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

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-eff less than or similar to 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 (similar to 13.5 M-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 <2Gyr 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-J to 79 M-J) - the number of companion brown dwarfs is proportional to the mass to the -1.2 +/- 0.2 power.