Abundance Analyses of the Field RV Tauri Variables: EP Lyrae, DY Orionis, AR Puppis, and R Sagittae

Abstract

Analyses of the photospheric compositions of the four Ðeld RV Tauri stars, EP Lyr, DY Ori, AR Pup, and R Sge, indicate that to varying degrees they have experienced fractionation processes that have preferentially depleted their atmospheres of elements with high condensation temperatures. The depletion, as indicated by, for instance, [S/Fe], is greatest for DY Ori, [S/Fe] \ 2.5, and least for R Sge, [S/Fe] \ 0.9. The initial composition, presumably indicated by the sulfur abundance, was nearly solar for AR Pup, R Sge, and DY Ori, while it was about 0.6 dex less than solar for EP Lyr. This implies that the RV Tauri stars as a group may not be as metal-poor as previously thoughtÈthey are instead "" metal-depleted.ÏÏ The Ðeld RV TauriÏs are not halo stars, but probably belong to the thick disk. This brings to seven the number of type II Cepheids that show such a trend ; the other three are IW Car and V1 in u Cen, RV Tauri stars, and ST Pup, a W Virginis star. The 12C/13C ratios for EP Lyr and DY Ori are 9 ^ 1 and 6 ^ 3, respectively, indicating that CN-cycled material has been mixed with their surface layers. This is consistent with the general consensus that RV Tau stars are in a post-AGB evolutionary stage. There is also evidence that EP Lyr has a stellar mass companion, but additional observations are required to calculate an orbit ; hence, EP Lyr could be a link to the group of metal-depleted, high-latitude AÈF supergiants, all of which are binaries.