BVI Photometry and the Luminosity Functions of the Globular Cluster M92

We present new BVI ground-based photometry and VI space-based photometry for the globular cluster M92 (NGC 6341) and examine luminosity functions in B, V, and I containing over 50,000 stars ranging from the tip of the red giant branch to several magnitudes below the main-sequence turnoff. Once corrected for completeness, the observed luminosity functions agree very well with theoretical models and do not show stellar excesses in any region of the luminosity function. Using reduced-χ² fitting, the new M92 luminosity function is shown to be an excellent match to the previously published luminosity function for M30. These points combine to establish that the "subgiant excess" found in previously published luminosity functions of Galactic globular clusters is due to deficiencies in the stellar models used at that time. Using up-to-date stellar models results in good agreement between observations and theory. Several statistical methods are presented to best determine the age of M92. These methods prove to be insensitive to the exact choice of metallicity within the published range. Using [Fe/H] = -2.17 to match recent studies we find an age of 14.2 ± 1.2 Gyr for the cluster.