366 research outputs found
New determination of abundances and stellar parameters for a set of weak G-band stars
Weak G-band (wGb) stars are very peculiar red giants almost devoided of
carbon and often mildly enriched in lithium. Despite their very puzzling
abundance patterns, very few detailed spectroscopic studies existed up to a few
years ago, preventing any clear understanding of the wGb phenomenon. We
recently proposed the first consistent analysis of published data for 28 wGb
stars and identified them as descendants of early A-type to late B-type stars,
without being able to conclude on their evolutionary status or the origin of
their peculiar abundance pattern.
We used newly obtained high-resolution and high SNR spectra for 19 wGb stars
in the southern and northern hemisphere to homogeneously derive their
fundamental parameters, metallicities, as well as the spectroscopic abundances
for Li, C, N, O, Na, Sr, and Ba. We also computed dedicated stellar evolution
models that we used to determine the masses and to investigate the evolutionary
status and chemical history of the stars in our sample. We confirm that the wGb
stars are stars in the mass range 3.2 to 4.2 M. We suggest that a large
fraction could be mildly evolved stars on the SGB currently undergoing the 1st
DUP, while a smaller number of stars are more probably in the core He burning
phase at the clump. After analysing their abundance pattern, we confirm their
strong N enrichment anti-correlated with large C depletion, characteristic of
material fully processed through the CNO cycle to an extent not known in other
evolved intermediate-mass stars. However, we demonstrate here that such a
pattern is very unlikely due to self-enrichment. In the light of the current
observational constraints, no solid self-consistent pollution scenario can be
presented either, leaving the wGb puzzle largely unsolved.Comment: 19 pages , 14 figures, accepted for publication in Astronomy &
Astrophysic
Towards a new full-sky list of radial velocity standard stars
The calibration of the Radial Velocity Spectrometer (RVS) onboard the ESA
Gaia satellite (to be launched in 2012) requires a list of standard stars with
a radial velocity (RV) known with an accuracy of at least 300 m/s. The IAU
Commission 30 lists of RV standard stars are too bright and not dense enough.
We describe the selection criteria due to the RVS constraints for building an
adequate full-sky list of at least 1000 RV standards from catalogues already
published in the literature. A preliminary list of 1420 candidate standard
stars is built and its properties are shown. An important re-observation
programme has been set up in order to ensure within it the selection of objects
with a good stability until the end of the Gaia mission (around 2018). The
present list of candidate standards is available at CDS and usable for many
other projects.Comment: Astronomy & Astrophysics, in press, 8 pages, 8 figure
Testing the binary hypothesis for the formation and shaping of planetary nebulae
There is no quantitative theory to explain why a high 80% of all planetary
nebulae are non-spherical. The Binary Hypothesis states that a companion to the
progenitor of a central star of planetary nebula is required to shape the
nebula and even for a planetary nebula to be formed at all. A way to test this
hypothesis is to estimate the binary fraction of central stars of planetary
nebulae and to compare it with that of the main sequence population.
Preliminary results from photometric variability and the infrared excess
techniques indicate that the binary fraction of central stars of planetary
nebulae is higher than that of the main sequence, implying that PNe could
preferentially form via a binary channel. This article briefly reviews these
results and current studies aiming to refine the binary fraction.Comment: EUROWD12 Proceeding
Planetary nebulae : getting closer to an unbiased binary fraction
Why 80% of planetary nebulae are not spherical is not yet understood. The
Binary Hypothesis states that a companion to the progenitor of the central star
of a planetary nebula is required to shape the nebula and even for a planetary
nebula to be formed at all. A way to test this hypothesis is to estimate the
binary fraction of central stars of planetary nebula and to compare it with the
main sequence population. Preliminary results from photometric variability and
infrared excess techniques indicate that the binary fraction of central stars
of planetary nebulae is higher than that of the putative main sequence
progenitor population, implying that PNe could be preferentially formed via a
binary channel. This article briefly reviews these results and future studies
aiming to refine the binary fraction.Comment: SF2A 2012 proceeding
The catalog of radial velocity standard stars for the Gaia RVS: status and progress of the observations
A new full-sky catalog of Radial Velocity standard stars is being built for
the determination of the Radial Velocity Zero Point of the RVS on board of
Gaia. After a careful selection of 1420 candidates matching well defined
criteria, we are now observing all of them to verify that they are stable
enough over several years to be qualified as reference stars. We present the
status of this long-term observing programme on three spectrographs : SOPHIE,
NARVAL and CORALIE, complemented by the ELODIE and HARPS archives. Because each
instrument has its own zero-point, we observe intensively IAU RV standards and
asteroids to homogenize the radial velocity measurements. We can already
estimate that ~8% of the candidates have to be rejected because of variations
larger than the requested level of 300 m/s.Comment: Proceedings of SF2A2010, S. Boissier, M. Heydari-Malayeri, R. Samadi
and D. Valls-Gabaud (eds), 3 pages, 2 figure
The SIMBAD astronomical database
Simbad is the reference database for identification and bibliography of
astronomical objects. It contains identifications, `basic data', bibliography,
and selected observational measurements for several million astronomical
objects. Simbad is developed and maintained by CDS, Strasbourg. Building the
database contents is achieved with the help of several contributing institutes.
Scanning the bibliography is the result of the collaboration of CDS with
bibliographers in Observatoire de Paris (DASGAL), Institut d'Astrophysique de
Paris, and Observatoire de Bordeaux. When selecting catalogues and tables for
inclusion, priority is given to optimal multi-wavelength coverage of the
database, and to support of research developments linked to large projects. In
parallel, the systematic scanning of the bibliography reflects the diversity
and general trends of astronomical research.
A WWW interface to Simbad is available at: http://simbad.u-strasbg.fr/SimbadComment: 14 pages, 5 Postscript figures; to be published in A&A
Fast robust control of linear systems subject to actuator saturation
This paper deals with the robust stability of soft variable-structure controls. More precisely, the control of linear plants subject to parametric uncertainty and actuator saturation is considered. Earlier works are summarized and new results are presented in this paper. It is shown that for all considered types of soft variable-structure controls, the robustness analysis leads to parameter-dependent Lyapunov inequalities. An overhead crane control is given as an illustrating example
Weak G-band stars on the H-R Diagram: Clues to the origin of Li anomaly
Weak G-band (WGB) stars are a rare class of cool luminous stars that present
a strong depletion in carbon, but also lithium abundance anomalies that have
been little explored in the literature since the first discovery of these
peculiar objects in the early 50's. Here we focus on the Li-rich WGB stars and
report on their evolutionary status. We explore different paths to propose a
tentative explanation for the lithium anomaly. Using archive data, we derive
the fundamental parameters of WGB (Teff, log g, log(L/Lsun)) using Hipparcos
parallaxes and recent temperature scales. From the equivalent widths of Li
resonance line at 6707 {\AA}, we uniformly derive the lithium abundances and
apply when possible NLTE corrections following the procedure described by Lind
et al. (2009). We also compute dedicated stellar evolution models in the mass
range 3.0 to 4.5 Msun, exploring the effects of rotation-induced and
thermohaline mixing. These models are used to locate the WGB stars in the H-R
diagram and to explore the origin of the abundance anomalies. The location of
WGB stars in the H-R diagram shows that these are intermediate mass stars of
masses ranging from 3.0 to 4.5 Msun located at the clump, which implies a
degeneracy of their evolutionary status between subgiant/red giant branch and
core helium burning phases. The atmospheres of a large proportion of WGB stars
(more than 50%) exhibit lithium abundances A(Li) \geq 1.4 dex similar to
Li-rich K giants. The position of WGB stars along with the Li-rich K giants in
the H-R diagram however indicates that both are well separated groups. The
combined and tentatively consistent analysis of the abundance pattern for
lithium, carbon and nitrogen of WGB stars seems to indicate that carbon
underabundance could be decorrelated from the lithium and nitrogen
overabundances.Comment: 13 pages, 3 figures, Accepted for publication in Astronomy and
Astrophysic
Li survey in giant stars: probing non-standard stellar physics
Lithium has long been known to be a good tracer of non-standard mixing processes occurring in stellar interiors. Here we present the results of a large survey aimed at determining the surface Li abundance in a sample of about 800 giant (RGB and AGB) stars with accurate Hipparcos parallaxes. We compare the observed Li behaviour with that predicted by stellar models including rotation and thermohaline mixin
- …