The minimum Jeans mass, brown dwarf companion IMF, and predictions for detection of Y-type dwarfs

¹ Dept. of Physics & Astronomy and Center for Astrobiology, University of California, Los Angeles, 475 Portola Plaza, Los Angeles, CA 90095–1547, USA e-mail: ben@astro.ucla.edu
² Department of Physics & Astronomy, The University of Georgia, Athens, GA 30605, USA e-mail: song@physast.uga.edu

Received: 23 April 2008
Accepted: 14 November 2008

Abstract

Cool L- and T-type objects were discovered first as companions to stars in 1988 and 1995, respectively. A certain example of the even cooler Y-type spectral class ( 500 K) has not been seen. Recent infrared-imaging observations of stars and brown dwarfs indicate that substellar companions with large semi-major axes and with masses less than the brown dwarf/giant planet dividing line (~13.5) are rare. Theoretical considerations of the Jeans mass fragmentation of molecular clouds are consistent with this minimum mass cutoff and also with the semi-major axis (hundreds of AU) characteristic of the lowest mass imaged companions. As a consequence, Y-class companions with large semi-major axes should be scarce around stars <2 Gyr old, and also around substellar primaries of all ages. By focusing on brown dwarf companions to young stellar primaries, it is possible to derive a first estimate of the brown dwarf IMF over the entire range of brown dwarf masses (13 to 79) – the number of companion brown dwarfs is proportional to the mass to the -1.2±0.2 power.