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

Abstract

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.