On the origin of brown dwarfs and free-floating planetary mass objects

Abstract

(abridged) Sub-stellar mass objects have fundamentally different binary properties than stars implying that they cannot have the same formation history. They are rather unfinished stars having either been ejected by their siblings from compact groups of accreting hydrostatic cores, or having lost their accretion envelopes through photo-evaporation by nearby O stars or through hyperbolic encounters with other accreting cores in their birth clusters. All three scenarios are studied with a view to understanding the observed smaller number of brown dwarfs (BDs) per star in the Taurus star-forming region as opposed to the larger number of BDs per star seen in the Orion Nebula cluster. It is found that both environments can produce the same number of ejected BDs per star if the true production rate is about one BD per four stars and the ejection velocity is about 2 km/s. This production rate is much lower than the number of BDs per star inferred for the Galactic field. Adding photo-evaporated and collisional BDs cannot reconcile this difference. The only possible solution would be to postulate that tranquil star-formation such as in Taurus is producing about one BD per star with ejection velocities of about 3 km/s, while Orion-Nebula-type clusters produce at most about one BD per three stars. Unless the Galactic-field census of BDs has been overestimated, this then would imply that low-mass star-forming regions may be the primary source of the field BDs, rather than rich clusters.