Gabriel Rockefeller

SIMULATING THE COMMON ENVELOPE PHASE OF A RED GIANT USING SMOOTHED-PARTICLE HYDRODYNAMICS AND UNIFORM-GRID CODES

We use three-dimensional hydrodynamical simulations to study the rapid infall phase of the common envelope (CE) interaction of a red giant branch star of mass equal to 0.88 M{sub Sun} and a companion star of mass ranging from 0.9 down to 0.1 M{sub Sun }. We first compare the results obtained using two different numerical techniques with different resolutions, and find very good agreement overall. We then compare the outcomes of those simulations with observed systems thought to have gone through a CE. The simulations fail to reproduce those systems in the sense that most of the envelope of the donor remains bound at the end of the simulations and the final orbital separations between the donors remnant and the companion, ranging from 26.8 down to 5.9 R{sub Sun }, are larger than the ones observed. We suggest that this discrepancy vouches for recombination playing an essential role in the ejection of the envelope and/or significant shrinkage of the orbit happening in the subsequent phase.

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.

CONVECTIVE-REACTIVE PROTON-12C COMBUSTION IN SAKURAI'S OBJECT (V4334 SAGITTARII) AND IMPLICATIONS FOR THE EVOLUTION AND YIELDS FROM THE FIRST GENERATIONS OF STARS

Depending on mass and metallicity as well as evolutionary phase, stars occasionally experience convectivereactive nucleosynthesis episodes. We specifically investigate the situation when nucleosynthetically unprocessed, H-rich material is convectively mixed with a He-burning zone, for example in convectively unstable shell on top of electron-degenerate cores in AGB stars, young white dwarfs or X-ray bursting neutron stars. Such episodes are frequently encountered in stellar evolution models of stars of extremely low or zero metal content, such as the first stars. We have carried out detailed nucleosynthesis simulations based on stellar evolution models and informed by hydrodynamic simulations. We focus on the convective-reactive episode in the very-late thermal pulse star Sakurai’s object (V4334 Sagittarii). Asplund et al. (1999) determined the abundances of 28 elements, many of which are highly non-solar, ranging from H, He and Li all the way to Ba and La, plus the C isotopic ratio. Our simulations show that the mixing evolution according to standard, one-dimensional stellar evolution models implies neutron densities in the He intershell (. few 10 11 cm -3 ) that are too low to obtain a significant neutron capture nucleosynthesis on the heavy elements. We have carried out 3D hydrodynamic He-shell flash convection simulations in 4 geometry to study the entrainment of H-rich material. Guided by these simulations we assume that the ingestion process of H into the He-shell convection zone leads only after some delay time to a sufficient entropy barrier that splits the convection zone into the original one driven by He-burning and a new one driven by the rapid burning of ingested H. By making such mixing assumptions that are motivated by our hydrodynamic simulations we obtain significantly higher neutron densities ( few 10 15 cm -3 ) and reproduce the key observed abundance trends found in Sakurai’s object. These include an overproduction of Rb, Sr and Y by about 2 orders of magnitude higher than the overproduction of Ba and La. Such a peculiar nucleosynthesis signature is impossible to obtain with the mixing predictions in our one-dimensional stellar evolution models. The simulated Li abundance and the isotopic ratio 12 C/ 13 C are as well in agreement with observations. Details of the observed heavy element abundances can be used as a sensitive diagnostic tool for the neutron density, for the neutron exposure and, in general, for the physics of the convective-reactive phases in stellar evolution. For example, the high elemental ratio Sc/Ca and the high Sc production indicate high neutron densities. The diagnostic value of such abundance markers depends on uncertain nuclear physics input. We determine how our results depend on uncertainties of nuclear reaction rates, for example for the 13 C(; n) 16 O reaction. Subject headings: stars: AGB and post-AGB — stars: abundances — stars: evolution — stars: interior — stars: individual (V4334 Sagittarii) — physical data and processes: hydrodynamics — physical data and processes: nuclear reactions, nucleosynthesis, abundances

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.

SNSPH : A parallel three-dimensional smoothed particle radiation hydrodynamics code

We provide a description of the SNSPH code—a parallel three-dimensional radiation hydrodynamics code implementing tree code gravity, smooth particle hydrodynamics, and flux-limited diffusion transport schemes. We provide descriptions of the physics and parallelization techniques for this code. We present performance results on a suite of code tests (both standard and new), showing the versatility of such a code, but focusing on what we believe are important aspects of modeling core-collapse supernovae.

Remnants of Binary White Dwarf Mergers

We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

56Ni Production in Double-degenerate White Dwarf Collisions

We present a comprehensive study of white dwarf collisions as an avenue for creating type Ia supernovae. Using a smooth particle hydrodynamics code with a 13-isotope, α-chain nuclear network, we examine the resulting 56Ni yield as a function of total mass, mass ratio, and impact parameter. We show that several combinations of white dwarf masses and impact parameters are able to produce sufficient quantities of 56Ni to be observable at cosmological distances. We find that the 56Ni production in double-degenerate white dwarf collisions ranges from sub-luminous to the super-luminous, depending on the parameters of the collision. For all mass pairs, collisions with small impact parameters have the highest likelihood of detonating, but 56Ni production is insensitive to this parameter in high-mass combinations, which significantly increases their likelihood of detection. We also find that the 56Ni dependence on total mass and mass ratio is not linear, with larger-mass primaries producing disproportionately more 56Ni than their lower-mass secondary counterparts, and symmetric pairs of masses producing more 56Ni than asymmetric pairs.

THE 12C + 12C REACTION AND THE IMPACT ON NUCLEOSYNTHESIS IN MASSIVE STARS

Despite much effort in the past decades, the C-burning reaction rate is uncertain by several orders of magnitude, and the relative strength between the different channels 12C(12C, α)20Ne, 12C(12C, p)23Na, and 12C(12C, n)23Mg is poorly determined. Additionally, in C-burning conditions a high 12C+12C rate may lead to lower central C-burning temperatures and to 13C(α, n)16O emerging as a more dominant neutron source than 22Ne(α, n)25Mg, increasing significantly the s-process production. This is due to the chain 12C(p, γ)13N followed by 13N(β +)13C, where the photodisintegration reverse channel 13N(γ, p)12C is strongly decreasing with increasing temperature. Presented here is the impact of the 12C+12C reaction uncertainties on the s-process and on explosive p-process nucleosynthesis in massive stars, including also fast rotating massive stars at low metallicity. Using various 12C+12C rates, in particular an upper and lower rate limit of ~50,000 higher and ~20 lower than the standard rate at 5 × 108 K, five 25 M ☉ stellar models are calculated. The enhanced s-process signature due to 13C(α, n)16O activation is considered, taking into account the impact of the uncertainty of all three C-burning reaction branches. Consequently, we show that the p-process abundances have an average production factor increased up to about a factor of eight compared with the standard case, efficiently producing the elusive Mo and Ru proton-rich isotopes. We also show that an s-process being driven by 13C(α, n)16O is a secondary process, even though the abundance of 13C does not depend on the initial metal content. Finally, implications for the Sr-peak elements inventory in the solar system and at low metallicity are discussed.

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.

The effect of 12C +12C rate uncertainties on the evolution and nucleosynthesis of massive stars

Over the last 40 years, the 12C +12C fusion reaction has been the subject of considerable experimental efforts to constrain uncertainties at temperatures relevant for stellar nucleosynthesis. Recent studies have indicated that the reaction rate may be higher than that currently used in stellar models. In order to investigate the effect of an enhanced carbon-burning rate on massive star structure and nucleosynthesis, new stellar evolution models and their yields are presented exploring the impact of three different 12C +12C reaction rates. Non-rotating stellar models considering five different initial masses, 15, 20, 25, 32 and 60 M⊙, at solar metallicity, were generated using the Geneva Stellar Evolution Code (genec) and were later post-processed with the NuGrid Multi-zone Post-Processing Network tool (mppnp). A dynamic nuclear reaction network of ∼1100 isotopes was used to track the s-process nucleosynthesis. An enhanced 12C +12C reaction rate causes core carbon burning to be ignited more promptly and at lower temperature. This reduces the neutrino losses, which increases the core carbon-burning lifetime. An increased carbon-burning rate also increases the upper initial mass limit for which a star exhibits a convective carbon core (rather than a radiative one). Carbon-shell burning is also affected, with fewer convective-shell episodes and convection zones that tend to be larger in mass. Consequently, the chance of an overlap between the ashes of carbon-core burning and the following carbon shell convection zones is increased, which can cause a portion of the ashes of carbon-core burning to be included in the carbon shell. Therefore, during the supernova explosion, the ejecta will be enriched by s-process nuclides synthesized from the carbon-core s-process. The yields were used to estimate the weak s-process component in order to compare with the Solar system abundance distribution. The enhanced rate models were found to produce a significant proportion of Kr, Sr, Y, Zr, Mo, Ru, Pd and Cd in the weak component, which is primarily the signature of the carbon-core s-process. Consequently, it is shown that the production of isotopes in the Kr–Sr region can be used to constrain the 12C +12C rate using the current branching ratio for α- and p-exit channels.