Abstract. We present the results of analysis of "snapshot" spectra of 253 metal-poor halo stars −3.8 ≤ [Fe/H] ≤ −1.5 obtained in the HERES survey. The snapshot spectra have been obtained with VLT/UVES and have typically S /N ∼ 54 per pixel (ranging from 17 to 308), R ∼ 20 000, λ = 3760-4980 Å. This sample represents the major part of the complete HERES sample of 373 stars; however, the CH strong content of the sample is not dealt with here. The spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy (SME) codes of Valenti & Piskunov. Elemental abundances of moderate precision (absolute rms errors of order 0.25 dex, relative rms errors of order 0.15 dex) have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where detectable. Of these elements, 14 are usually detectable at the 3σ confidence level for our typical spectra. The remainder can be detected in the least metal-poor stars of the sample, spectra with higher than average S /N, or when the abundance is enhanced. Among the sample of 253 stars, disregarding four previously known comparison stars, we find 8 r-II stars and 35 r-I stars. The r-II stars, including the two previously known examples CS 22892-052 and CS 31082-001, are centred on a metallicity of [Fe/H] = −2.81, with a very small scatter, on the order of 0.16 dex. The r-I stars are found across practically the entire metallicity range of our sample. We also find three stars with strong enhancements of Eu which are s-process rich. A significant number of new very metal-poor stars are confirmed: 49 stars with [Fe/H] < −3 and 181 stars with −3 < [Fe/H] < −2. We find one star with [Fe/H] < −3.5. We find the scatter in the abundance ratios of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to be similar to the estimated relative errors and thus the cosmic scatter to be small, perhaps even non-existent. The elements C, Sr, Y, Ba and Eu, and perhaps Zr, show scatter at [Fe/H] < ∼ −2.5 significantly larger than can be explained from the errors in the analysis, implying scatter which is cosmic in origin. Significant scatter is observed in abundance ratios between light and heavy neutron-capture elements at low metallicity and low levels of r-process enrichment.

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.

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.

We provide an in-depth analysis of the He II and H I absorption in the intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT telescope. Following up on our earlier study , Science, 293, 1112, we focus here on two major topics: (1) small-scale variability (∆z ≈ 10 −3 ) in the ratio η = N(He II)/N(H I); and (2) an observed correlation of high-η absorbers (soft radiation fields) with voids in the (H I) Lyα distribution. These effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc, together with radiative transfer through a filamentary IGM whose opacity variations control the penetration of 1-5 ryd radiation over 30-40 Mpc distances. Owing to photon statistics and backgrounds, we can measure optical depths over the ranges 0.1 < τ HeII < 2.3 and 0.02 < τ HI < 3.9, and reliably determine values of η ≈ 4τ HeII /τ HI over the range 0.1 to 460. Values η = 20 − 200 are consistent with models of photoionization by quasars with observed spectral indices α s = 0 − 3. Values η > 200 may require additional contributions from starburst galaxies, heavily filtered quasar radiation, or density variations. Regions with η < 30 may indicate the presence of local hard sources. We find that η is higher in "void" regions, where H I is weak or undetected and ∼80% of the path length has η > 100. These voids may be ionized by local soft sources (dwarf starbursts) or by QSO radiation softened by escape from the AGN cores or transfer through the "cosmic web". The apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag between the reionization epochs of H I (z HI ∼ 6.2 ± 0.2) and He II (z HeII ∼ 2.8 ± 0.2).

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).

Abstract.We assess the cosmological variability of the fine-structure constant α from the analysis of an ensemble of Fe  λ1608, λ2344, λ2374, λ2383, λ2587, and λ2600 absorption lines at the redshift z = 1.15 toward the QSO HE 0515-4414 by means of the standard many-multiplet (MM) technique and its revision based on linear regression (RMM). This is the first time the MM technique is applied to exceptional high-resolution and high signal-to-noise QSO spectra recorded with the UV-Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope (VLT). Our analysis results in ∆α/α MM = (0.1 ± 1.7) × 10and ∆α/α RMM = (−0.4 ± 1.9 ± 2.7 sys ) × 10 −6 , which are the most stringent bounds hitherto infered from an individual QSO absorption system. Our results support the null hypothesis ∆α/α = 0 at a significance level of 91 percent, whereas the support for the result ∆α/α = −5.7 × 10 −6 presented in former MM studies is 12 percent.

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