No abstract

How massive stars die-what sort of explosion and remnant each produces-depends chiefly on the masses of their helium cores and hydrogen envelopes at death. For single stars, stellar winds are the only means of mass loss, and these are a function of the metallicity of the star. We discuss how metallicity, and a simplified prescription for its effect on mass loss, affects the evolution and final fate of massive stars. We map, as a function of mass and metallicity, where black holes and neutron stars are likely to form and where different types of supernovae are produced. Integrating over an initial mass function, we derive the relative populations as a function of metallicity. Provided that single stars rotate rapidly enough at death, we speculate on stellar populations that might produce gamma-ray bursts and jet-driven supernovae.

Using a two-dimensional hydrodynamics code (PROMETHEUS), we explore the continued evolution of rotating helium stars, in which iron-core collapse does not produce a successful out-M a Z 10 M _ , going shock but instead forms a black hole of 2È3The model explored in greatest detail is the 14 M _ . helium core of a 35 main-sequence star. The outcome is sensitive to the angular momentum. M _ M _ For cm2 material falls into the black hole almost uninhibited. No outÑows are j 16 4 j/(1016 s~1) [ 3, expected. For the infalling matter is halted by centrifugal force outside 1000 km where neutrino j 16 Z 20, losses are negligible. The equatorial accretion rate is very low, and explosive oxygen burning may power a weak equatorial explosion. For however, a reasonable value for such stars, a compact 3 [ j 16 [ 20, disk forms at a radius at which the gravitational binding energy can be efficiently radiated as neutrinos or converted to beamed outÑow by magnetohydrodynamical (MHD) processes. These are the best candidates for producing gamma-ray bursts (GRBs). Here we study the formation of such a disk, the associated Ñow patterns, and the accretion rate for disk viscosity parameter a B 0.001 and 0.1. Infall along the rotational axis is initially uninhibited, and an evacuated channel opens during the Ðrst few seconds. Meanwhile the black hole is spun up by the accretion (to a B 0.9), and energy is dissipated in the disk by MHD processes and radiated by neutrinos. For the a \ 0.1 model, appreciable energetic outÑows develop between polar angles of 30¡ and 45¡. These outÑows, powered by viscous dissipation in the disk, have an energy of up to a few times 1051 ergs and a mass D1and are rich in 56Ni. They constitute M _ a supernova-like explosion by themselves. Meanwhile accretion through the disk is maintained for approximately 10È20 s but is time variable (^30%) because of hydrodynamical instabilities at the outer edge in a region where nuclei are experiencing photodisintegration. Because the efficiency of neutrino energy deposition is sensitive to the accretion rate, this instability leads to highly variable energy deposition in the polar regions. Some of this variability, which has signiÐcant power at 50 ms and overtones, may persist in the time structure of the burst. During the time followed, the average accretion rate for the standard a \ 0.1 and model is 0.07 s~1. The total energy deposited along the rotational j 16 \ 10 M _ axes by neutrino annihilation is (1È14) ] 1051 ergs, depending upon the evolution of the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of energy in the polar regions, at a constant rate of 5 ] 1050 ergs s~1 per pole, results in strong relativistic outÑow jets beamed to about 1% of the sky. These jets may be additionally modulated by instabilities in the sides of the "" nozzle ÏÏ through which they Ñow. The jets blow aside the accreting material, remain highly focused, and are capable of penetrating the star in D10 s. After the jet breaks through the surface of the sta...

Growing evidence suggests that the first generation of stars may have been quite massive (~100-300 M_sun). Could these stars have left a distinct nucleosynthetic signature? We explore the nucleosynthesis of helium cores in the mass range M_He=64 to 133 Msun, corresponding to main-sequence star masses of approximately 140 to 260 M_sun. Above M_He=133 M_sun, without rotation and using current reaction rates, a black hole is formed and no nucleosynthesis is ejected. For lighter helium core masses, ~40 to 63 M_sun, violent pulsations occur, induced by the pair instability and accompanied by supernova-like mass ejection, but the star eventually produces a large iron core in hydrostatic equilibrium. It is likely that this core, too, collapses to a black hole, thus cleanly separating the heavy element nucleosynthesis of pair instability supernovae from those of other masses, both above and below. Indeed, black hole formation is a likely outcome for all Population III stars with main sequence masses between about 25 M_sun and 140 M_sun (M_He = 9 to 63 M_sun) as well as those above 260 M_sun. Nucleosynthesis in pair-instability supernovae varies greatly with the mass of the helium core which determines the maximum temperature reached during the bounce. At the upper range of exploding core masses, a maximum of 57 M_sun of Ni56 is produced making these the most energetic and the brightest thermonuclear explosions in the universe. Integrating over a distribution of masses, we find that pair instability supernovae produce a roughly solar distribution of nuclei having even nuclear charge, but are remarkably deficient in producing elements with odd nuclear charge. Also, essentially no elements heavier than zinc are produced due to a lack of s- and r-processes.Comment: 20 pages, including 5 figures; accepted by Ap

Nucleosynthesis, light curves, explosion energies, and remnant masses are calculated for a grid of supernovae (SNe) resulting from massive stars with solar metallicity and masses from 9.0 to 120  M . The full evolution is followed using an adaptive reaction network of up to 2000 nuclei. A novel aspect of the survey is the use of a onedimensional neutrino transport model for the explosion. This explosion model has been calibrated to give the observed energy for SN 1987A, using five standard progenitors, and for the Crab SN using a 9.6  M progenitor. As a result of using a calibrated central engine, the final kinetic energy of the SN is variable and sensitive to the structure of each pre-SN star. Many progenitors with extended core structures do not explode, but become black holes (BHs), and the masses of exploding stars do not form a simply connected set. The resulting nucleosynthesis agrees reasonably well with the Sun provided that a reasonable contribution from SNe Iais also allowed, but with a deficiency of light s-process isotopes. The resulting neutron star initial mass function has a mean gravitational mass near 1.4  M . The average BH mass is about 9  M if only the helium core implodes, and 14  M if the entire pre-SN star collapses. Only ∼10% of SNe come from stars over 20  M , and some of these are Type Ib or Ic. Some useful systematics of Type IIp light curves are explored.

We present the first calculations to follow the evolution of all stable nuclei and their radioactive progenitors in stellar models computed from the onset of central hydrogen burning through explosion as Type II supernovae. Calculations are performed for Pop I stars of 15, 19, 20, 21, and 25 M ⊙ using the most recently available experimental and theoretical nuclear data, revised opacity tables, neutrino losses, and weak interaction rates, and taking into account mass loss due to stellar winds. A novel "adaptive" reaction network is employed with a variable number of nuclei (adjusted each time step) ranging from ∼ 700 on the main sequence to 2200 during the explosion. The network includes, at any given time, all relevant isotopes from hydrogen through polonium (Z = 84). Even the limited grid of stellar masses studied suggests that overall good agreement can be achieved with the solar abundances of nuclei between 16 O and 90 Zr. Interesting discrepancies are seen in the 20 M ⊙ model and, so far, only in that model, that are a consequence of the merging of the oxygen, neon, and carbon shells about a day prior to core collapse. We find that, in some stars, most of the "p-process" nuclei can be produced in the convective oxygen burning shell moments prior to collapse; in others, they are made only in the explosion. Serious deficiencies still exist in all cases for the p-process isotopes of Ru and Mo.

Over the past five years evidence has mounted that long-duration (> 2 s) γ-ray bursts (GRBs) the most brilliant of all astronomical explosionssignal the collapse of massive stars in our Universe. This evidence was originally based on the probable association of one unusual GRB with a supernova 1 , but now includes the association of GRBs with regions of massive star formation in distant galaxies 2,3 , the appearance of supernova-like 'bumps' in the optical afterglow light curves of several bursts 4-6 and lines of freshly synthesized elements in the spectra of a few X-ray afterglows 7 . These observations support, but do not yet conclusively demonstrate, the idea that long-duration GRBs are associated with the deaths of massive stars, presumably arising from core collapse. Here we report evidence that

Those massive stars that give rise to gamma-ray bursts (GRBs) during their deaths must be endowed with an unusually large amount of angular momentum in their inner regions, 1-2 orders of magnitude greater than the ones that make common pulsars. Yet the inclusion of mass loss and angular momentum transport by magnetic torques during the precollapse evolution is known to sap the core of the necessary rotation. Here we explore the evolution of very rapidly rotating massive stars, including stripped-down helium cores that might result from mergers or mass transfer in a binary, and single stars that rotate unusually rapidly on the main sequence. For the highest possible rotation rates (about 400 km s À1 ), a novel sort of evolution is encountered in which single stars mix completely on the main sequence, never becoming red giants. Such stars, essentially massive ''blue stragglers,'' produce helium-oxygen cores that rotate unusually rapidly. Such stars might comprise roughly 1% of all stars above 10 M and can, under certain circumstances, retain enough angular momentum to make GRBs. Because this possibility is very sensitive to mass loss, GRBs are much more probable in regions of low metallicity.