Detailed Abundance Analysis of the Brightest Star in Segue 2, the Least
  Massive Galaxy
  We present the first high resolution spectroscopic observations of one red
giant star in the ultra-faint dwarf galaxy Segue 2, which has the lowest total
mass (including dark matter) estimated for any known galaxy. These observations
were made using the MIKE spectrograph on the Magellan II Telescope at Las
Campanas Observatory. We perform a standard abundance analysis of this star,
SDSS J021933.13+200830.2, and present abundances of 21 species of 18 elements
as well as upper limits for 25 additional species. We derive [Fe/H] = -2.9, in
excellent agreement with previous estimates from medium resolution
spectroscopy. Our main result is that this star bears the chemical signatures
commonly found in field stars of similar metallicity. The heavy elements
produced by neutron-capture reactions are present, but they are deficient at
levels characteristic of stars in other ultra-faint dwarf galaxies and a few
luminous dwarf galaxies. The otherwise normal abundance patterns suggest that
the gas from which this star formed was enriched by metals from multiple Type
II supernovae reflecting a relatively well-sampled IMF. This adds to the
growing body of evidence indicating that Segue 2 may have been substantially
more massive in the past.
