Gauging the Helium Abundance of the Galactic Bulge RR Lyrae Stars
  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.
