S. A. Korotin

NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars

Aims. Aluminium is a key element to constrain the models of the chemical enrichment and the yields of the first supernovae. But obtaining precise Al abundances in extremely metal-poor (EMP) stars requires that the non-LTE effects be carefully taken into account. Methods. The NLTE profiles of the blue resonance aluminium lines have been computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI applied to an atomic model of the Al atom with 78 levels of Al I and 13 levels of Al II, and compared to the observations. Results. With these new determinations, all the stars of the sample show a ratio Al/Fe close to the solar value: [Al/Fe] = −0.06 ± 0.10 with a very small scatter. These results are compared to the models of the chemical evolution of the halo using different models of SN II and are compatible with recent computations. The sodium-rich giants are not found to be also aluminium-rich and thus, as expected, the convection in these giants only brings to the surface the products of the Ne-Na cycle.

Open clusters as key tracers of Galactic chemical evolution III. Element abundances in Berkeley 20, Berkeley 29, Collinder 261, and Melotte 66

Context. Galactic open clusters are since long recognized as one of the best tools for investigating the radial distribution of iron and other metals. Aims. We employed FLAMES at VLT to collect UVES spectra of bright giant stars in a large sample of open clusters, spanning a wide range of Galactocentric distances, ages, and metallicities. We present here the results for four clusters: Berkeleyxa020 and Berkeleyxa029, the two most distant clusters in the sample; Collinderxa0261, the oldest and the one with the minimum Galactocentric distance; Melottexa066. Methods. Equivalent width analysis was carried out using the spectral code MOOG and Kurucz model atmospheres to derive abundances ofxa0Fe, Al, Mg, Si, Ca, Ti, Cr, Ni, Ba; non-LTE Naxa0abundances were derived by direct line-profile fitting. Results. We obtain subsolar metallicities for the two anticenter clusters Bexa020 ([Fe/H]u2009=u2009-0.30, rmsu2009=u20090.02) and Bexa029 ([Fe/H]u2009=u2009-0.31, rmsu2009=u20090.03), and for Melxa066 ([Fe/H]u2009=u2009-0.33, rmsu2009=u20090.03), located in the third Galactic quadrant, while Crxa0261, located toward the Galactic center, has higher metallicity ([Fe/H]u2009=u2009+0.13, rmsu2009=u20090.05xa0dex). The α -elements Si, Ca andxa0Ti, and the Fe-peak elements Cr andxa0Ni are in general close to solar; the s -process element Ba is enhanced. Non-LTE computations of Naxa0abundances indicate solar scaled values, suggesting that the enhancement in Naxa0previously determined in giants in open clusters could be due to neglected non-LTE effects. Conclusions. Our results support the presence of a steep negative slope of the Fexa0radial gradient up to about 10–11xa0kpc from the Galactic center, while in the outer disk the [Fe/H]xa0distribution seems flat. All the elemental ratios measured are in very good agreement with those found for disk stars of similar metallicity and no trend with Galactocentric distance seems to be present.

Spectroscopic investigation of stars on the lower main sequence

Aims. The aim of this paper is to provide fundamental parameters and abundances with a high accuracy for a large sample of cool main sequence stars. This study is part of wider project, in which the metallicity distribution of the local thin disc is investigated from a complete sample of G and K dwarfs within 25 pc. Methods. The stars were observed at high resolution and a high signal-to-noise ratio with the ELODIE echelle spectrograph. The V sini were obtained with a calibration of the cross-correlation function. Effective temperatures were estimated by the line depth ratio method. Surface gravities (log g) were determined by two methods: parallaxes and ionization balance of iron. The Mg and Na abundances were derived using a non-LTE approximation. Abundances of other elements were obtained by measuring equivalent widths. Results. Rotational velocities, atmospheric parameters (Teff ,l ogg ,[ Fe/H], Vt ), and Li, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Co, Ni, and Zn abundances are provided for 131 stars. Among them, more than 30 stars are active stars with a fraction of BY Dra and RS CVn type stars for which spectral peculiarities were investigated. We find the mean abundances of the majority of elements in active and nonactive stars to be similar, except for Li, and possibly for Zn and Co. The lithium is reliably detected in 54% of active stars but only in 20% of nonactive stars. No correlation is found between Li abundances and rotational velocities. A possible anticorrelation of log A(Li) with the index of chromospheric activity GrandS is observed. Conclusions. Active and nonactive cool dwarfs show similar dependencies of most elemental ratios vs. [Fe/H]. This allows us to use such abundance ratios to study the chemical and dynamical evolution of the Galaxy. Among active stars, no clear correlation has been found between different indicators of activity for our sample stars.

Sodium enrichment of stellar atmospheres. I. Non-variable supergiants and bright giants

supergiants and bright giants have been observed in order to investigate the sodium enrichment of their atmospheres and its connection with stellar gravity. We present the equivalent widths of the 6154 A and 6160 AN ai lines measured from the program spectra, the results of eective temperature determinations, the NLTE sodium abundances, and the derived relation between the sodium overabundance and surface gravity.

Magellanic Clouds elemental abundances from F supergiants: Revisited results for the Large Magellanic Cloud

With an improved method of the LTE abundance analysis for most elements (Kovtyukh & Andrievsky 1999), the abundances have been re-investigated in nine F supergiants of the Large Magellanic Cloud. The NLTE calculations for carbon, oxygen and sodium, based on Kuruczs atmospheric models with an over-all metal deficiency, were carried out. The most important results are the following: - carbon abundance in supergiants is in much better agreement with the carbon abundance of the LMC Hii regions, oxygen remaining in fair agreement with Hii regions. The (C/O) ratio is low, lower than solar and the Hii ratios (a possible sign of the first dredge-up, or another large-scale mixing event in the supergiant atmospheres); - the NLTE computations do not show remarkable sodium overabundances in the LMC supergiants, in contrast with the Galactic supergiants; - the mean abundance of iron is found to be , slightly lower than the results of some previous determinations for the LMC, but in agreement with the recent result obtained by Korn et al. ([CITE]), who give . The relative to iron abundance of oxygen derived from nine F supergiants appears to be in excellent accordance with predictions based on the smooth model of the LMC evolution; - the ( α /Fe) ratios are comparable to those found in previous analyses. The surprisingly low abundance of Mg is even lower in the new analysis, but it is in good agreement with the relative magnesium abundance in LMC B-stars determined by Korn et al. ([CITE]). Sulphur in the program supergiants shows the same abundance within an error bar as in Hii regions; - the iron-group elements follow the solar distribution of the (M/Fe) ratios; - the heavy elements show enhanced abundances; - significantly larger values were obtained than in a previous study, resulting in supergiant masses of about 10-20 , which are in excellent agreement with evolutionary calculations.

The elemental abundance pattern of twenty λ Bootis candidate stars

Detailed elemental abundances were derived for twenty bona fide Bootis as well as two MK standard stars. Other than LTE abundances for ten elements (including C and O), NLTE values for Na were determined. The group of Bootis stars consists of non-magnetic, Population I, late B to early F-type dwarfs with a typical abundance pattern (Fe-peak elements being underabundant whereas C, N, O and S being almost solar abundant). Since classification resolution spectroscopy in the optical domain is not capable of determining the abundance of the light elements, a detailed abundance analysis is the ultimate test for the membership of an object to this group. Another important point is the detection of apparent spectroscopic binary systems in which two solar abundance objects mimic one metal-weak star, as proposed as a working hypothesis by Faraggiana & Bonifacio (1999). From twenty program stars we are able to confirm or establish the membership for nine objects (HD 23258, HD 36726, HD 40588, HD 74911, HD 84123, HD 91130, HD 106223, HD 111604 and HD 290799). Five stars (HD 90821, HD 98772, HD 103483, HD 108765 and HD 261904) can be definitely ruled out as being members of the Bootis group whereas no unambiguous decision can be drawn for another six stars (HD 66684, HD 105058, HD 120500, HD 141851, HD 201184 and HD 294253). One very important result is the apparent overabundances found for Na which cannot be explained by accretion or mass-loss alone.

Light element abundances in the young open clusters NGC 3293, NGC 4755 and NGC 6231: Tracers of stellar evolution

The abundances of He (LTE), C, N, and O (NLTE) were derived for 21 B stars in three young open clusters. Almost all the stars show subsolar CNO abundances. However, the mean oxygen abundance for each programme cluster appears to be in marginal agreement with the most recent revisions of the solar value. After consideration of the CN abundances in this sample, there is no clear evidence of internal mixing. Only three stars among the non-supergiants seem to show a nitrogen enhancement. Two of them have a fairly low projected equatorial velocity (admittedly, they may be rapid rotators seen pole-on); the third one is a denite fast rotator. In the lower gravity stars (three stars in this sample with log g< 3:0) some kind of mixing has apparently occurred. The supergiants do not dier signicantly from the other programme stars in their respective helium contents. The mean helium abundance for each cluster is close to the standard value, (He/H) 11:0.

The spectroscopic binaries 21 Her and γ Gem

In the framework of a search campaign for short-term oscillations of early-type stars we analysed recently obtained spectroscopic and photometric observations of the early A-type spectroscopic binaries 21 Her and Gem. From the radial velocities of 21 Her we derived an improved orbital period and a distinctly smaller eccentricity in comparison with the values known up to now. Moreover, fairly convincing evidence exists for an increase of the orbital period with time. In addition to the orbital motion we nd further periods in the orbital residuals. The longest period of 57: d 7 is most likely due to a third body which has the mass of a brown dwarf, whereas the period of 1: 48 could be related to the half rotational period of the star. For the spectral types we deduced A1 III for the primary and M for the secondary. Two further periods of 0: d 21 and 0: d 22 give hint to the existence of short-term pulsations in 21 Her. Their period dierence is of the order of the expected rotational period so that one possible explanation could be rotational splitting of nonradial pulsation modes. Because of the very strong aliasing of the data this nding has to be conrmed by observations having a more suitable time sampling, however. The analysis of photometric series and the Hipparcos photometry give no certain evidence for periodic light variations. For Gem, besides the orbital RV variation, no variations with amplitudes larger than about 100 m s 1 could be detected. The orbital elements of Gem are only slightly changed compared to the previously known orbital solution by including our new radial velocities, but their accuracy is improved. For some chemical elements we determined their abundances, NLTE values of C, O, and Na as well as LTE values of Mg, Sc, Fe, Cr, and Ti. We nd the abundances to be rather close to the solar values, only carbon shows a little underabundance.

Evolution of [O/Mg], [Na/Mg], [Al/Mg], and [K/Mg] in the Galaxy, from a NLTE analysis

M. Spite1, F. Spite1 P. Bonifacio 1,2 V. Hill 3 S. Andrievsky 4 R. Cayrel 1 P. François 1 and S. Korotin 4 Observatoire de Paris, GEPI, UMR8111 CNRS , France Osservatorio Astronomico di Trieste, INAF, Italy Universit de Nice Sophia-Antipolis, Observatoire de la Cte dAzur, UMR6202 CNRS, France Astronomical Observatory, Odessa National University, Isaac Newton Institute of Chile, Odessa branch,Ukraine

The ESO Large Programme “First Stars”

In ESO period 65 (April-September 2000) the large programme 165.N-0276, led by Roger Cayrel, began making use of UVES at the Kueyen VLT telescope. Known within the Team and outside as ``First Stars, it was aimed at obtaining high resolution, high signal-to-noise ratio spectra in the range 320 nm -- 1000 nm for a large sample of extremely metal-poor (EMP) stars identified from the HK objective prism survey.In this contribution we highlight the main results of the large programme.