Patrick A. Young

Constraints on the progenitor of cassiopeia A

We compare a suite of three-dimensional explosion calculations and stellar models incorporating advanced physics with observational constraints on the progenitor of Cassiopeia A. We consider binary and single stars from 16 to 40 M☉ with a range of explosion energies and geometries. The parameter space allowed by observations of nitrogen-rich high-velocity ejecta, ejecta mass, compact remnant mass, and 44Ti and 56Ni abundances individually and as an ensemble is considered. A progenitor of 15-25 M☉ that loses its hydrogen envelope to a binary interaction and undergoes an energetic explosion can match all the observational constraints.

Constraints on Type Ib/c Supernovae and Gamma‐Ray Burst Progenitors

Although there is strong support for the collapsar engine as the power source of long-duration gamma-ray bursts (GRBs), we still do not definitively know the progenitor of these explosions. Here we review the current set of progenitor scenarios for long-duration GRBs and the observational constraints on these scenarios. Examining these models, we find that single stars cannot be the only progenitor for long-duration GRBs. Several binary progenitors can match the solid observational constraints and also have the potential to match the trends that we are currently seeing in the observations. Type Ib/c supernovae are also likely to be produced primarily in binaries; we discuss the relationship between the progenitors of these explosions and those of the long-duration GRBs.Although there is strong support for the collapsar engine as the power source of long-duration gamma-ray bursts (GRBs), we still do not definitively know the progenitor of these explosions. Here we review the current set of progenitor scenarios for long-duration GRBs and the observational constraints on these scenarios. Examining these, we find that single-star models cannot be the only progenitor for long-duration GRBs. Several binary progenitors can match the solid observational constraints and also have the potential to match the trends we are currently seeing in the observations. Type Ib/c supernovae are also likely to be produced primarily in binaries; we discuss the relationship between the progenitors of these explosions and those of the long-duration GRBs.

Observational Tests and Predictive Stellar Evolution. II. Nonstandard Models

We examine contributions of second-order physical processes to the results of stellar evolution calculations that are amenable to direct observational testing. In the first paper in the series, we established baseline results using only physics that were common to modern stellar evolution codes. In this paper we establish how much of the discrepancy between observations and baseline models is due to particular elements of new physics in the areas of mixing, diffusion, equations of state, and opacities. We then consider the impact of the observational uncertainties on the maximum predictive accuracy achievable by a stellar evolution code. The Sun is an optimal case because of the precise and abundant observations and the relative simplicity of the underlying stellar physics. The standard model is capable of matching the structure of the Sun as determined by helioseismology and gross surface observables to better than a percent. Given an initial mass and surface composition within the observational errors, and no current observables as additional constraints for which the models can be optimized, it is not possible to predict the Suns current state to better than ~7%. Convectively induced mixing in radiative regions, terrestrially calibrated by multidimensional numerical hydrodynamic simulations, dramatically improves the predictions for radii, luminosity, and apsidal motions of eclipsing binaries while simultaneously maintaining consistency with observed light element depletion and turnoff ages in young clusters. Systematic errors in core size for models of massive binaries disappear with more complete mixing physics, and acceptable fits are achieved for all of the binaries without calibration of free parameters. The lack of accurate abundance determinations for binaries is now the main obstacle to improving stellar models using this type of test.

Stellar abundances in the solar neighborhood: The Hypatia Catalog

We compile spectroscopic abundance data from 84 literature sources for 50 elements across 3058 stars in the solar neighborhood, within 150 pc of the Sun, to produce the Hypatia Catalog. We evaluate the variability of the spread in abundance measurements reported for the same star by different surveys. We also explore the likely association of the star within the Galactic disk, the corresponding observation and abundance determination methods for all catalogs in Hypatia, the influence of specific catalogs on the overall abundance trends, and the effect of normalizing all abundances to the same solar scale. The resulting stellar abundance determinations in the Hypatia Catalog are analyzed only for thin-disk stars with observations that are consistent between literature sources. As a result of our large data set, we find that the stars in the solar neighborhood may reveal an asymmetric abundance distribution, such that a [Fe/H]-rich group near the midplane is deficient in Mg, Si, S, Ca, Sc II, Cr II, and Ni as compared to stars farther from the plane. The Hypatia Catalog has a wide number of applications, including exoplanet hosts, thick- and thin-disk stars, and stars with different kinematic properties.

The age and progenitor mass of Sirius B

The Sirius AB binary system has masses that are well determined from many decades of astrometric measurements. Because of the well-measured radius and luminosity of Sirius A, we employed the TYCHO stellar evolution code to determine the age of the Sirius AB binary system accurately, at 225-250 Myr. Note that this fit requires the assumption of solar abundance and the use of the new Asplund et al. primordial solar metallicity. No fit to Sirius As position is possible using the old Grevesse & Sauval scale. Because the Sirius B white dwarf parameters have also been determined accurately from space observations, the cooling age could be determined from recent calculations by Fontaine et al. or Wood to be 124 ± 10 Myr. The difference in the two ages yields the nuclear lifetime and mass of the original primary star, 5.056 M☉. This result yields, in principle, the most accurate data point at relatively high masses for the initial-to-final mass relation. However, the analysis relies on the assumption that the primordial abundance of the Sirius stars was solar, based on membership in the Sirius supercluster. A recent study suggests that its membership in the group is by no means certain.

NuGrid stellar data set. I. Stellar yields from H to Bi for stars with metallicities Z = 0.02 and Z = 0.01

We provide a set of stellar evolution and nucleosynthesis calculations that applies established physics assumptions simultaneously to low- and intermediate-mass and massive star models. Our goal is to provide an internally consistent and comprehensive nuclear production and yield database for applications in areas such as presolar grain studies. Our non-rotating models assume convective boundary mixing (CBM) where it has been adopted before. We include 8 (12) initial masses for Z = 0.01 (0.02). Models are followed either until the end of the asymptotic giant branch phase or the end of Si burning, complemented by simple analytic core-collapse supernova (SN) models with two options for fallback and shock velocities. The explosions show which pre-SN yields will most strongly be effected by the explosive nucleosynthesis. We discuss how these two explosion parameters impact the light elements and the s and p process. For low- and intermediate-mass models, our stellar yields from H to Bi include the effect of CBM at the He-intershell boundaries and the stellar evolution feedback of the mixing process that produces the ¹³C pocket. All post-processing nucleosynthesis calculations use the same nuclear reaction rate network and nuclear physics input. We provide a discussion of the nuclear production across the entire mass range organized by element group. The entirety of our stellar nucleosynthesis profile and time evolution output are available electronically, and tools to explore the data on the NuGrid VOspace hosted by the Canadian Astronomical Data Centre are introduced.

EXPLOSIVE NUCLEOSYNTHESIS FROM GAMMA-RAY BURST AND HYPERNOVA PROGENITORS: DIRECT COLLAPSE VERSUS FALLBACK

The collapsar engine behind long-duration gamma-ray bursts extracts the energy released from the rapid accretion of a collapsing star onto a stellar mass black hole. In a collapsing star, this black hole can form in two ways: the direct collapse of the stellar core into a black hole and the delayed collapse of a black hole caused by fallback in a weak supernova explosion. In the case of a delayed-collapse black hole, the strong collapsar-driven explosion overtakes the weak supernova explosion before shock breakout, and it is very difficult to distinguish this black hole formation scenario from the direct-collapse scenario. However, the delayed-collapse mechanism, with its double explosion, produces explosive nucleosynthetic yields that are very different from those in the direct-collapse scenario. We present one-dimensional studies of the nucleosynthetic yields from both black hole formation scenarios, deriving differences and trends in their nucleosynthetic yields.

Trends in 44Ti and 56Ni from Core-collapse Supernovae

We compare the yields of 44Ti and 56Ni produced from post-processing the thermodynamic trajectories from three different core-collapse models—a Cassiopeia A progenitor, a double shock hypernova progenitor, and a rotating two-dimensional explosion—with the yields from exponential and power-law trajectories. The peak temperatures and densities achieved in these core-collapse models span several of the distinct nucleosynthesis regions we identify, resulting in different trends in the 44Ti and 56Ni yields for different mass elements. The 44Ti and 56Ni mass fraction profiles from the exponential and power-law profiles generally explain the tendencies of the post-processed yields, depending on which regions are traversed by the model. We find that integrated yields of 44Ti and 56Ni from the exponential and power-law trajectories are generally within a factor two or less of the post-process yields. We also analyze the influence of specific nuclear reactions on the 44Ti and 56Ni abundance evolution. Reactions that affect all yields globally are the 3α, p(e–, νe)n and . The rest of the reactions are ranked according to their degree of impact on the synthesis of 44Ti. The primary ones include 44Ti(α, p)47V, 40Ca(α, γ)44Ti, 45V(p, γ)46Cr, 40Ca(α, p)43Sc, 17F(α, p)20Ne, 21Na(α, p)24Mg, 41Sc(p, γ)42Ti, 43Sc(p, γ)44Ti, 44Ti(p, γ)45V, and 57Ni(p, γ)58Cu, along with numerous weak reactions. Our analysis suggests that not all 44Ti need to be produced in an α-rich freeze-out in core-collapse events, and that reaction rate equilibria in combination with timescale effects for the expansion profile may account for the paucity of 44Ti observed in supernova remnants.

Spectra and light curves of failed supernovae

Astronomers have proposed a number of mechanisms to produce supernova explosions. Although many of these mechanisms are now not considered primary engines behind supernovae (SNe), they do produce transients that will be observed by upcoming ground-based surveys and NASA satellites. Here, we present the first radiation-hydrodynamics calculations of the spectra and light curves from three of these failed SNe: SNe with considerable fallback, accretion-induced collapse of white dwarfs, and energetic helium flashes (also known as type Ia SNe).

Late-Time Convection in the Collapse of a 23 M☉ Star

The results of a three-dimensional SNSPH simulation of the core collapse of a 23 M☉ star are presented. This simulation did not launch an explosion until over 600 ms after collapse, allowing an ideal opportunity to study the evolution and structure of the convection below the accretion shock to late times. This late-time convection allows us to study several of the recent claims in the literature about the role of convection: is it dominated by an l = 1 mode driven by vortical-acoustic (or other) instability, does it produce strong neutron star kicks, and, finally, is it the key to a new explosion mechanism? The convective region buffets the neutron star, imparting a 150-200 km s-1 kick. Because the l = 1 mode does not dominate the convection, the neutron star does not achieve large (>450 km s-1) velocities. Finally, the neutron star in this simulation moves but does not develop strong oscillations, the energy source for a recently proposed supernova engine. We discuss the implications these results have for supernovae, hypernovae (and gamma-ray bursts), and stellar-mass black holes.