The Metallicity of the Monoceros Stream
  We present low-resolution MMT Hectospec spectroscopy of 594 candidate
Monoceros stream member stars. Based on strong color-magnitude diagram
overdensities, we targeted three fields within the stream's footprint, with 178
deg < l < 203 deg and -25 deg < b < 25 deg. By comparing the measured iron
abundances with those expected from smooth Galactic components alone, we
measure, for the first time, the spectroscopic metallicity distribution
function for Monoceros. We find the stream to be chemically distinct from both
the thick disk and halo, with [Fe/H] = -1, and do not detect a trend in the
stream's metallicity with Galactic longitude. Passing from b = +25 deg to b =
-25 deg the median Monoceros metallicity trends upward by 0.1 dex, though
uncertainties in modeling sample contamination by the disk and halo make this a
marginal detection. In each field, we find Monoceros to have an intrinsic
[Fe/H] dispersion of 0.10-0.22 dex. From the CaII K line, we measure [Ca/Fe]
for a subsample of metal poor program stars with -1.1 < [Fe/H] < -0.5. In two
of three fields, we find calcium deficiencies qualitatively similar to
previously reported [Ti/Fe] underabundances in Monoceros and the Sagittarius
tidal stream. Further, using 90 spectra of thick disk stars in the Monoceros
pointings with b ~ +/-25 deg, we detect a 0.22 dex north/south metallicity
asymmetry coincident with known stellar density asymmetry at R_GC ~ 12 kpc and
|Z| ~ 1.7 kpc. Our median Monoceros [Fe/H] = -1.0 and its relatively low
dispersion naturally fit the expectation for an appropriately luminous M_V ~
-13 dwarf galaxy progenitor.
