Structure and Evolution of the Envelopes of Deeply Embedded Massive Young Stars

The physical structure of the envelopes around a sample of fourteen massive(1000-100,000 solar L) young stars is investigated on 100- 100,000 AU scalesusing maps and spectra in submillimeter continuum and lines of C17O, CS andH2CO. The total column densities and the temperature profiles are obtained byfitting self-consistent dust models to submillimeter photometry. Both themolecular line and dust emission data indicate density gradients ~r^{-alpha},with alpha=1.0-1.5, significantly flatter than the alpha=2.0 generally foundfor low-mass objects. This flattening may indicate that in massive youngstellar objects, nonthermal pressure is more important for the support againstgravitational collapse, while thermal pressure dominates for low-mass sources.We find alpha=2 for two hot core-type sources, but regard this as an upperlimit since in these objects, the CS abundance may be enhanced in the warm gasclose to the star.Comment: To be published in The Astrophysical Journal. 54 pages including 14 figures Revised version with references adde