indeed, these events may all have a common total energy near 10$^{52}$ erg.

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arXiv1999

Collapsars, Gamma-Ray Bursts, and Supernovae

A diverse range of phenomena is possible when a black hole experiences very rapid accretion from a disk due to the incomplete explosion of a massive presupernova star endowed with rotation. In the most extreme case, the outgoing shock fails promptly in a rotating helium star, a black hole and an accretion disk form, and a strong gamma-ray burst (GRB) results. However, there may also be more frequently realized cases where the black hole forms after a delay of from several tens of seconds to several hours as approximately 0.1 to 5 solar masses falls back into the collapsed remnant following a mildly successful supernova explosion. There, the same MHD mechanisms frequently invoked to produce GRBs would also produce jets in stars already in the process of exploding. The presupernova star could be a Wolf-Rayet star or a red or blue supergiant. Depending upon its initial pressure, the collimation of the jet may also vary since ``hot'' jets will tend to diverge and share their energy with the rest of the star. From these situations, one expects diverse outcomes ranging from GRBs with a large range of energies and durations, to asymmetric, energetic supernovae with weak GRBs. SN 1998bw may have been the explosion of a star in which fall back produced a black hole and a less collimated jet than in the case of prompt black hole formation.

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arXiv1999

Presupernova Evolution of Rotating Massive Stars I: Numerical Method and Evolution of the Internal Stellar Structure

The evolution of rotating stars with zero-age main sequence (ZAMS) masses in the range 8 to 25 M_sun is followed through all stages of stable evolution. The initial angular momentum is chosen such that the star's equatorial rotational velocity on the ZAMS ranges from zero to ~ 70 % of break-up. Redistribution of angular momentum and chemical species are then followed as a consequence of rotationally induced circulation and instablities. The effects of the centrifugal force on the stellar structure are included. Uncertain mixing efficiencies are gauged by observations. We find, as noted in previous work, that rotation increases the helium core masses and enriches the stellar envelopes with products of hydrogen burning. We determine, for the first time, the angular momentum distribution in typical presupernova stars along with their detailed chemical structure. Angular momentum loss due to (non-magnetic) stellar winds and the redistribution of angular momentum during core hydrogen burning are of crucial importance for the specific angular momentum of the core. Neglecting magnetic fields, we find angular momentum transport from the core to the envelope to be unimportant after core helium burning. We obtain specific angular momenta for the iron core and overlaying material of 1E16...1E17 erg s. These values are insensitive to the initial angular momentum. They are small enough to avoid triaxial deformations of the iron core before it collapses, but could lead to neutron stars which rotate close to break-up. They are also in the range required for the collapsar model of gamma-ray bursts. The apparent discrepancy with the measured rotation rates of young pulsars is discussed.

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arXiv1999

Formation Rates of Black Hole Accretion Disk Gamma-Ray Bursts

While many models have been proposed for GRBs, those currently favored are all based upon the formation of and/or rapid accretion into stellar mass black holes. We present population synthesis calculations of these models using a Monte Carlo approach in which the many uncertain parameters intrinsic to such calculations are varied. We estimate the event rate for each class of model as well as the propagation distance for those having significant delay between formation and burst production, i.e., double neutron star (DNS) mergers and black hole-neutron star (BH/NS) mergers. For reasonable assumptions regarding the many uncertainties in population synthesis, we calculate a daily event rate in the universe for i) merging neutron stars: ~100/day; ii) neutron-star black hole mergers: ~450/day; iii) collapsars: ~10,000/day; iv) helium star black hole mergers: ~1000/day; and v) white dwarf black hole mergers: ~20/day. The range of uncertainty in these numbers however, is very large, typically two to three orders of magnitude. These rates must additionally be multiplied by any relevant beaming factor and sampling fraction (if the entire universal set of models is not being observed). Depending upon the mass of the host galaxy, half of the DNS and BH/NS mergers will happen within 60kpc (for a Milky-Way massed galaxy) to 5Mpc (for a galaxy with negligible mass) from the galactic center. Because of the delay time, neutron star and black hole mergers will happen at a redshift 0.5 to 0.8 times that of the other classes of models. Information is still lacking regarding the hosts of short hard bursts, but we suggest that they are due to DNS and BH/NS mergers and thus will ultimately be determined to lie outside of galaxies and at a closer mean distance than long complex bursts (which we attribute to collapsars).

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arXiv1999

The Reaction Rate Sensitivity of Nucleosynthesis in Type II Supernovae

We explore the sensitivity of the nucleosynthesis of intermediate mass elements (28 < A < 80) in supernovae derived from massive stars to the nuclear reaction rates employed in the model. Two standard sources of reaction rate data (Woosley et al. 1978; and Thielemann et al. 1987) are employed in pairs of calculations that are otherwise identical. Both include as a common backbone the experimental reactions rates of Caughlan & Fowler (1988). Two stellar models are calculated for each of two main sequence masses: 15 and 25 solar masses. Each star is evolved from core hydrogen burning to a presupernova state carrying an appropriately large reaction network, then exploded using a piston near the edge of the iron core as described by Woosley & Weaver (1995). The final stellar yields from the models calculated with the two rate sets are compared and found to differ in most cases by less than a factor of two over the entire range of nuclei studied. Reasons for the major discrepancies are discussed in detail along with the physics underlying the two reaction rate sets employed. The nucleosynthesis results are relatively robust and less sensitive than might be expected to uncertainties in nuclear reaction rates, though they are sensitive to the stellar model employed.

astro-ph↗

arXiv1999

Merging White Dwarf/Black Hole Binaries and Gamma-Ray Bursts

The merger of compact binaries, especially black holes and neutron stars, is frequently invoked to explain gamma-ray bursts (GRB's). In this paper, we present three dimensional hydrodynamical simulations of the relatively neglected mergers of white dwarfs and black holes. During the merger, the white dwarf is tidally disrupted and sheared into an accretion disk. Nuclear reactions are followed and the energy release is negligible. Peak accretion rates are ~0.05 Msun/s (less for lower mass white dwarfs) lasting for approximately a minute. Many of the disk parameters can be explained by a simple analytic model which we derive and compare to our simulations. This model can be used to predict accretion rates for white dwarf and black hole (or neutron star) masses which are not simulated in this paper. Although the mergers studied here create disks with larger radii, and longer accretion times than those from the merger of double neutron stars, a larger fraction of the merging star's mass becomes part of the disk. Thus the merger of a white dwarf and a black hole could produce a long duration GRB. The event rate of these mergers may be as high as 1/Myr per galaxy.

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