On the Spectroscopic Determination of Atmospheric Parameters and O/Fe Abundances of RR Lyrae Stars

Abstract

Abstract In order to study how the conventional spectroscopic method based on the equivalent widths of the Fe I and Fe II lines effectively applies to determining the atmospheric parameters ($T_{\mathrm{eff}}, \log g, v_{\mathrm{t}}, [\mathrm{Fe/H}]$) of RR Lyrae variables and how accurately the abundances can be established from such constructed model atmospheres, we analyzed 15 high-dispersion spectrograms of RR Lyr, DX Del, DH Peg, and VY Ser taken at several different phases by using the HDS spectrograph of the Subaru Telescope, and examined the consistency of the resulting phase-to-phase abundances. Taking oxygen as the target element along with Fe, we determined its non-LTE abundance from the O I 6155–8 and 7771–5 triplets. It was found that consistent abundances were obtained for O as well as Fe to a level of $\sim 0.1 \,\mathrm{dex}$, irrespective of the pulsation phase, except for the special near-maximum high-temperature phase. This suggests that classical model atmospheres are reliably applicable to abundance determinations of RR Lyrae stars in most cases. While the oxygen abundances derived from O I 7771–5 well correlate with those from O I 6155–8, the former tends to be systematically larger by 0.1–0.2 dex than the latter, which may be interpreted as being due to the depth-dependence of the microturbulence increasing with height; i.e., the $v_{\mathrm{t}}$ value derived from deep-forming Fe lines of weak/medium-strength may not be simply applied to higher forming strong lines, such as O I 7771–5.