On the Use of Field RR Lyrae as Galactic Probes. II. A new $\Delta$S calibration to estimate their metallicity

TL;DR

This study presents a comprehensive spectroscopic survey of field RR Lyrae stars and introduces a new calibration method for estimating their metallicity using the ΔS technique, applicable over the entire pulsation cycle and including RRc variables.

Contribution

The paper develops a novel empirical ΔS calibration for RR Lyrae metallicity that incorporates spectra over the full pulsation cycle and includes RRc variables, covering a wide metallicity range.

Findings

The new calibration provides metallicity estimates consistent with literature.

RRc stars are on average 0.10 dex more metal-poor than RRab stars.

The metallicity distribution shows a well-defined tail approaching solar abundance.

Abstract

We performed the largest and most homogeneous spectroscopic survey of field RR Lyraes (RRLs). We secured $\approx$6,300 high resolution (HR, R$\sim$35,000) spectra for 143 RRLs (111 fundamental, RRab; 32 first overtone, RRc). The atmospheric parameters were estimated by using the traditional approach and the iron abundances were measured by using an LTE line analysis. The resulting iron distribution shows a well defined metal-rich tail approaching solar iron abundance. This suggests that field RRLs experienced a complex chemical enrichment in the early halo formation. We used these data to develop a new calibration of the $\Delta$S method. This diagnostic, based on the equivalent widths of CaII K and three Balmer (H$_{\delta,\gamma,\beta}$) lines, traces the metallicity of RRLs. For the first time the new empirical calibration: i) includes spectra collected over the entire pulsation cycle; ii) includes RRc variables; iii) relies on spectroscopic calibrators covering more than three dex in iron abundance; iv) provides independent calibrations based on one/two/three Balmer lines. The new calibrations were applied to both SEGUE-SDSS and degraded HR spectra totalling 6,451 low resolution (LR, R$\sim$2,000) spectra for 5,001 RRLs (3,439 RRab, 1,562 RRc). This resulted in an iron distribution with a median of -1.55$\pm$0.01 and $\sigma$=0.51 dex, in good agreement with literature values. We also found that RRc are 0.10 dex more metal-poor than RRab variables, and have a distribution with a smoother metal-poor tail. This finding supports theoretical prescriptions suggesting a steady decrease in the RRc number when moving from metal-poor to metal-rich stellar environments.