Probing the Faintest Stars in a Globular Star Cluster

Abstract

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.

arXiv comment12 pages, 4 figures. Full Resolution Figures in Science, 2006, 313, 936

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• Comparing the White Dwarf Cooling Sequences in 47 Tuc and NGC 6397
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• White dwarf atmosphere models with Ly-$\alpha$ opacity in the analysis of the white dwarf cooling sequence of NGC 6397
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• The M4 Core Project with HST --- I. Overview and First-Epoch
• Astrophysics in 2006
• Impact of distance determinations on Galactic structure. II. Old tracers
• The Number of Possible CETIs within Our Galaxy and the Communication Probability among These CETIs
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• A Glimpse into the Past: The Recent Evolution of Globular Clusters
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• The Near-Infrared Sky Surveyor