57 research outputs found
Mixing via Thermocompositional Convection in Hybrid C/O/Ne White Dwarfs
Convective overshooting in super asymptotic giant branch stars has been
suggested to lead to the formation of hybrid white dwarfs with carbon-oxygen
cores and oxygen-neon mantles. As the white dwarf cools, this core-mantle
configuration becomes convectively unstable and should mix. This mixing has
been previously studied using stellar evolution calculations, but these made
the approximation that convection did not affect the temperature profile of the
mixed region. In this work, we perform direct numerical simulations of an
idealized problem representing the core-mantle interface of the hybrid white
dwarf. We demonstrate that, while the resulting structure within the convection
zone is somewhat different than what is assumed in the stellar evolution
calculations, the two approaches yield similar results for the size and growth
of the mixed region. These hybrid white dwarfs have been invoked as progenitors
of various peculiar thermonuclear supernovae. This lends further support to the
idea that if these hybrid white dwarfs form then they should be fully mixed by
the time of explosion. These effects should be included in the progenitor
evolution in order to more accurately characterize the signatures of these
events.Comment: 12 pages, 7 figures; Accepted to Ap
Evolutionary models for R Coronae Borealis stars
We use Modules for Experiments in Stellar Astrophysics (MESA) to construct
stellar evolution models that reach a hydrogen-deficient, carbon-rich giant
phase like the R Coronae Borealis (R CrB) stars. These models use opacities
from OPAL and AESOPUS that cover the conditions in the cool, H-deficient,
CNO-enhanced envelopes of these stars. We compare models that begin from
homogeneous He stars with models constructed to reproduce the remnant structure
shortly after the merger of a He and a CO white dwarf (WD). We emphasize that
models originating from merger scenarios have a thermal reconfiguration phase
that can last up to 1 kyr post merger, suggesting some galactic
objects should be in this phase. We illustrate the important role of mass loss
in setting the lifetimes of the R CrB stars. Using AGB-like mass loss
prescriptions, models with CO WD primaries typically
leave the R CrB phase with total masses , roughly
independent of their total mass immediately post-merger. This implies that the
descendants of the R CrB stars may have a relatively narrow range in mass and
luminosity as extreme He stars and a relatively narrow range in mass as single
WDs.Comment: 15 pages, 13 figures; Accepted to Ap
Hot subdwarfs formed from the merger of two He white dwarfs
We perform stellar evolution calculations of the remnant of the merger of two
He white dwarfs (WDs). Our initial conditions are taken from hydrodynamic
simulations of double WD mergers and the viscous disc phase that follows. We
evolve these objects from shortly after the merger into their core He-burning
phase, when they appear as hot subdwarf stars. We use our models to quantify
the amount of H that survives the merger, finding that it is difficult for
of H to survive, with even less being
concentrated in the surface layers of the object. We also study the rotational
evolution of these merger remnants. We find that mass loss over the following the merger can significantly reduce the angular
momentum of these objects. As hot subdwarfs, our models have moderate surface
rotation velocities of . The properties of our
models are not representative of many apparently-isolated hot subdwarfs,
suggesting that those objects may form via other channels or that our modelling
is incomplete. However, a sub-population of hot subdwarfs are moderate-to-rapid
rotators and/or have He-rich atmospheres. Our models help to connect the
observed properties of these objects to their progenitor systems.Comment: 10 pages, 11 figures; Accepted to MNRA
Constraints on the Self-Gravity of Radiation Pressure via Big Bang Nucleosynthesis
Using standard big-bang nucleosynthesis and present, high-precision
measurements of light element abundances, we place constraints on the
self-gravity of radiation pressure in the early universe. The self-gravity of
pressure is strictly non-Newtonian, and thus the constraints we set are a
direct test of this aspect of general relativity.Comment: 4 pages, 1 figur
Electron Captures on as a Trigger for Helium Shell Detonations
White dwarfs (WDs) that accrete helium at rates , such as those in close binaries with sdB stars, can accumulate
large () helium envelopes which are likely to detonate.
We perform binary stellar evolution calculations of sdB+WD binary systems with
MESA, incorporating the important reaction chain (NCO), including a recent measurement for the rate. In large accreted helium shells, the NCO reaction
chain leads to ignitions at the dense base of the freshly accreted envelope, in
contrast to ignitions which occur away from the base of the shell. In
addition, at these accretion rates, the shells accumulate on a timescale
comparable to their thermal time, leading to an enhanced sensitivity of the
outcome on the accretion rate history. Hence, time dependent accretion rates
from binary stellar evolution are necessary to determine the helium layer mass
at ignition. We model the observed sdB+WD system
and find that the inclusion of these effects predicts ignition of a helium shell, nearly a factor of two larger than previous predictions.
A shell with this mass will ignite dynamically, a necessary condition for a
helium shell detonation.Comment: 11 pages, 9 figures; Accepted for publication in Ap
Carbon Shell or Core Ignitions in White Dwarfs Accreting from Helium Stars
White dwarfs accreting from helium stars can stably burn at the accreted rate
and avoid the challenge of mass loss associated with unstable Helium burning
that is a concern for many Type Ia supernovae scenarios. We study binaries with
helium stars of mass , which have
lost their hydrogen rich envelopes in an earlier common envelope event and now
orbit with periods () of several hours with non-rotating
and C/O WDs. The helium stars fill their Roche lobes (RLs) after
exhaustion of central helium and donate helium on their thermal timescales
(yr). As shown by others, these mass transfer rates coincide with
the steady helium burning range for WDs, and grow the WD core up to near the
Chandrasekhar mass () and a core carbon ignition. We show here,
however, that many of these scenarios lead to an ignition of hot carbon ashes
near the outer edge of the WD and an inward going carbon flame that does not
cause an explosive outcome. For hours, C/O WDs
with donor masses experience a shell carbon
ignition, while will fall below the steady
helium burning range and undergo helium flashes before reaching core C
ignition. Those with
will experience a core C ignition. We also calculate the retention fraction of
accreted helium when the accretion rate leads to recurrent weak helium flashes.Comment: 9 pages, 13 figure
The interplay of disk 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 (NS) and a black hole (BH). Our focus is the
interplay between material ejected dynamically during the merger, and the wind
launched on a viscous timescale by the remnant accretion disk. 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 disk
in axisymmetry. The wind blown from this disk is injected into a
three-dimensional computational domain where the dynamical ejecta is evolved.
We find that the wind can suppress fallback accretion on timescales longer than
~100 ms. Due to self-similar viscous evolution, the disk accretion at late
times nevertheless approaches a power-law time dependence .
This can power some late-time GRB 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 disk 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.Comment: Accepted by MNRAS with minor changes. New Figure 11 comparing an
extrapolation of the fallback and disk accretion rates to late time
Exploring the Carbon Simmering Phase: Reaction Rates, Mixing, and the Convective Urca Process
The neutron excess at the time of explosion provides a powerful discriminant
among models of Type Ia supernovae. Recent calculations of the carbon simmering
phase in single degenerate progenitors have disagreed about the final neutron
excess. We find that the treatment of mixing in convection zones likely
contributes to the difference. We demonstrate that in MESA models, heating from
exothermic weak reactions plays a significant role in raising the temperature
of the WD. This emphasizes the important role that the convective Urca process
plays during simmering. We briefly summarize the shortcomings of current models
during this phase. Ultimately, we do not pinpoint the difference between the
results reported in the literature, but show that the results are consistent
with different net energetics of the convective Urca process. This problem
serves as an important motivation for the development of models of the
convective Urca process suitable for incorporation into stellar evolution
codes.Comment: 10 pages, 7 figures; Accepted to Ap
Accretion-Induced Collapse From Helium Star + White Dwarf Binaries
Accretion-induced collapse (AIC) occurs when an O/Ne white dwarf (WD) grows
to nearly the Chandrasekhar mass (), reaching central densities
that trigger electron captures in the core. Using Modules for Experiments in
Stellar Astrophysics (), we present the first true binary
simulations of He star + O/Ne WD binaries, focusing on a He star
in a 3 hour orbital period with O/Ne WDs. The helium star
fills its Roche lobe after core helium burning is completed and donates helium
on its thermal timescale to the WD, /yr,
a rate high enough that the accreting helium burns stably on the WD. The
accumulated carbon/oxygen ashes from the helium burning undergo an unstable
shell flash that initiates an inwardly moving carbon burning flame. This flame
is only quenched when it runs out of carbon at the surface of the original O/Ne
core. Subsequent accumulation of fresh carbon/oxygen layers also undergo
thermal instabilities, but no mass loss is triggered, allowing , triggering the onset of AIC. We also discuss the
scenario of accreting C/O WDs that experience shell carbon ignitions to become
O/Ne WDs, and then, under continuing mass transfer, lead to AIC. Studies of the
AIC event rate using binary population synthesis should include all of these
channels, especially this latter channel, which has been previously neglected
but might dominate the rate.Comment: 9 pages, 8 figure
Neutronization During Carbon Simmering In Type Ia Supernova Progenitors
When a Type Ia supernova (SN Ia) progenitor first ignites carbon in its core,
it undergoes yr of convective burning prior to the
onset of thermonuclear runaway. This carbon simmering phase is important for
setting the thermal profile and composition of the white dwarf. Using the
\texttt{MESA} stellar evolution code, we follow this convective burning and
examine the production of neutron-rich isotopes. The neutron content of the SN
fuel has important consequences for the ensuing nucleosynthesis, and, in
particular, for the production of secondary Fe-peak nuclei like Mn and stable
Ni. These elements have been observed in the X-ray spectra of SN remnants like
Tycho, Kepler, and 3C 397, and their yields can provide valuable insights into
the physics of SNe Ia and the properties of their progenitors. We find that
weak reactions during simmering can at most generate a neutron excess of
. This is lower
than that found in previous studies that do not take the full density and
temperature profile of the simmering region into account. Our results imply
that the progenitor metallicity is the main contributor to the neutron excess
in SN Ia fuel for . Alternatively, at lower
metallicities, this neutron excess provides a floor that should be present in
any centrally-ignited SN~Ia scenario.Comment: 15 pages, 16 figures, 6 tables, accepted for publication in the Ap
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