406 research outputs found
Nucleosynthesis in Massive Stars With Improved Nuclear and Stellar Physics
We present the first calculations to follow the evolution of all stable
nuclei and their radioactive progenitors in stellar models computed from the
onset of central hydrogen burning through explosion as Type II supernovae.
Calculations are performed for Pop I stars of 15, 19, 20, 21, and 25 M_sun
using the most recently available experimental and theoretical nuclear data,
revised opacity tables, neutrino losses, and weak interaction rates, and taking
into account mass loss due to stellar winds. A novel ``adaptive'' reaction
network is employed with a variable number of nuclei (adjusted each time step)
ranging from about 700 on the main sequence to more than 2200 during the
explosion. The network includes, at any given time, all relevant isotopes from
hydrogen through polonium (Z=84). Even the limited grid of stellar masses
studied suggests that overall good agreement can be achieved with the solar
abundances of nuclei between 16O and 90Zr. Interesting discrepancies are seen
in the 20 M_sun model and, so far, only in that model, that are a consequence
of the merging of the oxygen, neon, and carbon shells about a day prior to core
collapse. We find that, in some stars, most of the ``p-process'' nuclei can be
produced in the convective oxygen burning shell moments prior to collapse; in
others, they are made only in the explosion. Serious deficiencies still exist
in all cases for the p-process isotopes of Ru and Mo.Comment: 53 pages, 17 color figures (3 as separate GIF images), slightly
extended discussion and references, accepted by Ap
The most massive progenitors of neutron stars: CXO J164710.2-455216
The evolution leading to the formation of a neutron star in the very young
Westerlund 1 star cluster is investigated. The turnoff mass has been estimated
to be 35 Msun, indicating a cluster age ~ 3-5 Myr. The brightest X-ray source
in the cluster, CXO J164710.2-455216, is a slowly spinning (10 s) single
neutron star and potentially a magnetar. Since this source was argued to be a
member of the cluster, the neutron star progenitor must have been very massive
(M_zams > 40 Msun) as noted by Muno et al. (2006). Since such massive stars are
generally believed to form black holes (rather than neutron stars), the
existence of this object poses a challenge for understanding massive star
evolution. We point out while single star progenitors below M_zams < 20 Msun
form neutron stars, binary evolution completely changes the progenitor mass
range. In particular, we demonstrate that mass loss in Roche lobe overflow
enables stars as massive as 50-80 Msun, under favorable conditions, to form
neutron stars. If the very high observed binary fraction of massive stars in
Westerlund 1 (> 70 percent) is considered, it is natural that CXO
J164710.2-455216 was formed in a binary which was disrupted in a supernova
explosion such that it is now found as a single neutron star. Hence, the
existence of a neutron star in a given stellar population does not necessarily
place stringent constraints on progenitor mass when binary interactions are
considered. It is concluded that the existence of a neutron star in Westerlund
1 cluster is fully consistent with the generally accepted framework of stellar
evolution.Comment: 5 pages of text and 4 figures (submitted to Astrophysical Journal
On the Binding Energy Parameter of Common Envelope Evolution. Dependency on the Definition of the Stellar Core Boundary during Spiral-in
According to the standard picture for binary interactions, the outcome of
binaries surviving the evolution through a common envelope (CE) and spiral-in
phase is determined by the internal structure of the donor star at the onset of
the mass transfer, as well as the poorly-known efficiency parameter, eta_CE},
for the ejection of the H-envelope of the donor. In this Research Note we
discuss the bifurcation point which separates the ejected, unprocessed H-rich
material from the inner core region of the donor (the central part of the star
which will later contract to form a compact object). We demonstrate that the
exact location of this point is very important for evaluating the binding
energy parameter, lambda, which is used to determine the post-CE orbital
separation. Here we compare various methods to define the bifurcation point
(core/envelope boundary) of evolved stars with masses 4, 7, 10 and 20 M_sun. We
consider the specific nuclear energy production rate profile, the change in the
mass-density gradient (Bisscheroux 1998), the inner region containing less than
10% hydrogen, the method suggested by Han et al. (1994) and the entropy
profile. We also calculated effective polytropic index profiles. The entropy
profile method measures the convective boundary (at the onset of flatness in
the specific entropy) which is not equivalent to the core boundary for RGB
stars. Hence, this method is not applicable for RGB stars, unless the actual
bifurcation point of a CE is located at the bottom of the outer convection zone
(resulting in larger values of lambda and larger post-CE orbital separations).
On the AGB, where highly degenerate and condensed cores are formed, we find
good agreement between the various methods, except for massive (20 M_sun)
stars.Comment: 4 pages, 1 figure, A&A in pres
Zolpidem is a potent stoichiometry-selective modulator of α1β3 GABAA receptors : evidence of a novel benzodiazepine site in the α1-α1 interface
Zolpidem is not a typical GABAA receptor hypnotic. Unlike benzodiazepines, zolpidem modulates tonic GABA currents in the rat dorsal motor nucleus of the vagus, exhibits residual effects in mice lacking the benzodiazepine binding site, and improves speech, cognitive and motor function in human patients with severe brain injury. The receptor by which zolpidem mediates these effects is not known. In this study we evaluated binary α1β3 GABAA receptors in either the 3α1:2β3 or 2α1:3β3 subunit stoichiometry, which differ by the existence of either an α1-α1 interface, or a β3-β3 interface, respectively. Both receptor stoichiometries are readily expressed in Xenopus oocytes, distinguished from each other by using GABA, zolpidem, diazepam and Zn2+. At the 3α1:2β3 receptor, clinically relevant concentrations of zolpidem enhanced GABA in a flumazenil-sensitive manner. The efficacy of diazepam was significantly lower compared to zolpidem. No modulation by either zolpidem or diazepam was detected at the 2α1:3β3 receptor, indicating that the binding site for zolpidem is at the α1-α1 interface, a site mimicking the classical α1-γ2 benzodiazepine site. Activating α1β3 (3α1:2β3) receptors may, in part, mediate the physiological effects of zolpidem observed under distinct physiological and clinical conditions, constituting a potentially attractive drug target
Evidence for a Neutron Star in the non-pulsating massive X-ray binary 4U2206+54
We present an analysis of archival RXTE and BeppoSAX data of the X-ray source
4U2206+54 . For the first time, high energy data (> 30 kev) are analyzed for
this source. The data are well described by comptonization models (CompTT and
BMC) in which seed photons with temperatures between 1.1 kev and 1.5 kev are
comptonized by a hot plasma at 50 kev thereby producing a hard tail which
extends up to, at least, 100 kev. We offer a new method of identification of
neutron star systems using a temperature - luminosity relation. If a given
X-ray source is characterized by a low bolometric luminosity and a relatively
high color blackbody temperature (>1 kev) it has necessarily to be a neutron
star rather than a black hole. From these arguments it is shown that the area
of the soft photon source must be small (r ~ 1 km) and that the accretion disk,
if present, must be truncated very far from the compact object. Here we report
on the possible existence of a cyclotron line around 30 kev. The presence of a
neutron star in the system is strongly favored by the available data.Comment: Accepted for publication in A&A. 9 pages, 7 figures. Submitted to
journal in November 200
Crossing the `Yellow Void' -- Spatially Resolved Spectroscopy of the Post- Red Supergiant IRC+10420 and Its Circumstellar Ejecta
IRC +10420 is one of the extreme hypergiant stars that define the empirical
upper luminosity boundary in the HR diagram. During their post--RSG evolution,
these massive stars enter a temperature range (6000-9000 K) of increased
dynamical instability, high mass loss, and increasing opacity, a
semi--forbidden region, that de Jager and his collaborators have called the
`yellow void'. We report HST/STIS spatially resolved spectroscopy of IRC +10420
and its reflection nebula with some surprising results. Long slit spectroscopy
of the reflected spectrum allows us to effectively view the star from different
directions. Measurements of the double--peaked Halpha emission profile show a
uniform outflow of gas in a nearly spherical distribution, contrary to previous
models with an equatorial disk or bipolar outflow. Based on the temperature and
mass loss rate estimates that are usually quoted for this object, the wind is
optically thick to the continuum at some and possibly all wavelengths.
Consequently the observed variations in apparent spectral type and inferred
temperature are changes in the wind and do not necessarily mean that the
underlying stellar radius and interior structure are evolving on such a short
timescale. To explain the evidence for simultaneous outflow and infall of
material near the star, we propose a `rain' model in which blobs of gas
condense in regions of lowered opacity outside the dense wind. With the
apparent warming of its wind, the recent appearance of strong emission, and a
decline in the mass loss rate, IRC +10420 may be about to shed its opaque wind,
cross the `yellow void', and emerge as a hotter star.Comment: To appear in the Astronomical Journal, August 200
Chapter 8 – Beyond authors’ rights
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How Massive Single Stars End their Life
How massive stars die -- what sort of explosion and remnant each produces --
depends chiefly on the masses of their helium cores and hydrogen envelopes at
death. For single stars, stellar winds are the only means of mass loss, and
these are chiefly a function of the metallicity of the star. We discuss how
metallicity, and a simplified prescription for its effect on mass loss, affects
the evolution and final fate of massive stars. We map, as a function of mass
and metallicity, where black holes and neutron stars are likely to form and
where different types of supernovae are produced. Integrating over an initial
mass function, we derive the relative populations as a function of metallicity.
Provided single stars rotate rapidly enough at death, we speculate upon stellar
populations that might produce gamma-ray bursts and jet-driven supernovae.Comment: 24 pages, 9 figues, submitted to Ap
Model atmospheres of chemically peculiar stars: Self-consistent empirical stratified model of HD24712
High-resolution spectra of some chemically peculiar stars clearly demonstrate
the presence of strong abundance gradients in their atmospheres. However, these
inhomogeneities are usually ignored in the standard scheme of model atmosphere
calculations, braking the consistency between model structure and
spectroscopically derived abundance pattern. In this paper we present first
empirical self-consistent stellar atmosphere model of roAp star HD24712, with
stratification of chemical elements included, and which is derived directly
from the observed profiles of spectral lines without time-consuming simulations
of physical mechanisms responsible for these anomalies. We used the LLmodels
stellar model atmosphere code and DDAFIT minimization tool for analysis of
chemical elements stratification and construction of self-consistent
atmospheric model. Empirical determination of Pr and Nd stratification in the
atmosphere of HD24712 is based on NLTE line formation for Prii/iii and Ndii/iii
with the use of the DETAIL code. Based on iterative procedure of stratification
analysis and subsequent re-calculation of model atmosphere structure we
constructed a self-consistent model of HD24712, i.e. the model which
temperature-pressure structure is consistent with results of stratification
analysis. It is shown that stratification of chemical elements leads to the
considerable changes in model structure as to compare with non-stratified
homogeneous case. We find that accumulation of REE elements allows for the
inverse temperature gradient to be present in upper atmosphere of the star with
the maximum temperature increase of about 600K.Comment: Comments: Accepted by A&A, 16 pages, 10 figures, 3 table
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