Infrared and optical studies of cool low-mass dwarfs

Infrared and optical studies of cool low-mass dwarfs are made in order to determine their properties. The goal of distinguishing low-mass stars from brown dwarfs is engaged by studies on a number of different fronts.(1) The first infrared spectral sequence of cool dwarfs from GL411 (M2V) to GD165B (>M 10V) enables derivation of reliable bolometric luminosities down to the hydrogen burning limit. The strength of the deep water absorption bands is used as the basis of a new reliable method to calculate effective temperatures. We find that GD165B is the only star in the sample that is a good brown dwarf candidate.(2) Expected temperatures, metallicities and gravities of cool dwarfs are compared to inferences from fitting synthetic to observed spectra at the peak of the energy dis­tribution. A good representation of the overall spectral features is found, although the study is limited by (i) the quality of atomic and molecular opacities and (ii) the complex behaviour of atomic lines having strengths which increase with decreasing metallicity down to around -1.5 dex. The comparisons suggest a similar spread in metallicities to that anticipated, although for our sample neither kinematic motion nor membership of a particular photometric class are, on their own, reliable indicators of metallicity.(3) A high resolution study of the strongest w ater vapour absorption band in cool dwarfs is compared to predictions from a preliminary ab initio com putation for water vapour incorporated into a stellar atm osphere calculation. The comparisons show th a t w ater vapour lines are formed relatively high in the photosphere at pressures about an order of magnitude lower than atomic lines. The overestimated pressure broadening of w ater vapour, and probably other molecular lines, is likely to explain much of the past discrepancy between observed and theoretical spectral energy distributions.(4) The discovery is reported of the reddest known brown dwarf candidate. Classi­fication as a star or a brown dwarf depends on the adopted age and the evolutionary model used. Its discovery within an infrared field survey places the first lower limit on the space density for objects fainter than Mbol = 14 and suggests that the luminosity function does not fall dramatically into the brown dwarf regime