Bulge K and M giants

Abstract

Several independent studies of the abundances and kinematics of K and M giants in the inner Galaxy (R < 2 kpc) are assembled to trace out this region's global properties. The mean metal abundance is [M/H] ≈ +0.3 at R = 0.5 kpc, and declines by about 1.2 dex out to R = 2 kpc. The line-of-sight velocity dispersion at R = 0.5 kpc is σr ≈ 115 km sec—1 for all population tracers, and declines by d log σr/d log R = —0.4. It now seems fairly clear that only the most metal-poor K giants become RR Lyrae variables, while the more metal-rich ones become late M stars. There is some evidence that the most metal-rich stars are in a flatter, more rapidly rotating system. Metal abundance ratios for a few K giants suggest that the inner Galaxy may have formed rapidly.