Unmixing the Galactic Halo with RR Lyrae tagging
  We show that tagging RR Lyrae stars according to their location in the
period-amplitude diagram can be used to shed light on the genesis of the
Galactic stellar halo. The mixture of RR Lyrae of ab type, separated into
classes along the lines suggested by Oosterhoff, displays a strong and coherent
evolution with Galactocentric radius. The change in the RR Lyrae composition
appears to coincide with the break in the halo's radial density profile at ~25
kpc. Using simple models of the stellar halo, we establish that at least three
different types of accretion events are necessary to explain the observed RRab
behavior. Given that there exists a correlation between the RRab class fraction
and the total stellar content of a dwarf satellite, we hypothesize that the
field halo RRab composition is controlled by the mass of the progenitor
contributing the bulk of the stellar debris at the given radius. This idea is
tested against a suite of cosmological zoom-in simulations of Milky Way-like
stellar halo formation. Finally, we study some of the most prominent stellar
streams in the Milky Way halo and demonstrate that their RRab class fractions
follow the trends established previously.
