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Session 71 - Globular Clusters.
Display session, Wednesday, January 15
We compare deep I-band luminosity functions of two globular clusters (NGC 6397 and NGC 6656) that have similar metal content, but have experienced very different dynamical histories, both in terms of internal dynamics and as a result of different interactions with the Galactic tidal field. Their luminosity functions measured at the half-mass radius are indistinguishable from one another within the statistical errors over a range of more than 5 mag. They both show an increase with decreasing luminosity up to a peak at M_I \simeq 8.5, and then drop all the way to the detection limit. We use the presently available theoretical mass-luminosity relations to convert these LFs into mass functions, and show that they all share one basic feature, namely an exponential rise with decreasing mass down to about \sim 0.20 M_ødot, followed by a plateau, irrespective of the adopted mass-luminosity relation. We conclude that an IMF that flattens out and possibly drops below \sim 0.2 M_ødot is the only viable hypothesis for the low metallicity globular clusters studied so far. At present, the study of low-mass stars in globular clusters is the best way to understand the bottom of the main sequence and, therefore, the mechanisms that determine its structure.
Program listing for Wednesday