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

GAIA RVS data reduction : the 6^{th} dimension

International audienceThis poster describes the current organisation of RVS data processing among the Gaia-DPAC (Data Processing & Analysis Consortium), with a particular focus on the French community's contribution

Diffusion in Binary Systems as a Key for AmFm Scenarios

International audienceNot Availabl

The puzzling high velocity G5 supergiant star HD 179821: new insight from Gaia DR2 data

International audienceHD 179821 is classified as G5 Ia star. From the IRAS colors and spectral energy distributions it was classified as a post-AGB star. But some studies classify it as a massive (30 to 19 M sun) post-red supergiant evolving to become a Type II supernova. Its mass and evolutionary status remained a hotly debated question even after several detailed spectroscopic studies as the distance was not known. We use the parallax of HD 179821 from the second Gaia data release, and deduce its distance 2959 ± 409 pc and its absolute magnitude MV =-5.7 ± 0.3. Using the absolute magnitude determined in this paper, we show that HD 179821 fits very well with post-AGB tracks in the H-R diagram. Our results clearly confirm that HD 179821 is a post-AGB star of mass in the range of 0.8 M sun. It is not a 30 M sun red supergiant. The progenitor mass of this post-AGB star may be in the range of 4 M sun but may be a bit more

The signature of diffusion in the binary system of omicron Leonis: a key for AmFm scenarios?

International audienceAbundances of 14 chemical elements have been derived for the two components of the double-lined AmFm spectroscopic binary o Leonis in order to set constraints on the main physical processes that lead to the Am star phenomenon.The tomographic image reconstruction and Fourier disentangling techniques were used in order to recover individual spectra from the composite spectra of the binary system observed in 9 nights using the SOPHIE spectrograph at the Observatoire de Haute Provence. Abundances have been derived by iteratively adjusting synthetic spectra to the normalized spectra of the two components by chi-squared minimization.The abundance determinations made in this paper clearly show that both components of the o Leonis binary system exhibit Am characteristics. Comparison with self-consistent evolutionary models including atomic diffusion and turbulence or mass loss shows a good agreement. This is in favor of the scenario in which element separation occurs relatively deep in the star

Searching for a variability of interstellar reddening in the line of sight of NGC 4833

International audienceThe globular cluster NGC 4833 lies near the galactic plane. It is known to exhibit a strong differential reddening caused by several galactic interstellar clouds in its line of sight. Based on two optical observation runs made in 2006 within and separated by a 6-month interval, we intend to detect the photometric variations (colors and magnitudes) of stars in NGC 4833 due to the varying interstellar reddening

Time variation of differential reddening towards NGC 4833

International audienc

HIP 21539 is not a past very close neighbour of the Sun

Aims. A previous study claimed that the star HIP 21539 passed close to the Sun, at a distance of 1.9 pc, around 0.14 Myr ago. We show that this is not the case.Methods. We redetermined the trajectory of the star relative to the Sun using a new accurate radial velocity from the HARPS spectrograph combined with the recent Gaia-TGAS astrometry. Results. With this new data, the closest approach of HIP 21539 to the Sun is now 17 pc, instead of 1.9 pc.Conclusions. At this distance, the star has not perturbed the Oort cloud

3D hydrodynamical model atmospheres: a tool to correct radial velocities and parallaxes for Gaia

International audienc