Common Origin of Two RR Lyrae Populations and the Double Red Clump in the Milky Way Bulge

Abstract

A recent survey toward the Milky Way bulge has discovered two sequences of RR Lyrae stars on the period-amplitude diagram with a maximum period-shift of Δlog P = 0.015 between the two populations. Here we show, from our synthetic horizontal-branch models, that this period-shift is most likely due to the small difference in helium abundance (ΔY = 0.012) between the first and second-generation stars (G1 and G2), as is the case in our models for the inner halo globular clusters with similar metallicity ([Fe/H] = -1.1). We further show that the observed double red clump (RC) in the bulge is naturally reproduced when these models are extended to solar metallicity following ΔY/ΔZ = 6 for G2, as would be expected from the chemical evolution models. Therefore, the two populations of RR Lyrae stars and the double RC observed in the bulge appear to be different manifestations of the same multiple population phenomenon in the metal-poor and metal-rich regimes respectively.

Accepted for publication in ApJ

References in corpus (6)

• Formation and Dynamical Evolution of Multiple Stellar Generations in Globular Clusters
• NGC 6441: another indication for a very high helium content in Globular Cluster stars
• CNO abundances in the globular clusters NGC 1851 and NGC 6752
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• Masses of RR Lyrae Stars with Different Chemical Abundances in the Galactic Field
• Multiple populations in globular clusters: Unified efforts from stellar evolution and chemical evolution models
• Distribution of red clump stars does not support the X-shaped Galactic bulge
• Are the double-mode bulge RR Lyrae stars with identical period-ratios the relic of a disrupted stellar system?