Morphologies of Ultracompact HII Regions in W49A and Sgr B2: Prevalence of Shells and a Modified Classification Scheme

Abstract

We have used Very Large Array (VLA) observations of the massive star forming regions W49A and Sgr B2, obtained with resolutions from 2\farcs0 to 0\farcs04, to classify the morphologies of nearly 100 ultracompact HII regions. These high resolution, multi-frequency, multi-configuration VLA observations motivate several modifications of the existing morphological classification scheme for UC HII regions. In this work, we describe the modified morphology scheme and the criteria used in source classification. In particular, we drop the ``core-halo'' classification, add a ``bipolar'' classification, and change the shell classification to ``shell-like''. We tally the percentage of each morphology found in the Sgr B2 and W49A regions and find broad agreement with the Galactic plane surveys in the distribution of morphologies for most types. However, we find that nearly a third of the sources in these regions are shell-like, which is a higher percentage by nearly a factor of ten than found in the surveys of Galactic plane star forming regions by Wood & Churchwell (1989a) and Kurtz et al. (1994). This difference may be due to physical differences in the environments of these two extreme star forming regions. Alternatively, differences in observational technique may be responsible.