Common Origin of Two RR Lyrae Populations and the Double Red Clump in
  the Milky Way Bulge
  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
{\Delta}log 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 ({\Delta}Y = 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
{\Delta}Y/{\Delta}Z = 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.
