Very-Low-Mass Stars in Globular Clusters

Summary

We discuss the low-mass ends of mass functions in globular clusters, and extrapolate them to estimate the number of brown dwarfs. Although the brown dwarfs can be quite numerous, they probably contain only a small fraction of the mass of a cluster. We show how the mass function can be pursued observationally down close to the hydrogen-burning limit, and how these observations can be used to derive an empirical mass-luminosity relation for this region. We mention briefly a projected microlensing observation that may actually reveal the presence of brown dwarfs in one cluster.

Introduction

This paper has three parts. First will be an estimate of how many brown dwarfs there ought to be in globular clusters, by following their observed mass functions as close as possible to the hydrogen-burning limit, and then naïvely extrapolating the mass function beyond that. Next will be a discussion of the H-burning limit and how we can try to locate it observationally, by pushing luminosity functions as faint as possible. This part will conclude with a demonstration of how the observations can guide the theoreticians toward more accurate models in that region, by telling us something about how the MLR must go. And finally we will give a brief description of a microlensing experiment that some one else has underway, that may actually tell us how many brown dwarfs one particular globular cluster contains.