Proto--Brown Dwarfs. I. Methods and Results for High-Latitude Clouds

Details Proto--Brown Dwarfs. I. Methods and Results for High-Latitude Clouds Proto--Brown Dwarfs. I. Methods and Results for High-Latitude Clouds

Nov 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...418..328p&link_type=abstract

Astrophysical Journal v.418, p.328

Astronomy and Astrophysics

Astronomy

29

Ism: Clouds, Ism: Individual Alphanumeric: L1257, Ism: Structure, Radio Continuum: Stars, Stars: Low-Mass, Brown Dwarfs

Scientific paper

We describe a method for finding substellar-mass self-gravitating clumps ("proto-brown dwarfs") in nearby molecular clouds, using millimeter-wave telescopes. This method represents a novel approach to looking for brown dwarfs and, for the nearest molecular clouds, has a greater mass sensitivity than traditional methods. We describe how the method can distinguish between various functional forms of the low-mass (<0.2 Msun) stellar initial mass function, which is currently poorly constrained.
We present results of a search for proto-brown dwarfs in high galactic latitude molecular clouds. The clouds have relatively low densities [n(H2) ≲ 104 cm-3], are at an average distance of 100 pc, and are apparently not forming stars. We find little unambiguous evidence for self-gravitating objects of any mass in them, although there are clearly clumps with masses as low as 3 MJupiter. We find two clumps in MBM 12(=L1457), the nearest molecular cloud to the sun, which have luminous masses within a factor of 3 of that needed to be gravitationally bound. One of these has a dense core which appears to be in hydrostatic equilibrium. If so, then it is the first known self-gravitating core in a high-latitude cloud.
The mean pressure in the clumps is P/k = 2.9 × 105 K cm-3, at least one order of magnitude higher than the general ISM pressure. We therefore conclude that either the clumps are transient structures in regions of typical ISM pressure, or the clouds themselves are short-lived objects in overpressured regions. In the latter case, the clouds will disperse in less than about 106 yr.
The search for pro to-brown dwarfs is being expanded to include known star-forming regions in the Ophiuchus and Taurus complexes. The results in Ophiuchus and Taurus will tightly constrain the functional form of the stellar initial mass function at very low masses regardless of whether or not any proto-brown dwarfs are found. The present results already exclude a single power-law initial mass function with index ≳2.

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