THE COMPOSITION OF HB STARS - RR LYRAE VARIABLES

We have used moderately high-resolution, high S/N spectra to study the chemical composition of ten field ab-type RR Lyrae stars. Variables h aving accurate photometric and radial-velocity data were selected, in order to derive a precise estimate of the atmospheric parameters indep endently of excitation and ionization equilibria. A new temperature sc ale was determined from literature Infrared Flux Method measures of su bdwarfs and the Kurucz (1992) 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 othe r determinations. The applicability of Kurucz (1992) 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 equ ilibria as well as the wings of H alpha. The comparison solar abundanc es were carefully determined. From a new analysis of weak Fe I lines w ith accurate gfs [Bard & Kock, A&A, 282, 1014 (1994)] we derived log e psilon(Fe).=7.52, in agreement with the Fe abundances determined from meteorites and Fe Il 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 c onditions. Fe lines satisfy very well the excitation and ionization eq uilibria. A comparison with statistical equilibrium computations shows that rather large collisional cross sections are required to reproduc e observations. If these cross sections are then used in the analysis of the formation of Fe lines in subdwarfs and RGB stars, no significan t departures from LTE are found for these stars, thus validating the v ery numerous LTE analyses. RR Lyraes share the typical abundance patte rn of other stars of similar [Fe/H]:alpha-elements are overabundant by similar to 0.4 dex and Mn is underabundant by similar to 0.6 dex in s tars 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]similar to-2.5). Significant departures from L TE are found for a few species: Nd II, Ce II, Y II, and Sc II are seve rely underabundant (similar to 0.5 dex) in metal-rich variables; Ti I and Cr I are slightly (similar to 0.1-0.2 dex) 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 Ly man Lines emission produced by the shock waves that propagate in the a tmosphere of these variables [Fokin, MNRAS, 256, 26 (1992)]. Departure s from LTE were considered in detail in the derivation of abundances f or the light elements (O and Na). Significant corrections were require d for the O I IR triplet and the Na D lines. The resulting pattern rep roduces that observed in less evolved field stars. We did not find any evidence for an O-Na anticorrelation among thesefield HE stars, sugge sting that the environment is Likely to be responsible for the anticor relation found in metal-poor globular cluster stars [Sneden et al. AJ, 104, 2121 (1992)]. We used our new [Fe/H] abundances, as well as valu es from Butler and co-workers (corrected to our system), and from high -resolution spectroscopy of globular clusters giants, to obtain a revi sed calibration of the low-resolution metallicity index Delta S [Prest on, ApJ, 130, 507 (1959)]:[Fe/H]=-0.194(+/-0.011)Delta S-0.08(+/-0.18) . Our new metallicity scale is stretched on both low and high metallic ity ends with respect to Butler's [ApJ, 200, 68 (1975)]. The error in [Fe/H] by Delta S observations is 0. 16 dex, well of the same order of high-resolution metallicity determinations. The slope of the calibrat ion obtained considering only stars with 4 <Delta S<10 is slightly sma ller 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 metallicit y calibration of the Ca II K line index [Clementini et al. AJ, 101, 21 68 (1991)]. Using the present new metallicities, and W'(K) values and relative errors from Clementini et al. (1991), a least-squares fit wei ghted 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 meta llicity dependence of the absolute magnitude of RR Lyrae stars, M(V). Using M(V) values from Fernley [AsrA, 284, L16 (1994)1 for the field s tars, and estimates from Liu & Janes [ApJ, 360, 561 (1990)1 and Storm etal. [AsrA, 290, 443 (1994)1 for the cluster variables, we found M(V) =0.20(+/-0.03)[Fe/H] + 1.06(+/-0.04) and M(V)-=O.19(+/-0.03)[Fe/H]+0.9 6(+/-0.04), the last being obtained by using M(V) estimates derived fo r a value of the conversion factor between observed and true pulsation velocity p=1.38 (Fernley 1994). The adoption of the new metallicity s cale does not yield significant changes in the slope and zero point of the M(V) vs [Fe/H] relation. Observations do not rule out the possibi lity 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 B aade-Wesselink M(V) determinations is required to definitely settle th is question. (C) 1995 American Astronomical Society.