The Dynamical Distance, RR Lyrae Absolute Magnitude, and Age of the Globular Cluster NGC 6266

Abstract

The internal proper motion dispersion of NGC 6266 was measured using Hubble Space Telescope Wide Field Planetary Camera 2 images with an epoch difference of eight years. The dispersion was found to be 0.041 ± 0.001 arcsec century ^-1. This value was then equated to the cluster's radial velocity dispersion of 13.7 ± 1.1 km s^-1 to yield a distance to NGC 6266 of 7054 ± 583 pc. Based on this distance we find that the NGC 6266 RR Lyrae stars have M_V = 0.51 ± 0.18 mag. This magnitude is in good agreement with that predicted by the M_V versus [Fe/H] relation found by Benedict et al. Using an average [Fe/H] of -1.25 for NGC 6266, their relation predicts M_V = 0.49 ± 0.06. Based on the RR Lyrae M_V versus age relation determined by Chaboyer et al., we estimate that NGC 6266 has an age of 11.4 ± 2.2 Gyr.