Gauging the Helium Abundance of the Galactic Bulge RR Lyrae Stars

TL;DR

This study estimates the helium abundance of RR Lyrae stars in the Galactic bulge using observational data and new models, finding they have primordial helium levels unlike the more helium-rich red clump giants.

Contribution

First estimate of helium content in bulge RR Lyrae stars using observational data and updated models, revealing primordial helium levels.

Findings

Bulge RR Lyrae stars have primordial helium abundance Y=0.245.

Most bulge RR Lyrae are consistent with primordial He, not enriched.

Red clump giants in the bulge are more helium-rich than RR Lyrae.

Abstract

We report the first estimate of the He abundance of the population of RR Lyrae stars in the Galactic bulge. This is done by comparing the recent observational data with the latest models. We use the large samples of ab type RR Lyrae stars found by OGLE IV in the inner bulge and by the VVV survey in the outer bulge. We present the result from the new models computed by Marconi et al. (2017), showing that the minimum Period for fundamental RR Lyrae pulsators depends on the He content. By comparing these models with the observations in a Period versus effective temperature plane, we find that the bulk of the bulge ab type RR Lyrae are consistent with primordial He abundance Y=0.245, ruling out a significant He-enriched population. This work demonstrates that the He content of the bulge RR Lyrae is different from that of the bulk of the bulge population as traced by the red clump giants, that appear to be significantly more He-rich.