We present an abundance analysis based on high resolution spectra of 8 stars
selected to span the full range in metallicity in the Draco dwarf spheroidal
galaxy. We find [Fe/H] for the sample stars ranges from -1.5 to -3.0 dex.
Combining our sample with previously published work for a total of 14 luminous
Draco giants, we show that the abundance ratios [Na/Fe], [Mg/Fe] and [Si/Fe]
for the Draco giants overlap those of Galactic halo giants at the lowest [Fe/H]
probed, but are significantly lower for the higher Fe-metallicity Draco stars.
For the explosive alpha-elements Ca and Ti, the abundance ratios for Draco
giants with [Fe/H] > -2.4 dex are approximately constant and slightly
sub-solar, well below values characteristic of Galactic halo stars. The
s-process contribution to the production of heavy elements begins at
significantly lower Fe-metallicity than in the Galactic halo.
Using a toy model we compare the behavior of the abundance ratios within the
sample of Draco giants with those from the literature of Galactic globular
clusters, and the Carina and Sgr dSph galaxies. The differences appear to be
related to the timescale for buildup of the heavy elements, with Draco having
the slowest rate.
We note the presence of a Draco giant with [Fe/H] < -3.0 dex in our sample,
and reaffirm that the inner Galactic halo could have been formed by early
accretion of Galactic satellite galaxies and dissolution of young globular
clusters, while the outer halo could have formed from those satellite galaxies
accreted later.Comment: Submitted to ApJ Dec 22, 2008; accepted June 4, 2009. 75 pages
including 22 figures and 9 table
