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 H₂CO 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 H₂CO radiative transport demonstrate that a rapid radial decline of density, ρ(R) ∝ R-3/2 to R⁻², from a density of 10⁶ in the core to l0 cm⁻³ in the envelope, occurs over radial distances from 0.06 to 0.60 pc. These models require a decline in H₂CO 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 Msun), the H₂CO 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 appears to be inadequate to prevent cloud contraction and eventual star formation. The unusual 2 cm H2CO emission toward ρ Oph B is found to be spatially extended (8' × 4'). The lack of far-infrared emission from this cold dense (>10⁶ cm⁻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 ρ Oph B the stage has been set for the possible formation of one or more massive protostars.
No takes yet. Share an insight, caveat, or question.
Loren et al. (1983) studied this question.