The G19.6−0.2 Region of Star Formation: Molecular and Ionized Environs

Abstract

We present VLA observations, made with angular resolutions of 2''-3'', of the ionized gas (H66α recombination line and 1.6 GHz continuum) and of the molecular gas ([2, 2] inversion transition of NH3) toward the star formation region known as G19.6-0.2. The observations of the ionized gas indicate the presence of five distinct H II regions that are excited by individual stars, implying the presence of a cluster of O-B stars. Three of the individual H II regions have ringlike structures, with sizes ranging from 0.1 to 0.4 pc, and have clumpy and inhomogeneous distributions of gas. Some of the ultracompact structures detected toward these regions with higher angular resolution (~04) probably are associated with partially ionized clumps, which are externally ionized, and embedded within larger H II regions. The brightest compact H II region within the complex shows a cometary-like morphology, which may be due to the expansion of ionized gas in an anisotropic medium. The ammonia observations show that the molecular emission arises from three distinct structures: a dense (~9 × 106 cm-3) clump, located near the center of the cluster of H II regions, which has a size of ~0.05 pc and exhibits very broad line widths (~9.5 km s-1), and two less dense (~1 - 2 × 105 cm-3) clumps, one of which is detected in absorption toward the northernmost H II region. The physical relationship between these molecular structures and the ionized gas is discussed. We find that the cometary-like and the most compact H II region within the complex are intimately associated with the densest ammonia clump. The mean velocity of the ionized gas is blueshifted with respect to the mean velocity of the molecular clump by 3.5 km s-1. We suggest that the star that excites this H II region was formed at the edge of the densest molecular clump. The ionized gas flowing toward the observer is undergoing expansion into the lower density interclump medium (champagne flow). In the opposite direction, the H II region is driving a shock into the dense molecular clump, compressing it and further increasing its density. The less dense ammonia clumps are probably heated and compressed by the shock fronts driven by the more extended H II regions.