The Density Structure of Highly Compact HII Regions


J. Franco (1), S. Kurtz (1), P. Hofner (2,3), L. Testi (4), G. Garcia-Segura (1), and M. Martos (1)

(1) Instituto de Astronom\'{\i}a, UNAM, Apdo. Postal 70-264, 04510 M\'exico D.F., M\'exico

(2) Physics Department, University of Puerto Rico at Rio Piedras, P.O. Box 23343, San Juan, Puerto Rico 00931

(3) Arecibo Observatory, NAIC/Cornell University, HC3 Box 53995, Arecibo, Puerto Rico 00612

(4) Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy


We report the density structure of the ultracompact HII regions G35.20-1.74, G9.62+0.19-E, and G75.78+0.34-H2O. The density profiles are derived from radio continuum emission at wavelengths from 6 to 0.3cm. In the case of G35.20-1.74, a cometary UC HII region with a core and a tail, the spectrum of the core varies as Snu~nu^(0.6), implying that the density structure is ne~r^(-2). The emission from the tail has a flatter spectrum, indicating that the density gradient is also negative, but shallower. For the case of G9.62+0.19, which is an HII region complex with several components, the spectrum of the region designated component E is Snu~nu^(0.95), corresponding to ne~r^(-2.5). The steepest spectral index, Snu~nu^(1.4), is for the super UC HII region G75.78+0.34-H2O; its density stratification may be as steep as ne~r^(-4). The actual density gradient may be smaller, owing to an exponential (rather than a power-law) density distribution or to the effects of finite spatial extent. The contribution from dust emission and some of the possible implications of these density distributions are briefly discussed.

Mantained by: Leonardo Testi