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Aims .Over the past decades, libraries of stellar spectra have been used in a large variety of science cases, including as sources of reference spectra for a given object or a given spectral type. Despite the existence of large libraries and the increasing number of projects of large-scale spectral surveys, there is to date only one very high-resolution spectral library offering spectra from a few hundred objects from the southern hemisphere (UVES-POP). We aim to extend the sample, offering a finer coverage of effective temperatures and surface gravity with a uniform collection of spectra obtained in the northern hemisphere. Methods .Between 2010 and 2020, we acquired several thousand echelle spectra of bright stars with the Mercator-HERMES spectrograph located in the Roque de Los Muchachos Observatory in La Palma, whose pipeline offers high-quality data reduction products. We have also developed methods to correct for the instrumental response in order to approach the true shape of the spectral continuum. Additionally, we have devised a normalisation process to provide a homogeneous normalisation of the full spectral range for most of the objects. Results .We present a new spectral library consisting of 3256 spectra covering 2043 stars. It combines high signal-to-noise and high spectral resolution over the entire range of effective temperatures and luminosity classes. The spectra are presented in four versions: raw, corrected from the instrumental response, with and without correction from the atmospheric molecular absorption, and normalised (including the telluric correction)., SCOPUS: ar.j, info:eu-repo/semantics/published

We carried out an extensive observational study of the Slowly Pulsating B (SPB) star, HD 25558. The ≈2000 spectra obtained at different observatories, the ground-based and MOST satellite light curves revealed that this object is a double-lined spectroscopic binary with an orbital period of about nine years. The observations do not allow the inference of an orbital solution. We determined the physical parameters of the components, and found that both lie within the SPB instability strip. Accordingly, both show line-profile variations due to stellar pulsations. 11 independent frequencies were identified in the data. All the frequencies were attributed to one of the two components based on pixel-by-pixel variability analysis of the line profiles. Spectroscopic and photometric mode identification was also performed for the frequencies of both stars. These results suggest that the inclination and rotation of the two components are rather different. The primary is a slow rotator with ≈6 d period, seen at ≈60° inclination, while the secondary rotates fast with ≈1.2 d period, and is seen at ≈20° inclination. Spectropolarimetric measurements revealed that the secondary component has a magnetic field with at least a few hundred Gauss strength, while no magnetic field can be detected in the primary. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

We carried out an extensive observational study of the Slowly Pulsating B (SPB) star, HD 25558. The ≈2000 spectra obtained at different observatories, the ground-based and MOST satellite light curves revealed that this object is a double-lined spectroscopic binary with an orbital period of about nine years. The observations do not allow the inference of an orbital solution. We determined the physical parameters of the components, and found that both lie within the SPB instability strip. Accordingly, both show line-profile variations due to stellar pulsations. 11 independent frequencies were identified in the data. All the frequencies were attributed to one of the two components based on pixel-by-pixel variability analysis of the line profiles. Spectroscopic and photometric mode identification was also performed for the frequencies of both stars. These results suggest that the inclination and rotation of the two components are rather different. The primary is a slow rotator with ≈6 d period, seen at ≈60° inclination, while the secondary rotates fast with ≈1.2 d period, and is seen at ≈20° inclination. Spectropolarimetric measurements revealed that the secondary component has a magnetic field with at least a few hundred Gauss strength, while no magnetic field can be detected in the primary. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

Hot subdwarfs represent a group of low-mass helium-burning stars formed through binarystar interactions and include some of the most chemically peculiar stars in the Galaxy. Stellar evolution theory suggests that they should have helium-rich atmospheres but, because radiation causes hydrogen to diffuse upwards, a majority are extremely helium poor. Questions posed include: when does the atmosphere become chemically stratified and at what rate? The existence of several helium-rich subdwarfs suggests further questions: are there distinct subgroups of hot subdwarf, or do hot subdwarfs change their surface composition in the course of evolution? Recent analyses have revealed remarkable surface chemistries amongst the helium-rich subgroup. In this paper, we analyse high-resolution spectra of nine intermediate helium-rich hot subdwarfs. We report the discovery that two stars, HE 2359-2844 and HE 1256-2738, showan atmospheric abundance of lead which is nearly 10 000 times that seen in the Sun. This is measured from optical Pb IV absorption lines never previously seen in any star. The lead abundance is 10 to 100 times that measured in normal hot-subdwarf atmospheres from ultraviolet spectroscopy. HE 2359-2844 also shows zirconium and yttrium abundances similar to those in the zirconium star LS IV-14°116. The new discoveries are interpreted in terms of heavily stratified atmospheres and the general picture of a surface chemistry in transition from a new-born helium-rich subdwarf to a normal helium-poor subdwarf. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

Hot subdwarfs represent a group of low-mass helium-burning stars formed through binarystar interactions and include some of the most chemically peculiar stars in the Galaxy. Stellar evolution theory suggests that they should have helium-rich atmospheres but, because radiation causes hydrogen to diffuse upwards, a majority are extremely helium poor. Questions posed include: when does the atmosphere become chemically stratified and at what rate? The existence of several helium-rich subdwarfs suggests further questions: are there distinct subgroups of hot subdwarf, or do hot subdwarfs change their surface composition in the course of evolution? Recent analyses have revealed remarkable surface chemistries amongst the helium-rich subgroup. In this paper, we analyse high-resolution spectra of nine intermediate helium-rich hot subdwarfs. We report the discovery that two stars, HE 2359-2844 and HE 1256-2738, showan atmospheric abundance of lead which is nearly 10 000 times that seen in the Sun. This is measured from optical Pb IV absorption lines never previously seen in any star. The lead abundance is 10 to 100 times that measured in normal hot-subdwarf atmospheres from ultraviolet spectroscopy. HE 2359-2844 also shows zirconium and yttrium abundances similar to those in the zirconium star LS IV-14°116. The new discoveries are interpreted in terms of heavily stratified atmospheres and the general picture of a surface chemistry in transition from a new-born helium-rich subdwarf to a normal helium-poor subdwarf. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society., SCOPUS: ar.j, info:eu-repo/semantics/published

Context. The old Galactic halo stars hold the fossil record of the interstellar medium chemical composition at the time of their formation. Most of the stars studied so far are relatively near to the Sun, this prompts the study of more distant stars, both to increase the size of the sample and to search for possible variations of abundance patterns at greater distances. Aims. The purpose of our study is to determine the chemical composition of a sample of 16 candidate extremely metal-poor (EMP) dwarf stars, extracted from the Sloan Digital Sky Survey (SDSS). There are two main purposes: in the first place to verify the reliability of the metallicity estimates derived from the SDSS spectra; in the second place to see if the abundance trends found for the brighter nearer stars studied previously also hold for this sample of fainter, more distant stars. Methods. We used the UVES at the VLT to obtain high-resolution spectra of the programme stars. The abundances were determined by an automatic analysis with the MyGIsFOS code, with the exception of lithium, for which the abundances were determined from the measured equivalent widths of the Li i resonance doublet. Results. All candidates are confirmed to be EMP stars, with [Fe/H] =-3.0. The chemical composition of the sample of stars is similar to that of brighter and nearer samples. We measured the lithium abundance for 12 stars and provide stringent upper limits for three other stars, for a fourth star the upper limit is not significant, owing to the low signal-to noise ratio of the spectrum. The "meltdown" of the Spite plateau is confirmed, but some of the lowest metallicity stars of the sample lie on the plateau. Conclusions. The concordance of the metallicities derived from high-resolution spectra and those estimated from the SDSS spectra suggests that the latter may be used to study the metallicity distribution of the halo. The abundance pattern suggests that the halo was well mixed for all probed metallicities and, SCOPUS: ar.j, SCOPUS: ar.j, info:eu-repo/semantics/published

Subluminous B stars come in a variety of flavours including single stars, close and wide binaries, and pulsating and non-pulsating variables. A majority have helium-poor surfaces (helium by number n He < 1per cent), whilst a minority have extremely helium rich surfaces (n He > 90per cent). A small number have an intermediate surface helium abundance (˜10-30per cent), accompanied by peculiar abundances of other elements. The questions posed are (i) whether these abundance peculiarities are associated with radiatively driven and time-dependent stratification of elements within the photosphere as the star evolves from a helium-enriched progenitor to become a normal helium-poor sdB star and (ii) whether these phenomena occur only in single sdB stars or are also associated with sdB stars in binaries. We present a fine analysis of the bright intermediate helium sdB star CPD-20°1123 (Albus 1) which shows it to be cool, for a hot subdwarf, with T eff˜ 23000K and with a surface helium abundance ˜17per cent by number. Other elements do not show extraordinary anomalies; in common with majority sdB stars, carbon and oxygen are substantially depleted, whilst nitrogen is enriched. Magnesium through sulphur appear to be depleted by ˜0.5dex, but chlorine and argon are substantially enhanced. We also present a series of radial velocity measurements which show the star to be a close binary with an orbital period of 2.3d, suggesting it to be a post-common-envelope system. The discovery of an intermediate helium rich sdB star in a close binary in addition to known and apparently single exemplars supports the view that these are very young sdB stars in which radiatively driven stratification of the photosphere is incomplete. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS., SCOPUS: ar.j, info:eu-repo/semantics/published

Subluminous B stars come in a variety of flavours including single stars, close and wide binaries, and pulsating and non-pulsating variables. A majority have helium-poor surfaces (helium by number n He < 1per cent), whilst a minority have extremely helium rich surfaces (n He > 90per cent). A small number have an intermediate surface helium abundance (˜10-30per cent), accompanied by peculiar abundances of other elements. The questions posed are (i) whether these abundance peculiarities are associated with radiatively driven and time-dependent stratification of elements within the photosphere as the star evolves from a helium-enriched progenitor to become a normal helium-poor sdB star and (ii) whether these phenomena occur only in single sdB stars or are also associated with sdB stars in binaries. We present a fine analysis of the bright intermediate helium sdB star CPD-20°1123 (Albus 1) which shows it to be cool, for a hot subdwarf, with T eff˜ 23000K and with a surface helium abundance ˜17per cent by number. Other elements do not show extraordinary anomalies; in common with majority sdB stars, carbon and oxygen are substantially depleted, whilst nitrogen is enriched. Magnesium through sulphur appear to be depleted by ˜0.5dex, but chlorine and argon are substantially enhanced. We also present a series of radial velocity measurements which show the star to be a close binary with an orbital period of 2.3d, suggesting it to be a post-common-envelope system. The discovery of an intermediate helium rich sdB star in a close binary in addition to known and apparently single exemplars supports the view that these are very young sdB stars in which radiatively driven stratification of the photosphere is incomplete. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS., SCOPUS: ar.j, info:eu-repo/semantics/published

Context. The old Galactic halo stars hold the fossil record of the interstellar medium chemical composition at the time of their formation. Most of the stars studied so far are relatively near to the Sun, this prompts the study of more distant stars, both to increase the size of the sample and to search for possible variations of abundance patterns at greater distances. Aims. The purpose of our study is to determine the chemical composition of a sample of 16 candidate extremely metal-poor (EMP) dwarf stars, extracted from the Sloan Digital Sky Survey (SDSS). There are two main purposes: in the first place to verify the reliability of the metallicity estimates derived from the SDSS spectra; in the second place to see if the abundance trends found for the brighter nearer stars studied previously also hold for this sample of fainter, more distant stars. Methods. We used the UVES at the VLT to obtain high-resolution spectra of the programme stars. The abundances were determined by an automatic analysis with the MyGIsFOS code, with the exception of lithium, for which the abundances were determined from the measured equivalent widths of the Li i resonance doublet. Results. All candidates are confirmed to be EMP stars, with [Fe/H] =-3.0. The chemical composition of the sample of stars is similar to that of brighter and nearer samples. We measured the lithium abundance for 12 stars and provide stringent upper limits for three other stars, for a fourth star the upper limit is not significant, owing to the low signal-to noise ratio of the spectrum. The "meltdown" of the Spite plateau is confirmed, but some of the lowest metallicity stars of the sample lie on the plateau. Conclusions. The concordance of the metallicities derived from high-resolution spectra and those estimated from the SDSS spectra suggests that the latter may be used to study the metallicity distribution of the halo. The abundance pattern suggests that the halo was well mixed for all probed metallicities and, SCOPUS: ar.j, SCOPUS: ar.j, info:eu-repo/semantics/published

Helium-rich subdwarf B (He-sdB) stars represent a small group of low-mass hot stars with luminosities greater than those of conventional sdB stars, and effective temperatures lower than those of subdwarf O (sdO) stars. By measuring their surface chemistry, we aim to explore the connection between He-sdB stars, He-rich sdO stars and normal sdB stars. LSIV-14°116 is a relatively intermediate He-sdB star, also known to be a photometric variable. High-resolution blue-optical spectroscopy was obtained with the Anglo-Australian Telescope. Analysis of the spectrum shows LSIV-14°116 to have effective temperature T eff= 34000 ± 500K, surface gravity logg= 5.6 ± 0.2 and surface helium abundance n He= 0.16 ± 0.03 by number. This places the star slightly above the standard extended horizontal branch, as represented by normal sdB stars. The magnesium and silicon abundances indicate the star to be metal poor relative to the Sun. A number of significant but unfamiliar absorption lines were identified as being due to germanium, strontium, yttrium and zirconium. After calculating oscillator strengths (for Ge, Y and Zr), the photospheric abundances of these elements were established to range from 3 dex (Ge) to 4 dex (Sr, Y and Zr) above solar. The most likely explanation is that these overabundances are caused by radiatively driven diffusion forming a chemical cloud layer in the photosphere. It is conjectured that this cloud formation could be mediated by a strong magnetic field. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS., SCOPUS: ar.j, info:eu-repo/semantics/published

Helium-rich subdwarf B (He-sdB) stars represent a small group of low-mass hot stars with luminosities greater than those of conventional sdB stars, and effective temperatures lower than those of subdwarf O (sdO) stars. By measuring their surface chemistry, we aim to explore the connection between He-sdB stars, He-rich sdO stars and normal sdB stars. LSIV-14°116 is a relatively intermediate He-sdB star, also known to be a photometric variable. High-resolution blue-optical spectroscopy was obtained with the Anglo-Australian Telescope. Analysis of the spectrum shows LSIV-14°116 to have effective temperature T eff= 34000 ± 500K, surface gravity logg= 5.6 ± 0.2 and surface helium abundance n He= 0.16 ± 0.03 by number. This places the star slightly above the standard extended horizontal branch, as represented by normal sdB stars. The magnesium and silicon abundances indicate the star to be metal poor relative to the Sun. A number of significant but unfamiliar absorption lines were identified as being due to germanium, strontium, yttrium and zirconium. After calculating oscillator strengths (for Ge, Y and Zr), the photospheric abundances of these elements were established to range from 3 dex (Ge) to 4 dex (Sr, Y and Zr) above solar. The most likely explanation is that these overabundances are caused by radiatively driven diffusion forming a chemical cloud layer in the photosphere. It is conjectured that this cloud formation could be mediated by a strong magnetic field. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS., SCOPUS: ar.j, info:eu-repo/semantics/published

We recently presented (Sbordone et al. 2009a) the largest sample to date of lithium abundances in extremely metal-poor (EMP) Halo dwarf and Turn-Off (TO) stars. One of the most crucial aspects in estimating Li abundances is the Teff determination, since the Li I 670.8 nm doublet is highly temperature sensitive. In this short contribution we concentrate on the Teff determination based on Halpha wings fitting, and on its sensitivity to the chosen stellar gravity., DOI: 10.1017/S1743921310004448, info:eu-repo/semantics/published

We present the largest sample available to date of lithium abundances in extremely metal poor (EMP) Halo dwarfs. Four Teff estimators are used, including IRFM and Halpha wings fitting against 3D hydrodynamical synthetic profiles. Lithium abundances are computed by means of 1D and 3D-hydrodynamical NLTE computations. Below [Fe/H]~-3, a strong positive correlation of A(Li) with [Fe/H] appears, not influenced by the choice of the Teff estimator. A linear fit finds a slope of about 0.30 dex in A(Li) per dex in [Fe/H], significant to 2-3 sigma, and consistent within 1 sigma among all the Teff estimators. The scatter in A(Li) increases significantly below [Fe/H]~-3. Above, the plateau lies at = 2.199 ± 0.086. If the primordial A(Li) is the one derived from standard Big Bang Nucleosynthesis (BBN), it appears difficult to envision a single depletion phenomenon producing a thin, metallicity independent plateau above [Fe/H] = -2.8, and a highly scattered, metallicity dependent distribution below. Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Keywords (in text query field) Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints, DOI: 10.1017/S1743921310000244, info:eu-repo/semantics/published

Extreme helium stars are hydrogen‐deficient supergiants of spectral type A and B. They are believed to result from mergers in double degenerate systems, if one white dwarf is of C/O‐type and the other of He‐type. We calculate a new grid of model atmospheres accounting for metal line blanketing with ATLAS12 and compare it to the benchmark code STERNE. Synthetic spectra are calculated accounting for NLTE effects for many ions and applied to high‐quality optical spectra of the prototype extreme helium star BD+10°2179.© 2010 American Institute of Physics, 17 European White Dwarf Workshop, Tubingen (Germany), 16-20 August 2010, info:eu-repo/semantics/published

info:eu-repo/semantics/published

Approximately 20% of very metal-poor stars ([Fe/H] < -2.0) are strongly enhanced in carbon ([C/Fe] > +1.0). Such stars are referred to as carbon-enhanced metal-poor (CEMP) stars. We present a chemical abundance analysis based on high resolution spectra acquired with UVES at the VLT of three dwarf CEMP stars: SDSS J1349-0229, SDSS J0912+0216 and SDSS J1036+1212. These very metal-poor stars, with [Fe/H] < -2.5, were selected from our ongoing survey of extremely metal-poor dwarf candidates from the SDSS. Among these CEMPs, SDSS J1349-0229 has been identified as a carbon star ([C/O] > +1.0). First and second peak s-process elements, as well as second peak r-process elements have been detected in all stars. In addition, elements from the third r-process peak were detected in one of the stars, SDSS J1036+1212. We present the abundance results of these stars in the context of neutron-capture nucleosynthesis theories., DOI: 10.1017/S1743921310000359, info:eu-repo/semantics/published

As part of an extended study of the surface abundances of extremely helium-rich hot subdwarfs, high-resolution optical échelle spectra of the three helium-rich subdwarf B stars LB 1766, SB 21 and BPS 22940-0009 are presented. Opacity-sampled line-blanketed model atmospheres have been used to derive atmospheric properties and surface abundances. All three stars are moderately metal-poor compared with the Sun ([Fe/H]˜-0. 4). The first two stars are nitrogen-rich He-sdBs, while the third is a carbon-rich He-sdB. The surface composition and locus of single N-rich He-sdBs are currently best explained by the merger of two helium white dwarfs. C-rich He-sdBs require further investigation. © 2010 Springer Science+Business Media B.V., SCOPUS: ar.j, SCOPUS: ar.j, info:eu-repo/semantics/published

Context. Oxygen is the third most common element in the Universe. The measurement of oxygen lines in metal-poor unevolved stars, in particular near-UV OH lines, can provide invaluable information about the properties of the Early Galaxy. Aims. Near-UV OH lines constitute an important tool to derive oxygen abundances in metal-poor dwarf stars. Therefore, it is important to correctly model the line formation of OH lines, especially in metal-poor stars, where 3D hydrodynamical models commonly predict cooler temperatures than plane-parallel hydrostatic models in the upper photosphere. Methods. We have made use of a grid of 52 3D hydrodynamical model atmospheres for dwarf stars computed with the code CO 5BOLD, extracted from the more extended CIFIST grid. The 52 models cover the effective temperature range 5000-6500 K, the surface gravity range 3.5-4.5 and the metallicity range-3 < [Fe/H] < 0. Results. We determine 3D-LTE abundance corrections in all 52 3D models for several OH lines and Fe i lines of different excitation potentials. These 3D-LTE corrections are generally negative and reach values of roughly-1 dex (for the OH 3167 with excitation potential of approximately 1 eV) for the higher temperatures and surface gravities. Conclusions. We apply these 3D-LTE corrections to the individual O abundances derived from OH lines of a sample the metal-poor dwarf stars reported in Israelian et al. (1998, ApJ, 507, 805), Israelian et al. (2001, ApJ, 551, 833) and Boesgaard et al. (1999, AJ, 117, 492) by interpolating the stellar parameters of the dwarfs in the grid of 3D-LTE corrections. The new 3D-LTE [O/Fe] ratio still keeps a similar trend as the 1D-LTE, i.e. increasing towards lower [Fe/H] values. We applied 1D-NLTE corrections to 3D Fei abundances and still see an increasing [O/Fe] ratio towards lower metallicites. However, the Galactic [O/Fe] ratio must be revisited once 3D-NLTE corrections become available for OH and Fe lines for a grid of 3D hydrodynamical model atmos, info:eu-repo/semantics/published

Context. The primordial nature of the Spite plateau is at odds with the WMAP satellite measurements, implying a primordial Li production at least three times higher than observed. It has also been suggested that A(Li) might exhibit a positive correlation with metallicity below [Fe/H] ~ -2.5. Previous samples studied comprised few stars below [Fe/H] = -3. Aims. We present VLT-UVES Li abundances of 28 halo dwarf stars between [Fe/H] = -2.5 and -3.5, ten of which have [Fe/H] <-3. Methods. We determined stellar parameters and abundances using four different Teff scales. The direct infrared flux method was applied to infrared photometry. Ha wings were fitted with two synthetic grids computed by means of 1D LTE atmosphere models, assuming two different self-broadening theories. A grid of Ha profiles was finally computed by means of 3D hydrodynamical atmosphere models. The Li i doublet at 670.8 nm has been used to measure A(Li) by means of 3D hydrodynamical NLTE spectral syntheses. An analytical fit of A(Li)3D, NLTE as a function of equivalent width, Teff, log g, and [Fe/H] has been derived and is made available. Results. We confirm previous claims that A(Li) does not exhibit a plateau below [Fe/H] = -3. We detect a strong positive correlation with [Fe/H] that is insensitive to the choice of Teff estimator. From a linear fit, we infer a steep slope of about 0.30 dex in A(Li) per dex in [Fe/H], which has a significance of 2-3s. The slopes derived using the four T eff estimators are consistent to within 1s. A significant slope is also detected in the A(Li)-Teff plane, driven mainly by the coolest stars in the sample (Teff < 6250), which appear to be Li-poor. However, when we remove these stars the slope detected in the A(Li)-[Fe/H] plane is not altered significantly. When the full sample is considered, the scatter in A(Li) increases by a factor of 2 towards lower metallicities, while the plateau appears very thin above [Fe/H] = -2.8. At this metallicity, the plateau lies at, SCOPUS: ar.j, SCOPUS: ar.j, info:eu-repo/semantics/published

Context. The origin of carbon-enhanced metal-poor stars enriched with both s and r elements is highly debated. Detailed abundances of these types of stars are crucial to understand the nature of their progenitors. Aims. The aim of this investigation is to study in detail the abundances of SDSS J1349-0229, SDSS J0912+0216 and SDSS J1036+1212, three dwarf CEMP stars, selected from the Sloan Digital Sky Survey. Methods. Using high resolution VLT/UVES spectra (R ~ 30000) we determine abundances for Li, C, N, O, Na, Mg, Al, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni and 21 neutron-capture elements. We made use of CO5BOLD 3D hydrodynamical model atmospheres in the analysis of the carbon, nitrogen and oxygen abundances. NLTE corrections for Ci and Oi lines were computed using the Kiel code. Results. We classify SDSS J1349-0229 and SDSS J0912+0216 as CEMP-r+s stars. SDSS J1036+1212 belongs to the class CEMP-no/s, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date. Radial-velocity variations have been observed in SDSS J1349-0229, providing evidence that it is a member of a binary system. Conclusions. The chemical composition of the three stars is generally compatible with mass transfer from an AGB companion. However, many details remain difficult to explain. Most notably of those are the abundance of Li at the level of the Spite plateau in SDSS J1036+1212 and the large over-abundance of the pure r-process element Eu in all three stars. © ESO, 2010., SCOPUS: ar.j, info:eu-repo/semantics/published

We present FLAMES/GIRAFFE spectroscopy obtained with the Very Large Telescope (VLT). Using these observations, we have been able (for the first time) to observe the Lii doublet in the main-sequence (MS) stars of a globular cluster. We also observed Li in a sample of subgiant (SG) stars of the same B - V colour. Our final sample is composed of 84 SG and 79 MS stars. In spite of the fact that SG and MS stars span the same temperature range, we find that the equivalent widths of the Lii doublet in SG stars are systematically greater than in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis, which makes use of both 1D and 3D model atmospheres. We find that SG stars show, on average, a higher Li abundance, by 0.1 dex, than MS stars. We also detect a positive slope of Li abundance with effective temperature: the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide unambiguous evidence that the Li abundance changes with evolutionary state. The physical mechanisms that contribute to this are not yet clear, since none of the proposed models seem to describe accurately the observations. Whether such a mechanism can explain the cosmological lithium problem is still an open question. © International Astronomical Union 2010., info:eu-repo/semantics/published

The connection between helium-rich hot subdwarfs of spectral types O and B (He-sdB) has been relatively unexplored since the latter were found in significant numbers in the 1980s. In order to explore this connection further, we have analysed the surface composition of six He-sdB stars, including LB 1766, LB 3229, SB 21 (= Ton-S 137 = BPS 29503-0009), BPS 22940-0009, BPS 29496-0010 and BPS 22956-0094. Opacity-sampled line-blanketed model atmospheres have been used to derive atmospheric properties and elemental abundances. All the stars are moderately metal poor compared with the Sun ([ Fe/H ]˜-0.5). Four stars are nitrogen rich, two of these are carbon rich and at least four appear to be neon rich. The data are insufficient to rule out binarity in any of the sample. The surface composition and locus of the N-rich He-sdBs are currently best explained by the merger of two helium white dwarfs, or possibly by the merger of a helium white dwarf with a post-sdB white dwarf. C-rich He-sdBs require further investigation. © 2010 The Authors. Journal compilation © 2010 RAS., SCOPUS: ar.j, info:eu-repo/semantics/published

We present the largest sample available to date of lithium abundances in extremely metal poor (EMP) Halo dwarfs. Four Teff estimators are used, including IRFM and Halpha wings fitting against 3D hydrodynamical synthetic profiles. Lithium abundances are computed by means of 1D and 3D-hydrodynamical NLTE computations. Below [Fe/H]~-3, a strong positive correlation of A(Li) with [Fe/H] appears, not influenced by the choice of the Teff estimator. A linear fit finds a slope of about 0.30 dex in A(Li) per dex in [Fe/H], significant to 2-3 sigma, and consistent within 1 sigma among all the Teff estimators. The scatter in A(Li) increases significantly below [Fe/H]~-3. Above, the plateau lies at = 2.199 ± 0.086. If the primordial A(Li) is the one derived from standard Big Bang Nucleosynthesis (BBN), it appears difficult to envision a single depletion phenomenon producing a thin, metallicity independent plateau above [Fe/H] = -2.8, and a highly scattered, metallicity dependent distribution below. Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Keywords (in text query field) Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints, DOI: 10.1017/S1743921310000244, info:eu-repo/semantics/published

Approximately 20% of very metal-poor stars ([Fe/H] < -2.0) are strongly enhanced in carbon ([C/Fe] > +1.0). Such stars are referred to as carbon-enhanced metal-poor (CEMP) stars. We present a chemical abundance analysis based on high resolution spectra acquired with UVES at the VLT of three dwarf CEMP stars: SDSS J1349-0229, SDSS J0912+0216 and SDSS J1036+1212. These very metal-poor stars, with [Fe/H] < -2.5, were selected from our ongoing survey of extremely metal-poor dwarf candidates from the SDSS. Among these CEMPs, SDSS J1349-0229 has been identified as a carbon star ([C/O] > +1.0). First and second peak s-process elements, as well as second peak r-process elements have been detected in all stars. In addition, elements from the third r-process peak were detected in one of the stars, SDSS J1036+1212. We present the abundance results of these stars in the context of neutron-capture nucleosynthesis theories., DOI: 10.1017/S1743921310000359, info:eu-repo/semantics/published

Extreme helium stars are hydrogen‐deficient supergiants of spectral type A and B. They are believed to result from mergers in double degenerate systems, if one white dwarf is of C/O‐type and the other of He‐type. We calculate a new grid of model atmospheres accounting for metal line blanketing with ATLAS12 and compare it to the benchmark code STERNE. Synthetic spectra are calculated accounting for NLTE effects for many ions and applied to high‐quality optical spectra of the prototype extreme helium star BD+10°2179.© 2010 American Institute of Physics, 17 European White Dwarf Workshop, Tubingen (Germany), 16-20 August 2010, info:eu-repo/semantics/published

info:eu-repo/semantics/published

We present FLAMES/GIRAFFE spectroscopy obtained with the Very Large Telescope (VLT). Using these observations, we have been able (for the first time) to observe the Lii doublet in the main-sequence (MS) stars of a globular cluster. We also observed Li in a sample of subgiant (SG) stars of the same B - V colour. Our final sample is composed of 84 SG and 79 MS stars. In spite of the fact that SG and MS stars span the same temperature range, we find that the equivalent widths of the Lii doublet in SG stars are systematically greater than in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis, which makes use of both 1D and 3D model atmospheres. We find that SG stars show, on average, a higher Li abundance, by 0.1 dex, than MS stars. We also detect a positive slope of Li abundance with effective temperature: the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide unambiguous evidence that the Li abundance changes with evolutionary state. The physical mechanisms that contribute to this are not yet clear, since none of the proposed models seem to describe accurately the observations. Whether such a mechanism can explain the cosmological lithium problem is still an open question. © International Astronomical Union 2010., info:eu-repo/semantics/published

The connection between helium-rich hot subdwarfs of spectral types O and B (He-sdB) has been relatively unexplored since the latter were found in significant numbers in the 1980s. In order to explore this connection further, we have analysed the surface composition of six He-sdB stars, including LB 1766, LB 3229, SB 21 (= Ton-S 137 = BPS 29503-0009), BPS 22940-0009, BPS 29496-0010 and BPS 22956-0094. Opacity-sampled line-blanketed model atmospheres have been used to derive atmospheric properties and elemental abundances. All the stars are moderately metal poor compared with the Sun ([ Fe/H ]˜-0.5). Four stars are nitrogen rich, two of these are carbon rich and at least four appear to be neon rich. The data are insufficient to rule out binarity in any of the sample. The surface composition and locus of the N-rich He-sdBs are currently best explained by the merger of two helium white dwarfs, or possibly by the merger of a helium white dwarf with a post-sdB white dwarf. C-rich He-sdBs require further investigation. © 2010 The Authors. Journal compilation © 2010 RAS., SCOPUS: ar.j, info:eu-repo/semantics/published

Context. Oxygen is the third most common element in the Universe. The measurement of oxygen lines in metal-poor unevolved stars, in particular near-UV OH lines, can provide invaluable information about the properties of the Early Galaxy. Aims. Near-UV OH lines constitute an important tool to derive oxygen abundances in metal-poor dwarf stars. Therefore, it is important to correctly model the line formation of OH lines, especially in metal-poor stars, where 3D hydrodynamical models commonly predict cooler temperatures than plane-parallel hydrostatic models in the upper photosphere. Methods. We have made use of a grid of 52 3D hydrodynamical model atmospheres for dwarf stars computed with the code CO 5BOLD, extracted from the more extended CIFIST grid. The 52 models cover the effective temperature range 5000-6500 K, the surface gravity range 3.5-4.5 and the metallicity range-3 < [Fe/H] < 0. Results. We determine 3D-LTE abundance corrections in all 52 3D models for several OH lines and Fe i lines of different excitation potentials. These 3D-LTE corrections are generally negative and reach values of roughly-1 dex (for the OH 3167 with excitation potential of approximately 1 eV) for the higher temperatures and surface gravities. Conclusions. We apply these 3D-LTE corrections to the individual O abundances derived from OH lines of a sample the metal-poor dwarf stars reported in Israelian et al. (1998, ApJ, 507, 805), Israelian et al. (2001, ApJ, 551, 833) and Boesgaard et al. (1999, AJ, 117, 492) by interpolating the stellar parameters of the dwarfs in the grid of 3D-LTE corrections. The new 3D-LTE [O/Fe] ratio still keeps a similar trend as the 1D-LTE, i.e. increasing towards lower [Fe/H] values. We applied 1D-NLTE corrections to 3D Fei abundances and still see an increasing [O/Fe] ratio towards lower metallicites. However, the Galactic [O/Fe] ratio must be revisited once 3D-NLTE corrections become available for OH and Fe lines for a grid of 3D hydrodynamical model atmos, info:eu-repo/semantics/published

As part of an extended study of the surface abundances of extremely helium-rich hot subdwarfs, high-resolution optical échelle spectra of the three helium-rich subdwarf B stars LB 1766, SB 21 and BPS 22940-0009 are presented. Opacity-sampled line-blanketed model atmospheres have been used to derive atmospheric properties and surface abundances. All three stars are moderately metal-poor compared with the Sun ([Fe/H]˜-0. 4). The first two stars are nitrogen-rich He-sdBs, while the third is a carbon-rich He-sdB. The surface composition and locus of single N-rich He-sdBs are currently best explained by the merger of two helium white dwarfs. C-rich He-sdBs require further investigation. © 2010 Springer Science+Business Media B.V., SCOPUS: ar.j, SCOPUS: ar.j, info:eu-repo/semantics/published

Context. The origin of carbon-enhanced metal-poor stars enriched with both s and r elements is highly debated. Detailed abundances of these types of stars are crucial to understand the nature of their progenitors. Aims. The aim of this investigation is to study in detail the abundances of SDSS J1349-0229, SDSS J0912+0216 and SDSS J1036+1212, three dwarf CEMP stars, selected from the Sloan Digital Sky Survey. Methods. Using high resolution VLT/UVES spectra (R ~ 30000) we determine abundances for Li, C, N, O, Na, Mg, Al, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni and 21 neutron-capture elements. We made use of CO5BOLD 3D hydrodynamical model atmospheres in the analysis of the carbon, nitrogen and oxygen abundances. NLTE corrections for Ci and Oi lines were computed using the Kiel code. Results. We classify SDSS J1349-0229 and SDSS J0912+0216 as CEMP-r+s stars. SDSS J1036+1212 belongs to the class CEMP-no/s, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date. Radial-velocity variations have been observed in SDSS J1349-0229, providing evidence that it is a member of a binary system. Conclusions. The chemical composition of the three stars is generally compatible with mass transfer from an AGB companion. However, many details remain difficult to explain. Most notably of those are the abundance of Li at the level of the Spite plateau in SDSS J1036+1212 and the large over-abundance of the pure r-process element Eu in all three stars. © ESO, 2010., SCOPUS: ar.j, info:eu-repo/semantics/published

Context. The primordial nature of the Spite plateau is at odds with the WMAP satellite measurements, implying a primordial Li production at least three times higher than observed. It has also been suggested that A(Li) might exhibit a positive correlation with metallicity below [Fe/H] ~ -2.5. Previous samples studied comprised few stars below [Fe/H] = -3. Aims. We present VLT-UVES Li abundances of 28 halo dwarf stars between [Fe/H] = -2.5 and -3.5, ten of which have [Fe/H] <-3. Methods. We determined stellar parameters and abundances using four different Teff scales. The direct infrared flux method was applied to infrared photometry. Ha wings were fitted with two synthetic grids computed by means of 1D LTE atmosphere models, assuming two different self-broadening theories. A grid of Ha profiles was finally computed by means of 3D hydrodynamical atmosphere models. The Li i doublet at 670.8 nm has been used to measure A(Li) by means of 3D hydrodynamical NLTE spectral syntheses. An analytical fit of A(Li)3D, NLTE as a function of equivalent width, Teff, log g, and [Fe/H] has been derived and is made available. Results. We confirm previous claims that A(Li) does not exhibit a plateau below [Fe/H] = -3. We detect a strong positive correlation with [Fe/H] that is insensitive to the choice of Teff estimator. From a linear fit, we infer a steep slope of about 0.30 dex in A(Li) per dex in [Fe/H], which has a significance of 2-3s. The slopes derived using the four T eff estimators are consistent to within 1s. A significant slope is also detected in the A(Li)-Teff plane, driven mainly by the coolest stars in the sample (Teff < 6250), which appear to be Li-poor. However, when we remove these stars the slope detected in the A(Li)-[Fe/H] plane is not altered significantly. When the full sample is considered, the scatter in A(Li) increases by a factor of 2 towards lower metallicities, while the plateau appears very thin above [Fe/H] = -2.8. At this metallicity, the plateau lies at, SCOPUS: ar.j, SCOPUS: ar.j, info:eu-repo/semantics/published

We present a detailed analysis of the carbon and nitrogen abundances of two dwarf carbon-enhanced metal-poor (CEMP) stars: SDSS J1349-0229 and SDSS J0912+0216. We also report the oxygen abundance of SDSS J1349-0229. These stars are metal-poor, with [Fe/H] < -2.5, and were selected from our ongoing survey of extremely metal-poor dwarf candidates from the Sloan Digital SkySurvey (SDSS). The carbon, nitrogen and oxygen abundances rely on molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by `classical' 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. Using CO5BOLD 3D hydrodynamical model atmospheres and the Linfor3D line formation code, molecular lines of CH, NH, OH and C2 were computed, and 3D carbon, nitrogen and oxygen abundances were determined. The resulting carbon abundances were compared to abundances derived using atomic CI lines in 1D LTE and NLTE. There is not a good agreement between the carbon abundances determined from C2 bands and from the CH band, and molecular lines do not agree with the atomic CI lines. Although this may be partly due to uncertainties in the transition probabilities of the molecular bands it certainly has to do with the temperature structure of the outer layers of the adopted model atmosphere. We explore the influence of the 3D model properties on the molecular abundance determination. In particular, the choice of the number of opacity bins used in the model calculations and its subsequent effects on the temperature structure and molecular line formation is discussed. (Abridged), Poster presented at IAU JD 10, Rio de Janeiro, 10-11 August 2009, info:eu-repo/semantics/published

Photometric data of the dwarf and subgiant stars of the globular cluster NGC 6397. We also provide the signal-to-noise of the spectra, the 3D and 1D Halpha-based effective temperatures, 3D Li abundances, and the equivalent widths and errors: dEWa: Error of the equivalent width measurements estimated from a fitting routine that uses as free parameters the velocity shift, the continuum location, and the equivalent width of the Li line. dEWb: Error of the equivalent width associated to the signal-to-noise ratio and the wavelength dispersion of the spectra, derived using Cayrel's formula (Cayrel, 1988, IAU Symp. 132: The Impact of Very High S/N Spectroscopy on Stellar Physics, 132, 345)., VizieR On-line Data Catalog: J/A+A/505/L13, info:eu-repo/semantics/published

Context. Most globular clusters are believed to host a single stellar population. They can thus be considered a good place to study the Spite plateau and to search for possible evolutionary modifications of the Li content. Aims. We want to determine the Li content of subgiant (SG) and main sequence (MS) stars of the old, metal-poor globular cluster NGC 6397. This work was aimed not only at studying possible Li abundance variations but also to investigate the cosmological Li discrepancy.Methods. Here, we present FLAMES/GIRAFFE observations of a sample of 84 SG and 79 MS stars in NGC 6397 selected in a narrow range of B-V colour and, therefore, effective temperatures. We determine both effective temperatures and Li abundances using three-dimensional hydrodynamical model atmospheres for all the MS and SG stars of the sample. Results. We find a significant difference in the Li abundance between SG stars and MS stars, the SG stars having an abundance higher by almost 0.1 dex on average. We also find a decrease in the lithium abundance with decreasing effective temperature, both in MS and SG stars, albeit with a significantly different slope for the two classes of stars. This suggests that the lithium abundance in these stars is, indeed, altered by some process, which is temperature-dependent. Conclusions. The lithium abundance pattern observed in NGC 6397 is different from what is found among field stars, casting some doubt on the use of globular cluster stars as representative of Population II with respect to the lithium abundance. None of the available theories of Li depletion appears to satisfactorily describe our observations. © 2009 ESO., info:eu-repo/semantics/published

Context. Balmer lines serve as important indicators of stellar effective temperatures in late-type stellar spectra. One of their modelling uncertainties is the influence of convective flows on their shape. Alms. We aim to characterize the influence of convection on the wings of Balmer lines. Methods. We perform a differential comparison of synthetic Balmer line profiles obtained from 3D hydrodynamical model atmospheres and ID hydrostatic standard ones. The model parameters are appropriate for F, G, K dwarf and subgiant stars of metallicity ranging from solar to 10-3 solar. Results. The shape of the Balmer lines predicted by 3D models can never be exactly reproduced by a ID model, irrespective of its effective temperature. We introduce the concept of a 3D temperature correction, as the effective temperature difference between a 3D model and a ID model which provides the closest match to the 3D profile. The temperature correction is different for the different members of the Balmer series and depends on the adopted mixing-length parameter OMU in the ID model. Among the investigated models, the 3D correction ranges from -300 K to +300 K. Horizontal temperature fluctuations tend to reduce the 3D correction. Conclusions. Accurate effective temperatures cannot be derived from the wings of Balmer lines, unless the effects of convection are properly accounted for. The 3D models offer a physically well justified way of doing so. The use of ID models treating convection with the mixing-length theory do not appear to be suitable for this purpose. In particular, there are indications that it is not possible to determine a single value of aMLT which will optimally reproduce the Balmer lines for any choice of atmospheric parameters. The investigation of a more extended grid and direct comparison with observed Balmer profiles will be carried out in the near future. ©ESO 2009., info:eu-repo/semantics/published

Balmer lines are recognized as accurate indicators of the effective temperature of late- type stars. The influence of convection on the shape of Balmer line profiles has been investigated using LTE 3D hydrodynamical model atmospheres and 'classical' LTE 1D stellar atmospheres, where convection is modeled within the simplistic picture of mixing-length theory. Models and line profiles computed with the CO5 BOLD and Linfor3D codes have been used to determine the effective temperatures of the Sun and three well known metal-poor stars HD84937, HD74000, and HD140283. Our 3D fit provides the best fit thus far for the solar Ha temperature using the Barklem theory. The resulting (3D-1D) Teff biases related to the different treatment of convection in the 1D and 3D models are presented. © 2009 American Institute of Physics., 15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun; St. Andrews; United Kingdom; 21 July 2008 through 25 July 2008, info:eu-repo/semantics/published

We present a detailed analysis of the carbon and nitrogen abundances of two dwarf carbon-enhanced metal-poor (CEMP) stars: SDSS J1349-0229 and SDSS J0912+0216. We also report the oxygen abundance of SDSS J1349-0229. These stars are metal-poor, with [Fe/H] < -2.5, and were selected from our ongoing survey of extremely metal-poor dwarf candidates from the Sloan Digital SkySurvey (SDSS). The carbon, nitrogen and oxygen abundances rely on molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by `classical' 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. Using CO5BOLD 3D hydrodynamical model atmospheres and the Linfor3D line formation code, molecular lines of CH, NH, OH and C2 were computed, and 3D carbon, nitrogen and oxygen abundances were determined. The resulting carbon abundances were compared to abundances derived using atomic CI lines in 1D LTE and NLTE. There is not a good agreement between the carbon abundances determined from C2 bands and from the CH band, and molecular lines do not agree with the atomic CI lines. Although this may be partly due to uncertainties in the transition probabilities of the molecular bands it certainly has to do with the temperature structure of the outer layers of the adopted model atmosphere. We explore the influence of the 3D model properties on the molecular abundance determination. In particular, the choice of the number of opacity bins used in the model calculations and its subsequent effects on the temperature structure and molecular line formation is discussed. (Abridged), Poster presented at IAU JD 10, Rio de Janeiro, 10-11 August 2009, info:eu-repo/semantics/published

Photometric data of the dwarf and subgiant stars of the globular cluster NGC 6397. We also provide the signal-to-noise of the spectra, the 3D and 1D Halpha-based effective temperatures, 3D Li abundances, and the equivalent widths and errors: dEWa: Error of the equivalent width measurements estimated from a fitting routine that uses as free parameters the velocity shift, the continuum location, and the equivalent width of the Li line. dEWb: Error of the equivalent width associated to the signal-to-noise ratio and the wavelength dispersion of the spectra, derived using Cayrel's formula (Cayrel, 1988, IAU Symp. 132: The Impact of Very High S/N Spectroscopy on Stellar Physics, 132, 345)., VizieR On-line Data Catalog: J/A+A/505/L13, info:eu-repo/semantics/published

Context. Most globular clusters are believed to host a single stellar population. They can thus be considered a good place to study the Spite plateau and to search for possible evolutionary modifications of the Li content. Aims. We want to determine the Li content of subgiant (SG) and main sequence (MS) stars of the old, metal-poor globular cluster NGC 6397. This work was aimed not only at studying possible Li abundance variations but also to investigate the cosmological Li discrepancy.Methods. Here, we present FLAMES/GIRAFFE observations of a sample of 84 SG and 79 MS stars in NGC 6397 selected in a narrow range of B-V colour and, therefore, effective temperatures. We determine both effective temperatures and Li abundances using three-dimensional hydrodynamical model atmospheres for all the MS and SG stars of the sample. Results. We find a significant difference in the Li abundance between SG stars and MS stars, the SG stars having an abundance higher by almost 0.1 dex on average. We also find a decrease in the lithium abundance with decreasing effective temperature, both in MS and SG stars, albeit with a significantly different slope for the two classes of stars. This suggests that the lithium abundance in these stars is, indeed, altered by some process, which is temperature-dependent. Conclusions. The lithium abundance pattern observed in NGC 6397 is different from what is found among field stars, casting some doubt on the use of globular cluster stars as representative of Population II with respect to the lithium abundance. None of the available theories of Li depletion appears to satisfactorily describe our observations. © 2009 ESO., info:eu-repo/semantics/published

Context. Balmer lines serve as important indicators of stellar effective temperatures in late-type stellar spectra. One of their modelling uncertainties is the influence of convective flows on their shape. Alms. We aim to characterize the influence of convection on the wings of Balmer lines. Methods. We perform a differential comparison of synthetic Balmer line profiles obtained from 3D hydrodynamical model atmospheres and ID hydrostatic standard ones. The model parameters are appropriate for F, G, K dwarf and subgiant stars of metallicity ranging from solar to 10-3 solar. Results. The shape of the Balmer lines predicted by 3D models can never be exactly reproduced by a ID model, irrespective of its effective temperature. We introduce the concept of a 3D temperature correction, as the effective temperature difference between a 3D model and a ID model which provides the closest match to the 3D profile. The temperature correction is different for the different members of the Balmer series and depends on the adopted mixing-length parameter OMU in the ID model. Among the investigated models, the 3D correction ranges from -300 K to +300 K. Horizontal temperature fluctuations tend to reduce the 3D correction. Conclusions. Accurate effective temperatures cannot be derived from the wings of Balmer lines, unless the effects of convection are properly accounted for. The 3D models offer a physically well justified way of doing so. The use of ID models treating convection with the mixing-length theory do not appear to be suitable for this purpose. In particular, there are indications that it is not possible to determine a single value of aMLT which will optimally reproduce the Balmer lines for any choice of atmospheric parameters. The investigation of a more extended grid and direct comparison with observed Balmer profiles will be carried out in the near future. ©ESO 2009., info:eu-repo/semantics/published

Balmer lines are recognized as accurate indicators of the effective temperature of late- type stars. The influence of convection on the shape of Balmer line profiles has been investigated using LTE 3D hydrodynamical model atmospheres and 'classical' LTE 1D stellar atmospheres, where convection is modeled within the simplistic picture of mixing-length theory. Models and line profiles computed with the CO5 BOLD and Linfor3D codes have been used to determine the effective temperatures of the Sun and three well known metal-poor stars HD84937, HD74000, and HD140283. Our 3D fit provides the best fit thus far for the solar Ha temperature using the Barklem theory. The resulting (3D-1D) Teff biases related to the different treatment of convection in the 1D and 3D models are presented. © 2009 American Institute of Physics., 15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun; St. Andrews; United Kingdom; 21 July 2008 through 25 July 2008, info:eu-repo/semantics/published

The effects of diffusion in the atmospheres of subdwarf B stars have been investigated using a model atmosphere code which includes diffusion self-consistently. The impact of helium abundance profiles on the atmospheres of sdBs are presented. An examination of helium spectral lines in the models revealed the sensitivity of the profiles to abundance gradients. Possible diagnostics for detecting helium stratification in the atmospheres of sdBs are discussed., ¨rpceedo,g pthe the conference on "Hot Subdwarf Stars and Related Objects", held 23-27 July, 2007, Otto-Friedrich-Universität, Bamberg (Germany). Edited by Ulrich Heber, C. Simon Jeffery and Ralf Napiwotzki., info:eu-repo/semantics/published

We report the discovery of a new class of hydrogen-deficient stars: white dwarfs with an atmosphere primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch (AGB) evolution, although these objects might be the cooler counter-part of the unique and extensively studied PG 1159 star H1504+65. These stars, together with H1504+65, might thus form a new evolutionary post-AGB sequence., Proceedings of conference on "Hydrogen-Deficient Stars", held 17-21 September, 2007 at Eberhard Karls University, Tübingen (Germany). Edited by Klaus Werner and Thomas Rauch., info:eu-repo/semantics/published

We present a detailed analysis of all the known hot DQ white dwarfs in the Fourth Data Release of the Sloan Digital Sky Survey (SDSS) recently found to have carbon-dominated atmospheres. Our spectroscopic and photometric analysis reveals that these objects all have effective temperatures between ~18,000 and 24,000 K. The surface composition is found to be completely dominated by carbon, as revealed by the absence of Hß and He I ?14471 lines (or a determination of trace amounts in a few cases). We find that the surface gravity of all objects but one seems to be "normal" and around log g = 8.0, while one is likely near log g = 9.0. The presence of a weak magnetic field is directly detected by spectropolarimetry in one object and is suspected in two others. We propose that these strange stars could be cooled-down versions of the weird PG 1159 star H1504+65 and form a new family of hydrogen- and helium-deficient objects following the post-AGB phase. Finally, we present the results of full nonadiabatic calculations dedicated specifically to each of the hot DQs that show that only SDSS J142625.70+575218.4 is expected to exhibit luminosity variations. This result is in excellent agreement with recent observations by Montgomery et al. who find that J142625.70+575218.4 is the only pulsator among six hot DQ white dwarfs surveyed in 2008 February. © 2008. The American Astronomical Society. All rights reserved., info:eu-repo/semantics/published

Approximately 20% of very metal-poor stars ([Fe/H] < -2.0) are strongly enhanced in carbon ([C/Fe] > +1.0). Such stars are referred to as carbon-enhanced metal-poor (CEMP) stars. A variety of abundance patterns are found among CEMP stars. Strong overabundances of nitrogen are common, and overabundances of neutron capture elements are often, however not always, present. The variety of abundance patterns among CEMP stars strongly suggests that this population of stars comprises several astrophysical origins. We are conducting a high-resolution follow-up of candidate EMP stars extracted from the Sloan Digital Sky Survey (SDSS; York et al. 2000) using UVES at the VLT. Three of the programme stars, SDSS J0912+0216, SDSS J1036+1212 and SDSS J1349-0229, where deliberately targetted as CEMP stars since a strong G band was evident from the SDSS spectra and the weakness of the Ca II K line testified their very low metallicity. The UVES high resolution follow-up confirmed the original findings ([Fe/H] < -2.50) and allowed a more detailed investigation of their chemical composition. We determined the carbon abundance from molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by classical 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. 3D carbon abundances were determined for all three stars, using CO5BOLD 3D hydrodynamical model atmospheres. 3D effects on the carbon abundance are found to be quite significant for these stars, with 3D corrections of up to -0.7 dex. Two of the stars, SDSS J0912+0216 and SDSS J1349-0229 exhibit an overabundance of neutron capture elements which classifies them as CEMP-s. Star SDSS J1036+1212, instead belongs to the elusive class of CEMP-no/s stars, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date. © Copyright owned by the author(, 10th Symposium on Nuclei in the Cosmos, NIC 2008; Mackinac Island, MI; United States; 27 July 2008 through 1 August 2008; Code 100690, info:eu-repo/semantics/published

Standard one-dimensional (1D) model atmospheres rely on the assumption of radiative equilibrium in the non-convective part of the stellar photosphere. However, gas-dynamical effects can lead to dramatic deviations from radiative equilibrium conditions, especially in metal-poor stellar atmospheres. These can be taken into account in 3D stellar atmosphere models representing the detailed interplay of hydrodynamics and radiation. During the last two years efforts have been invested to compute such 3D models for metal-poor atmospheres with the CO 5 BOLD code within the CIFIST (Cosmological Impact of the FIrst STars) Team, an European Union funded research group dedicated to the study of metal-poor stars. Based on the available models we will give an account of the radiation-hydrodynamical processes at work, and discuss consequences for the temperature scale and abundance analysis of metal-poor stars. © 2008 American Institute of Physics., First Stars III; Santa Fe, NM; United States; 15 July 2007 through 20 July 2007, info:eu-repo/semantics/published

Context. The stable element hafnium (Hf) and the radioactive element thorium (Th) were recently suggested as a suitable pair for radioactive dating of stars. The applicability of this elemental pair needs to be established for stellar spectroscopy.Aims. We aim at a spectroscopic determination of the abundance of Hf and Th in the solar photosphere based on a COBOLD 3D hydrodynamical model atmosphere. We put this into a wider context by investigating 3D abundance corrections for a set of G- and F-type dwarfs.Methods. High-resolution, high signal-to-noise solar spectra were compared to line synthesis calculations performed on a solar COBOLD model. For the other atmospheres, we compared synthetic spectra of COBOLD 3D and associated 1D models.Results. For Hf we find a photospheric abundance (Hf) = 0.87 0.04, in good agreement with a previous analysis, based on 1D model atmospheres. The weak Th II 401.9 nm line constitutes the only Th abundance indicator available in the solar spectrum. It lies in the red wing of a Ni-Fe blend exhibiting a non-negligible convective asymmetry. Accounting for the asymmetry-related additional absorption, we obtain (Th) = 0.08 0.03, consistent with the meteoritic abundance, and about 0.1 dex lower than obtained in previous photospheric abundance determinations. Conclusions. Only for the second time, to our knowledge, has a non-negligible effect of convective line asymmetries on an abundance derivation been highlighted. Three-dimensional hydrodynamical simulations should be employed to measure Th abundances in dwarfs if similar blending is present, as in the solar case. In contrast, 3D effects on Hf abundances are small in G- to mid F-type dwarfs and sub-giants, and 1D model atmospheres can be conveniently used. © 2008 ESO., info:eu-repo/semantics/published

We give a progress report on the activities within the CIFIST Team related to the search for extremely metal-poor (EMP) stars in the Sloan Digital Sky Survey's (SDSS) spectroscopic catalogue. So far, the search has provided 25 candidates with metallicities around or smaller than -3. For 15 candidates, high-resolution spectroscopy with UVES at the VLT has confirmed their EMP status. Work is under way to extend the search to the SDSS's photometric catalogue by augmenting the SDSS photometry and by gauging the capabilities of X-shooter when going to significantly fainter targets. © 2008 The Royal Swedish Academy of Sciences., A Stellar Journey: A symposium in Celebration of Bengt Gustafsson's 65th Birthday; Uppsala; Sweden; 23 June 2008 through 27 June 2008; Code 78224, info:eu-repo/semantics/published

We present our current sample of Lithium abundances in 28 low metallicity dwarf and Turn Off (TO) stars ([Fe/H] between -2.5 and -3.5), based on high resolution, high signal to noise echelle spectra. Nine new stars have been added to the Bonifacio et al. [1] sample, and the full sample has been reanalyzed in order to take into account the effect of two different possible temperature scales. In fact, the Li abundance measurement based on the 670.8 nm line is highly sensitive to temperature, and Teff scales are still poorly calibrated at low metallicities. First, the effective temperature has been derived from Ha profile fitting, and second, directly from the star's infrared flux. The two methods offer similar precision but are affected by different uncertainties and systematics. The infrared flux method (IRFM) leads to a larger Teff dispersion than the Ha profile fitting, while also producing an offset of about 150 K towards hotter temperatures. This leads to a contraction of the metallicity scale of the sample, which encompasses [Fe/H]=-3.7 to -2.5 when using Ha calibrated temperatures, and [Fe/H]=-3.4 to -2.5 when using IRFM. The higher average IRFM temperature increases somewhat the mean Li abundance, changing from A(Li)Ha=2.10 to A(Li)IRFM=2.18. © 2008 American Institute of Physics., First Stars III; Santa Fe, NM; United States; 15 July 2007 through 20 July 2007, info:eu-repo/semantics/published