2001ApJ...553..254O

Mid-infrared (7.8-13.5µm) imaging and low-resolution spectroscopic observations were made for the massive star-forming region W51 IRS 2 with spatial resolution of 1". IRS 2 was resolved into seven sources based on the continuum and the emission line maps. The nature of these sources was investigated using their thermal dust emission, the 9.7 µm absorption feature, and/or three fine-structure emission lines of [Ne II] at 12.8 µm, [Ar III] at 8.99 µm, and [S IV] at 10.51 µm. Four of the sources were identified as ultracompact H II regions and one as an embedded protostar candidate. The spectral types of the ionizing sources are derived from the line ratios to be all around O9, which is much later than those (O5.5 and O7.5) derived from the radio continuum fluxes. The observed line ratios of (ultra-)compact H II regions in IRS 2 and in the literature follow a single excitation track, but it is different from that predicted from model calculations. We propose two possible resolutions for the discrepancy: (1) the employed models for the stellar atmosphere are not sufficiently accurate in the UV range, or (2) the (ultra-)compact H II regions are ionized by stellar clusters.