Search

PLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2 R_(Earth)) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5 %, 10 %, 10 % for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO's target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile at the beginning of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases.

Stellar mergers and accretion events have been crucial in shaping the evolution of the Milky Way (MW). These events have been dynamically identified and chemically characterised using red giants and main-sequence stars. RR Lyrae (RRL) variables can play a crucial role in tracing the early formation of the MW since they are ubiquitous, old (t$\ge$10 Gyr) low-mass stars and accurate distance indicators. We exploited Data Release 3 of the GALAH survey to identify 78 field RRLs suitable for chemical analysis. Using synthetic spectra calculations, we determined atmospheric parameters and abundances of Fe, Mg, Ca, Y, and Ba. Most of our stars exhibit halo-like chemical compositions, with an iron peak around [Fe/H]$\approx -$1.40, and enhanced Ca and Mg content. Notably, we discovered a metal-rich tail, with [Fe/H] values ranging from $-$1 to approximately solar metallicity. This sub-group includes almost ~1/4 of the sample, it is characterised by thin disc kinematics and displays sub-solar $\alpha$-element abundances, marginally consistent with the majority of the MW stars. Surprisingly, they differ distinctly from typical MW disc stars in terms of the s-process elements Y and Ba. We took advantage of similar data available in the literature and built a total sample of 535 field RRLs for which we estimated kinematical and dynamical properties. We found that metal-rich RRLs (1/3 of the sample) likely represent an old component of the MW thin disc. We also detected RRLs with retrograde orbits and provided preliminary associations with the Gaia-Sausage-Enceladus, Helmi, Sequoia, Sagittarius, and Thamnos stellar streams., Comment: Accepted for publication in MNRAS. 29 pages, 20 figures

We present new empirical infrared Period-Luminosity-Metallicity (PLZ) and Period-Wesenheit-Metallicity (PWZ) relations for RR Lyrae based on the latest Gaia EDR3 parallaxes. The relations are provided in the WISE $W1$ and $W2$ bands, as well as in the $W(W1, V - W1)$ and $W(W2, V - W2)$ Wesenheit magnitudes. The relations are calibrated using a very large sample of Galactic halo field RR Lyrae stars with homogeneous spectroscopic [Fe/H] abundances (over 1,000 stars in the $W1$ band), covering a broad range of metallicities ($-2.5 \lesssim \textrm{[Fe/H]} \lesssim 0.0$). We test the performance of our PLZ and PWZ relations by determining the distance moduli of both galactic and extragalactic stellar associations: the Sculptor dwarf spheroidal galaxy in the Local Group (finding $\bar{\mu}_{0}=19.47 \pm 0.06$), the Galactic globular clusters M4 ($\bar{\mu}_{0}=11.16 \pm 0.05$) and the Reticulum globular cluster in the Large Magellanic Cloud ($\bar{\mu}_{0}=18.23 \pm 0.06$). The distance moduli determined through all our relations are internally self-consistent (within $\lesssim$ 0.05 mag) but are systematically smaller (by $\sim$ 2-3$\sigma$) than previous literature measurements taken from a variety of methods/anchors. However, a comparison with similar recent RR Lyrae empirical relations anchored with EDR3 likewise shows to varying extents a systematically smaller distance modulus for PLZ/PWZ RR Lyrae relations., Comment: Accepted by ApJ, 14 pages, 5 Figures, 2 Tables

We present new period-$\phi_{31}$-[Fe/H] relations for first overtone RRL stars (RRc), calibrated over a broad range of metallicities ($-2.5 < \textrm{[Fe/H]}< 0.0$) utilizing the largest currently available set of Galactic halo field RRL with homogeneous spectroscopic metallicities. Our relations are defined in the optical (ASAS-SN $V$-band) and, inaugurally, in the infrared (WISE $W1$ and $W2$ bands). Our $V$-band relation can reproduce individual RRc spectroscopic metallicities with a dispersion of 0.30 dex over the entire metallicity range of our calibrator sample (an RMS smaller than what we found for other relations in literature including non-linear terms). Our infrared relation has a similar dispersion in the low and intermediate metallicity range ($\textrm{[Fe/H]} < -0.5$) but tends to underestimate the [Fe/H] abundance around solar metallicity. We tested our relations by measuring both the metallicity of the Sculptor dSph and a sample of Galactic globular clusters, rich in both RRc and RRab stars. The average metallicity we obtain for the combined RRL sample in each cluster is within $\pm 0.08$ dex of their spectroscopic metallicities. The infrared and optical relations presented in this work will enable deriving reliable photometric RRL metallicities in conditions where spectroscopic measurements are not feasible; e.g., in distant galaxies or reddened regions (observed with upcoming Extremely Large Telescopes and the James Webb Space Telescope), or in the large sample of new RRL that will be discovered in large-area time-domain photometric surveys (such as LSST and the Roman space telescope)., Comment: Accepted by ApJ, 22 pages, 9 Figures, 3 Tables

The multiplicity fraction of stars, down to the substellar regime, is a parameter of fundamental importance for stellar formation, evolution, and planetology. The census of multiple stars in the solar neighborhood is however incomplete. Our study is aimed at detecting companions of Hipparcos catalog stars from the proper motion anomaly (PMa) they induce on their host star, namely, the difference between their long-term Hipparcos-Gaia and short-term Gaia proper motion vectors. We also aim to detect resolved, gravitationally bound companions of the Hipparcos stars (117,955 stars) and of the Gaia EDR3 stars closer than 100 pc (542,232 stars). In order to identify gravitationally bound visual companions of our sample, we searched the EDR3 catalog for common proper-motion (CPM) candidates. The detection of tangential velocity anomalies with a median accuracy of 26 cm/s per parsec of distance is demonstrated. This improvement by a factor 2.5 in accuracy, as compared to DR2, results in PMa detection limits on companions that are well into the planetary mass regime for many targets. We identify 37,515 Hipparcos stars with a significant PMa (S/N>3), namely, a fraction of 32% and 12,914 (11%) hosting CPM bound candidate companions. After including the EDR3 renormalised unit weight error (RUWE>1.4) as an additional indicator, 50,720 stars of the Hipparcos catalog (43%) exhibit at least one signal of binarity. Among the EDR3 stars within 100 pc, we find CPM bound candidate companions for 39,490 stars (7.3%). The search for companions using a combination of the PMa, CPM, and RUWE indicators improves the exhaustivity of the multiplicity survey. The detection of CPM companions of very bright stars (heavily saturated on the Gaia detectors) that are classical benchmark objects for stellar physics provides a useful proxy for estimating their distance with a higher accuracy than with Hipparcos., Comment: 27 pages, 25 figures. Astronomy & Astrophysics, in press. The PMa and CPM catalogs are available for download from https://sites.lesia.obspm.fr/pierre-kervella/2021/10/26/gaiaedr3/ and will be available from the CDS/VizieR service

Accurate metallicities of RR Lyrae are extremely important in constraining period-luminosity-metallicity relationships (PLZ), particularly in the near-infrared. We analyse 69 high-resolution spectra of Galactic RR Lyrae stars from the Southern African Large Telescope (SALT). We measure metallicities of 58 of these RR Lyrae stars with typical uncertainties of 0.13 dex. All but one RR Lyrae in this sample has accurate ({\sigma}_parallax ~ 10%) parallax from Gaia. Combining these new high resolution spectroscopic abundances with similar determinations from the literature for 93 stars, we present new PLZ relationships in WISE W1 and W2 magnitudes, and the Wesenheit magnitudes W(W1,V-W1) and W(W2,V-W2)., Comment: 16 pages, 13 figures, accepted in MNRAS

We present newly-calibrated period-$\phi_{31}$-[Fe/H] relations for fundamental mode RR Lyrae stars in the optical and, for the first time, mid-infrared. This work's calibration dataset provides the largest and most comprehensive span of parameter space to date with homogeneous metallicities from $-3<\textrm{[Fe/H]}<0.4$ and accurate Fourier parameters derived from 1980 ASAS-SN ($V$-band) and 1083 WISE (NEOWISE extension, $W1$ and $W2$ bands) RR Lyrae stars with well-sampled light curves. We compare our optical period-$\phi_{31}$-[Fe/H] with those available in the literature and demonstrate that our relation minimizes systematic trends in the lower and higher metallicity range. Moreover, a direct comparison shows that our optical photometric metallicities are consistent with both those from high-resolution spectroscopy and globular clusters, supporting the good performance of our relation. We found an intrinsic scatter in the photometric metallicities (0.41 dex in the $V$-band and 0.50 dex in the infrared) by utilizing large calibration datasets covering a broad metallicity range. This scatter becomes smaller when optical and infrared bands are used together (0.37 dex). Overall, the relations derived in this work have many potential applications, including large-area photometric surveys with JWST in the infrared and LSST in the optical., Comment: Accepted by ApJ, 29 pages, 14 Figures, 4 Tables

We have been monitoring the metal-poor Population II giant, HD 122563, for radial velocity variations since 2016 using the SONG telescope on Tenerife. We have detected the global seismic quantity, numax, which provides information related to the stellar parameters. By combining these data with complementary data, we derive a new precise surface gravity, radius and distance to the star. Our results are corroborated by using the parallax from Gaia DR2. We present these results and some of their implications., Comment: Proceedings of the French Astronomy Society Meeting, SF2A, June 2019; v2: typo in reported numax in Table, authors corrected

The nearby metal-poor giant HD122563 is an important astrophysical laboratory for which to test stellar atmospheric and interior physics. It is also a benchmark star for which to calibrate methods to apply to large scale surveys. Recently it has been remeasured using various methodologies given the new high precision instruments at our disposal. However, inconsistencies in the observations and models have been found. In order to better characterise this star we have been measuring its radial velocities since 2016 using the Hertzsprung telescope (SONG network node). In this work we report the first detections of sun-like oscillations in this star, and to our knowledge, a detection in the most metal-poor giant to date. We apply the classical seismic scaling relation to derive a new surface gravity of $\log g_{\nu} = 1.39 \pm 0.01$ dex. Constraints on the mass imposed by its PopII giant classification then yield a radius of $30.8 \pm 1.0$ R$_{\odot}$. By coupling this with recent interferometric measurements we infer a distance to the star of 306 $\pm$ 9 pc. Data from the Gaia mission corroborates the distance hypothesis ($d_{\rm GDR2}$ = 290 $\pm$ 5 pc), and thus the updated fundamental parameters. We confirm the validity of the seismic scaling relation without corrections for surface gravity in metal-poor and evolved star regimes. The small discrepancy of 0.04 dex reduces to 0.02 dex by applying corrections to the scaling relations. The new constraints on the HR diagram ($L_{\odot} = 381 \pm 26$) reduce the disagreement between the stellar parameters and evolution models, however, a discrepancy still exists. Fine-tuned stellar evolution calculations show that this can be reconciled by changing the mixing-length parameter by an amount (--0.35) that is in agreement with predictions from recent 3D simulations and empirical results., Comment: Accepted for publication in A&A

The census of stellar and substellar companions of nearby stars is largely incomplete. It is, however, fundamentally important in the understanding of the stellar and planetary formation and evolution mechanisms. We aim to characterize the presence of physical companions in orbit around nearby stars. Orbiting secondary bodies influence the proper motion (PM) of their parent star through their gravitational reflex motion. Using the Hipparcos catalog and Gaia's second data release (GDR2), we determined the long-term PM of the stars common to these two catalogs. We then searched for a PM anomaly (PMa) between the long-term PM vector and the GDR2 (or Hipparcos) measurements, indicative of the presence of a perturbing secondary object. We focussed our analysis on the 6741 nearby stars located within 50 pc, and we also present a catalog of the PMa for >99% of the Hipparcos catalog (>117000 stars). 30% of the stars studied present a PMa greater than 3 sigmas. The PMa allows us to detect orbiting companions, or set stringent limits on their presence. We present a few applications to interesting targets. We set upper limits of 0.1-0.3 M_Jup to potential planets orbiting Proxima between 1 and 10 au (P_orb=3 to 100 years). We confirm that Proxima is gravitationally bound to alpha Cen. We recover the masses of the known companions of epsilon Eri, epsilon Ind, Ross 614 and beta Pic. We also detect a possible planet of a few Jovian masses orbiting tau Ceti. The combination of the GDR2 with Hipparcos results in very high accuracy PMa vectors. The detection of tangential velocity anomalies at a median accuracy of sigma(Delta v_tan)=1.0 m/s per parsec of distance opens the possibility to identify long period orbital companions otherwise inaccessible. Gaia's complementarity to radial velocity and transit techniques already appears to be remarkably powerful., Comment: 23 pages, 31 figures. Revised mass limits on companions with very long orbital periods, and computations extended to the full Hipparcos catalog. Accepted for publication in Astronomy & Astrophysics

The second Gaia data release (Gaia-DR2) contains, beyond the astrometry, three-band photometry for 1.38 billion sources. We have used these three broad bands to infer stellar effective temperatures, Teff, for all sources brighter than G=17 mag with Teff in the range 3000-10 000 K (161 million sources). Using in addition the parallaxes, we infer the line-of-sight extinction, A_G, and the reddening, E[BP-RP], for 88 million sources. Together with a bolometric correction we derive luminosity and radius for 77 million sources. These quantities as well as their estimated uncertainties are part of Gaia-DR2. Here we describe the procedures by which these quantities were obtained, including the underlying assumptions, comparison with literature estimates, and the limitations of our results. Typical accuracies are of order 324 K (Teff), 0.46 mag (A_G), 0.23 mag (E[BP-RP]), 15% (luminosity), and 10% (radius). Being based on only a small number of observable quantities and limited training data, our results are necessarily subject to some extreme assumptions that can lead to strong systematics in some cases (not included in the aforementioned accuracy estimates). One aspect is the non-negativity contraint of our estimates, in particular extinction. Yet in several regions of parameter space our results show very good performance, for example for red clump stars and solar analogues. Large uncertainties render the extinctions less useful at the individual star level, but they show good performance for ensemble estimates. We identify regimes in which our parameters should and should not be used and we define a "clean" sample. Despite the limitations, this is the largest catalogue of uniformly-inferred stellar parameters to date. More precise and detailed astrophysical parameters based on the full BP/RP spectrophotometry are planned as part of the third Gaia data release., Comment: A&A in press

High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized on the 15th and 16th of January, 2015 in Nice with the support of the Action Specific in Haute Resolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la Cote d'Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41., Comment: White Paper prospective. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41. v2 includes some corrections to text

We present stellar properties (mass, age, radius, distances) of 57 stars from a seismic inference using full-length data sets from Kepler. These stars comprise active stars, planet-hosts, solar-analogs, and binary systems. We validate the distances derived from the astrometric Gaia-Tycho solution. Ensemble analysis of the stellar properties reveals a trend of mixing-length parameter with the surface gravity and effective temperature. We derive a linear relationship with the seismic quantity $\langle r_{02} \rangle$ to estimate the stellar age. Finally, we define the stellar regimes where the Kjeldsen et al (2008) empirical surface correction for 1D model frequencies is valid., Comment: 4-page proceedings from Seismology of the Sun and the Distant Stars 2016, TASC/KASC, Azores, Portugal, corrected references in v2

We investigate the role of hydrogen collisions in NLTE spectral line synthesis, and introduce a new general empirical recipe to determine inelastic charge transfer (CT) and bound-bound hydrogen collisional rates. This recipe is based on fitting the energy functional dependence of published quantum collisional rate coefficients of several neutral elements (BeI, NaI, MgI, AlI, SiI and CaI) using simple polynomial equations. We perform thorough NLTE abundance calculation tests using our method for four different atoms, Na, Mg, Al and Si, for a broad range of stellar parameters. We then compare the results to calculations computed using the published quantum rates for all the corresponding elements. We also compare to results computed using excitation collisional rates via the commonly used Drawin equation for different fudge factors, SH, applied. We demonstrate that our proposed method is able to reproduce the NLTE abundance corrections performed with the quantum rates for different spectral types and metallicities for representative NaI and AlI lines to within $\le$0.05 dex and %\le%0.03 dex, respectively. For MgI and SiI lines, the method performs better for the cool giants and dwarfs, while larger discrepancies up to 0.2 dex could be obtained for some lines for the subgiants and warm dwarfs. We obtained larger NLTE correction differences between models incorporating Drawin rates relative to the quantum models by up to 0.4 dex. These discrepancies are potentially due to ignoring either or both CT and ionization collisional processes by hydrogen in our Drawin models. Our empirical fitting method performs well in its ability to reproduce, within narrow uncertainties, the abundance corrections computed with models incorporating quantum collisional rates. It could possibly be extended to other transitions or in the absence of published quantum calculations, to other elements as well., Comment: 15 pages, published in A&A at https://doi.org/10.1051/0004-6361/201630352

The Kepler space telescope yielded unprecedented data for the study of solar-like oscillations in other stars. The large samples of multi-year observations posed an enormous data analysis challenge that has only recently been surmounted. Asteroseismic modeling has become more sophisticated over time, with better methods gradually developing alongside the extended observations and improved data analysis techniques. We apply the latest version of the Asteroseismic Modeling Portal (AMP) to the full-length Kepler data sets for 57 stars and the Sun, comprising planetary hosts, binaries, solar-analogs, and active stars. From an analysis of the derived stellar properties for the full sample, we identify a variation of the mixing-length parameter with atmospheric properties. We also derive a linear relation between the stellar age and a characteristic frequency separation ratio. In addition, we find that the empirical correction for surface effects suggested by Kjeldsen and coworkers is adequate for solar-type stars that are not much hotter (Teff < 6200 K) or significantly more evolved (logg > 4.2, > 80 muHz) than the Sun. Precise parallaxes from the Gaia mission and future observations from TESS and PLATO promise to improve the reliability of stellar properties derived from asteroseismology., Comment: A&A accepted

Context. delta Vel contains the brightest eclipsing binary in the southern sky (delta Vel A), and a nearby third star located ~0.6" away (delta Vel B). The proximity of delta Vel B (usable as a reference) makes it a particularly well-suited target to detect the astrometric displacement of the center of light of the eclipsing pair. Aims. We obtained NACO astrometric observations with two goals: (1) to confirm the orientation of the orbital plane of the eclipsing pair on the sky determined by interferometry (Paper III) and (2) demonstrate the capabilities of narrow-angle adaptive optics astrometry on a simple system with predictable astrometric properties. Methods. We measured the angular separation vector between the eclipsing binary delta Vel A and the visual companion delta Vel B from narrow-band images at 2.17 microns obtained with the VLT/NACO adaptive optics system. Based on these observations and our previous determination of the orbital parameters of the wide binary delta Vel A-B, we derive the apparent displacement of the center-of-light of the eclipsing pair at 11 epochs over its orbital cycle. Results. We detect the astrometric wobble of the center of light of the delta Vel A pair relatively to B with a typical measurement precision of ~50 microarcseconds per epoch, for a total amplitude of the measured displacement of ~2 milliarcseconds. Conclusions. The detected wobble is in agreement with our model presented in Paper III and confirms the orientation of the Aab orbital plane on the sky. The residual dispersion compared to our model is 110 microarcseconds rms, that we tentatively attribute to photometric variability of the fast rotating A-type components Aa and/or Ab in the BrGamma line. Based on these results, we conclude that in favorable conditions (bright source with only two resolved components, small angular separation), narrow-angle astrometry with adaptive optics on an 8-meter class telescope can reach an accuracy of 50 to 100 microarcseconds., Comment: 8 pages, accepted for publication in A\&A

We investigate NLTE effects for magnesium and calcium in the atmospheres of late-type giant and supergiant stars. The aim of this paper is to provide a grid of NLTE/LTE equivalent width ratios W/W* of Mg and Ca lines for the following range of stellar parameters: Teff in [3500, 5250] K, log g in [0.5, 2.0] dex and [Fe/H] in [-4.0, 0.5] dex. We use realistic model atoms with the best physics available and taking into account the fine structure. The Mg and Ca lines of interest are in optical and near IR ranges. A special interest concerns the lines in the Gaia spectrograph (RVS) wavelength domain [8470, 8740] A. The NLTE corrections are provided as function of stellar parameters in an electronic table as well as in a polynomial form for the Gaia/RVS lines., Comment: 27 pages, 19 figures and 11 tables. Also on-line data; MNRAS (2011) 0000

- Context: The triple stellar system delta Vel system presents a significant infrared excess, whose origin is still being debated. A large infrared bow shock has been discovered using Spitzer/MIPS observations. Although it appears as a significant contributor to the measured IR excess, the possibility exists that a circumstellar IR excess is present around the stars of the system. - Aims: The objective of the present VISIR and NACO observations is to identify whether one of the stars of the delta Vel system presents a circumstellar photometric excess in the thermal IR domain and to quantify it. - Methods: We observed delta Vel using the imaging modes of the ESO/VLT instruments VISIR (in BURST mode) and NACO to resolve the A-B system (0.6" separation) and obtain the photometry of each star. We also obtained one NACO photometry epoch precisely at the primary (annular) eclipse of delta Vel Aa by Ab. - Results: Our photometric measurements with NACO (2.17 mic), complemented by the existing visible photometry allowed us to reconstruct the spectral energy distribution of the three stars. We then compared the VISIR photometry (8.6-12.8 mic) to the expected photospheric emission from the three stars at the corresponding wavelengths. - Conclusions: We can exclude the presence of a circumstellar thermal infrared excess around delta Vel A or B down to a few percent level. This supports the conclusions of Gaspar et al. (2008) that the IR excess of delta Vel has an interstellar origin, although a cold circumstellar disk could still be present. In addition, we derive the spectral types of the three stars Aa, Ab, and B (respectively A2IV, A4V and F8V), and we estimate the age of the system around 400-500 Myr., Comment: 8 pages, 9 figures, A&A, in press

Context: The main sequence binary star 61 Cyg (K5V+K7V) is our nearest stellar neighbour in the northern hemisphere. This proximity makes it a particularly well suited system for very high accuracy interferometric radius measurements. Aims: Our goal is to constrain the poorly known evolutionary status and age of this bright binary star. Methods: We obtained high accuracy interferometric observations in the infrared K' band, using the CHARA/FLUOR instrument. We then computed evolutionary models of 61 Cyg A & B with the CESAM2k code. As model constraints, we used a combination of observational parameters from classical observation methods (photometry, spectroscopy) as well as our new interferometric radii. Results: The measured limb darkened disk angular diameters are theta_LD(A) = 1.775 +/- 0.013 mas and theta_LD(B) = 1.581 +/- 0.022 mas, respectively for 61 Cyg A and B. Considering the high accuracy parallaxes available, these values translate into photospheric radii of R(A) = 0.665 +/- 0.005 Rsun and R(B) = 0.595 +/- 0.008 Rsun. The new radii constrain efficiently the physical parameters adopted for the modeling of both stars, allowing us to predict asteroseismic frequencies based on our best-fit models. Conclusions: The CESAM2k evolutionary models indicate an age around 6 Gyrs and are compatible with small values of the mixing length parameter. The measurement of asteroseismic oscillation frequencies in 61 Cyg A & B would be of great value to improve the modeling of this important fiducial stellar system, in particular to better constrain the masses., Comment: Accepted for publication in A&A, 8 pages

Context: The nearby pair of solar-type stars Alpha Centauri is a favorable target for an imaging search for extrasolar planets. Indications exist that the gravitational mass of Alpha Cen B could be higher than its modeled mass, the difference being consistent with a substellar companion of a few tens of Jupiter masses. However, Alpha Centauri usually appears in star catalogues surrounded by a large void area, due to the strong diffused light. Aims: We searched for faint comoving companions to Alpha Cen located at angular distances of the order of a few tens of arcseconds, up to 2-3 arcmin. As a secondary objective, we built a catalogue of the detected background sources. Methods: In order to complement our adaptive optics search at small angular distances (Paper I), we used atmosphere limited CCD imaging from the NTT-SUSI2 instrument in the Bessel V, R, I, and Z bands. Results: We present the results of our search in the form of a catalogue of the detected objects inside a 5.5 arcmin box around this star. A total of 4313 sources down to mV~24 and mI~22 were detected from this wide-field survey. We extracted the infrared photometry of part of the detected sources from archive images of the 2MASS survey (JHK bands). We investigate briefly the nature of the detected sources, many of them presenting extremely red color indices (V-K > 14). Conclusions: We did not detect any companion to Alpha Centauri between 100 and 300 AU, down to a maximum mass of ~15 times Jupiter. We also mostly exclude the presence of a companion more massive than 30 MJup between 50 and 100 AU., Comment: Accepted for publication as a Research Note in A&A

Context: Alpha Centauri is our closest stellar neighbor, at a distance of only 1.3 pc, and its two main components have spectral types comparable to the Sun. This is therefore a favorable target for an imaging search for extrasolar planets. Moreover, indications exist that the gravitational mass of Alpha Cen B is higher than its modeled mass, the difference being consistent with a substellar companion of a few tens of Jupiter masses. Aims: We searched for faint comoving companions to Alpha Cen B. As a secondary objective, we built a catalogue of the detected background sources. Methods: We used the NACO adaptive optics system of the VLT in the J, H, and Ks bands to search for companions to Alpha Cen B. This instrument allowed us to achieve a very high sensitivity to point-like sources, with a limiting magnitude of m\_Ks ~ 18 at 7" from the star. We complemented this data set with archival coronagraphic images from the HST-ACS instrument to obtain an accurate astrometric calibration. Results: Over the observed area, we did not detect any comoving companion to Alpha Cen B down to an absolute magnitude of 19-20 in the H and Ks bands. However, we present a catalogue of 252 background objects within about 15" of the star. This catalogue fills part of the large void area that surrounds Alpha Cen in sky surveys due to the strong diffused light. We also present a model of the diffused light as a function of angular distance for the NACO instrument, that can be used to predict the background level for bright star observations. Conclusions: According to recent numerical models, the limiting magnitude of our search sets the maximum mass of possible companions to 20-30 times Jupiter, between 7 and 20 AU from Alpha Cen B., Comment: 15 pages, accepted for publication in A&A

The effects of background line opacity (line-blocking) in statistical equilibrium calculations for Fe in late-type stellar atmospheres have been investigated using an extensive and up-to-date model atom with radiative data primarily from the IRON Project. The background metal line opacities have been computed using data from the MARCS stellar model atmospheres. While accounting for this line opacity is important at solar metallicity, the differences between calculations including and excluding line-blocking at low metallicity are insignificant for the non-local thermodynamic equilibrium (non-LTE) abundance corrections for Fe I lines. The line-blocking has no impact on the non-LTE effects of Fe II lines. The dominant uncertainty in Fe non-LTE calculations for metal-poor stars is still the treatment of the inelastic H I collisions, which have here been included using scaling factors to the classical Drawin formalism, and whether or not thermalisation of the high Fe I levels to Fe II ground state should be enforced. Without such thermalisation, the Fe I non-LTE abundance corrections are substantial in metal-poor stars: about 0.3 dex with efficient (i.e. Drawin-like) H I collisions and about 0.5 dex without. Without both thermalisation and H I collisions, even Fe II lines show significant non-LTE effects in such stars., Comment: Accepted for publication in A&A, 8 pages, 12 figures (here degraded)

We report the first results of a systematic search near the plane of the Galaxy for the so called very active stars (VAS), which are characterized by a hard X-ray spectrum and activity in the radio domain. Candidates with hard X-ray binary-like spectra have been selected from the Bright ROSAT Source Catalogue in the Zone of Avoidance ($| b | < 20{^o}$) and were tentatively identified in GB6/PMM/NVSS radio surveys. Most of them were observed with the ATCA and VLA. Precise radio coordinates have led to unambiguous optical identification for 60 candidates, and a sub-sample of five of themhas been observed with the VLT. Also some discovery and confirmatory spectra were obtained with the AAT (4-m) and BTA (6-m). Spectroscopy with moderate dispersion, made with the FORS1 spectrograph of the VLT has revealed two stellar objects (one of them, VASC J1628-41, is definitivelya binary VAS), one new AGN and two featureless spectrum sources. One of these objects, VASC J1353-66, shows a marginal evidence of proper motion, which, if confirmed, would imply the discovery of a new type of galactic source., Comment: to appear in A&A, 7 figures

A 10-night asteroseismic observation programme has been conducted in January 2004 with the spectrometer Harps at the ES0 3.6-m telescope. The selected target, the 6th magnitude F5V star HD 49933, was chosen among the prime candidates of Corot, the European space mission dedicated to characterize stellar oscillations mode with high precision photometry measurements. This star shows important line profiles variations, indicating a surprisingly high activity with respect to its low rotation rate. However, with the help of tools developed for disentangling the signatures of activity and oscillations, we are able to observe its oscillation spectrum in the frequency range [1.2, 2.2 mHz]. We measure the large separation (88.7+-0.4 microHz) and the maximum amplitude (around 0.4+-0.1 m/s rms), respectively in agreement and marginal agreement with the predicted values., Comment: accepted in A&A Letters

Using VLTI/VINCI angular diameter measurements, we constrain the evolutionary status of three asteroseismic targets: the stars $\delta$ Eri, $\xi$ Hya, $\eta$ Boo. Our predictions of the mean large frequency spacing of these stars are in agreement with published observational estimations. Looking without success for a companion of $\delta$ Eri we doubt on its classification as an RS CVn star., Comment: other location: http://www.obs-nice.fr/pichon/science.html ; Accepted for publication in Astron. Astrophys