6,186 research outputs found
Study of extremely reddened AGB stars in the Galactic bulge
Context. Extremely reddened AGB stars lose mass at high rates of >10^-5
Msun/yr. This is the very last stage of AGB evolution, in which stars in the
mass range 2.0--4.0 Msun (for solar metallicity) should have been converted to
C stars already. The extremely reddened AGB stars in the Galactic bulge are
however predominantly O-rich, implying that they might be either low-mass stars
or stars at the upper end of the AGB mass range. Aims. To determine the mass
range of the most reddened AGB stars in the Galactic bulge. Methods. Using
Virtual Observatory tools, we constructed spectral energy distributions of a
sample of 37 evolved stars in the Galactic bulge with extremely red IRAS
colours. We fitted DUSTY models to the observational data to infer the
bolometric fluxes. Applying individual corrections for interstellar extinction
and adopting a common distance, we determined luminosities and mass-loss rates,
and inferred the progenitor mass range from comparisons with AGB evolutionary
models. Results. The observed spectral energy distributions are consistent with
a classification as reddened AGB stars, except for two stars, which are
proto-planetary nebula candidates. For the AGB stars, we found luminosities in
the range 3000--30,000 Lsun and mass-loss rates 10^-5--3x10^-4 Msun/yr. The
corresponding mass range is 1.1--6.0 Msun assuming solar metallicity.
Conclusions. Contrary to the predictions of the evolutionary models, the
luminosity distribution is continuous, with many O-rich AGB stars in the mass
range in which they should have been converted into C stars already. We suspect
that bulge AGB stars have higher than solar metallicity and therefore may avoid
the conversion to C-rich. The presence of low-mass stars in the sample shows
that their termination of the AGB evolution also occurs during a final phase of
very high mass-loss rate, leading to optically thick circumstellar shells
Cosmological Magnetic Fields from Gauge-Mediated Supersymmetry-Breaking Models
We study the generation of primordial magnetic fields, coherent over
cosmologically interesting scales, by gravitational creation of charged scalar
particles during the reheating period. We show that magnetic fields consistent
with those detected by observation may obtained if the particle mean life
\tau_s is in the range 10^{-14} sec \leq \tau_s \leq 10{-7} sec. We apply this
mechanism to minimal gauge mediated supersymmetry-breaking models, in the case
in which the lightest stau \tilde\tau_1 is the next-to-lightest supersymmetric
particle. We show that, for a large range of phenomenologically acceptable
values of the supersymmetry-breaking scale \sqrt{F}, the generated primordial
magnetic field can be strong enough to seed the galactic dynamo.Comment: 12 pages, Latex. Final version accepted for publication in Phys.
Lett.
An infrared study of galactic OH/IR stars. I. An optical/near-IR atlas of the Arecibo sample
In this paper we present optical and near-infrared finding charts, accurate
astrometry (~1") and single-epoch near-infrared photometry for 371 IRAS
sources, 96% of those included in the so-called Arecibo sample of OH/IR stars
(Eder et al. 1988; Lewis et al. 1990a; Chengalur et al. 1993). The main
photometric properties of the stars in the sample are presented and discussed
as well as the problems found during the process of identification of the
optical/near-infrared counterparts. In addition, we also identify suitable
reference stars in each field to be used for differential photometry purposes
in the future.
We find that 39% of the sources (144 in number) have no optical counterpart,
8 of them being invisible even at near infrared wavelengths. The relative
distribution of sources with and without optical counterpart in the IRAS
two-colour diagram and their characteristic near infrared colours are
interpreted as the consequence of the increasing thickness of their
circumstellar shells. Among the objects not detected at near infrared
wavelengths four non-variable sources are proposed to be heavily obscured
post-AGB stars which have just very recently left the AGB. Eight additional
objects with unusually bright and/or blue near-infrared colours are identified
as candidate post-AGB stars and/or proto-planetary nebulae.Comment: 28 pages, 9 figures, for associated finding charts see:
http://www.edpsciences.org/articles/aa/full/2005/08/aa1709/FINDING_CHARTS/are
cibo_index.htm
Generalized information entropies depending only on the probability distribution
Systems with a long-term stationary state that possess as a spatio-temporally
fluctuation quantity can be described by a superposition of several
statistics, a "super statistics". We consider first, the Gamma, log-normal and
-distributions of . It is assumed that they depend only on , the
probability associated with the microscopic configuration of the system. For
each of the three distributions we calculate the Boltzmann factors and
show that they coincide for small variance of the fluctuations. For the Gamma
distribution it is possible to calculate the entropy in a closed form,
depending on , and to obtain then an equation relating with . We also propose, as other examples, new entropies close related with the
Kaniadakis and two possible Sharma-Mittal entropies. The entropies presented in
this work do not depend on a constant parameter but on . For the
-Gamma distribution and its corresponding Boltzmann factor
and the associated entropy, we show the validity of the saddle-point
approximation. We also briefly discuss the generalization of one of the four
Khinchin axioms to get this proposed entropy.Comment: 13 pages, 3 figure
Existence of global-in-time solutions to a generalized Dirac-Fock type evolution equation
We consider a generalized Dirac-Fock type evolution equation deduced from
no-photon Quantum Electrodynamics, which describes the self-consistent
time-evolution of relativistic electrons, the observable ones as well as those
filling up the Dirac sea. This equation has been originally introduced by Dirac
in 1934 in a simplified form. Since we work in a Hartree-Fock type
approximation, the elements describing the physical state of the electrons are
infinite rank projectors. Using the Bogoliubov-Dirac-Fock formalism, introduced
by Chaix-Iracane ({\it J. Phys. B.}, 22, 3791--3814, 1989), and recently
established by Hainzl-Lewin-Sere, we prove the existence of global-in-time
solutions of the considered evolution equation.Comment: 12 pages; more explanations added, some final (minor) corrections
include
HII Shells Surrounding Wolf-Rayet stars in M31
We present the results of an ongoing investigation to provide a detailed view
of the processes by which massive stars shape the surrounding interstellar
medium (ISM), from pc to kpc scales. In this paper we have focused on studying
the environments of Wolf-Rayet (WR) stars in M31 to find evidence for WR
wind-ISM interactions, through imaging ionized hydrogen nebulae surrounding
these stars.
We have conducted a systematic survey for HII shells surrounding 48 of the 49
known WR stars in M31. There are 17 WR stars surrounded by single shells, or
shell fragments, 7 stars surrounded by concentric limb brightened shells, 20
stars where there is no clear physical association of the star with nearby
H-alpha emission, and 4 stars which lack nearby H-alpha emission. For the 17+7
shells above, there are 12 which contain one or two massive stars (including a
WR star) and that are <=40 pc in radius. These 12 shells may be classical WR
ejecta or wind-blown shells. Further, there may be excess H-alpha point source
emission associated with one of the 12 WR stars surrounded by putative ejecta
or wind-blown shells. There is also evidence for excess point source emission
associated with 11 other WR stars. The excess emission may arise from
unresolved circumstellar shells, or within the extended outer envelopes of the
stars themselves.
In a few cases we find clear morphological evidence for WR shells interacting
with each other. In several H-alpha images we see WR winds disrupting, or
punching through, the walls of limb-brightened HII shells.Comment: 20 pages, 4 figures (in several parts: some .jpg and others .ps),
accepted to AJ (appearing Oct, 1999
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