A SPECTROSCOPIC STUDY OF SUPERGIANTS WITH INFRARED EXCESSES - PULSATING RV TAU STARS

We use high-resolution CCD spectra from the LYNX echelle spectrometer of the 6-m telescope to determine the fundamental parameters T-eff and logg and the detailed chemical composition for four pulsating RV Tau stars in the Galactic field-AC Her, U Mon, RV Tau, and Al CMi-by the m ethod of model atmospheres. Based on high-resolution spectra, we have studied the chemical composition of RV Tan, the prototype of this clas s of pulsating variables, for the first time, Our main conclusion is t hat the group of stars under consideration is inhomogeneous in metalli city ([Fe/H] ranges from +0.05 to -1.16 dex) and elemental abundances. The pattern of chemical composition for RV Tau suggests that this sta r belongs to the young Galactic population (the mean abundance of the iron-group elements Ti, V, Cr, and Fe for RV Tau is [X/H]. = -0.04 dex ) and that there are no traces of selective fractionation of chemical elements. The atmospheric chemical composition of AC Her was modified by nucleosynthesis (C/O > 1) and by selective fractionation on dust gr ains of the circumstellar envelope: while thr:easily condensable iron- group elements are appreciably underabundant, the atmospheric Zn and S abundances of AC Her differ only slightly from their solar values: [Z n/Ee]. = +0.68 and [S/Fe]. = +0.50. The atmosphere of AC Her exhibits appreciable lanthanide overabundances: the mean value of [X/Fe]. for L a, Ce, Pr, Nd, Sm, and Eu is 0.41. The chemical composition of AI CMi is rather typical of post-AGB metal-poor supergiants with. a great und erabundance of the s-process elements. The four stars all show a great sodium overabundance, which is commonly observed in high-luminosity s tars over a wide range of masses, The great sodium overabundance can b e a product of the synthesis during the previous evolutionary stage of the star and partly a result of systematic errors in the abundance an alysis due to failure to take into account non-LTE effects. We offer a n explanation for the phenomenon of semiregular light variations in RV Tau stars.