A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities
  The Monoceros Ring (also known as the Galactic Anticenter Stellar Structure)
and A13 are stellar overdensities at estimated heliocentric distances of $d
\sim 11$ kpc and 15 kpc observed at low Galactic latitudes towards the
anticenter of our Galaxy. While these overdensities were initially thought to
be remnants of a tidally-disrupted satellite galaxy, an alternate scenario is
that they are composed of stars from the Milky Way (MW) disk kicked out to
their current location due to interactions between a satellite galaxy and the
disk. To test this scenario, we study the stellar populations of the Monoceros
Ring and A13 by measuring the number of RR Lyrae and M giant stars associated
with these overdensities. We obtain low-resolution spectroscopy for RR Lyrae
stars in the two structures and measure radial velocities to compare with
previously measured velocities for M giant stars in the regions of the
Monoceros Ring and A13, to assess the fraction of RR Lyrae to M giant stars
($f_{RR:MG}$) in A13 and Mon/GASS. We perform velocity modeling on 153 RR Lyrae
stars (116 in the Monoceros Ring and 37 in A13) and find that both structures
have very low $f_{RR:MG}$. The results support a scenario in which stars in A13
and Mon/GASS formed in the MW disk. We discuss a possible association between
Mon/GASS, A13, and the Triangulum-Andromeda overdensity based on their similar
velocity distributions and $f_{RR:MG}$.
