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We describe a method for finding substellar mass self-gravitating clumps (``proto-brown dwarfs'') in nearby molecular clouds, using mm-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 show how the method can distinguish between various functional forms of the low-mass ($< 0.2~{\rm M_{\sun}}$) 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 (HLCs) and the nearest star-forming regions. The HLCs have relatively low densities (n($H_2$)~$\lapprox 10^4$~cm$^{-3}$), are at an average distance of 100 pc, and are apparently not forming stars. The mass sensitivity of our search was extremely high, capable of detecting masses well below the main-sequence hydrogen burning limit. Despite this high sensitivity, we have found little unambiguous evidence for self-gravitating objects of any mass in the HLCs, although there are clumps with masses as low as $3~{\rm M_{Jupiter}}$. In MBM12\thinspace (=L1457), the nearest molecular cloud to the sun, we found two clumps which have luminous masses within a factor of 3 of that needed to be gravitationally bound. Neither is of substellar mass, however. One of them 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.
In both molecular line (single-dish and interferometer) and continuum searches in the Ophiuchus star-forming region, we found no proto-brown dwarfs. This is surprising, as even if the initial mass function were flat, we would expect to have found several. We speculate on the reasons for the negative result.
