Molecular clouds on the threshold of star formation: the radial density profile of the cores of the RHO ophiuchi and R coronae australis clouds
In a cold, dense, dynamically evolved cloud core that lacks embedded massive stars, the density distribution should reflect a stage of cloud evolution at the threshold of star formation. The radial density distribution in three such dense massive cores, located within two nearby regions of recent star formation, has been determined from extensive maps of two H/sub 2/CO transitions. The appearance of the maps of emission and absorption of the 2 cm line confirms the predictions of models for the excitation of this transition. Detailed models of the H/sub 2/CO radiative transport demonstrate that a rapid radial decline of density, rho(R)proportionalR/sup -3//sup ///sup 2/ to R/sup -2/, from a density of 10/sup 6/ cm/sup -3/ in the core to 10/sup 4/ cm/sup -3/ in the envelope, occurs over radial distances from 0.06 to 0.60 pc. These models require a decline in H/sub 2/CO abundance with increasing density in these cold cores, perhaps a result of condensation of gas onto grains. For the observed values of core radius (0.06-0.09 pc) and core mass (19-110 M/sub sun/), the H/sub 2/CO line widths indicate that the velocity dispersion due to rotation or turbulence is too small to stabilize the cloud against gravitational collapse. The observed magnetic field strength also apears to be inadequate to prevent cloud contraction and eventual star formation. The unusual 2 cm H/sub 2/CO emission toward rho Oph B is found to be spatially extended (8' x 4'). The lack of far-infrared emission from this cold dense (>10/sup 6/ cm/sup -3/) region indicates a lack of stars of high luminosity embedded within or near the surface of the cloud. The combination of cold, very dense gas in a centrally condensed region together with a lack of internal support means that within rho Oph B the stage has been set for the possible formation of one or more massive protostars.
- Research Organization:
- Millimeter Wave Observatory, McDonald Observatory, and the Electrical Engineering Research Laboratory, The University of Texas at Austin
- OSTI ID:
- 5471391
- Journal Information:
- Astrophys. J.; (United States), Vol. 270:2
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GENERAL PHYSICS
FORMALDEHYDE
ABUNDANCE
ENERGY-LEVEL TRANSITIONS
NEBULAE
EMISSION SPECTRA
INTERSTELLAR GRAINS
MAPS
RADIATION TRANSPORT
STAR ACCRETION
ALDEHYDES
ORGANIC COMPOUNDS
PARTICLES
SPECTRA
STAR EVOLUTION
640105* - Astrophysics & Cosmology- Galaxies
640102 - Astrophysics & Cosmology- Stars & Quasi-Stellar
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