Low-mass Stars and Brown Dwarfs

The lowest mass stars M < 0.2 Mo despite their extremely low luminosity, are significant players in the galaxy owing to their sheer numbers. The census of stars with mass near the hydrogen burning limit (M < 0.08 Mo) is incomplete at present since, even in the solar neighborhood, such stars are elusive due to their intrinsically low luminosities. Current observational constraints permit a population of these objects that may exceed the known population of stars - a possibility supported by dynamical estimates of the mass of the galactic disk. I have been involved in constraining the numbers of such objects through infrared imaging of nearby stars searching for extremely low-mass binary companions and investigating the identity of candidates through visible and infrared-wavelength spectroscopy. The advent of large-area infrared surveys like 2MASS have revolutionized this field by identifying hundreds of candidate brown dwarfs, dozens of which are so cool that methane forms in their atmosphere betraying unequivocally their sub-stellar nature.