Brown dwarfs in open clusters

This thesis presents the results of the work undertaken during the course of this PhD.;Firstly, the results of a large, deep survey of 5.65 square degrees of the Taurus Dark Clouds are presented. Using a combination of three colour-magnitude diagrams, 165 brown dwarf candidates are identified from RIZ photometry. The presence of deep TiO absorption bands and weak Halpha emission in follow-up far-optical spectra revealed 3 borderline brown dwarfs, and the presence of gravity sensitive H 2O absorption features and weak NaI lines in follow-up infrared spectra confirmed 3 more brown dwarfs.;Secondly, the results of a detailed study of brown dwarfs in the Pleiades are presented. The proper motion for 48 brown dwarf candidates is calculated to determine their membership status, with 37 being confirmed as genuine members. Theoretical models indicate that these objects have masses in the range 0.087-0.031 M⊙ (+/-0.015 M⊙ ). The effects on the IMF of the cluster are then described. In addition, infrared spectra for 12 of the candidates are presented, with all but 1 of these showing the broad H2O absorption features typical of brown dwarfs.;Finally, the results of an extremely large scale survey of the Hyades are presented. Data from the USNO and 2MASS catalogues are analysed in terms of proper motion and photometry in order to compile a complete list of the very low mass and brown dwarf cluster members. Unfortunately, no brown dwarfs are found. The core radius of the cluster is calculated for objects of typical brown dwarf mass, and their escape velocity is found to be greater than that of the cluster. Therefore any substellar objects originally formed within the Hyades would have since been ejected.