Photometry and spectroscopy of low mass stars and brown dwarfs.

This thesis describes an investigation of the faintest known proper motion members of the Pleiades star cluster. The primary aim of the work is an attempt to determine whether the lowest mass members of the cluster are brown dwarfs. A brief introduction outlines the fundamental properties of brown dwarfs and discusses the relation of research in this area to other branches of astronomy. The second chapter describes the basic principles of astronomical photometry and spectroscopy, the instruments used and the methods of data reduction. Chapter three gives a review of the current theory and observations of brown dwarfs. The results are presented in chapters four, five and six. RIJHK photometry for a sample of 62 objects is presented. The 1,1 - K diagram is shown to be a good discriminant of binary status for low mass cluster objects and using this diagram two single brown dwarf candidates and seven potential brown dwarf containing binaries are identified. FOS optical spectroscopy of 31 objects is used in an investigation of temperature indicators for cool stars. In addition gravity sensitive features indicate that the surface gravity of the Pleiades objects is lower than equivalent main sequence objects. Also discussed are the chromospheric and coronal activity of cluster members, and evidence is presented showing a turnover in both at ~ 0.3M⊕, where the objects become fully convective. CGS4 1-2.5 micron spectra are used in a futher investigation of methods of determining effective temperature and in searching for evidence of low mass companions. One object shows evidence for a low mass companion of ~ 0.035M⊕. Chapter seven discusses the evidence from these results that the faintest members observed are brown dwarfs and evidence from other workers that they are not. This chapter also gives suggestions for future work. Appendices compile the theoretical models and data used in the project.