52 research outputs found
Common-Envelope Evolution: the Nucleosynthesis in Mergers of Massive Stars
We study the merging of massive stars inside a common envelope for binary
systems consisting of a red supergiant with a mass of 15-20 Msun and a
main-sequence companion of 1-5 Msun. We are particularly interested in the
stage when the secondary, having overfilled its Roche lobe inside the common
envelope, starts to transfer mass to the core of the primary at a very high
mass-transfer rate and the subsequent nucleo-synthesis in the core-impact
region. Using a parametrized model for the structure of the envelope at this
stage, we perform 2-dimensional hydrodynamical calculations with the Munich
Prometheus code to calculate the dynamics of the stream emanating from the
secondary and its impact on the core of the primary. We find that, for the
lower end of the estimated mass-transfer rate, low-entropy, hydrogen-rich
material can penetrate deep into the primary core where nucleosynthesis through
the hot CNO cycle can take place and that the associated neutron exposure may
be sufficiently high for significant s-processing. For mass-transfer rates at
the high end of our estimated range and higher densities in the stream, the
stream impact can lead to the dredge-up of helium, but the neutron production
is too low for significant s-processing.Comment: 5 pages, 2 figures, to appear in the proceeding of ``Binary and
Multiple Star Systems'' (Bormio (Italy), June 2000
Hydrodynamical Simulations of the Stream-Core Interaction in the Slow Merger of Massive Stars
We present detailed simulations of the interaction of a stream emanating from
a mass-losing secondary with the core of a massive supergiant in the slow
merger of the two stars inside a common envelope. The dynamics of the stream
can be divided into a ballistic phase, starting at the L_1 point, and a
hydrodynamical phase where the stream interacts strongly with the core.
Considering the merger of a 1 and 5Msun star with a 20Msun evolved supergiant,
we present two-dimensional hydrodynamical simulations using the PROMETHEUS code
to demonstrate how the penetration depth and post-impact conditions depend on
the initial properties of stream material (e.g. entropy, angular momentum,
stream width) and the properties of the core (e.g. density structure and
rotation rate). Using these results, we present a fitting formula for the
entropy generated in the stream--core interaction and a recipe for the
determination of the penetration depth based on a modified Bernoulli integral.Comment: 13 pages, 8 figures, submitted to MNRA
Gamma-Ray Bursts from tidally spun-up Wolf-Rayet stars?
The collapsar model requires rapidly rotating Wolf-Rayet stars as progenitors
of long gamma-ray bursts. However, Galactic Wolf-Rayet stars rapidly lose
angular momentum due to their intense stellar winds. We investigate whether the
tidal interaction of a Wolf-Rayet star with a compact object in a binary system
can spin up the Wolf-Rayet star enough to produce a collapsar. We compute the
evolution of close Wolf-Rayet binaries, including tidal angular momentum
exchange, differential rotation of the Wolf-Rayet star, internal magnetic
fields, stellar wind mass loss, and mass transfer. The Wolf-Rayet companion is
approximated as a point mass. We then employ a population synthesis code to
infer the occurrence rates of the various relevant binary evolution channels.
We find that the simple scenario -- i.e., the Wolf-Rayet star being tidally
spun up and producing a collapsar -- does not occur at solar metallicity and
may only occur with low probability at low metallicity. It is limited by the
widening of the binary orbit induced by the strong Wolf-Rayet wind or by the
radius evolution of the Wolf-Rayet star that most often leads to a binary
merger. The tidal effects enhance the merger rate of Wolf-Rayet stars with
black holes such that it becomes comparable to the occurrence rate of long
gamma-ray bursts.Comment: 9 pages, 11 figures, accepted for publication in A&
Differences in Walking Pattern during 6-Min Walk Test between Patients with COPD and Healthy Subjects
BACKGROUND: To date, detailed analyses of walking patterns using accelerometers during the 6-min walk test (6MWT) have not been performed in patients with chronic obstructive pulmonary disease (COPD). Therefore, it remains unclear whether and to what extent COPD patients have an altered walking pattern during the 6MWT compared to healthy elderly subjects. METHODOLOGY/PRINCIPAL FINDINGS: 79 COPD patients and 24 healthy elderly subjects performed the 6MWT wearing an accelerometer attached to the trunk. The accelerometer features (walking intensity, cadence, and walking variability) and subject characteristics were assessed and compared between groups. Moreover, associations were sought with 6-min walk distance (6MWD) using multiple ordinary least squares (OLS) regression models. COPD patients walked with a significantly lower walking intensity, lower cadence and increased walking variability compared to healthy subjects. Walking intensity and height were the only two significant determinants of 6MWD in healthy subjects, explaining 85% of the variance in 6MWD. In COPD patients also age, cadence, walking variability measures and their interactions were included were significant determinants of 6MWD (total variance in 6MWD explained: 88%). CONCLUSIONS/SIGNIFICANCE: COPD patients have an altered walking pattern during 6MWT compared to healthy subjects. These differences in walking pattern partially explain the lower 6MWD in patients with COPD
On type Ia supernovae and the formation of single low-mass white dwarfs
There is still considerable debate over the progenitors of type Ia supernovae
(SNe Ia). Likewise, it is not agreed how single white dwarfs with masses less
than ~0.5 Msun can be formed in the field, even though they are known to exist.
We consider whether single low-mass white dwarfs (LMWDs) could have been formed
in binary systems where their companions have exploded as a SN Ia. In this
model, the observed single LMWDs are the remnants of giant-branch donor stars
whose envelopes have been stripped off by the supernova explosion. We
investigate the likely remnants of SNe Ia, including the effects of the
explosion on the envelope of the donor star. We also use evolutionary arguments
to examine alternative formation channels for single LMWDs. In addition, we
calculate the expected kinematics of the potential remnants of SNe Ia. SN Ia in
systems with giant-branch donor stars can naturally explain the production of
single LMWDs. It seems difficult for any other formation mechanism to account
for the observations, especially for those single LMWDs with masses less than
~0.4 Msun. Independent of those results, we find that the kinematics of one
potentially useful population containing single LMWDs is consistent with our
model. Studying remnant white-dwarf kinematics seems to be a promising way to
investigate SN Ia progenitors. The existence of single LMWDs appears to
constitute evidence for the production of SNe Ia in binary systems with a
red-giant donor star. Other single white dwarfs with higher space velocities
support a second, probably dominant, population of SN Ia progenitors which
contained main-sequence or subgiant donor stars at the time of explosion. The
runaway stars LP400-22 and US 708 suggest the possibility of a third formation
channnel for some SNe Ia in systems where the donor stars are hot subdwarfs.Comment: Accepted for publication in Astronomy & Astrophysic
Analysis of stellar spectra with 3D and NLTE models
Models of radiation transport in stellar atmospheres are the hinge of modern
astrophysics. Our knowledge of stars, stellar populations, and galaxies is only
as good as the theoretical models, which are used for the interpretation of
their observed spectra, photometric magnitudes, and spectral energy
distributions. I describe recent advances in the field of stellar atmosphere
modelling for late-type stars. Various aspects of radiation transport with 1D
hydrostatic, LTE, NLTE, and 3D radiative-hydrodynamical models are briefly
reviewed.Comment: 21 pages, accepted for publication as a chapter in "Determination of
Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E.
Niemczura, B. Smalley, W. Pyc
On the formation and evolution of black-hole binaries
We present the results of a systematic study of the formation and evolution
of binaries containing black holes and normal-star companions with a wide range
of masses. We first reexamine the standard formation scenario for close
black-hole binaries, where the spiral-in of the companion in the envelope of a
massive star causes the ejection of the envelope. We estimate the formation
rates for different companion masses and different assumptions about the
common-envelope structure and other model parameters. We find that black-hole
binaries with intermediate- and high-mass secondaries can form for a wide range
of assumptions, while black-hole binaries with low-mass secondaries can only
form with apparently unrealistic assumptions (in agreement with previous
studies). We then present detailed binary evolution sequences for black-hole
binaries with secondaries of 2 to 17 Msun and demonstrate that in these systems
the black hole can accrete appreciably even if accretion is Eddington limited
(up to 7 Msun for an initial black-hole mass of 10 Msun) and that the black
holes can be spun up significantly in the process. We discuss the implications
of these calculations for well-studied black-hole binaries (in particular GRS
1915+105), ultra-luminous X-ray sources and Cygnus X-1. Finally, we discuss how
some of the assumptions in the standard model could be relaxed to allow the
formation of low-mass, short-period black-hole binaries which appear to be very
abundant in Nature. (Abstract abridged)Comment: 21 pages, 9 figures, accepted by MNRAS, Figs. 2a/2b and 5 in very
reduced forma
Evolutionary Binary Sequences for Low- and Intermediate-Mass X-ray Binaries
We present the results of a systematic study of the evolution of low- and
intermediate-mass X-ray binaries (LMXBs and IMXBs). Using a standard
Henyey-type stellar-evolution code and a standard model for binary
interactions, we have calculated 100 binary evolution sequences containing a
neutron star and a normal-type companion star, where the initial mass of the
secondary ranges from 0.6 to 7\Ms and the initial orbital period from hr to d. This grid of models samples the entire range of parameters
one is likely to encounter for LMXBs and IMXBs. The sequences show an enormous
variety of evolutionary histories and outcomes, where different mass-transfer
mechanisms dominate in different phases. Very few sequences resemble the
classical evolution of cataclysmic variables, where the evolution is driven by
magnetic braking and gravitational radiation alone. Many systems experience a
phase of mass transfer on a thermal timescale and may briefly become detached
immediately after this phase (for the more massive secondaries). In agreement
with previous results (\markcite{Tauris1}Tauris & Savonije 1999), we find that
all sequences with (sub-)giant donors up to \sim 2\Ms are stable against
dynamical mass transfer. Sequences where the secondary has a radiative envelope
are stable against dynamical mass transfer for initial masses up to \sim
4\Ms. (abridged)Comment: Submitted to ApJ; 25 pages in emulateapj styl
A Comprehensive Study of Neutron Star Retention in Globular Clusters
(abridged) Observations of very high speeds among pulsars in the Galactic disk present a puzzle regarding neutron stars in globular clusters. The inferred characteristic speed of single pulsars in the Galaxy is times as large as the central escape speed from the most massive globular clusters. It is then reasonable to ask why any pulsars are seen in globular clusters, whereas, in fact, quite a large number have been detected and as many as are thought to be present in some of the richest clusters. We use a Monte Carlo approach to generate a population of massive primordial binaries. If we utilize the convention assumptions regarding mass transfer and neutron star kicks, we find that < 5% of the neutron stars initially formed in a massive cluster can be retained. We suggest that this fraction is too low to account for what is observed, and we speculate on possible alternative solutions to the retention problem
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