[Ne II] 12.8 Micron Images of Four Galactic Ultracompact H II Regions: Ionized Neon Abundance as a Tracer of the Ionizing Stars

We present the results of ground-based imaging spectroscopy of the [Ne II] 12.8 μm line emitted from the ultracompact (UC) H II regions W51d, G45.12+0.13, G35.20-1.74, and Monoceros R2, with 2'' spatial resolution. We found that the overall distribution of the [Ne II] emission is generally in good agreement with the radio (5 or 15 GHz) VLA distribution for each source. The Ne+ abundance ([Ne+/H+]) distributions are also derived from the [Ne II] and the radio maps. As for G45.12+0.13 and W51d, the Ne+ abundance decreases steeply from the outer part of the map toward the radio peak. On the other hand, the Ne+ abundance distributions of G35.20-1.74 and Mon R2 appear rather uniform. These results can be interpreted by the variation of ionizing structures of neon, which is determined primarily by the spectral type of the ionizing stars. We have evaluated the effective temperature of the ionizing star by comparing the Ne+ abundance averaged over the whole observed region with that calculated by H II region models based on recent non-LTE stellar atmosphere models: 39,100 K (O7.5 V-O8 V) for W51d; 37,200 K (O8 V-O8.5 V) for G45.12+0.13; 35,000-37,600 K (O8 V-O9 V) for G35.20-1.74; and ≤34,000 K (≤B0 V) for Mon R2. These effective temperatures are consistent with those inferred from the observed Ne+ abundance distributions.