The extended giant branch of the Andromeda II dwarf spheroidal galaxy

Abstract

One, and possibly two, carbon stars and an M giant are reported in And II, a dwarf spheroidal companion of M31. Infrared photometry indicates an extended asymptotic giant branch in And II, reaching -4.5 bolometric magnitude for m - M = 24.3. These results can be understood in terms of the third dredge-up mechanism in asymptotic giant branch (AGB) stars, if And II has spent approximately half its life as a gas-rich star-forming irregular galaxy. But the alternative possibility that these luminous late-type stars could be a consequence of binary mass transfer cannot be ruled out. In either event, the presence of AGB carbon stars may be a general characteristic of dwarf spheroidal galaxies, as they are also found in most of the spheroidal systems that surround the Milky Way.