The fundamental parameters of the roAp star γ\gamma Equulei

Physical processes working in the stellar interiors as well as the evolution of stars depend on some fundamental stellar properties, such as mass, radius, luminosity, and chemical abundances. A classical way to test stellar interior models is to compare the predicted and observed location of a star on theoretical evolutionary tracks in a H-R diagram. This requires the best possible determinations of stellar mass, radius, luminosity and abundances. To derive its fundamental parameters, we observed the well-known rapidly oscillating Ap star, $\gamma$ Equ, using the visible spectro-interferometer VEGA installed on the optical CHARA array. We computed the calibrated squared visibility and derived the limb-darkened diameter. We used the whole energy flux distribution, the parallax and this angular diameter to determine the luminosity and the effective temperature of the star. We obtained a limb-darkened angular diameter of 0.564~$\pm$~0.017~mas and deduced a radius of $R$~=~2.20~$\pm$~0.12~${\rm R_{\odot}}$. Without considering the multiple nature of the system, we derived a bolometric flux of $(3.12\pm 0.21)\times 10^{-7}$ erg~cm$^{-2}$~s$^{-1}$ and an effective temperature of 7364~$\pm$~235~K, which is below the effective temperature that has been previously determined. Under the same conditions we found a luminosity of $L$~=~12.8~$\pm$~1.4~${\rm L_{\odot}}$. When the contribution of the closest companion to the bolometric flux is considered, we found that the effective temperature and luminosity of the primary star can be, respectively, up to $\sim$~100~K and up to $\sim$~0.8~L$_\odot$ smaller than the values mentioned above.These new values of the radius and effective temperature should bring further constraints on the asteroseismic modelling of the star.Comment: Accepted by A&

The Hα\alpha line forming region of AB Aur spatially resolved at sub-AU with the VEGA/CHARA spectro-interferometer

A crucial issue in star formation is to understand the physical mechanism by which mass is accreted onto and ejected by a young star. The visible spectrometer VEGA on the CHARA array can be an efficient means of probing the structure and the kinematics of the hot circumstellar gas at sub-AU. For the first time, we observed the Herbig Ae star AB Aur in the H$\alpha$ emission line, using the VEGA low spectral resolution on two baselines of the array. We computed and calibrated the spectral visibilities between 610 nm and 700 nm. To simultaneously reproduce the line profile and the visibility, we used a 1-D radiative transfer code that calculates level populations for hydrogen atoms in a spherical geometry and synthetic spectro-interferometric observables. We clearly resolved AB Aur in the H$\alpha$ line and in a part of the continuum, even at the smallest baseline of 34 m. The small P-Cygni absorption feature is indicative of an outflow but could not be explained by a spherical stellar wind model. Instead, it favors a magneto-centrifugal X-disk or disk-wind geometry. The fit of the spectral visibilities could not be accounted for by a wind alone, so we considered a brightness asymmetry possibly caused by large-scale nebulosity or by the known spiral structures, inducing a visibility modulation around H$\alpha$. Thanks to the unique capabilities of VEGA, we managed to simultaneously record for the first time a spectrum at a resolution of 1700 and spectral visibilities in the visible range on a target as faint as $m_{V}$ = 7.1. It was possible to rule out a spherical geometry for the wind of AB Aur and provide realistic solutions to account for the H$\alpha$ emission compatible with magneto-centrifugal acceleration. The study illustrates the advantages of optical interferometry and motivates observations of other bright young stars to shed light on the accretion/ejection processes

A new interferometric study of four exoplanet host stars : {\theta} Cygni, 14 Andromedae, {\upsilon} Andromedae and 42 Draconis

Studying exoplanet host stars is of the utmost importance to establish the link between the presence of exoplanets around various types of stars and to understand the respective evolution of stars and exoplanets. Using the limb-darkened diameter (LDD) obtained from interferometric data, we determine the fundamental parameters of four exoplanet host stars. We are particularly interested in the F4 main-sequence star, {\theta} Cyg, for which Kepler has recently revealed solar-like oscillations that are unexpected for this type of star. Furthermore, recent photometric and spectroscopic measurements with SOPHIE and ELODIE (OHP) show evidence of a quasi-periodic radial velocity of \sim150 days. Models of this periodic change in radial velocity predict either a complex planetary system orbiting the star, or a new and unidentified stellar pulsation mode. We performed interferometric observations of {\theta} Cyg, 14 Andromedae, {\upsilon} Andromedae and 42 Draconis for two years with VEGA/CHARA (Mount Wilson, California) in several three-telescope configurations. We measured accurate limb darkened diameters and derived their radius, mass and temperature using empirical laws. We obtain new accurate fundamental parameters for stars 14 And, {\upsilon} And and 42 Dra. We also obtained limb darkened diameters with a minimum precision of \sim 1.3%, leading to minimum planet masses of Msini=5.33\pm 0.57, 0.62 \pm 0.09 and 3.79\pm0.29 MJup for 14 And b, {\upsilon} And b and 42 Dra b, respectively. The interferometric measurements of {\theta} Cyg show a significant diameter variability that remains unexplained up to now. We propose that the presence of these discrepancies in the interferometric data is caused by either an intrinsic variation of the star or an unknown close companion orbiting around it.Comment: 10 pages + 2 pages appendix, 16 figures, accepted for publication in A&

Report of IERS components: GRGS combination centre

IERS Annual Report 2005, edited by W. R. Dick and B. Richter, Verlag des Bundesamts für Kartographie und Geodäsie Frankfurt am Main 2007, p 137

« Global combination from space geodetic techniques

International audienceWe have demonstrated the possibility of deriving both station coordinates and Earth Orientation Parameters (EOP) using weekly normal equations derived from the processing of different space-geodetic techniques: VLBI, SLR, LLR, DORIS and GPS. The work is performed in the frame of a joint project within the GRGS, Groupe de Recherches de Géodésie Spatiale, federation of French institutes. Observations of the different techniques: VLBI, SLR, LLR, DORIS and GPS are processed separately at the different institutes where in addition expertise can be found for the specific technique. Before such products are adopted as references, different problems concerning the stabilization of the terrestrial reference frame have to be solved in addition to upgrading the individual techniques processing to the best possible level of accuracy

« Global combination from space geodetic techniques

We have demonstrated the possibility of deriving both station coordinates and Earth Orientation Parameters (EOP) using weekly normal equations derived from the processing of different space-geodetic techniques: VLBI, SLR, LLR, DORIS and GPS. The work is performed in the frame of a joint project within the GRGS, Groupe de Recherches de Géodésie Spatiale, federation of French institutes. Observations of the different techniques: VLBI, SLR, LLR, DORIS and GPS are processed separately at the different institutes where in addition expertise can be found for the specific technique. Before such products are adopted as references, different problems concerning the stabilization of the terrestrial reference frame have to be solved in addition to upgrading the individual techniques processing to the best possible level of accuracy

Performances and first science results with the VEGA/CHARA visible instrument

International audienceThis paper presents the current status of the VEGA (Visible spEctroGraph and polArimeter) instrument installed at the coherent focus of the CHARA Array, Mount Wilson CA. Installed in september 2007, the first science programs have started during summer 2008 and first science results are now published. Dedicated to high angular (0.3mas) and high spectral (R=30000) astrophysical studies, VEGA main objectives are the study of circumstellar environments of hot active stars or interactive binary systems and a large palette of new programs dedicated to fundamental stellar parameters. We will present successively the main characteristics of the instrument and its current performances in the CHARA environment, a short summary of two science programs and finally we will develop some studies showing the potential and difficulties of the 3 telescopes mode of VEGA/CHARA

A high angular and spectral resolution view into the hidden companion of

The enigmatic binary, ε Aur, is yielding its parameters as a result of new methods applied to the recent eclipse, including optical spectro-interferometry with the VEGA beam combiner at the CHARA Array. VEGA/CHARA visibility measurements from 2009 to 2011 indicate the formation of emission wings of Hα in an expanding zone almost twice the photospheric size of the F star, namely, in a stellar wind. These may be caused by shocks in the atmosphere from large scale convective or multi-periodic pulsation modes emerging from the star. During the total eclipse phase in 2010, when the disk was in the line of sight, we saw broadening of the Hα absorption and a less steep drop of the visibility curve, consistent with the addition of neutral hydrogen in the line of sight but extended above and below the plane of the interferometrically imaged disk itself. This provides a unique constraint on the scale height of the gaseous component of the disk material, and, based on some additional assumptions, points to a mass of the central object being 2.4 to 5.5 M⊙ for a distance of 650 pc or 3.8 to 9.1 M⊙ for a distance of 1050 pc. These results can be tested during coming observing seasons as the star moves from eclipse phase toward quadrature

Kinematics and geometrical study of the Be stars 48 Persei and

Context. Five different physical processes might be responsible for the formation of decretion disks around Be stars: fast rotation of the star, stellar pulsations, binarity, stellar winds, and magnetic fields. Our observations indicate that fast rotation seems to produce a disk in Keplerian rotation, at least in the specific case of the two stars observed. We do not know if this observational result is a generality or not. Aims. We measure the size, orientation, shape, and kinematics of the disks around 2 Be stars, namely 48 Per and ψ Per. Methods. We used the VEGA/CHARA interferometer with a spectral resolution of 5000 to obtain spectrally dispersed visibility modulus and phases within the Hα emission line. Results. We were able to estimate the disk extension in the continuum and in the Hα line, as well as flattening, for both stars. Both stars rotate at nearly a critical rotation, but while the disk of 48 Per seems to be in Keplerian rotation, our preliminary data suggest that the disk of ψ Per is possibly faster than Keplerian, similarly to what has been found for κ CMa with observations carried out in the near-IR. However, more data is needed to confirm the fast rotation of the disk. Conclusions. Assuming a simple uniform disk model for the stellar photosphere in the continuum and a Gaussian brightness distribution in the line emission region, we obtain a ratio of the disk diameter over the photospheric diameter of 8 for 48 Per and 11 for and ψ Per. We also found that the major axis of 48 Per is parallel to the polarization angle and not perpendicular to it as previously observed for many Be stars, including ψ Per. This might be due to the optical thickness of the disk, which is also responsible for the incoherent scattering of a non negligible part of the Hα line emission. To our knowledge, this is the first time that this effect has been measured in a Be star