Metal-Rich RRc stars in the Carnegie RR Lyrae Survey

TL;DR

This study uses a stacking method to analyze low S/N spectra from the Carnegie RR Lyrae Survey, revealing a wide range of metallicities in RRc stars and distinguishing them from delta Scuti stars through heavy element overabundances.

Contribution

The paper introduces a spectroscopic discriminant based on heavy element overabundance to differentiate RRc stars from delta Scuti stars and extends metallicity measurements for RRc stars.

Findings

RRc stars have metallicities from [Fe/H] ~ -2.5 to -0.5.

Heavy element overabundance distinguishes delta Scuti stars from RRc stars.

Metal-rich RRc stars are as common as metal-rich RRab stars in the Galactic field.

Abstract

We describe and employ a stacking procedure to investigate abundances derived from the low S/N spectra obtained in the Carnegie RR Lyrae Survey (CARRS; Kollmeier et al. 2013). We find iron metallicities that extend from [Fe/H] ~ -2.5 to values at least as large as [Fe/H] ~ -0.5 in the 274-spectrum CARRS RRc data set. We consider RRc sample contamination by high amplitude solar metallicity delta Scuti stars (HADS) at periods less than 0.3 days, where photometric discrimination between RRc and delta Scuti stars has proven to be problematic. We offer a spectroscopic discriminant, the well-marked overabundance of heavy elements, principally [Ba/H], that is a common, if not universal, characteristic of HADS of all periods and axial rotations. No bona fide RRc stars known to us have verified heavy-element overabundances. Three out of 34 stars in our sample with [Fe/H] > -0.7 exhibit anomalously strong features of Sr, Y, Zr, Ba, and many rare earths. However, carbon is not enhanced in these three stars, and we conclude that their elevated n-capture abundances have not been generated in interior neutron-capture nucleosynthesis. Contamination by HADS appears to be unimportant, and metal-rich RRc stars occur in approximately the same proportion in the Galactic field as do metal-rich RRab stars. An apparent dearth of metal-rich RRc is probably a statistical fluke. Finally we show that RRc stars have a similar inverse period-metallicity relationship as has been found for RRab stars.