563 research outputs found
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
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
PREFACE: The Milky Way Unravelled by Gaia: GREAT Science from the Gaia Data Releases
International audiencePREFAC
Calibration of the Gaia RVS from ground-based observations of candidate standard stars
International audienceThe Radial Velocity Spectrometer (RVS) on board of Gaia will perform a large spectroscopic survey to determine the radial velocities of some 1.5 × 10^8 stars. We present the status of ground-based observations of a sample of 1420 candidate standard stars designed to calibrate the RVS. Each candidate star has to be observed several times before Gaia launch (and at least once during the mission) to ensure that its radial velocity remains stable during the whole mission. Observations are performed with the high-resolution spectrographs SOPHIE, NARVAL and CORALIE, completed with archival data of the ELODIE and HARPS instruments. The analysis shows that about 7% of the current catalogue exhibits variations larger than the adopted threshold of 300 m s^{-1}. Consequently, those stars should be rejected as reference targets, due to the expected accuracy of the Gaia RVS. Emphasis is also put here on our observations of bright asteroids to calibrate the ground-based velocities by a direct comparison with celestial mechanics. It is shown that the radial velocity zero points of SOPHIE, NARVAL and CORALIE are consistent with each other, within the uncertainties. Despite some scatter, their temporal variations remain small with respect to our adopted stability criterion
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Commissioning the Cryogenic System of the First LHC Sector
The LHC machine, composed of eight sectors with superconducting magnets and accelerating cavities requires a complex cryogenic system providing high cooling capacities (18 kW equivalent at 4.5 K and 2.4  W at 1.8 K per sector produced in large cold boxes and distributed via 3.3-km cryogenic transfer lines). After individual reception tests of the cryogenic subsystems (cryogen storages, refrigerators, cryogenic transfer lines and distribution boxes) performed since 2000, the commissioning of the cryogenic system of the first LHC sector has been under way since November 2006. After a brief introduction to the LHC cryogenic system and its specificities, the commissioning is reported detailing the preparation phase (pressure and leak tests, circuit conditioning and flushing), the cool-down sequences including the handling of cryogenic fluids, the magnet powering phase and finally the warm-up. Preliminary conclusions on the commissioning of the first LHC sector will be drawn with the review of the critical points already solved or still pending. The last part of the paper reports on the first operational experience of the LHC cryogenic system in the perspective of the commissioning of the remaining LHC sectors and the beam injection test
Using a Hipparcos derived HR diagram to limit the metallicity scatter of stars in the Hyades -- Are Stars Polluted?
Hipparcos parallaxes and proper motions have made it possible to construct
Hertzsprung-Russell (HR) diagrams of nearby clusters with unprecedented
accuracy. The standard deviation of high fidelity non-binary non-variable stars
about a model stellar evolution isochrone in the Hyades cluster is about 0.04
magnitudes. We use this deviation to estimate an upper limit on the scatter in
metallicities in stars in this cluster. From the gradient of the isochrones
evolution in the HR diagram we estimate an upper limit for the scatter of
metallicities Delta [Fe/H] <~ 0.03 dex, a smaller limit than has been measured
previously spectroscopically. This suggests that stars in open clusters are
formed from gas that is nearly homogeneous in its metallicity. We consider the
hypothesis that processes associated with planet formation can pollute the
convection zone of stars. If the position on the HR diagram is insensitive to
the metallicity of the convection zone and atmosphere, then stars which have
very polluted convection zones can be identified from a comparison between
their metallicity and position on the HR diagram. Alternatively if the
pollution of the star by metals results in a large change in the position of
the star on the HR diagram in a direction perpendicular to the isochrone, then
the low scatter of stars in the Hyades can be used to place constraints on
quantity of high-Z material that could have polluted the stars.Comment: submitted to A
New membership determination and proper motions of NGC 1817. Parametric and non-parametric approach
We have calculated proper motions and re-evaluated the membership
probabilities of 810 stars in the area of two NGC objects, NGC 1817 and NGC
1807. We have obtained absolute proper motions from 25 plates in the reference
system of the Tycho-2 Catalogue. The plates have a maximum epoch difference of
81 years; and they were taken with the double astrograph at Zo-Se station of
Shanghai Observatory, which has an aperture of 40 cm and a plate scale of 30
arcsec/mm. The average proper motion precision is 1.55 mas/yr. These proper
motions are used to determine the membership probabilities of stars in the
region, based on there being only one very extended physical cluster: NGC 1817.
With that aim, we have applied and compared parametric and non-parametric
approaches to cluster/field segregation. We have obtained a list of 169
probable member stars.Comment: 11 pages, 8 figures, A&A in pres
Age determination for 269 DR2 Open Clusters
. Gaia Second Data Release provides precise astrometry and
photometry for more than 1.3 billion sources. This catalog opens a new era
concerning the characterization of open clusters and test stellar models,
paving the way for a better understanding of the disc properties. . The
aim of the paper is to improve the knowledge of cluster parameters, using only
the unprecedented quality of the Gaia photometry and astrometry. . We
make use of the membership determination based on the precise Gaia astrometry
and photometry. We apply anautomated Bayesian tool, BASE-9, to fit stellar
isochrones on the observed G, GBP, GRP magnitudes of the high probability
member stars. . We derive parameters such as age, distance modulus and
extinction for a sample of 269 open clusters, selecting only low reddening
objects and discarding very young clusters, for which techniques other than
isochrone-fitting are more suitable for estimating ages.Comment: 14 pages, 11 figures. Submitte
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