The Chemical Evolution of the Monoceros Ring/Galactic Anticenter Stellar
  Structure
  The origin of the Galactic Anticenter Stellar Structure (GASS) or "Monoceros
ring" --- a low latitude overdensity at the edge of the Galactic disk spanning
at least the second and third Galactic quadrants --- remains controversial.
Models for the origin of GASS fall generally into scenarios where it is either
a part (e.g., warp) of the Galactic disk or it represents tidal debris from the
disruption of a Milky Way (MW) satellite galaxy. To further constrain models
for the origin of GASS, we derive chemical abundance patterns from high
resolution spectra for 21 M giants spatially and kinematically identified with
it. The abundances of the (mostly) $\alpha$ element titanium and s-process
elements yttrium and lanthanum for these GASS stars are found to be lower at
the same [Fe/H] than those for MW stars, but similar to those of stars in the
Sagittarius stream, other dwarf spheroidal galaxies, and the Large Magellanic
Cloud. This demonstrates that GASS stars have a chemical enrichment history
typical of dwarf galaxies --- and unlike those of typical MW stars (at least MW
stars near the Sun). Nevertheless, these abundance results cannot definitively
rule out the possibility that GASS was dynamically created out of a previously
formed, outer MW disk because $\Lambda$CDM-based structure formation models
show that galactic disks grow outward by accretion of dwarf galaxies. On the
other hand, the chemical patterns seen in GASS stars do provide striking
verification that accretion of dwarf galaxies has indeed happened at the edge
of the MW disk.
