50 research outputs found
Producing type Iax supernovae from a specific class of helium-ignited WD explosions?
It has recently been proposed that one sub-class of type Ia supernovae (SNe
Ia) is sufficiently both distinct and common to be classified separately from
the bulk of SNe Ia, with a suggested class name of "type Iax supernovae" (SNe
Iax), after SN 2002cx. However, their progenitors are still uncertain. We study
whether the population properties of this class might be understood if the
events originate from a subset of sub-Chandrasekhar mass explosions. In this
potential progenitor population, a carbon--oxygen white dwarf (CO WD)
accumulates a helium layer from a non-degenerate helium star; ignition of that
helium layer then leads to ignition of the CO WD. We incorporated detailed
binary evolution calculations for the progenitor systems into a binary
population synthesis model to obtain rates and delay times for such events. The
predicted Galactic event rate of these explosions is ~1.5\times10^{-3}{yr}^{-1}
according to our standard model, in good agreement with the measured rates of
SNe Iax. In addition, predicted delay times are ~70Myr-800Myr, consistent with
the fact that most of SNe Iax have been discovered in late-type galaxies. If
the explosions are assumed to be double-detonations -- following current model
expectations -- then based on the CO WD masses at explosion we also estimate
the distribution of resulting SN brightness (-13 \gtrsim M_{bol} \gtrsim
-19mag), which can reproduce the empirical diversity of SNe Iax. We speculate
on why binaries with non-degenerate donor stars might lead to SNe Iax if
similar systems with degenerate donors do not. We suggest that the high mass of
the helium layer necessary for ignition at the lower accretion rates typically
delivered from non-degenerate donors might be necessary to produce SN
2002cx-like characteristics, perhaps even by changing the nature of the CO
ignition.Comment: 8 pages, 10 figures, 1 table, accepted for publication in Astronomy
and Astrophysic
Globular cluster formation efficiencies from black-hole X-ray binary feedback
We investigate a scenario in which feedback from black-hole X-ray binaries
(BHXBs) sometimes begins inside young star clusters before strong supernova
feedback. Those BHXBs could reduce the gas fraction inside embedded young
clusters whilst maintaining virial equilibrium, which may help globular
clusters (GCs) to stay bound when supernova-driven gas ejection subsequently
occurs. Adopting a simple toy model with parameters guided by BHXB population
models, we produce GC formation efficiencies consistent with
empirically-inferred values. The metallicity dependence of BHXB formation could
naturally explain why GC formation efficiency is higher at lower metallicity.
For reasonable assumptions about that metallicity dependence, our toy model can
produce a GC metallicity bimodality in some galaxies without a bimodality in
the field-star metallicity distribution.Comment: Accepted to ApJ Letters on 19th July. 6 pages. The definitive version
is available from: http://iopscience.iop.org/2041-8205/809/1/L16
Sub-Chandrasekhar White Dwarf Mergers as the Progenitors of Type Ia Supernovae
Type Ia supernovae (SNe Ia) are generally thought to be due to the thermonuclear explosions of carbonâoxygen
white dwarfs (COWDs) with masses near the Chandrasekhar mass. This scenario, however, has two long-standing
problems. First, the explosions do not naturally produce the correct mix of elements, but have to be finely tuned
to proceed from subsonic deflagration to supersonic detonation. Second, population models and observations
give formation rates of near-Chandrasekhar WDs that are far too small. Here, we suggest that SNe Ia instead
result from mergers of roughly equal-mass CO WDs, including those that produce sub-Chandrasekhar mass
remnants. Numerical studies of such mergers have shown that the remnants consist of rapidly rotating cores that
contain most of the mass and are hottest in the center, surrounded by dense, small disks. We argue that the disks
accrete quickly, and that the resulting compressional heating likely leads to central carbon ignition. This ignition
occurs at densities for which pure detonations lead to events similar to SNe Ia. With this merger scenario, we
can understand the type Ia rates and have plausible reasons for the observed range in luminosity and for the
bias of more luminous supernovae toward younger populations. We speculate that explosions of WDs slowly
brought to the Chandrasekhar limitâwhich should also occurâare responsible for some of the âatypicalâ SNe Ia
Episodic mass ejections from common-envelope objects
After the initial fast spiral-in phase experienced by a common-envelope
binary, the system may enter a slow, self-regulated phase, possibly lasting
100s of years, in which all the energy released by orbital decay can be
efficiently transported to the surface, where it is radiated away. If the
remaining envelope is to be removed during this phase, this removal must occur
through some as-yet-undetermined mechanism. We carried out 1-d hydrodynamic
simulations of a low-mass red giant undergoing a synthetic common-envelope
event in such a slow spiral-in phase, using the stellar evolutionary code MESA.
We simulated the heating of the envelope due to frictional dissipation from a
binary companion's orbit in multiple configurations and investigated the
response of the giant's envelope. We find that our model envelopes become
dynamically unstable and develop large-amplitude pulsations, with periods in
the range 3-20 years and very short growth time-scales of similar order. The
shocks and associated rebounds that emerge as these pulsations grow are in some
cases strong enough to dynamically eject shells of matter of up to 0.1
, % of the mass of the envelope, from the stellar
surface at above escape velocity. These ejections are seen to repeat within a
few decades, leading to a time-averaged mass-loss rate of order
which is sufficiently high to
represent a candidate mechanism for removing the entire envelope over the
duration of the slow spiral-in phase.Comment: 24 pages, 15 figures. This article has been accepted for publication
in Monthly Notices of the Royal Astronomical Society, published by Oxford
University Pres
Luminous Blue Variables and superluminous supernovae from binary mergers
Evidence suggests that the direct progenitor stars of some core-collapse
supernovae (CCSNe) are luminous blue variables (LBVs), perhaps including some
`superluminous supernovae' (SLSNe). We examine models in which massive stars
gain mass soon after the end of core hydrogen burning. These are mainly
intended to represent mergers following a brief contact phase during early Case
B mass transfer, but may also represent stars which gain mass in the
Hertzsprung Gap or extremely late during the main-sequence phase for other
reasons. The post-accretion stars spend their core helium-burning phase as blue
supergiants (BSGs), and many examples are consistent with being LBVs at the
time of core collapse. Other examples are yellow supergiants at explosion. We
also investigate whether such post-accretion stars may explode successfully
after core collapse. The final core properties of post-accretion models are
broadly similar to those of single stars with the same initial mass as the
pre-merger primary star. More surprisingly, when early Case B accretion does
affect the final core properties, the effect appears likely to favour a
successful SN explosion, i.e., to make the core properties more like those of a
lower-mass single star. However, the detailed structures of these cores
sometimes display qualitative differences to any single-star model we have
calculated. The rate of appropriate binary mergers may match the rate of SNe
with immediate LBV progenitors; for moderately optimistic assumptions we
estimate that the progenitor birthrate is ~1% of the CCSN rate.Comment: Accepted to The Astrophysical Journal. 24 page
STROOPWAFEL: Simulating rare outcomes from astrophysical populations, with application to gravitational-wave sources
Gravitational-wave observations of double compact object (DCO) mergers are
providing new insights into the physics of massive stars and the evolution of
binary systems. Making the most of expected near-future observations for
understanding stellar physics will rely on comparisons with binary population
synthesis models. However, the vast majority of simulated binaries never
produce DCOs, which makes calculating such populations computationally
inefficient. We present an importance sampling algorithm, STROOPWAFEL, that
improves the computational efficiency of population studies of rare events, by
focusing the simulation around regions of the initial parameter space found to
produce outputs of interest. We implement the algorithm in the binary
population synthesis code COMPAS, and compare the efficiency of our
implementation to the standard method of Monte Carlo sampling from the birth
probability distributions. STROOPWAFEL finds 25-200 times more DCO
mergers than the standard sampling method with the same simulation size, and so
speeds up simulations by up to two orders of magnitude. Finding more DCO
mergers automatically maps the parameter space with far higher resolution than
when using the traditional sampling. This increase in efficiency also leads to
a decrease of a factor 3-10 in statistical sampling uncertainty for the
predictions from the simulations. This is particularly notable for the
distribution functions of observable quantities such as the black hole and
neutron star chirp mass distribution, including in the tails of the
distribution functions where predictions using standard sampling can be
dominated by sampling noise.Comment: Accepted. Data and scripts to reproduce main results is publicly
available. The code for the STROOPWAFEL algorithm will be made publicly
available. Early inquiries can be addressed to the lead autho