Alexander Heger

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.

Nucleosynthesis in rotating massive stars

Abstract Observational evidence for rotationally induced mixing in massive stars is summarized. From these observations and the models required to explain them, we conclude that rotation will increase the primary metal yields of massive stars, enhance the production of H-burning secondary products (e.g. 14 N and 26 Al), and reduce the initial stellar mass limit for Type II supernova explosions. For the first time, these features are described quantitatively in the context of new evolutionary models for mass losing, rotating stars. These calculations include the effects of the centrifugal force on the structure as well as angular momentum transport and chemical element diffusion. The chemical yields of these models are presented and compared to those of other models evolved without rotation. Our models also indicate the presence of qualitatively new nucleosynthesis channels which may result in primary 14 N production in the H-burning shell and primary neutron processing in the He-burning shell of rotating stars. Implications for the supernova explosion and neutron star remnant are briefly described.

The pre-supernova evolution of rotating massive stars

Massive stars are born rotating rigidly with a significant fraction of critical rotation at the surface. Consequently, rotationally-induced circulation and instabilities lead to chemical mixing in regions that would otherwise be stable, as well as a redistribution of angular momentum. Differential rotation also winds up magnetic fields, causing instabilities that can power a dynamo and magnetic stresses that lead to additional angular momentum transport. We follow the evolution of typical massive stars, their structure and angular momentum distribution, from the zero-age main sequence until iron core collapse. Without the action of magnetic fields, the resulting angular momentum is sufficiently large to significantly affect the explosion mechanism and neutron star formation. Sub-millisecond pulsars result that could encounter the r-mode instability. In helium cores massive enough, at least at low metalicity, the angular momentum is also sufficiently great to form a centrifugally supported accretion disk around a central black hole, powering the engine of the collapsar model for GRBs. Including current estimates of the effect of magnetic fields still allows the formation of rapidly rotating (fV 5 10 IDS) pulsars, but might leave too little angular momentum for collapsars.

Fates of the most massive primordial stars

We present our results of numerical simulations of the most massive primordial stars. For the extremely massive non-rotating Pop III stars over 300M⊙, they would simply die as black holes. But the Pop III stars with initial masses 140 - 260M⊙ may have died as gigantic explosions called pair-instability supernovae (PSNe). We use a new radiation-hydrodynamics code CASTRO to study evolution of PSNe. Our models follow the entire explosive burning and the explosion until the shock breaks out from the stellar surface. In our simulations, we find that fluid instabilities occurred during the explosion. These instabilities are driven by both nuclear burning and hydrodynamical instability. In the red supergiant models, fluid instabilities can lead to significant mixing of supernova ejecta and alter the observational signature.

Evolution and explosion of Wolf-Rayet stars

We investigate the pre-supernova evolution of Wolf-Rayet stars. We discuss whether the separation of hydrogen-free, core collapse supernovae into Type Ic and Type Ib supernovae is related to the occurrence of ‘Case BB mass transfer’ in massive close binaries, especially since the new, smaller WR mass loss rates do not favor helium-poor progenitor models from massive single stars. We also discuss the influence of rotation on the formation, evolution and explosion of WR stars using new models for rotating massive stars that have been computed from zero age to core collapse. We compute the spin-down of (non-magnetic) WR stars due to their strong mass loss, and compare pulsar spin rates with our predictions. Finally, we discuss implications of our results for the rotation rate of Type Ib/c supernova progenitors in general, and for SN 1998bw and the ‘collapsar’ model for γ-ray bursts in particular.