1998ApJ...507..263W

We have observed [Ne II] 12.814 µm line emission from two well-studied ultracompact H II regions, G29.96-0.02 and W51d, and obtained its spatially resolved line images. Spatial distributions of the line emission were compared with the VLA 15 GHz continuum maps to derive the Ne⁺ abundance, which puts strong constraint on the effective temperature or the spectral type of the ionizing star(s). We show that the total neon abundance of G29.96-0.02 is close to the local interstellar abundance of neon, and the dominant species of neon is Ne⁺ over the whole H II region. When the ionic ratio derived from observed fluxes of [Ne III] 36.0 µm and [Ne II] 12.8 µm is compared with a theoretical calculation based on the recent model of non-LTE stellar atmosphere, it implies that the ionizing star(s) have an effective temperature of ∼34,000 K or spectral type of O9.5 in the main sequence. On the other hand, the derived Ne⁺ abundance of W51d is significantly lower than the local interstellar abundance of neon. In addition, we found the Ne⁺ abundance decreases remarkably toward the center of W51d. These results suggest that the dominant species of neon would be Ne⁺⁺ in W51d, and the ionizing star(s) has significantly high effective temperature (∼39,000 K). Alternatively, the [Ne II] line could suffer from enormous extinction due to dust associated with this ultracompact H II region.