560 research outputs found
Double-core evolution and the formation of neutron-star binaries with compact companions
We present the results of a systematic exploration of an alternative
evolutionary scenario to form double neutron-star binaries, first proposed by
Brown (1995), which does not involve a neutron star passing through a common
envelope. In this scenario, the initial binary components have very similar
masses, and both components have left the main sequence before they evolve into
contact; preferably the primary has already developed a CO core. We have
performed population synthesis simulations to study the formation of double
neutron star binaries via this channel and to predict the orbital properties
and system velocities of such systems. We obtain a merger rate for DNSs in this
channel in the range of 0.1 - 12/Myr. These rates are still subject to
substantial uncertainties such as the modelling of the contact phase.Comment: MNRAS, accepte
On the Possibility of Tidal Formation of Binary Planets Around Ordinary Stars
The planet formation process and subsequent planet migration may lead to
configurations resulting in strong dynamical interactions among the various
planets. Well-studied possible outcomes include collisions between planets,
scattering events that eject one or more of the planets, and a collision of one
or more of the planets with the parent star. In this work we consider one other
possibility that has seemingly been overlooked in the various scattering
calculations presented in the literature: the tidal capture of two planets
which leads to the formation of a binary planet (or binary brown dwarf) in
orbit about the parent star. We carry out extensive numerical simulations of
such dynamical and tidal interactions to explore the parameter space for the
formation of such binary planets. We show that tidal formation of binary
planets is possible for typical planet masses and distances from the host star.
The detection (or lack thereof) of planet-planet binaries can thus be used to
constrain the properties of planetary systems, including their mutual spacing
during formation, and the fraction of close planets in very eccentric orbits
which are believed to form by a closely related process.Comment: 11 pages, 10 Figures, submitted to Ap
Cosmological Implications of the Second Parameter of Type Ia Supernovae
Theoretical models predict that the initial metallicity of the progenitor of
a Type Ia supernova (SN Ia) affects the peak of the supernova light curve. This
can cause a deviation from the standard light curve calibration employed when
using SNe Ia as standardizable distance candles and, if there is a systematic
evolution of the metallicity of SN Ia progenitors, could affect the
determination of cosmological parameters. Here we show that this metallicity
effect can be substantially larger than has been estimated previously, when the
neutronisation in the immediate pre-explosion phase in the CO white dwarf is
taken into account, and quantitatively assess the importance of metallicity
evolution for determining cosmological parameters. We show that, in principle,
a moderate and plausible amount of metallicity evolution could mimic a
lambda-dominated, flat Universe in an open, lambda-free Universe. However, the
effect of metallicity evolution appears not large enough to explain the high-z
SN Ia data in a flat Universe, for which there is strong independent evidence,
without a cosmological constant. We also estimate the systematic uncertainties
introduced by metallicity evolution in a lambda-dominated, flat Universe. We
find that metallicity evolution may limit the precision with which Omega_m and
w can be measured and that it will be difficult to distinguish evolution of the
equation of state of dark energy from metallicity evolution, at least from SN
Ia data alone.Comment: 10 pages, 6 figures, constructive comments welcom
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
Squeezars: Tidally powered stars orbiting a massive black hole
We propose that there exists a class of transient sources, "squeezars", which
are stars caught in highly eccentric orbits around a massive (m<10^8 Mo) black
hole (MBH), whose atypically high luminosity (up to a significant fraction of
their Eddington luminosity) is powered by tidal interactions with the MBH.
Their existence follows from the presence of a mass sink, the MBH, in the
galactic center, which drives a flow of stars into nearly radial orbits to
replace those it has destroyed. We consider two limits for the stellar response
to tidal heating: surface heating with radiative cooling ("hot squeezars") and
bulk heating with adiabatic expansion ("cold squeezars"), and calculate the
evolution of the squeezar orbit, size, luminosity and effective temperature.
The squeezar formation rate is only ~0.05 that of tidal disruption flares, but
squeezar lifetimes are many orders of magnitude longer, and so future
observations of squeezars in nearby galaxies can probe the tidal process that
feeds MBHs and the effects of extreme tides on stars. The mean number of
squeezars orbiting the Galactic MBH is estimated at 0.1-1.Comment: ApJ Lett. accepted. 4 pp. 1 fi
The C-flash and the ignition conditions of type Ia supernovae
Thanks to a stellar evolution code able to compute through the
C-flash we link the binary population synthesis of single degenerate
progenitors of type Ia supernovae (SNe Ia) to their physical condition at the
time of ignition. We show that there is a large range of possible ignition
densities and we detail how their probability distribution depends on the
accretion properties. The low density peak of this distribution qualitatively
reminds of the clustering of the luminosities of Branch-normal SNe Ia. We
tighten the possible range of initial physical conditions for explosion models:
they form a one-parameter family, independent of the metallicity. We discuss
how these results may be modified if we were to relax our hypothesis of a
permanent Hachisu wind or if we were to include electron captures.Comment: 10 pages, 14 figures, MNRAS accepte
The (54Fe+58Ni)/56Ni ratio as a second parameter for Type Ia supernova properties
A variation of the relative content of (54Fe+58Ni) versus 56Ni may be
responsible for the observed scatter of Type Ia Supernovae (SNe Ia) about a
mean relation between their intrinsic brightness and the shape of their light
curve. Synthetic light curves are computed of parametrised Chandrasekhar-mass
explosion models of constant kinetic energy, where the ejecta are divided into
an inner NSE zone, composed of (54Fe+58Ni) inside and 56Ni outside, an outer
zone with Intermediate Mass Elements and a CO zone. Both the size of the NSE
zone and the fraction of (54Fe+58Ni) v. 56Ni are varied systematically. Models
with the same original NSE content but different (54Fe+58Ni)/56Ni ratios reach
different peak brightness but have similar light curve shapes. Synthetic
spectra indicate that the V-band decline rate is not affected by the
(54Fe+58Ni)/56Ni ratio. While the 56Ni mass and the total NSE mass are the
dominant parameters determining the peak luminosity and the shape of the light
curve, respectively, a variation in the (54Fe+58Ni)/56Ni ratio, which may
depend on the metallicity of the progenitor (Timmes, Brown & Truran 2003) is
likely to account for a significant part of the observed scatter of local SNe
Ia about the mean brightness--decline rate relation.Comment: 7 pages, 2 figures; accepted by MNRA
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 ~1.25 M [subscript â], as the product of e-capture SNe. We associate the remaining 10 neutron stars, which have a mean mass of ~1.35 M [subscript â], 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
Shrinking binary and planetary orbits by Kozai cycles with tidal friction
At least two arguments suggest that the orbits of a large fraction of binary
stars and extrasolar planets shrank by 1-2 orders of magnitude after formation:
(i) the physical radius of a star shrinks by a large factor from birth to the
main sequence, yet many main-sequence stars have companions orbiting only a few
stellar radii away, and (ii) in current theories of planet formation, the
region within ~0.1 AU of a protostar is too hot and rarefied for a Jupiter-mass
planet to form, yet many "hot Jupiters" are observed at such distances. We
investigate orbital shrinkage by the combined effects of secular perturbations
from a distant companion star (Kozai oscillations) and tidal friction. We
integrate the relevant equations of motion to predict the distribution of
orbital elements produced by this process. Binary stars with orbital periods of
0.1 to 10 days, with a median of ~2 d, are produced from binaries with much
longer periods (10 d to 10^5 d), consistent with observations indicating that
most or all short-period binaries have distant companions (tertiaries). We also
make two new testable predictions: (1) For periods between 3 and 10 d, the
distribution of the mutual inclination between the inner binary and the
tertiary orbit should peak strongly near 40 deg and 140 deg. (2) Extrasolar
planets whose host stars have a distant binary companion may also undergo this
process, in which case the orbit of the resulting hot Jupiter will typically be
misaligned with the equator of its host star.Comment: Submitted to ApJ; 18 pages, 10 figure
Supernova Kicks and Misaligned Microquasars
The low-mass X-ray binary microquasar GRO J1655-40 is observed to have a
misalignment between the jets and the binary orbital plane. Since the current
black hole spin axis is likely to be parallel to the jets, this implies a
misalignment between the spin axis of the black hole and the binary orbital
plane. It is likely the black holes formed with an asymmetric supernova which
caused the orbital axis to misalign with the spin of the stars. We ask whether
the null hypothesis that the supernova explosion did not affect the spin axis
of the black hole can be ruled out by what can be deduced about the properties
of the explosion from the known system parameters. We find that this null
hypothesis cannot be disproved but we find that the most likely requirements to
form the system include a small natal black hole kick (of a few tens of km/s)
and a relatively wide pre-supernova binary. In such cases the observed close
binary system could have formed by tidal circularisation without a common
envelope phase.Comment: Accepted for publication in MNRA
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