The Luminosity Function of the Globular Cluster NGC 6397 near the Limit of Hydrogen Burning

Abstract

Second-epoch Hubble Space Telescope observations of NGC 6397 have led to the measurement of proper motions accurate enough to separate the faintest cluster stars from the field, thus extending the luminosity function of this globular cluster far enough to approach the limit of hydrogen burning on the main sequence. We isolate a sample of 1385 main-sequence stars, from just below the turnoff down to I=24.5 (M_I~=12.5), which corresponds to a mass of less than 0.1 m_solar for the metallicity of this cluster. Below I=22 (M_I~=10), the main-sequence luminosity function drops rapidly, in a manner similar to that predicted by theoretical models of low-mass stars near the hydrogen-burning limit.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.