5,062 research outputs found
The warm circumstellar envelope and wind of the G9 IIb star HR 6902
IUE observations of the eclipsing binary system HR 6902 obtained at various
epochs spread over four years indicate the presence of warm circumstellar
material enveloping the G9 IIb primary. The spectra show Si IV and C IV
absorption up to a distance of 3.3 giant radii (R_g}. Line ratio diagnostics
yields an electron temperature of ~ 78000 K which appears to be constant over
the observed height range.
Applying a least square fit absorption line analysis we derive column
densities as a function of height. We find that the inner envelope (< 3 R_g) of
the bright giant is consistent with a hydrostatic density distribution. The
derived line broadening velocity of ~ 70 kms^{-1} is sufficient to provide
turbulent pressure support for the required scale height. However, an improved
agreement with observations over the whole height regime including the emission
line region is obtained with an outflow model. We demonstrate that the common
beta power-law as well as a P \propto rho wind yield appropriate fit models.
Adopting a continuous mass outflow we obtain a mass-loss rate of M_loss= (0.8 -
3.4)*10^{-11} M_{sun}yr^{-1} depending on the particular wind model.Comment: 11 pages, 8 figures, submitted to Astronomy Astrophysics main Journa
The QSO evolution derived from the HBQS and other complete QSO surveys
An ESO Key programme dedicated to an Homogeneous Bright QSO Survey (HBQS) has
been completed. 327 QSOs (Mb<-23, 0.3<z<2.2) have been selected over 555 deg^2
with 15<B<18.75. For B<16.4 the QSO surface density turns out to be a factor
2.2 higher than what measured by the PG survey, corresponding to a surface
density of 0.013+/-.006 deg^{-2}. If the Edinburgh QSO Survey is included, an
overdensity of a factor 2.5 is observed, corresponding to a density of
0.016+/-0.005 deg^{-2}. In order to derive the QSO optical luminosity function
(LF) we used Monte Carlo simulations that take into account of the selection
criteria, photometric errors and QSO spectral slope distribution. The LF can be
represented with a Pure Luminosity Evolution (L(z)\propto(1+z)^k) of a two
power law both for q_0=0.5 and q_0=0.1. For q_0=0.5 k=3.26, slower than the
previous Boyle's (1992) estimations of k=3.45. A flatter slope beta=-3.72 of
the bright part of the LF is also required. The observed overdensity of bright
QSOs is concentrated at z<0.6. It results that in the range 0.3<z<0.6 the
luminosity function is flatter than observed at higher redshifts. In this
redshift range, for Mb<-25, 32 QSOs are observed instead of 19 expected from
our best-fit PLE model. This feature requires a luminosity dependent luminosity
evolution in order to satisfactorily represent the data in the whole 0.3<z<2.2
interval.Comment: Invited talk in "Wide Field Spectroscopy" (20-24 May 1996, Athens),
eds. M. Kontizas et al. 6 pages and 3 eps figures, LaTex file, uses epfs.sty
and crckapb.sty (included
Representations of p-brane topological charge algebras
The known extended algebras associated with p-branes are shown to be
generated as topological charge algebras of the standard p-brane actions. A
representation of the charges in terms of superspace forms is constructed. The
charges are shown to be the same in standard/extended superspace formulations
of the action.Comment: 22 pages. Typos fixed, refs added. Minor additions to comments
sectio
Finding the First Stars: The Hamburg/ESO Objective Prism Survey
We report on a search for extremely metal-poor ([Fe/H]<-3.0) turnoff stars in
the Hamburg/ESO objective prism survey (HES). Metal-poor stars are selected by
automatic spectral classification. Extensive simulations show that the
selection efficiency for turnoff stars of [Fe/H]25% at B<16.5. Since
the HES is more than 1 mag deeper than the HK survey of Beers et al. (1992),
the HES offers the possibility to efficiently increase the total number of
metal-poor stars by at least a factor of 4.Comment: To appear in: Proceedings of ESO/MPA conference "The First Stars". 2
pages, 1 figur
Static Critical Behavior of the Spin-Freezing Transition in the Geometrically Frustrated Pyrochlore Antiferromagnet Y2Mo2O7
Some frustrated pyrochlore antiferromagnets, such as Y2Mo2O7, show a
spin-freezing transition and magnetic irreversibilities below a temperature Tf
similar to what is observed nonlinear magnetization measurements on Y2Mo2O7
that provide strong evidence that there is an underlying thermodynamic phase
transition at Tf, which is characterized by critical exponents \gamma \approx
2.8 and \beta \approx 0.8. These values are typical of those found in random
spin glasses, despite the fact that the level of random disorder in Y2Mo2O7 is
immeasurably small.Comment: Latex file, calls for 4 encapsulated postscript figures (included).
Submitted to Phys. Rev. Letters
On the Metallicity-Color Relations and Bimodal Color Distributions in Extragalactic Globular Cluster Systems
We perform a series of numerical experiments to study how the nonlinear
metallicity--color relations predicted by different stellar population models
affect the color distributions observed in extragalactic globular cluster
systems. % We present simulations in the bandpasses based on five
different sets of simple stellar population (SSP) models. The presence of
photometric scatter in the colors is included as well. % We find that unimodal
metallicity distributions frequently ``project'' into bimodal color
distributions. The likelihood of this effect depends on both the mean and
dispersion of the metallicity distribution, as well as of course on the SSP
model used for the transformation. % Adopting the Teramo-SPoT SSP models for
reference, we find that optical--to--near-IR colors should be favored with
respect to other colors to avoid the bias effect in globular cluster color
distributions discussed by \citet{yoon06}. In particular, colors such as \vh\
or \vk are more robust against nonlinearity of the metallicity--color relation,
and an observed bimodal distribution in such colors is more likely to indicate
a true underlying bimodal metallicity distribution. Similar conclusions come
from the simulations based on different SSP models, although we also identify
exceptions to this result.Comment: ApJ accepte
Subdwarf B Stars from the ESO Supernova Ia Progenitor Survey -- Observation versus Theory
Original paper can be found at: http://www.astrosociety.org/pubs/cs/328.html--Copyright Astronomical Society of the PacificWe present the analysis of a high-quality sample of optical spectra for 76 sdB stars from the ESO Supernova Ia Progenitor Survey. Effective temperature, surface gravity, and photospheric helium abundance were derived from line profile fits. We demonstrate that our subsample of 52 single-lined sdB stars is a useful tool to compare observation and theory. The predictions of population synthesis models for close binary evolution are compared to our data. We show that the simulations cover the observed parameter range of sdBs, but fail to reproduce the observed distribution in detail
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