Discovery of carbon stars in the Draco dwarf spheroidal galaxy

Abstract

A grating prism survey of the Draco dwarf spheroidal galaxy that is 97% areally complete has led to the discovery of three carbon stars. Membership of these stars in Draco is firmly established from luminosity, proper motion, and radial velocity considerations. Optical and preliminary infrared photometry suggest that the Draco carbon stars are more closely related to the CH stars in omega Cen than to the luminous carbon stars found in the Fornax and Carina dwarfs. One of the carbon stars possesses an unusual emission line spectrum which might be indicative of either a very hot degenerate companion or a preplanetary evolutionary stage. Carbon stars have now been located in all four of the dwarf spheroidals that have been examined using transmission grating prism techniques. The rarity of these stars in galactic globulars, systems with which the dwarf spheroidals are often compared, indicates a fundamental population difference whose cause is not yet fully understood.