Context. The new VISual and Infrared Telescope for Astronomy (VISTA) has started operations. Over its first five years it will be collecting data for six public surveys, one of which is the near-infrared Y JK s VISTA survey of the Magellanic Clouds system (VMC). This survey comprises the Large Magellanic Cloud (LMC), the Small Magellanic Cloud, the Magellanic Bridge connecting the two galaxies and two fields in the Magellanic Stream. Aims. This paper provides an overview of the VMC survey strategy and presents first science results. The main goals of the VMC survey are the determination of the spatially-resolved star-formation history and the three-dimensional structure of the Magellanic system. The VMC survey is therefore designed to reach stars as faint as the oldest main sequence turn-off point and to constrain the mean magnitude of pulsating variables such as RR Lyrae stars and Cepheids. This paper focuses on observations of VMC fields in the LMC obtained between November 2009 and March 2010. These observations correspond to a completeness of 7% of the planned LMC fields. Methods. The VMC data are comprised of multi-epoch observations which are executed following specific time constraints. The data were reduced using the VISTA Data Flow System pipeline with source catalogues, including astrometric and photometric corrections, produced and made available via the VISTA Science Archive. The VMC data will be released to the astronomical community following the European Southern Observatory's Public Survey policy. The analysis of the data shows that the sensitivity in each wave band agrees with expectations. Uncertainties and completeness of the data are also derived. Results. The first science results, aimed at assessing the scientific quality of the VMC data, include an overview of the distribution of stars in colour-magnitude and colour-colour diagrams, the detection of planetary nebulae and stellar clusters, and the K s band light-curves of variable stars. Conclusions. The VMC survey represents a tremendous improvement, in spatial resolution and sensitivity, on previous panoramic observations of the Magellanic system in the near-infrared, providing a powerful complement to deep observations at other wavelengths.

We report the discovery of a 16 th magnitude star, HE 0437−5439, with a heliocentric radial velocity of +723 ± 3 km s −1 . A quantitative spectral analysis of high-resolution optical spectra obtained with the VLT and the UVES * Based on observations collected at the European Southern Observatory, La Silla and Paranal, Chile (Proposal No. 68.D-0192 and 70.D-0334). spectrograph shows that HE 0437−5439 is a main sequence B-type star with T eff =20 350 K, log(g) = 3.77, solar within a factor of a few helium abundance and metal content, rotating at v sin(i) = 54 km s −1 . Using appropriate evolutionary tracks we derive a mass of 8 M ⊙ and a corresponding distance of 61 kpc. Its galactic rest frame velocity is at least 563 km s −1 , almost twice the local Galactic escape velocity, indicating that the star is unbound to the Galaxy. Numerical kinematical experiments are carried out to constrain its place of birth. It has been suggested that such hyper-velocity stars can be formed by the tidal disruption of a binary through interaction with the super-massive black hole at the Galactic center (GC). HE 0437−5439 needs about 100 Myrs to travel from the GC to its present position, much longer than its main sequence lifetime of 25 Myrs. This can only be reconciled if HE 0437−5439 is a blue straggler star. In this case, the predicted proper motion is so small that it can only be measured by future space missions. Since the star is much closer to the Large Magellanic Cloud (LMC, 18 kpc) than to the GC, it can reach its position from the center of the LMC. The proper motion predicted in this case is about 2 mas yr −1 (relative to the LMC), large enough to be measurable with conventional techniques from the ground. The LMC origin could also be tested by a high-precision abundance analysis.

For many years we are witnessing a lively debate on the existence and extent of convective overshooting, mainly in the cores of main-sequence stars. This is an important issue, since even a small amount of overshooting increases considerably the mass of the finally hydrogen exhausted core and lenghthens the main-sequence lifetime correspondingly. The available evolutionary calculations assume either moderate overshooting, d/Hp = 0.25, (d = overshooting distance, Hp = pressure scale height; Maeder & Meynet 1988) or strong overshooting, d/Hp ≈ 0.50 (Bertelli et al. 1986). Presently theory is unable to quantify the exact amount of overshooting, and one has to resort to empirical determinations.Recently, Stothers (1991) collected all available information from the literature on stellar parameters and evolutionary calculations and concluded that, within the errors, d/Hp = 0 is an acceptable result, with a conservative upper limit of d/Hp < 0.2. However, such an approach is hampered by observational errors (like distance or temperature uncertainties, rotation) that are difficult to quantify and that may mask any definitive result. Detailed investigations of detached binaries may help in this matter (Andersen et al. 1990) but the number of suitable binary systems is probably not very large.

Aims. We address the origin and evolutionary status of hot subdwarf stars by studying the optical spectral properties of 58 subdwarf O (sdO) stars. Combining them with the results of our previously studied subdwarf B (sdB) stars, we aim at investigating possible evolutionary links. Methods. We analyse high-resolution (R > 18 000), high-quality optical spectra of sdO stars obtained with the ESO VLT UVES echelle spectrograph in the course of the ESO Supernova Ia Progenitor Survey (SPY). Effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously by fitting the profiles of hydrogen and helium lines using dedicated synthetic spectra calculated from an extensive grid of NLTE model atmospheres. Results. We find spectroscopic or photometric evidence for cool companions to eight sdO stars, as well as a binary consisting of two sdO stars. A clear correlation between helium abundances and the presence of carbon and/or nitrogen lines is found: below solar helium abundance, no sdO star shows C or N lines. In contrast, C and/or N lines are present in the spectra of all sdO stars with supersolar helium abundance. We thus use the solar helium abundance to divide our sample into helium-deficient and helium-enriched sdO stars. While helium-deficient sdO stars are scattered in a wide range of the T eff -log (g)-diagram, most of the helium-enriched sdO stars cluster in a narrow region at temperatures between 40 000 and 50 000 K and gravities between log g = 5.5 and 6.0.Conclusions. An evolutionary link between sdB stars and sdO stars appears plausible only for the helium-deficient sdO stars. They probably have evolved away from the extreme horizontal branch; i.e., they are the likely successors to sdB stars. In contrast, the atmospheric properties of helium-enriched sdO stars cannot be explained with canonical single-star evolutionary models. Alternative scenarios for both single-star (late hot flasher) and binary evolution (white-dwarf merger; post-RGB evolution) fail to reproduce the observed properties of helium-enriched sdO stars in detail. While we regard the post-RGB scenario as inappropriate, the white-dwarf merger and the late hot-flasher scenarios remain viable to explain the origin of helium-enriched sdO stars.

Context. We present a detailed spectroscopic analysis of the stars with helium-dominated spectra in the ESO Supernova Ia Progenitor Survey (SPY). Aims. Atmospheric parameters, masses, and abundances of trace hydrogen are determined and discussed in the context of spectral evolution of white dwarfs. Methods. The spectra are compared with theoretical model atmospheres using a χ 2 fitting technique, leading to determinations of effective temperature, surface gravity, and hydrogen abundance. Results. Our final sample contains 71 objects, of which 6 are new detections and 14 are reclassified from DB to DBA because of the presence of H lines. One is a cool DO with weak He ii lines, 2 are composite DB+dM. 55% of the DB sample show hydrogen and are thus DBA, a significantly higher fraction than found before. Conclusions. The large incidence of DBA, and the derived total hydrogen masses are compatible with the scenario that DBs "reappear" around 30 000 K from the DB gap by mixing and diluting a thin hydrogen layer of the order of 10 −15 M . This hydrogen mass is then during the evolution continuously increased by interstellar accretion. There are indications that the accretion rate increases smoothly with age or decreasing temperature, a trend which continuous even below the current low temperature limit (Dufour 2006). A remaining mystery is the low accretion rate of H compared to that of Ca observed in the DBZA, but a stellar wind extending down to the lowest temperatures with decreasing strength might be part of the solution.

Context. The ESO Supernova Ia Progenitor Survey (SPY) took high-resolution spectra of more than 1000 white dwarfs and pre-white dwarfs. About two thirds of the stars observed are hydrogen-dominated DA white dwarfs. Here we present a catalog and detailed spectroscopic analysis of the DA stars in the SPY. Aims. Atmospheric parameters effective temperature and surface gravity are determined for normal DAs. Double-degenerate binaries, DAs with magnetic fields or dM companions, are classified and discussed. Methods. The spectra are compared with theoretical model atmospheres using a χ 2 fitting technique. Results. Our final sample contains 615 DAs, which show only hydrogen features in their spectra, although some are double-degenerate binaries. 187 are new detections or classifications. We also find 10 magnetic DAs (4 new) and 46 DA+dM pairs (10 new).

We present the results of a radial velocity (RV) survey of 46 subdwarf B (sdB) and 23 helium-rich subdwarf O (He-sdO) stars. We detected 18 (39%) new sdB binary systems, but only one (4%) He-sdO binary. Orbital parameters of nine sdB and sdO binaries, derived from follow-up spectroscopy, are presented. Our results are compared with evolutionary scenarios and previous observational investigations.Comment: To appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics and Space Science, Kluver Academic Publishers, edited by P.F.L. Maxte

Abstract. We present the results of a spectral analysis of a large sample of subdwarf B stars selected from follow-up observations of candidates from the Hamburg Quasar Survey. Fundamental parameters (effective temperature, gravity, and helium abundance) were determined by matching synthetic line profiles calculated from model atmospheres to all hydrogen and helium absorption lines present in the observed optical spectra. The derived helium abundances are compared with the atmospheric parameters to search for possible trends. We discovered a correlation between the helium abundance and the effective temperature: the larger the temperature, the larger the photospheric helium content of sdB stars. Additionally, a separation into two sequences of sdB stars in the effective temperature -helium abundance plane is detected. We compared our analysis results with data from the literature. The stars from our sample are found to be somewhat more luminous. This can only partly be explained by NLTE effects. Three apparently normal B stars were discovered, which could be massive stars far away from the galactic plane (7-19 kpc). Radial velocities were measured for 23 stars from which we discovered a new radial velocity variable sdB star.