Low mass stars and brown dwarfs in open clusters

This thesis describes several open cluster searches for very low mass stars (VLM) and brown dwarfs (BD) (M/M? < 0.08).;With recent developments in charged coupled devices, it has become possible to survey open clusters to well into the BD regime. The principal aim of this work is to employ these new instruments to conduct such open cluster surveys, to investigate the properties of VLM and BD populations.;The thesis begins with a short introduction, which describes the fundamental properties of brown dwarfs, relates research in this field to other areas of astronomy, and summarises the project aims and thesis structure. The second chapter gives a review of the current theoretical and observational status of the field, and the third chapter describes the principles and methods used to carry out the photometry presented in this thesis.;The results are presented in chapters (4), (5), (6) and (7). An RIZ survey of Praesepe produces a sample of 26 candidate VLM and BDs. K band photometric follow up of 11 of these highlights one as a binary system that almost certainly contains at least one brown dwarf. K photometry is still needed for the remaining 15. The RIZ survey also finds a possible steepening of Praesepe's mass function into the BD regime. The Praesepe infrared sequence compares very well with the latest non gray model predictions, and the binary fraction in Praesepe is found to be essentially the same as for The Pleiades.;A catalogue of Pleiades stars is used to model the cluster. King profiles are plotted for 4 mass bins and these profiles suggest that the cluster is relaxed. Virial analysis is then used to show that the cluster contains not more than 131M of dark matter (BDs) to add to the observed mass of 735M.;Further deep surveys in The Pleiades, Praesepe and the Hyades result in 88 new Pleiades BD candidates, 141 Praesepe VLM candidates and 33 Hyades BD candidates. These new candidate lists, as well as finder charts are given in the appendices.