K3-50A: An ultracompact HII region ionized by a massive stellar cluster

We have made imaging and spectroscopic observations of the ultracompact H II region K3-50A with a spatial resolution of 0.4 using a new mid-infrared instrument, the Cooled Mid-Infrared Camera and Spectrometer, on the 8.2 m Subaru Telescope. The spectra show thermal dust emission, 9.7 mum silicate absorption, and fine-structure line emissions of [Ne II] at 12.8 mum, [Ar III] at 8.99 mum, and [S IV] at 10.1 mum. From the maps of the continuum, line emissions, and/or derived dust parameters, we identify eight mid-infrared sources in K3-50A. Especially the central [Ne II] emission has been resolved into two peaks clearly. The ionization condition is investigated with the line flux ratios I([Ar III])/I([Ne II]) and I([S IV])/I([Ne II]). It is suggested that the spectral types of the ionizing stars in K3-50A correspond to B0-O8 V, which is much later than O5.5 V, the type estimated from radio continuum observations under a single-star assumption. The three line fluxes suggest a number of Ne+ ions greater than what is ionized by a single star of any spectral type, but the numbers of Ar2+ and S3+ are similar to that formed by a single O8-O9 V star and that by a single O7-O9 V star, respectively. From these features as well as the dust temperature and the correspondence of each identified source with the near-infrared source, we propose that K3-50A is excited by at least two (possibly three) ionizing stars. This is the first convincing example that a massive stellar cluster is ionizing an ultracompact H II region.