S. M. Andrievsky

Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood

A number of studies of abundance gradients in the galactic disk have been performed in recent years. The results obtained are rather disparate: from no detectable gradient to a rather signicant slope of about 0: 1d ex kpc 1 . The present study concerns the abundance gradient based on the spectroscopic analysis of a sample of classical Cepheids. These stars enable one to obtain reliable abundances of a variety of chemical elements. Additionally, they have well determined distances which allow an accurate determination of abundance distributions in the galactic disc. Using 236 high resolution spectra of 77 galactic Cepheids, the radial elemental distribution in the galactic disc between galactocentric distances in the range 6{11 kpc has been investigated. Gradients for 25 chemical elements (from carbon to gadolinium) are derived. The following results were obtained in this study. Almost all investigated elements show rather flat abundance distributions in the middle part of galactic disc. Typical values for iron-group elements lie within an interval from 0:02 to 0:04 dex kpc 1 (in particular, for iron we obtained d(Fe/H)/dRG = 0:029 dex kpc 1 ). Similar gradients were also obtained for O, Mg, Al, Si, and Ca. For sulphur we have found a steeper gradient ( 0:05 dex kpc 1 ). For elements from Zr to Gd we obtained (within the error bars) a near to zero gradient value. This result is reported for the rst time. Those elements whose abundance is not expected to be altered during the early stellar evolution (e.g. the iron-group elements) show at the solar galactocentric distance (El/H) values which are essentially solar. Therefore, there is no apparent reason to consider our Sun as a metal-rich star. The gradient values obtained in the present study indicate that the radial abundance distribution within 6{11 kpc is quite homogeneous, and this result favors a galactic model including a bar structure which may induce radial flows in the disc, and thus may be responsible for abundance homogenization.

Using Cepheids to determine the galactic abundance gradient II. Towards the galactic center

Based on spectra obtained at the Anglo-Australian Observatory, we present a discussion of the metallic- ity of the galactic disc derived using Cepheids at galactocentric distances of 4{6 kpc. Our new results together with previous gradient determination (Paper I) show that the overall abundance distribution within the galactocentric distances 4{11 kpc cannot be represented by a single gradient value. The distribution is more likely bimodal: it is flatter in the solar neighbourhood with a small gradient, and steepens towards the galactic center. The steepening begins at a distance of about 6.6 kpc.

The Distribution of the Elements in the Galactic Disk

This paper reports on the spectroscopic investigation of 54 Cepheids, deriving parameters and abundances. These Cepheids extend previous samples by about 35% in number and increase the amount of the Galactic disk coverage, especially in the direction of l ≈ 120°. We find that there exists in the Galactic disk at that longitude and at a solar distance of about 3-4 kpc a region that has enhanced abundances, Fe/H] ≈ +0.2, with respect to the local region. A simple linear fit to all Cepheid data now extant yields a gradient d[Fe/H]/dRG = -0.068 ± 0.003 dex kpc-1. After consideration of the spatial abundance inhomogeneities in the sample, we conclude that the best current estimate of the overall gradient is d[Fe/H]/dRG = -0.06 dex kpc-1.

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.

Reddenings of FGK supergiants and classical Cepheids from spectroscopic data

Accurate and homogeneous atmospheric parameters (T eff , log g, V t , [Fe/H]) are derived for 74 FGK non-variable supergiants from high-resolution, high signal-to-noise ratio, echelle spectra. Extremely high precision for the inferred effective temperatures (10-40K) is achieved by using the line-depth ratio method. The new data are combined with atmospheric values for 164 classical Cepheids, observed at 675 different pulsation phases, taken from our previously published studies. The derived values are correlated with unreddened B - V colours compiled from the literature for the investigated stars in order to obtain an empirical relationship of the form (B - V) o = 57.984 - 10.3587(log T eff ) 2 + 1.67572(logT eff ) 3 - 3.356 log g + 0.0321V t + 0.2615[Fe/H] + 0.8833(logg)(logTeff). The expression is used to estimate colour excesses E(B - V) for individual supergiants and classical Cepheids, with a precision of ±0.05 mag for supergiants and Cepheids with n = 1-2 spectra, reaching ±0.025 mag for Cepheids with n > 2 spectra, matching uncertainties for the most sophisticated photometric techniques. The reddening scale is also a close match to the system of space reddenings for Cepheids. The application range is for spectral types FO-KO and luminosity classes I and II.

Phase-dependent Variation of the Fundamental Parameters of Cepheids. III. Periods between 3 and 6 Days

We present the results of a detailed multiphase spectroscopic analysis of six classical Cepheids with pulsation periods between 3 and 6 days. For each star we have derived phased values of effective temperature, surface gravity, microturbulent velocity, and elemental abundances. We show that the elemental abundance results for these Cepheids are consistent for all pulsational phases.

Phase-dependent Variation of the Fundamental Parameters of Cepheids. II. Periods Longer than 10 Days

We present the results of a detailed multiphase spectroscopic analysis of 14 classical Cepheids with pulsation periods longer than 10 days. For each star, we have derived phased values of effective temperature, surface gravity, microturbulent velocity, and elemental abundances. We show that the elemental abundance results for these Cepheids are consistent for all pulsational phases.

PHASE-DEPENDENT VARIATION OF THE FUNDAMENTAL PARAMETERS OF CEPHEIDS. IV. s-CEPHEIDS

In this fourth of a series of papers presenting the results of a detailed multiphase spectroscopic analysis of classical Cepheids, we report on fundamental parameter variation in a group of so-called s-Cepheids. For each star, we have derived phased values of effective temperature, surface gravity, microturbulent velocity, and elemental abundances. Dependences between these parameters, amplitudes of their variation, and pulsational period are discussed in a framework of such dependences obtained in our previous works for 30 classical Cepheids. Our temperature data reveal that five of the s-Cepheids have mean temperatures that deviate substantially from the remainder of the Cepheid data. The temperature deviations place these s-Cepheids near the edges of the fundamental instability strip. While these deviations cannot differentiate between fundamental and overtone pulsation modes, the deviations are undoubtedly related to the root cause of the small amplitudes found in these stars.

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.