Lithium-rich stars in the Sloan Digital Sky Survey

We report the discovery of 23 lithium-rich post-main-sequence stars, identified from moderate-resolution SDSS spectroscopy and confirmed with high-resolution spectra taken at the Hobby-Eberly Telescope. These new Li-rich stars cover a broad range in mass and evolutionary phase, including bright giants and post-AGB stars. The process responsible for preserving or producing excess lithium in a small fraction of evolved stars remains unclear.Comment: 5 pages, XII International Symposium on Nuclei in the Cosmos, August 5-12, 2012, Cairns, Australia. To appear in Proceedings of Scienc

Comparing the Ca II H and K Emission Lines in Red Giant Stars

Measurements of the asymmetry of the emission peaks in the core of the Ca II H line for 105 giant stars are reported. The asymmetry is quantified with the parameter V/R, defined as the ratio between the maximum number of counts in the blueward peak and the redward peak of the emission profile. The Ca II H and K emission lines probe the differential motion of certain chromospheric layers in the stellar atmosphere. Data on V/R for the Ca II K line are drawn from previous papers and compared to the analogous H line ratio, the H and K spectra being from the same sets of observations. It is found that the H line V/R value is +0.04 larger, on average, than the equivalent K line ratio, however, the difference varies with B-V color. Red giants cooler than B-V = 1.2 are more likely to have the H line V/R larger than the K line V/R, whereas the opposite is true for giants hotter than B-V = 1.2. The differences between the Ca II H and K line asymmetries could be caused by the layers of chromospheric material from which these emission features arise moving with different velocities in an expanding outflow.Comment: 36 pages, 12 figures, 2 tables. Accepted to PASP. Corrected a typo in Table

The Nature of the Red Giant Branches in the Ursa Minor and Draco Dwarf Spheroidal Galaxies

Spectra for stars located redward of the fiducial red giant branches of the Ursa Minor and Draco dwarf spheroidal galaxies have been obtained with the Hobby-Eberly telescope and the Marcario Low Resolution Spectrometer. From a comparison of our radial velocities with those reported in previous medium-resolution studies, we find an average difference of 10 km/s with a standard deviation of 11 km/s. On the basis of these radial velocities, we confirm the membership of five stars in Ursa Minor, and find two others to be nonmembers. One of the confirmed members is a known carbon star which lies redward of RGB; three others are previously unidentified carbon stars. The fifth star is a red giant which was found previously by Shetrone et al. (2001) to have [Fe/H] =-1.68+/-0.11 dex. In Draco, we find eight nonmembers, confirm the membership of one known carbon star, and find two new members. One of these stars is a carbon star, while the other shows no evidence for C2 bands or strong atomic bands, although the signal-to-noise ratio of the spectrum is low. Thus, we find no evidence for a population of stars more metal-rich than [Fe/H] \~ -1.45 dex in either of these galaxies. Indeed, our spectroscopic survey suggests that every candidate suspected of having a metallicity in excess of this value based on its position in the color-magnitude diagram is, in actuality, a carbon star. Based on the census of 13 known carbon stars in these two galaxies, we estimate of the carbon star specific frequency to be e(dSph) ~ 2.4E-5/Lsolarv, 25-100 times higher than that of Galactic globular clusters.Comment: 8 pages including 3 figures accepted in the Publications of the Astronomical Society of the Pacific. This work is based on observations obtained with the Hobby-Eberly Telescop

Chemical Abundances of the Leo II Dwarf Galaxy

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 dSph galaxy. We find that for spectra with SNR > 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 RGB 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.Comment: Accepted to A