K3-50A: An Ultracompact H II Region Ionized by a Massive Stellar Cluster

Details K3-50A: An Ultracompact H II Region Ionized by a Massive Stellar Cluster K3-50A: An Ultracompact H II Region Ionized by a Massive Stellar Cluster

Feb 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apj...584..368o&link_type=abstract

The Astrophysical Journal, Volume 584, Issue 1, pp. 368-384.

Astronomy and Astrophysics

Astronomy

25

Ism: Dust, Extinction, Ism: H Ii Regions, Infrared: Ism, Ism: Individual: Alphanumeric: K3-50A, Ism: Lines And Bands, Stars: Formation

Scientific paper

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 μm silicate absorption, and fine-structure line emissions of [Ne II] at 12.8 μm, [Ar III] at 8.99 μm, and [S IV] at 10.51 μm. 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.
Based on data collected with the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

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