PROTO-BROWN DWARFS. Ⅱ. RESULTS IN THE OPHIUCHUS AND TAURUS MOLECULAR CLOUDS

摘要

We expand our proto-brown dwarf search to include two nearby star-forming regions in the Ophiuchus and Taurus molecular clouds. In both molecular line and continuum searches in the Ophiuchus B and Barnard 18 (in Taurus) star-forming regions, we find no clear-cut evidence of any proto-brown dwarfs. This lack of proto-brown dwarfs is surprising; even if the initial mass function (IMF) (given by dN/dM ∝ M~(-α)) were flat (α = 1), we would expect to have found ～ 10 objects. We do find, however, a few candidate objects near our detection limit (～ 0.02 solar mass) which deserve further scrutiny. We find 21 gravitationally bound clumps distributed in mass with a spectral index α = 1.1 ± 0.2. However, there are fewer low-mass clumps (approx < 0.1 solar mass) than would be expected from extrapolation of any reasonable mass function, including the well-known giant molecular cloud clump mass spectrum. If the IMF follows the I clump mass spectrum below 0.08 solar mass, as it does at higher masses, our results in Oph B imply that, unless some undetermined process causes the production of many more low-mass clumps, the IMF is falling at masses below 0.08 solar mass, even if all our candidate objects turn out to be true proto-brown dwarfs. This work presents evidence that there is a mass range (0.1-0.25 solar mass) where the molecular cloud clump mass spectrum has the same slope stellar IMF (α ～ 1). This is an indication that there is indeed a direct relationship between clump mass and subsequent stellar mass at the scale of a few tenths of a solar mass.