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Description
We have used moderately high-resolution, high S/N spectra to study the chemical composition of ten field ab-type RR Lyrae stars. Variables having accurate photometric and radial-velocity data were selected, in order to derive a precise estimate of the atmospheric parameters independently of excitation and ionization equilibria. A new temperature scale was determined from literature "Infrared Flux Method" measures of subdwarfs and the Kurucz (1992) (priv. com.) model atmospheres, and used to calibrate colors for both dwarfs and RR Lyraes. Photometric reddening estimates for the program stars were carefully examined, and compared with other determinations. The applicability of Kurucz (1992) (priv. com.) model atmospheres in the analysis of RR Lyraes at minimum light was analyzed: we found that they are able to reproduce colors, excitation, and ionization equilibria as well as the wings of Halpha. The comparison solar abundances were carefully determined. From a new analysis of weak Fe I lines with accurate gfs [Bard & Kock, A&A, 282, 1014 (1994)] we derived log epsilon(Fe)_Sun=7.52, in agreement with the Fe abundances determined from meteorites and Fe II lines. We derived abundances for 21 species. Main results are: The metal abundances of the program stars span the range -2.50<[Fe/H]<+0.17. Lines of most elements are found to form in LTE conditions. Fe lines satisfy very well the excitation and ionization equilibria. A comparison with statistical equilibrium computations shows that rather large collisional cross sections are required to reproduce observations. If these cross sections are then used in the analysis of the formation of Fe lines in subdwarfs and RGB stars, no significant departures from LTE are found for these stars, thus validating the very numerous LTE analyses. RR Lyraes share the typical abundance pattern of other stars of similar [Fe/H]: alpha-elements are overabundant by ~0.4dex and Mn is underabundant by ~0.6dex in stars with [Fe/H]<-1. Solar scaled abundances are found for most of the other species, except for the low Ba abundance in the extremely metal-poor star X Ari ([Fe/H]~-2.5). Significant departures from LTE are found for a few species: Nd II, Ce II, Y II, and Sc II are severely underabundant (~0.5dex) in metal-rich variables; Ti I and Cr I are slightly (~0.1-0.2dex) underabundant in metal-poor stars. These effects are attributed to overionization. We suggest that the photoionization of the alkaline earth-like ions is due to Lyman lines emission produced by the shock waves that propagate in the atmosphere of these variables [Fokin (1992MNRAS.256...26F)]. Departures from LTE were considered in detail in the derivation of abundances for the light elements (O and Na). Significant corrections were required for the O I IR triplet and the Na D lines. The resulting pattern reproduces that observed in less evolved field stars. We did not find any evidence for an O-Na anticorrelation among these field HB stars, suggesting that the environment is likely to be responsible for the anticorrelation found in metal-poor globular cluster stars [Sneden et al. =1992AJ....104.2121S]. We used our new [Fe/H] abundances, as well as values from Butler and co-workers (corrected to our system), and from high- resolution spectroscopy of globular clusters giants, to obtain a revised calibration of the low-resolution metallicity index Delta(S) [Preston =1959ApJ...130..507P]: [Fe/H]=-0.194(+/-0.011)Delta(S)-0.08(+/-0.18). Our new metallicity scale is stretched on both low and high metallicity ends with respect to Butler's [1975ApJ...200...68B]. The error in [Fe/H] by Delta(S) observations is 0.16dex, well of the same order of high-resolution metallicity determinations. The slope of the calibration obtained considering only stars with 4<Delta(S)<10 is slightly smaller than that obtained using all stars. While this difference is only barely significant, it might point out the presence of a nonlinearity of the Delta(S) vs [Fe/H] relation, as suggested by Manduca [ApJ, 245, 258 (1981)]. The new [Fe/H] values were used to update the metallicity calibration of the Ca II K line index [Clementini et al. =1991AJ....101.2168C]. Using the present new metallicities, and W'(K) values and relative errors from Clementini et al. (1991), a least-squares fit weighted both in W'(K) and [Fe/H] gives [Fe/H]=0.65(+/-0.17)W'(K)-3.49(+/-0.39). Finally, our new metallicity scale was used to revise the metallicity dependence of the absolute magnitude of RR Lyrae stars, M_V. Using M_V values from Fernley [1994A&A...284L..16F] for the field stars, and estimates from Liu & Janes [1990ApJ...360..561L] and Storm et al. [1994A&A...290..443S] for the cluster variables, we found M_V=0.20(+/-0.03) [Fe/H]+1.06(+/-0.04) and M_V=0.19(+/-0.03)[Fe/H]+0.96(+/-0.04), the last being obtained by using M_V estimates derived for a value of the conversion factor between observed and true pulsation velocity p=1.38 (Fernley 1994). The adoption of the new metallicity scale does not yield significant changes in the slope or zero point of the M_V vs [Fe/H] relation. Observations do not rule out the possibility that the slope of the M_V vs [Fe/H] relation might be different for metal-poor and metal-rich variables. However, a larger sample of Baade-Wesselink M_V determinations is required to definitely settle this question.
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