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.