Evolutionary Oddities in Old Disk Population Clusters

Abstract

With a luminosity zero point fixed by the kinematics of old disk superclusters (HR 1614, t = 6 Gyr, [Fe/H] = +0.1 dex) and groups (Arcturus, t = 14 Gyr, [Fe/H] = -0.65 dex), the luminosities and colors of evolved old disk stars, especially red horizontal branch (RHB), early asymptotic branch [AGB(1)], thermally pulsing asymptotic giant branch [AGB(2)], and sdOB stars in old disk clusters (NGC 6791, 47 Tuc, M71, M67, Mel 66, NGC 2420, NGC 2204, and NGC 2443) are discussed. (1) The RHB stars in the old disk all have MV = +0.7 ± 0.1 (MK = -1.3 ± 0.1) mag. (2) Large-amplitude red variables (LARVs) with quasi-stable periods and light curves are old disk stars on AGB(2). (3) AGB(1) objects include CH stars and semiregular (SRa) variables. (4) The populous and overabundant cluster NGC 6791 may be the only disk cluster with sdOB stars, populating the lower portion of the bifurcated extended horizontal branch that is usual in most blue tailed and high-density halo clusters. (5) Post–red giant branch (RGB) stars in old disk clusters show a B - V (b - y) defect when compared with RGB stars, possibly because of a change in the character of the atmospheres. (6) If the bulk of the LARVs are pulsating in the fundamental mode, R Vir (P = 145 days) is possibly a first-overtone pulsator. (7) The overabundant old disk clusters are within the solar circle, with Liller 1 being at the Galactic center. (8) Several probable RHB stars at the south Galactic pole are identified. (9) The period-age relation, combined with the known spatial distribution of Galactic LARVs, leads to a relation between age and scale height of distribution that monotonically increases with age, leaving no obvious reason for a bifurcation of the population.