EDP Sciences
Free Access
Volume 493, Number 3, January III 2009
Page(s) 1149 - 1154
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361:200810038
Published online 20 November 2008

A&A 493, 1149-1154 (2009)
DOI: 10.1051/0004-6361:200810038

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

B. Zuckerman1 and I. Song2

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

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 ($T_{\rm eff} \la$ 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 ${M}_{\rm J}$) 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 ${M}_{\rm J}$ to 79 ${M}_{\rm J}$) – the number of companion brown dwarfs is proportional to the mass to the -1.2$\pm$0.2 power.

Key words: stars: planetary systems -- stars: low-mass, brown dwarfs

© ESO 2009