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

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

New Neutron-Capture Site in Massive Pop III and Pop II Stars as a Source for Heavy Elements in the Early Galaxy

We propose a new neutron-capture site in early metal-poor and metal-free stars of $\sim 20$--$30 \,\mathrm{M}_\odot$ that results from proton ingestion in the He shell during late stages of the stars' lives. Most of the neutron capture occurs in the first $\lesssim 10^6\,$s following proton ingestion when $^{13}{\rm C}(\alpha,\mathrm{n})^{16}\mathrm{O}$ produces neutron densities typical of the intermediate neutron-capture process. This phase may be followed by another lasting $\gtrsim 10^7\,$s with $^{17}\mathrm{O}(\alpha,\mathrm{n})^{20}\mathrm{Ne}$ producing much lower neutron densities typical of the slow neutron-capture process. We explore the dependence of the proposed neutron-capture nucleosynthesis on the amount and time of proton ingestion, the initial metallicity, and the ensuing supernova shock. We obtain a range of heavy element abundance patterns including those attributed to the slow neutron-capture process or a combination of the slow and rapid neutron-capture processes. Our results can account for the observed ubiquity of heavy elements such as Sr and Ba in the early Galaxy and explain puzzling abundance patterns of these elements in at least some very metal-poor (VMP) stars including those of the carbon-enhanced varieties. In the latter case, the explanation by the single site proposed here differs from the existing paradigm that attributes various classes of VMP stars to enrichment by multiple different sites.Comment: 21 pages, 12 figures, Accepted for publication in the Astrophysical Journa

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 Remarkable Deaths of 9 - 11 Solar Mass Stars

The post-helium burning evolution of stars from 7 to 11 solar masses is complicated by the lingering effects of degeneracy and off-center ignition. Here stars in this mass range are studied using a standard set of stellar physics. Two important aspects of the study are the direct coupling of a reaction network of roughly 220 nuclei to the structure calculation at all stages and the use of a sub grid model to describe the convective bounded flame that develops during neon and oxygen burning. Below 9.0 solar masses, degenerate oxygen-neon cores form that may become either white dwarfs or electron-capture supernovae. Above 10.3 solar masses the evolution proceeds "normally" to iron-core collapse, without composition inversions or degenerate flashes. Emphasis here is upon the stars in between which typically ignite oxygen burning off center. After oxygen burns in a convectively bounded flame, silicon burning ignites in a degenerate flash that commences closer to the stellar center and with increasing violence for stars of larger mass. In some cases the silicon flash is so violent that it could lead to the early ejection of the hydrogen envelope. This might have interesting observable consequences. For example, the death of a 10.0 solar mass star could produce two supernova-like displays, a faint low energy event due to the silicon flash, and an unusually bright supernova many months later as the low energy ejecta from core collapse collides with the previously ejected envelope. The potential relation to the Crab supernova is discussed.Comment: Submitted to Astrophysical Journal January 5, 2015; revised May 8, 201