The Metallicity Dependence of the Minimum Mass for Core-Collapse Supernovae

Understanding the progenitors of core collapse supernovae and their population statistics is a key ingredient for many current studies in astronomy but as yet this remains elusive. Using the MESA stellar evolution code we study the dependence of the lower mass limit for making core collapse supernovae (SNe) as function of initial stellar metallicity. We find that this mass limit is smallest at approximately [Z] = -2 with a value of ~ 8.3 Msun. At [Z] = 0 the limit is ~ 9.5 Msun and continues to rise with higher metallicity. As a consequence, for a fixed initial mass function the supernova rate may be 20% to 25% higher at [Z] = -2. This affects the association of observed SN rates as a probe for the cosmological star formation rate, rate predictions for supernova surveys, and population synthesis studies.Comment: 13 pages, 1 figure, 1 table, submitted to ApJ

Combined Nucleosynthetic Yields of Multiple First Stars

Modern numerical simulations of the formation of the first stars predict that the first stars formed in multiples. In those cases, the chemical yields of multiple supernova explosions may have contributed to the formation of a next generation star. We match the chemical abundances of the oldest observed stars in the universe to a database of theoretical supernova models, to show that it is likely that the first stars formed from the ashes of two or more progenitors.Comment: 3 pages, 2 figures, NIC 2016 Conference Proceeding

The quest for blue supergiants: binary merger models for the evolution of the progenitor of SN 1987A

We present the results of a detailed, systematic stellar evolution study of binary mergers for blue supergiant (BSG) progenitors of Type II supernovae. In particular, these are the first evolutionary models that can simultaneously reproduce nearly all observational aspects of the progenitor of SN 1987A, \text{Sk}-69\,^{\circ}202, such as its position in the HR diagram, the enrichment of helium and nitrogen in the triple-ring nebula, and its lifetime before its explosion. The merger model, based on the one proposed by Podsiadlowski 1992 et al. and Podsiadlowski 2007 et al., consists of a main sequence secondary star that dissolves completely in the common envelope of the primary red supergiant at the end of their merger. We empirically explore a large initial parameter space, such as primary masses ($15\,\text{M}_{\odot}$, $16\,\text{M}_{\odot}$, and $17\,\text{M}_{\odot}$), secondary masses ($2\,\text{M}_{\odot}$, $3\,\text{M}_{\odot}$, ..., $8\,\text{M}_{\odot}$) and different depths up to which the secondary penetrates the He core of the primary during the merger. The evolution of the merged star is continued until just before iron-core collapse and the surface properties of the 84 pre-supernova models ($16\,\text{M}_{\odot}-23\,\mathrm{M}_{\odot}$) computed have been made available in this work. Within the parameter space studied, the majority of the pre-supernova models are compact, hot BSGs with effective temperature $>12\,\text{kK}$ and radii of $30\,\text{R}_{\odot}-70\,\mathrm{R}_{\odot}$ of which six match nearly all the observational properties of \text{Sk}-69\,^{\circ}202.Comment: Submitted to MNRAS. 21 pages, 11 figures, 7 table

Long Gamma-Ray Transients from Collapsars

In the collapsar model for common gamma-ray bursts, the formation of a centrifugally supported disk occurs during the first $\sim$10 seconds following the collapse of the iron core in a massive star. This only occurs in a small fraction of massive stellar deaths, however, and requires unusual conditions. A much more frequent occurrence could be the death of a star that makes a black hole and a weak or absent outgoing shock, but in a progenitor that only has enough angular momentum in its outermost layers to make a disk. We consider several cases where this is likely to occur - blue supergiants with low mass loss rates, tidally-interacting binaries involving either helium stars or giant stars, and the collapse to a black hole of very massive pair-instability supernovae. These events have in common the accretion of a solar mass or so of material through a disk over a period much longer than the duration of a common gamma-ray burst. A broad range of powers is possible, $10^{47}$ to $10^{50}\,$erg s$^{-1}$, and this brightness could be enhanced by beaming. Such events were probably more frequent in the early universe where mass loss rates were lower. Indeed this could be one of the most common forms of gamma-ray transients in the universe and could be used to study first generation stars. Several events could be active in the sky at any one time. A recent example of this sort of event may have been the SWIFT transient Sw-1644+57.Comment: submitted to Astrophysical Journa

Reducing Uncertainties in the Production of the Gamma Emitting Nuclei 26Al, 44Ti, and 60Fe in Core Collapse Supernovae by Using Effective Helium Burning Rates

We have used effective reaction rates (ERR) for the helium burning reactions to predict the yield of the gamma-emitting nuclei 26Al, 44Ti, and 60Fe in core col- lapse supernovae. The variations in the predicted yields for values of the reaction rates allowed by the ERR are much smaller than obtained previously, and smaller than other uncertainties. A "filter" for supernova nucleosynthesis yields based on pre-supernova structure was used to estimate the effect of failed supernovae on the initial mass function-averaged yields; this substantially reduced the yields of all these isotopes, but the predicted yield ratio 60Fe/26Al was little affected. The robustness of this ratio is promising for comparison with data, but it is larger than observed in nature; possible causes for this discrepancy are discussed.Comment: 16 pages, 3 figure