183 research outputs found
On the role of recombination in common-envelope ejections
The energy budget in common-envelope events (CEEs) is not well understood,
with substantial uncertainty even over to what extent the recombination energy
stored in ionised hydrogen and helium might be used to help envelope ejection.
We investigate the reaction of a red-giant envelope to heating which mimics
limiting cases of energy input provided by the orbital decay of a binary during
a CEE, specifically during the post-plunge-in phase during which the spiral-in
has been argued to occur on a time-scale longer than dynamical. We show that
the outcome of such a CEE depends less on the total amount of energy by which
the envelope is heated than on how rapidly the energy was transferred to the
envelope and on where the envelope was heated. The envelope always becomes
dynamically unstable before receiving net heat energy equal to the envelope's
initial binding energy. We find two types of outcome, both of which likely lead
to at least partial envelope ejection: "runaway" solutions in which the
expansion of the radius becomes undeniably dynamical, and superficially
"self-regulated" solutions, in which the expansion of the stellar radius stops
but a significant fraction of the envelope becomes formally dynamically
unstable. Almost the entire reservoir of initial helium recombination energy is
used for envelope expansion. Hydrogen recombination is less energetically
useful, but is nonetheless important for the development of the dynamical
instabilities. However, this result requires the companion to have already
plunged deep into the envelope; therefore this release of recombination energy
does not help to explain wide post-common-envelope orbits.Comment: 17 pages, 10 figures, submitted to MNRAS. Comments are welcom
Eclipsing binaries in extrasolar planet transit surveys: the case of SuperWASP
Using a comprehensive binary population synthesis scheme, we investigate the
statistical properties of a sample of eclipsing binaries that is detectable by
an idealised extrasolar planet transit survey with specifications broadly
similar to those of the SuperWASP (Wide Angle Search for Planets) project. In
this idealised survey the total number of detectable single stars in the
Galactic disc is of the order of 10^6-10^7, while, for a flat initial mass
ratio distribution, the total number of detectable eclipsing binaries is of the
order of 10^4-10^5. The majority of the population of detectable single stars
is made up of main-sequence stars (60%), horizontal-branch stars (20%), and
giant-branch stars (10%). The largest contributions to the population of
detectable eclipsing binaries stem from detached double main-sequence star
binaries (60%), detached giant-branch main-sequence star binaries (20%), and
detached horizontal-branch main-sequence star binaries (10%). The ratio of the
number of eclipsing binaries to the number of single stars detectable by the
idealised SuperWASP survey varies by less than a factor of 2.5 across the sky,
and decreases with increasing Galactic latitude. It is found to be largest in
the direction of the Galactic longitude l=-7.5deg and the Galactic latitude
b=-22.5deg. We also show that the fractions of systems in different subgroups
of eclipsing binaries are sensitive to the adopted initial mass ratio
distribution, which is one of the poorest constrained input parameters in
present-day binary population synthesis calculations. This suggests that once
statistically meaningful results from transit surveys are available, they will
be able to significantly improve the predictive power of population synthesis
studies of interacting binaries and related objects. (abridged)Comment: Accepted for publication in MNRA
Observations of Doppler Boosting in Kepler Lightcurves
Among the initial results from Kepler were two striking lightcurves, for KOI
74 and KOI 81, in which the relative depths of the primary and secondary
eclipses showed that the more compact, less luminous object was hotter than its
stellar host. That result became particularly intriguing because a substellar
mass had been derived for the secondary in KOI 74, which would make the high
temperature challenging to explain; in KOI 81, the mass range for the companion
was also reported to be consistent with a substellar object. We re-analyze the
Kepler data and demonstrate that both companions are likely to be white dwarfs.
We also find that the photometric data for KOI 74 show a modulation in
brightness as the more luminous star orbits, due to Doppler boosting. The
magnitude of the effect is sufficiently large that we can use it to infer a
radial velocity amplitude accurate to 1 km/s. As far as we are aware, this is
the first time a radial-velocity curve has been measured photometrically.
Combining our velocity amplitude with the inclination and primary mass derived
from the eclipses and primary spectral type, we infer a secondary mass of
0.22+/-0.03 Msun. We use our estimates to consider the likely evolutionary
paths and mass-transfer episodes of these binary systems.Comment: 8 pages, 4 figures, ApJ 715, 51 (v4 is updated to match the published
version, including a note added in proof with measured projected rotational
velocities)
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
The Cosmic Carbon Footprint of Massive Stars Stripped in Binary Systems
The cosmic origin of carbon, a fundamental building block of life, is still uncertain. Yield predictions for massive stars are almost exclusively based on single-star models, even though a large fraction interact with a binary companion. Using the MESA stellar evolution code, we predict the amount of carbon ejected in the winds and supernovae of single and binary-stripped stars at solar metallicity. We find that binary-stripped stars are twice as efficient at producing carbon (1.5–2.6 times, depending on choices regarding the slope of the initial mass function and black hole formation). We confirm that this is because the convective helium core recedes in stars that have lost their hydrogen envelope, as noted previously. The shrinking of the core disconnects the outermost carbon-rich layers created during the early phase of helium burning from the more central burning regions. The same effect prevents carbon destruction, even when the supernova shock wave passes. The yields are sensitive to the treatment of mixing at convective boundaries, specifically during carbon-shell burning (variations up to 40%), and improving upon this should be a central priority for more reliable yield predictions. The yields are robust (variations less than 0.5%) across our range of explosion assumptions. Black hole formation assumptions are also important, implying that the stellar graveyard now explored by gravitational-wave detections may yield clues to better understand the cosmic carbon production. Our findings also highlight the importance of accounting for binary-stripped stars in chemical yield predictions and motivates further studies of other products of binary interactions
Models of Ultraluminous X-Ray Sources with Intermediate-Mass Black Holes
We have computed models for ultraluminous X-ray sources ("ULXs") consisting
of a black-hole accretor of intermediate mass ("IMBH"; e.g., ~1000 Msun) and a
captured donor star. For each of four different sets of initial donor masses
and orbital separations, we computed 30,000 binary evolution models using a
full Henyey stellar evolution code. To our knowledge this is the first time
that a population of X-ray binaries this large has been carried out with other
than approximation methods, and it serves to demonstrate the feasibility of
this approach to large-scale population studies of mass-transfer binaries. In
the present study, we find that in order to have a plausible efficiency for
producing active ULX systems with IMBHs having luminosities > 10^{40} ergs/sec,
there are two basic requirements for the capture of companion/donor stars.
First, the donor stars should be massive, i.e., > 8 Msun. Second, the initial
orbital separations, after circularization, should be close, i.e., < 6-30 times
the radius of the donor star when on the main sequence. Even under these
optimistic conditions, we show that the production rate of IMBH-ULX systems may
fall short of the observed values by factors of 10-100.Comment: 5 pages, 2 figures, submitted to Ap
Aspherical supernova explosions and formation of compact black hole low-mass X-ray binaries
It has been suggested that black-hole low-mass X-ray binaries (BHLMXBs) with
short orbital periods may have evolved from BH binaries with an
intermediate-mass secondary, but the donor star seems to always have higher
effective temperatures than measured in BHLMXBs (Justham, Rappaport &
Podsiadlowski 2006). Here we suggest that the secondary star is originally an
intermediate-mass (\sim 2-5 M_{\sun}) star, which loses a large fraction of
its mass due to the ejecta impact during the aspherical SN explosion that
produced the BH. The resulted secondary star could be of low-mass (\la 1
M_{\sun}). Magnetic braking would shrink the binary orbit, drive mass transfer
between the donor and the BH, producing a compact BHLMXB.Comment: 4 pages, accepted for publication in MNRAS Letter
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