1,568 research outputs found
Forming short-period Wolf-Rayet X-ray binaries and double black holes through stable mass transfer
We show that black-hole High-Mass X-ray Binaries (HMXBs) with O- or B-type
donor stars and relatively short orbital periods, of order one week to several
months may survive spiral in, to then form Wolf-Rayet (WR) X-ray binaries with
orbital periods of order a day to a few days; while in systems where the
compact star is a neutron star, HMXBs with these orbital periods never survive
spiral-in. We therefore predict that WR X-ray binaries can only harbor black
holes. The reason why black-hole HMXBs with these orbital periods may survive
spiral in is: the combination of a radiative envelope of the donor star, and a
high mass of the compact star. In this case, when the donor begins to overflow
its Roche lobe, the systems are able to spiral in slowly with stable Roche-lobe
overflow, as is shown by the system SS433. In this case the transferred mass is
ejected from the vicinity of the compact star (so-called "isotropic
re-emission" mass loss mode, or "SS433-like mass loss"), leading to gradual
spiral-in. If the mass ratio of donor and black hole is , these systems
will go into CE evolution and are less likely to survive. If they survive, they
produce WR X-ray binaries with orbital periods of a few hours to one day.
Several of the well-known WR+O binaries in our Galaxy and the Magellanic
Clouds, with orbital periods in the range between a week and several months,
are expected to evolve into close WR-Black-Hole binaries,which may later
produce close double black holes. The galactic formation rate of double black
holes resulting from such systems is still uncertain, as it depends on several
poorly known factors in this evolutionary picture. It might possibly be as high
as per year.Comment: MNRAS in pres
Spin transport in a unitary Fermi gas close to the BCS transition
We consider spin transport in a two-component ultracold Fermi gas with
attractive interspecies interactions close to the BCS pairing transition. In
particular, we consider the spin-transport relaxation rate and the
spin-diffusion constant. Upon approaching the transition, the scattering
amplitude is enhanced by pairing fluctuations. However, as the system
approaches the transition, the spectral weight for excitations close to the
Fermi level is decreased by the formation of a pseudogap. To study the
consequence of these two competing effects, we determine the spin-transport
relaxation rate and the spin-diffusion constant using both a Boltzmann approach
and a diagrammatic approach. The former ignores pseudogap physics and finite
lifetime effects. In the latter, we incorporate the full pseudogap physics and
lifetime effects, but we ignore vertex corrections, so that we effectively
calculate single-particle relaxation rates instead of transport relaxation
rates. We find that there is qualitative agreement between these two approaches
although the results for the transport coefficients differ quantitatively.Comment: 9 pages, 10 figure
Interaction effects on dynamic correlations in non-condensed Bose gases
We consider dynamic, i.e., frequency-dependent, correlations in non-condensed
ultracold atomic Bose gases. In particular, we consider the single-particle
correlation function and its power spectrum. We compute this power spectrum for
a one-component Bose gas, and show how it depends on the interatomic
interactions that lead to a finite single-particle relaxation time. As another
example, we consider the power spectrum of spin-current fluctuations for a
two-component Bose gas and show how it is determined by the spin-transport
relaxation time.Comment: 9 pages, 3 figure
Vortex-lattice pinning in two-component Bose-Einstein condensates
We investigate the vortex-lattice structure for single- and two-component
Bose-Einstein condensates in the presence of an optical lattice, which acts as
a pinning potential for the vortices. The problem is considered in the
mean-field quantum-Hall regime, which is reached when the rotation frequency
of the condensate in a radially symmetric trap approaches the (radial)
trapping frequency and the interactions between the atoms are weak. We
determine the vortex-lattice phase diagram as a function of optical-lattice
strength and geometry. In the limit of strong pinning the vortices are always
pinned at the maxima of the optical-lattice potential, similar to the
slow-rotation case. At intermediate pinning strength, however, due to the
competition between interactions and pinning energy, a structure arises for the
two-component case where the vortices are pinned on lines of minimal potential
The VLT-FLAMES Tarantula Survey XXII. Multiplicity properties of the B-type stars
We investigate the multiplicity properties of 408 B-type stars observed in
the 30 Doradus region of the Large Magellanic Cloud with multi-epoch
spectroscopy from the VLT-FLAMES Tarantula Survey (VFTS). We use a
cross-correlation method to estimate relative radial velocities from the helium
and metal absorption lines for each of our targets. Objects with significant
radial-velocity variations (and with an amplitude larger than 16 km/s) are
classified as spectroscopic binaries. We find an observed spectroscopic binary
fraction (defined by periods of 0.1) for the B-type
stars, f_B(obs) = 0.25 +/- 0.02, which appears constant across the field of
view, except for the two older clusters (Hodge 301 and SL 639). These two
clusters have significantly lower fractions of 0.08 +/- 0.08 and 0.10 +/- 0.09,
respectively. Using synthetic populations and a model of our observed epochs
and their potential biases, we constrain the intrinsic multiplicity properties
of the dwarf and giant (i.e. relatively unevolved) B-type stars in 30 Dor. We
obtain a present-day binary fraction f_B(true) = 0.58 +/- 0.11, with a flat
period distribution. Within the uncertainties, the multiplicity properties of
the B-type stars agree with those for the O stars in 30 Dor from the VFTS.Comment: Accepted by A&
The evolution of rotating very massive stars with LMC composition
We present a dense model grid with tailored input chemical composition
appropriate for the Large Magellanic Cloud. We use a one-dimensional
hydrodynamic stellar evolution code, which accounts for rotation, transport of
angular momentum by magnetic fields, and stellar wind mass loss to compute our
detailed models. We calculate stellar evolution models with initial masses of
70-500 Msun and with initial surface rotational velocities of 0-550 km/s,
covering the core-hydrogen burning phase of evolution. We find our rapid
rotators to be strongly influenced by rotationally induced mixing of helium,
with quasi-chemically homogeneous evolution occurring for the fastest rotating
models. Above 160 Msun, homogeneous evolution is also established through mass
loss, producing pure helium stars at core hydrogen exhaustion independent of
the initial rotation rate. Surface nitrogen enrichment is also found for slower
rotators, even for stars that lose only a small fraction of their initial mass.
For models above 150 MZAMS, and for models in the whole considered mass range
later on, we find a considerable envelope inflation due to the proximity of
these models to their Eddington limit. This leads to a maximum zero-age main
sequence surface temperature of 56000 K, at 180 Msun, and to an evolution of
stars in the mass range 50-100 Msun to the regime of luminous blue variables in
the HR diagram with high internal Eddington factors. Inflation also leads to
decreasing surface temperatures during the chemically homogeneous evolution of
stars above 180 Msun. The cool surface temperatures due to the envelope
inflation in our models lead to an enhanced mass loss, which prevents stars at
LMC metallicity from evolving into pair-instability supernovae. The
corresponding spin-down will also prevent very massive LMC stars to produce
long-duration gamma-ray bursts, which might, however, originate from lower
masses.Comment: 21 pages, 25 figure
Solubility isotope effects in aqueous solutions of methane
The isotope effect on the Henry's law coefficients of methane in
aqueous solution (H/D and C-12/C-13 substitution) are interpreted using
the statistical mechanical theory of condensed phase isotope effects.
The missing spectroscopic data needed for the implementation of the
theory were obtained either experimentally (infrared measurements), by
computer simulation (molecular dynamics technique), or estimated using
the Wilson's GF matrix method. The order of magnitude and sign of both
solute isotope effects can be predicted by the theory. Even a crude
estimation based on data from previous vapor pressure isotope effect
studies of pure methane at low temperature can explain the inverse
effect found for the solubility of deuterated methane in water. (C)
2002 American Institute of Physics
The VLT-FLAMES Tarantula Survey X: Evidence for a bimodal distribution of rotational velocities for the single early B-type stars
Aims: Projected rotational velocities (\vsini) have been estimated for 334
targets in the VLT-FLAMES Tarantula survey that do not manifest significant
radial velocity variations and are not supergiants. They have spectral types
from approximately O9.5 to B3. The estimates have been analysed to infer the
underlying rotational velocity distribution, which is critical for
understanding the evolution of massive stars.
Methods: Projected rotational velocities were deduced from the Fourier
transforms of spectral lines, with upper limits also being obtained from
profile fitting. For the narrower lined stars, metal and non-diffuse helium
lines were adopted, and for the broader lined stars, both non-diffuse and
diffuse helium lines; the estimates obtained using the different sets of lines
are in good agreement. The uncertainty in the mean estimates is typically 4%
for most targets. The iterative deconvolution procedure of Lucy has been used
to deduce the probability density distribution of the rotational velocities.
Results: Projected rotational velocities range up to approximately 450 \kms
and show a bi-modal structure. This is also present in the inferred rotational
velocity distribution with 25% of the sample having \ve100\,\kms
and the high velocity component having \ve\,\kms. There is no
evidence from the spatial and radial velocity distributions of the two
components that they represent either field and cluster populations or
different episodes of star formation. Be-type stars have also been identified.
Conclusions: The bi-modal rotational velocity distribution in our sample
resembles that found for late-B and early-A type stars. While magnetic braking
appears to be a possible mechanism for producing the low-velocity component, we
can not rule out alternative explanations.Comment: to be publisged in A&
The VLT-FLAMES Tarantula Survey XXI. Stellar spin rates of O-type spectroscopic binaries
The initial distribution of spin rates of massive stars is a fingerprint of
their elusive formation process. It also sets a key initial condition for
stellar evolution and is thus an important ingredient in stellar population
synthesis. So far, most studies have focused on single stars. Most O stars are
however found in multiple systems. By establishing the spin-rate distribution
of a sizeable sample of O-type spectroscopic binaries and by comparing the
distributions of binary sub-populations with one another as well as with that
of presumed single stars in the same region, we aim to constrain the initial
spin distribution of O stars in binaries, and to identify signatures of the
physical mechanisms that affect the evolution of the massive stars spin rates.
We use ground-based optical spectroscopy obtained in the framework of the
VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial
rotational velocities (\vrot) for components of 114 spectroscopic binaries in
30 Doradus. The \vrot\ values are derived from the full-width at half-maximum
(FWHM) of a set of spectral lines, using a FWHM vs. \vrot\ calibration that we
derive based on previous line analysis methods applied to single O-type stars
in the VFTS sample. The overall \vrot\ distribution of the primary stars
resembles that of single O-type stars in the VFTS, featuring a low-velocity
peak (at \vrot < 200 kms) and a shoulder at intermediate velocities (200 <
\vrot < 300 kms). The distributions of binaries and single stars however
differ in two ways. First, the main peak at \vrot \sim100 kms is broader and
slightly shifted toward higher spin rates in the binary distribution compared
to that of the presumed-single stars. Second, the \vrot distribution of
primaries lacks a significant population of stars spinning faster than 300 kms
while such a population is clearly present in the single star sample.Comment: 16 pages, 16 figures, paper accepted in Astronomy & Astrophysic
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