Mid-infrared fine structure line studies of ultracompact HII regions
Ultracompact HII (UCHII) regions form around young massive stars when the stars are still embedded in their natal molecular clouds. Studies show that UCHII regions have long lifetimes and various morphologies that are not expected by the classic Str¨omgren model of HII regions. Both theoretical and observational efforts are required to solve these problems in order to answer one of the basic questions in astrophysics - how are stars formed? We present high resolution mid-infrared fine structure line observations of several galactic compact and UCHII regions. The primary goal of this study is to find the connection between kinematic structure and morphology of the UCHII regions. We scan-mapped these regions using a mid-infrared cross-dispersed spectrograph, TEXES, in several fine structure lines with high spectral resolution (R∼100,000). Organized bulk motion of ionized gas is present in every object we observed. Observations indicate that morphology is generally not a good indicator of the underlying gas kinematics and the vicinity of massive star formation regions is not uniform. The classic Str¨omgren sphere model is not consistent with the observed kinematical structures. Gas bulk motion and acceleration along the ionization front are common in observed HII regions. We compare the observed velocity structures of a few UCHII regions in our sample with predictions of stellar wind bow shock models. Similarities between the observations and the models show that ionized gas is flowing along tilted approximately paraboloidal surfaces formed by ram pressures of the stellar winds and ambient medium. Our results show that pressure gradient can have a significant effect on acceleration of the ionized gas in UCHII regions, thus should not be neglected in models for UCHII regions. Using the ionization code - CLOUDY (Ferland et al. 1998), we try to derive the physical properties of two UCHII regions:G29.96 -0.02 and NGC 7538A from a few fine structure line fluxes we measured. This method provides a systematic way to determine the chemical abundances and the effective temperatures of the ionizing stars.