|Affiliation||Institute of Astronomy, Cambridge CB 30HA UK|
|Links||Pleiades / Blanco 1 / NGC 2516|
My thesis work deals with the initial mass function (IMF), especially in the brown dwarf regime which was still unexplored a few years ago. This function describes the number of formed stars per mass bin in a given environment and is therefore directly linked to the stellar and substellar formation processes. However the theoretical framework is far from being satisfactory: the brown dwarf formation mecanism is still discussed and the origin of the IMF remains unknown. In order to improve this situation, I determined the mass function of three young open clusters around the stellar/substellar boundary. This allows me to investigate the IMF universality and to compare the observations with the various formation scenarios which have been proposed to date.
After a general introduction on the IMF and brown dwarfs, I present the results I obtained for the Pleiades cluster and for two other clusters of similar age, Blanco 1 and NGC 2516. Very low mass star and brown dwarf candidates are identfied from deep wide-field surveys on the basis of their photometric properties. Their cluster membership is then assessed thanks to other observations (photometric and spectroscopic, optical and infrared). From these samples the mass function have been determined and it appears that they are all coherent.
Then, I compare these results with those obtained for other regions having different age and density. Taking into account the cluster dynamical evolution as well as the binarity effect, I find that the observations suggest the IMF is universal in the stellar domain. At fainter masses however, differences have been reported and I discuss the impact of such a result on the theories of star formation.
Finally, I describe the different scenarios proposed to explain the brown dwarf formation and I compare their predictions to the observations. N-Body numerical simulations modelling a cluster dynamical evolution allow me to constrain the kinematic of brown dwarfs at birth. Despite this study two competing scenarios cannot be distinguished and I propose observational tests to do it.