Infrared ionic line emission in W33

We have mapped the central parsec of the H II region W33 in the infrared fine-structure lines of [Ne II] (12.8 mu m) and [Ar III] (8.99 mu m), and have measured spectra of [Ar Il] (6.99 mu m) at selected locations. This is the first observation of the 6.99 mu m line from the ground. The spatial distribution of the infrared lines resembles that of the radio continuum, except for one region where the extinction is apparently very high (even at 12.8 mu m). The abundances of neon and argon in W33 are a factor of 4 lower than solar. We have analyzed the ratios of the mapped lines to derive the types of the exciting stars, their ages, and other features of the source. The cluster contains three distinct subsources, one of which appears to be a cometary and another a shell-like ultracompact H II region, although they are larger and less dense than most objects of this class. The stars are of types O6 to O7 and less than a few million years old. We suggest that star formation in this object has been proceeding simultaneously in at least three centers, and that the H II regions have not yet expanded to form one source.