Proper Motions of Very Low Mass Stars and Brown Dwarfs in Open Clusters

Summary

Open clusters are a rich source of very low mass stars and brown dwarfs of a single known metallicity, age, and distance. Proper motion surveys enable candidate members within these clusters to be identified with a reasonably high degree of confidence. The nearby clusters are therefore a challenging test-bed for the latest evolutionary models of these ellusive objects. In this talk, I will review the progress that has been made recently in pushing proper motion surveys through very low mass ranges into the substellar régime, and I examine the prospects for extending these surveys to other clusters and to lower masses.

Introduction

Open clusters provide the astronomer with a rich source of objects for studying stellar structure over the full mass range of stable, hydrogen burning stars; furthermore, stellar evolution can be studied as the higher mass stars evolve away from, and as the low mass stars contract onto, the main sequence. Moreover, open cluster studies of objects that have too low a mass to stabilise on the hydrogen burning main sequence (i.e. brown dwarfs) have recently come of age, so now it is possible to study the physics of coeval objects having masses ranging over three orders of magnitude (and luminosities over eight orders of magnitude). Properties of very low mass (VLM) stars being studied in open clusters include lithium evolution, angular momentum evolution, spotting and variability, choronal activity, the binary fraction, and, most fundamentally, the mass function.