Near-Infrared Spectroscopy of G29.96-0.02: The First Spectral Classification of the Ionizing Star of an Ultracompact H II Region

We have obtained the first classification spectrum and present the first direct spectral classification of the ionizing star of an ultracompact H II region. The ultracompact H II region is G29.96-0.02, a well-studied object with a metallicity value roughly twice that of the Sun. The near-infrared K-band spectrum of the ionizing star exhibits C IV and N III emission and He II absorption, but lines of H I and He I are obliterated by nebular emission. We determine that the star has a spectral type of O5-O7 or possibly O8. We critically evaluate limits on the properties of the star and find that it is compatible with zero-age main-sequence properties only if it is binary and if a significant fraction of the bolometric luminosity can escape from the region. G29.96-0.02 will now be an excellent test case for nebular models, as the properties of the ionizing star are independently constrained.