Josiah Schwab

FURTHER EVIDENCE FOR THE BIMODAL DISTRIBUTION OF NEUTRON-STAR MASSES

We use a collection of 14 well-measured neutron-star masses to strengthen the case that a substantial fraction of these neutron stars were formed via electron-capture (e-capture) supernovae (SNe) as opposed to Fe core-collapse SNe. The e-capture SNe are characterized by lower resultant gravitational masses and smaller natal kicks, leading to lower orbital eccentricities when the e-capture SN has led to the formation of the second neutron star in a binary system. Based on the measured masses and eccentricities, we identify four neutron stars, which have a mean post-collapse gravitational mass of {approx}1.25 M {sub sun}, as the product of e-capture SNe. We associate the remaining 10 neutron stars, which have a mean mass of {approx}1.35 M {sub sun}, with Fe core-collapse SNe. If the e-capture SN occurs during the formation of the first neutron star, then this should substantially increase the formation probability for double neutron stars, given that more systems will remain bound with the smaller kicks. However, this does not appear to be the case for any of the observed systems and we discuss possible reasons for this.

The viscous evolution of white dwarf merger remnants

The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multidimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an alpha-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al., which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towards spherical states on time-scales of hours, even though a significant fraction of the mass is initially rotationally supported. The viscous evolution unbinds only a very small amount of mass (less than or similar to 10(-5)M(circle dot)). Viscous heating causes some of the systems we study with He WD secondaries to reach conditions of nearly-dynamical burning. It is thus possible that the post-merger viscous phase triggers detonation of the He envelope in some WD mergers, potentially producing a Type Ia supernova via a double-detonation scenario. Our calculations provide the proper initial conditions for studying the long-term thermal evolution of WD merger remnants. This is important for understanding WD mergers as progenitors of Type Ia supernovae, neutron stars, R Coronae Borealis stars and other phenomena.

Thermal runaway during the evolution of ONeMg cores towards accretion-induced collapse

We study the evolution of degenerate electron cores primarily composed of the carbon burning products 16 O, 20 Ne, and 24 Mg (hereafter ONeMg cores) that are undergoing compression. Electron capture reactions on A = 20 and A = 24 isotopes reduce the electron fraction and heat the core. We develop and use a new capability of the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code that provides a highly accurate implementation of these key reactions. These new accurate rates and the ability of MESA to perform extremely small spatial zoning demonstrates a thermal runaway in the core triggered by the temperature and density sensitivity of the 20 Ne electron capture reactions. Both analytics and numerics show that this thermal runaway does not trigger core convection, but rather leads to a centrally concentrated (r < km) thermal runaway that will subsequently launch an oxygen deagration wave from the center of the star. We use MESA to perform a parameter study that quanties the inuence of the

The interplay of disc wind and dynamical ejecta in the aftermath of neutron star–black hole mergers

We explore the evolution of the different ejecta components generated during the merger of a neutron star and a black hole. Our focus is the interplay between material ejected dynamically during the merger, and the wind launched on a viscous time-scale by the remnant accretion disc. These components are expected to contribute to an electromagnetic transient and to produce r-process elements, each with a different signature when considered separately. Here we introduce a two-step approach to investigate their combined evolution, using two-and three-dimensional hydrodynamic simulations. Starting from the output of a merger simulation, we identify each component in the initial condition based on its phase-space distribution, and evolve the accretion disc in axisymmetry. The wind blown from this disc is injected into a three-dimensional computational domain where the dynamical ejecta is evolved. We find that the wind can suppress fallback accretion on time-scales longer than similar to 100 ms. Because of self-similar viscous evolution, the disc accretion at late times nevertheless approaches a power-law time dependence alpha t(-2.2). This can power some late-time gamma-ray burst engine activity, although the available energy is significantly less than in traditional fallback models. Inclusion of radioactive heating due to the r-process does not significantly affect the fallback accretion rate or the disc wind. We do not find any significant modification to the wind properties at large radius due to interaction with the dynamical ejecta. This is a consequence of the different expansion velocities of the two components.

The evolution and fate of super-Chandrasekhar mass white dwarf merger remnants

We present stellar evolution calculations of the remnant of the merger of two carbon-oxygen white dwarfs (CO WDs). We focus on cases that have a total mass in excess of the Chandrasekhar mass. After the merger, the remnant manifests as an

Carbon Shell or Core Ignitions in White Dwarfs Accreting from Helium Stars

L \sim 3 \times 10^4 L_\odot

NEUTRONIZATION DURING CARBON SIMMERING IN TYPE IA SUPERNOVA PROGENITORS

source for

WAIT FOR IT: POST-SUPERNOVA WINDS DRIVEN BY DELAYED RADIOACTIVE DECAYS

\sim 10^4

Accretion-Induced Collapse From Helium Star + White Dwarf Binaries

yr. A dusty wind may develop, leading these sources to be self-obscured and to appear similar to extreme AGB stars. Roughly

Convection Destroys the Core/Mantle Structure in Hybrid C/O/Ne White Dwarfs

\sim 10