Chemical Abundances in the Leading Arm of the Magellanic Stream
  The Leading Arm (LA) of the Magellanic Stream is a vast debris field of H I
clouds connecting the Milky Way and the Magellanic Clouds. It represents an
example of active gas accretion onto the Galaxy. Previously only one chemical
abundance measurement had been made in the LA. Here we present chemical
abundance measurements using Hubble Space Telescope/Cosmic Origins Spectrograph
Green Bank Telescope spectra of four sightlines passing through the LA, and
three nearby sightlines that may trace outer fragments of the LA. We find low
oxygen abundances, ranging from 4.0(+4.0,-2.0) percent solar to 12.6(+6.2,-4.1)
percent solar, in the confirmed LA directions, with the lowest values found in
the region known as LA III, farthest from the LMC. These abundances are
substantially lower than the single previous measurement, S/H=35+/-7 percent
solar (Lu et al. 1998), but are in agreement with those reported in the SMC
filament of the trailing Stream, supporting a common origin in the SMC (not the
LMC) for the majority of the LA and the trailing Stream. This provides
important constraints for models of the formation of the Magellanic System.
Finally, the HVCs in two of the three nearby sightlines show H I columns,
kinematics, and oxygen abundances consistent with LA membership. This suggests
that the LA is larger than traditionally thought, extending at least 20 degrees
further to the Galactic northwest.
