SHETRONE M.D., SIEGEL M.H., COOK D.O. and BOSLER T.
Abstract (from CDS):
We use previously published moderate-resolution spectra in combination with stellar atmosphere models to derive the first measured chemical abundance ratios in the Leo II dwarf Spheroidal (dSph) galaxy. We find that for spectra with signal-to-noise ratio greater than 24, we are able to measure abundances from weak Ti, Fe, and Mg lines located near the calcium infrared triplet (CaT). We also quantify and discuss discrepancies between the metallicities measured from Fe I lines and those estimated from the CaT features. We find that while the most metal-poor ([Fe/H] ←2.0]) Leo II stars have Ca and Ti abundance ratios similar to those of Galactic globular clusters, the more metal-rich stars show a gradual decline of Ti, Mg, and Ca abundance ratio with increasing metallicity. Finding these trends in this distant and apparently dynamically stable dSph galaxy supports the hypothesis that the slow chemical enrichment histories of the dSph galaxies is universal, independent of any interaction with the Milky Way. Combining our spectroscopic abundances with published broadband photometry and updated isochrones, we are able to approximate stellar ages for our bright red giant branch stars to a relative precision of 2-3 Gyr. While the derived age-metallicity relationship of Leo II hints at some amount of slow enrichment, the data are still statistically consistent with no enrichment over the history of Leo II.
galaxies: dwarf - galaxies: evolution - galaxies: individual: Leo II - stars: abundances