Kinematics of Ultracompact H II Regions Revealed: High Spatial and Spectral Resolution Mapping of the 12.8 Micron [Ne II] Line in Monoceros R2

We present the first results of a study of the kinematics and morphology of ultracompact H II regions by using a new observational technique. We used very high spatial and spectral resolution observations (θbm = 1.″4, ΔV = 3.4 km s-1) of the [Ne II] line at 12.8 μm to study the ionized gas in Monoceros R2. The [Ne II] emission shows an H II region with highest emission measure in a ~24'' diameter shell, surrounded on all sides by neutral material. [Ne II] line widths are as narrow as 8 km s-1 at some positions. In places where the lines are complex and broader, the additional width is most likely due to overlap of narrower features along the line of sight. The narrow features themselves, however, are broader than the purely thermal width. The global kinematics suggest that the 24'' shell is expanding at ≈10 km s-1. This interpretation leads to a dynamical age for the H II region of less than 104 yr. However, the spectral profiles toward the brightest part of the nebula (on the southeast side of the shell) are not consistent with a simple expansion picture. Both the 24'' shell and the bright southeast ridge can be part of a common kinematic pattern in which material flows from the bottom to the rim of a bowl-like feature. High-resolution observations of mid-IR fine-structure transitions offer great promise as a probe of the kinematics and morphology of ionized regions around very young massive stars. Once appropriate theoretical modeling and observations of a larger sample of ultracompact H II regions are in place, it should be possible to determine the physics behind the observed systematic motions in sources such as Mon R2.