The Hydrogen-Burning Limit in the Globular Cluster NGC 6397 {AR part}

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Hst Proposal Id #8736 Stellar Populations

Scientific paper

We propose a major enhancement of an earlier study of the bottom of the main sequence of NGC 6397, the globular cluster with the smallest distance modulus. In earlier work the lowest part of the MS had been lost among the numerically dominant field stars; but accurate astrometry, over a baseline of a few years, now allows an excellent proper-motion separation of faint cluster stars from the field. The purified CMD follows the main sequence to its ``end" {i.e., the terminal plunge of the LF}. Just as the MS CMD gives a mass-radius relation, we show in a new way how this LF can give a mass-luminosity relation; both of these offer unique checks on theory. Our single WFPC2 field, however, had only a small number of stars in this range, too few to set firm restraints on the theories. We propose now to increase the number of such stars by a large factor by {1} getting 2nd-epoch images for three more fields in the cluster and, {2} in an accompanying AR proposal, remeasuring our previous images, and others that exist, to the deeper limit that we know can be attained. The number and the magnitudes of these faintest stars will greatly strengthen the constraints that we place on structure and atmosphere theories of lower-main-sequence stars. In each field we will also measure the anisotropy of internal stellar motions, which is predicted to be large in a collapsed-core cluster such as this one. as this one.

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