Review on low-mass stars and brown dwarfs

In this review, we examine the successes and weaknesses of modern low-mass star and brown dwarf theory.(1) We first focus on the mechanical (equation of state) and thermal (atmosphere) properties and on the evolution. We then examine the current shortcomings of the theory and we discuss recent observational analysis which have suggested discrepancies between models and observations.(2) We then examine the stellar and brown dwarf IMF and suggest that a power-law above the average thermal Jeans mass (about 1 Msol) rolling over a lognormal form below this limit adequately reproduces the observations of field and young cluster stellar and brown dwarf distributions. This yields a reasonably accurate estimate of the stellar and brown dwarf Galactic census. Finally (3) we argue that the combination of turbulence driven fragmentation at large scale and gravity at small scales provides an appealing solution for the general star and brown dwarf formation mechanism. It also provides a physical ground for the aforementioned power-law + lognormal form for the IMF, whereas a series of different power laws lacks such a physical motivation. At last, we argue that the deuterium-burning limit as the distinction between stars and planets has no physical foundation in this modern star formation scheme. Opacity limited fragmentation extending down to a few (< 10) jupiter masses, due to shocks, anisotropy or magnetic fields, provides a much more robust limit, even though difficult to determine accurately. Therefore, the various "direct" detections of exoplanets claimed recently in the literature are most likely regular low-mass brown dwarfs and the direct detection of an extrasolar planet remains for now elusive.