New preprint :: NGC 6397
Probing the Faintest Stars in a Globular Star Cluster
Authors: Harvey B. Richer, Jay Anderson, James Brewer, Saul Davis, Gregory G. Fahlman, Brad M.S. Hansen, Jarrod Hurley, Jasonjot S. Kalirai, Ivan R. King, David Reitzel, R. Michael Rich, Michael M. Shara, Peter B. Stetson
Comments: 12 pages, 4 figures. Full Resolution Figures in Science, 2006, 313, 936
Journal-ref: Science (2006), 313, 936
NGC 6397 is the second closest globular star cluster to the Sun. Using 5 days of time on the Hubble Space Telescope, we have constructed the deepest ever color-magnitude diagram for this cluster. We see a clear truncation in each of its two major stellar sequences. Faint red main sequence stars run out well above our observational limit and near to the theoretical prediction for the lowest mass stars capable of stable hydrogen-burning in their cores. We also see a truncation in the number counts of faint blue stars, namely white dwarfs. This reflects the limit to which the bulk of the white dwarfs can cool over the lifetime of the cluster. There is also a turn towards bluer colors in the least luminous of these objects. This was predicted for the very coolest white dwarfs with hydrogen-rich atmospheres as the formation of H2 causes their atmospheres to become largely opaque to infrared radiation due to collision-induced absorption.