Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........15p&link_type=abstract
PhD Dissertation, Maryland Univ. College Park, MD United States
Astronomy and Astrophysics
Astronomy
2
Molecular Clouds, Brown Dwarf Stars, Stellar Mass, Mass Spectra, Clumps, Interstellar Matter, Telescopes, Millimeter Waves, Brightness Temperature, Spectral Line Width, Molecular Spectra, Emission Spectra
Scientific paper
We develop a new technique to find the progenitors of brown dwarfs, or 'proto-brown dwarfs,' in nearby molecular clouds. By using mm-wave telescopes, this method can achieve greater mass sensitivity than traditional approaches such as astrometry and infrared imaging. Our method can distinguish between various forms of the low-mass (less than 0.2 solar mass) stellar initial mass function (IMF), which currently is not well-constrained. We present results of a search for proto-brown dwarfs in two types of molecular cloud - those at high galactic latitudes, and those in the disk which are forming stars. The high latitude clouds (HLC's) have relatively low densities, and are apparently not forming stars. Our analysis of several HLC's gives little evidence for self-gravitating objects of any mass, although there are clumps with masses as low as 3 M(Jupiter). In molecular line and continuum searches in the Ophiuchus B and Barnard 18 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 IMF slope were flat, we would expect to have found approx. 10 objects. We do find a few candidate objects near our detection limit (approx. 0.02 solar mass) which deserve further scrutiny. We find fewer low-mass clumps (approx. less than O.l solar mass) than expected from extrapolation of any reasonable mass function. If the IMF follows the clump mass spectrum below 0.08 solar mass, as it seems to at higher masses, our results 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. We therefore predict that future searches for brown dwarfs will find very few, regardless of the depth of the searches. We present analyses of two HLC's, MBM 12 and MBM 41-44. The properties of 400 clumps - position, size, mass, line width, brightness temperature - are measured. We find the clump mass spectrum in MBM 12 deviates below 0.3 solar mass from the 'universal' GMC mass spectrum N(Mf) approx. M-0 5. This is the first molecular cloud known which has a mass spectrum which differs significantly and unambiguously from the GMC spectrum.
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