On the Progenitors of Collapsars

We study the evolution of stars that may be the progenitors of common (long-soft) GRBs. Bare rotating helium stars, presumed to have lost their envelopes due to winds or companions, are followed from central helium ignition to iron core collapse. Including realistic estimates of angular momentum transport (Heger, Langer, & Woosley 2000) by non-magnetic processes and mass loss, one is still able to create a collapsed object at the end with sufficient angular momentum to form a centrifugally supported disk, i.e., to drive a collapsar engine. However, inclusion of current estimates of magnetic torques (Spruit 2002) results in too little angular momentum for collapsars.Comment: 3 pages, 5 figures, in Proc. Woods Hole GRB meeting, ed. Roland Vanderspe

The Central Engines of Gamma-Ray Bursts

Leading models for the "central engine" of long, soft gamma-ray bursts (GRBs) are briefly reviewed with emphasis on the collapsar model. Growing evidence supports the hypothesis that GRBs are a supernova-like phenomenon occurring in star forming regions, differing from ordinary supernovae in that a large fraction of their energy is concentrated in highly relativistic jets. The possible progenitors and physics of such explosions are discussed and the important role of the interaction of the emerging relativistic jet with the collapsing star is emphasized. This interaction may be responsible for most of the time structure seen in long, soft GRBs. What we have called "GRBs" may actually be a diverse set of phenomena with a key parameter being the angle at which the burst is observed. GRB 980425/SN 1988bw and the recently discovered hard x-ray flashes may be examples of this diversity.Comment: 8 pages, Proc. Woods Hole GRB meeting, Nov 5 - 9 WoodsHole Massachusetts, Ed. Roland Vanderspe

Fallback and Black Hole Production in Massive Stars

The compact remnants of core collapse supernovae - neutron stars and black holes - have properties that reflect both the structure of their stellar progenitors and the physics of the explosion. In particular, the masses of these remnants are sensitive to the density structure of the presupernova star and to the explosion energy. To a considerable extent, the final mass is determined by the ``fallback'', during the explosion, of matter that initially moves outwards, yet ultimately fails to escape. We consider here the simulated explosion of a large number of massive stars (10 to 100 \Msun) of Population I (solar metallicity) and III (zero metallicity), and find systematic differences in the remnant mass distributions. As pointed out by Chevalier(1989), supernovae in more compact progenitor stars have stronger reverse shocks and experience more fallback. For Population III stars above about 25 \Msun and explosion energies less than $1.5 \times 10^{51}$ erg, black holes are a common outcome, with masses that increase monotonically with increasing main sequence mass up to a maximum hole mass of about 35 \Msun. If such stars produce primary nitrogen, however, their black holes are systematically smaller. For modern supernovae with nearly solar metallicity, black hole production is much less frequent and the typical masses, which depend sensitively on explosion energy, are smaller. We explore the neutron star initial mass function for both populations and, for reasonable assumptions about the initial mass cut of the explosion, find good agreement with the average of observed masses of neutron stars in binaries. We also find evidence for a bimodal distribution of neutron star masses with a spike around 1.2 \Msun (gravitational mass) and a broader distribution peaked around 1.4 \Msun.Comment: Accepted for publication in Ap

Nucleosynthesis and Remnants in Massive Stars of Solar Metallicity

Hans Bethe contributed in many ways to our understanding of the supernovae that happen in massive stars, but, to this day, a first principles model of how the explosion is energized is lacking. Nevertheless, a quantitative theory of nucleosynthesis is possible. We present a survey of the nucleosynthesis that occurs in 32 stars of solar metallicity in the mass range 12 to 120 solar masses. The most recent set of solar abundances, opacities, mass loss rates, and current estimates of nuclear reaction rates are employed. Restrictions on the mass cut and explosion energy of the supernovae based upon nucleosynthesis, measured neutron star masses, and light curves are discussed and applied. The nucleosynthetic results, when integrated over a Salpeter initial mass function (IMF), agree quite well with what is seen in the sun. We discuss in some detail the production of the long lived radioactivities, 26Al and 60Fe, and why recent model-based estimates of the ratio 60Fe/26Al are overly large compared with what satellites have observed. A major source of the discrepancy is the uncertain nuclear cross sections for the creation and destruction of these unstable isotopes.Comment: 22 pages, 8 figures; to appear in Physics Reports, Bethe Centennial Volum

Evolution and Explosion of Very Massive Primordial Stars

While the modern stellar IMF shows a rapid decline with increasing mass, theoretical investigations suggest that very massive stars (>100 solar masses) may have been abundant in the early universe. Other calculations also indicate that, lacking metals, these same stars reach their late evolutionary stages without appreciable mass loss. After central helium burning, they encounter the electron-positron pair instability, collapse, and burn oxygen and silicon explosively. If sufficient energy is released by the burning, these stars explode as brilliant supernovae with energies up to 100 times that of an ordinary core collapse supernova. They also eject up to 50 solar masses of radioactive Ni56. Stars less massive than 140 solar masses or more massive than 260 solar masses should collapse into black holes instead of exploding, thus bounding the pair-creation supernovae with regions of stellar mass that are nucleosynthetically sterile. Pair-instability supernovae might be detectable in the near infrared out to redshifts of 20 or more and their ashes should leave a distinctive nucleosynthetic pattern.Comment: 7 pages, including 4 figures; in. proc. MPA/ESO/MPE/USM Joint Astronomy Conference "Lighthouses of the Universe: The Most Luminous Celestial Objects and their use for Cosmology