The Galactic Bulge exploration III.: Calcium Triplet Metallicities for RR Lyrae Stars

TL;DR

This study calibrates calcium triplet spectroscopy to determine metallicities of RR Lyrae stars, improving accuracy and applying it to Galactic bulge stars to analyze their population characteristics.

Contribution

It provides a new calibration method for RR Lyrae metallicities using Calcium triplet spectra, enhancing the precision of spectroscopic measurements.

Findings

Gaia RVS spectra yield RR Lyrae metallicities with 0.25 dex uncertainty.

Most bulge RR Lyrae stars are low-energy, likely belonging to an in-situ metal-poor component.

The metallicity distribution suggests a dominant old, metal-poor population in the Galactic bulge.

Abstract

RR Lyrae stars are excellent tracers of stellar populations for old, metal-poor components in the Milky Way Galaxy and the Local Group. Their luminosities have a metallicity-dependence, but determining spectroscopic [Fe/H] metallicities for RR Lyrae stars, especially at distances outside the solar neighbourhood, is challenging. Using 40 RRLs with metallicities derived from both Fe(II) and Fe(I) abundances, we verify the calibration between the [Fe/H] of RR Lyrae stars from the Calcium triplet. Our calibration is applied to all RR Lyrae stars with Gaia RVS spectra in Gaia DR3 as well as to 80 stars in the inner Galaxy from the BRAVA-RR survey. The co-added Gaia RVS RR Lyrae spectra provide RR Lyrae metallicities with an uncertainty of 0.25~dex, which is a factor of two improvement over the Gaia photometric RR Lyrae metallicities. Within our Galactic bulge RR Lyrae star sample, we find a dominant fraction with low energies without a prominent rotating component. Due to the large fraction of such stars, we interpret these stars as belonging to the $in-situ$ metal-poor Galactic bulge component, although we can not rule out that a fraction of these belong to an ancient accretion event such as Kraken/Heracles.