The Density Structure of Highly Compact H [CSC]ii[/CSC] Regions

Abstract

We report the density structure of the ultracompact (UC) H ii regions G35.2021.74, G9.6210.19-E, and G75.7810.34-H O. The density profiles are derived from radio continuum emission at wavelengths from 6 to 2 0.3 cm. In the case of G35.2021.74, a cometary UC H ii region with a core and a tail, the spectrum of the core varies as , implying that the density structure is . The emission from the tail has a flatter spectrum, 0.6 22 S / n n / r n e indicating that the density gradient is also negative but shallower. For the case of G9.62 10.19, which is an H ii region complex with several components, the spectrum of the region designated component E is , cor0.95 S / n n responding to . The steepest spectral index, , is for the super UC H ii region G75.7810.34-H O; 22.5 1.4 n / rS / n e n 2 its density stratification may be as steep as . The actual density gradient may be smaller, owing to an 24 n / r e 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. Stellar groups form in the dense cores of molecular clouds. The structure of these clouds has been unveiled with optical, infrared, and radio observations over the last three decades, and a great effort has been made to understand the connections between cloud properties and star-forming activity. In particular, knowledge of the gas density structure is required to determine fundamental properties, such as the mass and stability of star-forming cores. Extinction studies can be used to derive the density profiles of clouds, but given the large opacities involved, they can only probe the outermost gas layers. Tracers occurring at radio frequencies, on the other hand, can penetrate deeper into the cloud and can reveal the density stratification of dark clouds and cloud cores. The information obtained from extinction and molecular-line studies shows that molecular clouds are centrally condensed. For instance, moderate-density (10 3 ‐10 5 cm ) envelopes sur23 rounding higher density (10 7 cm ) cores suggest the existence 23