K3-50A: An Ultracompact H II 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 04 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 ?m silicate absorption, and fine-structure line emissions of [Ne II] at 12.8 ?m, [Ar III] at 8.99 ?m, and [S IV] at 10.51 ?m. 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.

[1]  J. Houck,et al.  Abundances in five nearby galactic H II regions from infrared forbidden lines , 1982 .

[2]  S. Harris.  High-resolution Radio Observations of the Sources Near K3–50 , 1975 .

[3]  E. Churchwell,et al.  The morphologies and physical properties of ultracompact H II regions , 1989 .

[4]  E. Howard,et al.  The Rotating Gas Toroid Surrounding the K3-50A Ionized Bipolar Outflow , 1997 .

[5]  R. Russell,et al.  Detection of the N II 122 and 205 micron lines - Densities in G333.6-0.2 , 1993 .

[6]  Paolo Persi,et al.  The Impact of Large Scale Near-IR Sky Surveys , 1997 .

[7]  J. F. Mccarthy,et al.  Abundances of argon, sulfur, and neon in six galactic H II regions from infrared forbidden lines , 1981 .

[8]  V. Petrosian.  INFRARED LINE EMISSION FROM H II REGIONS. , 1970 .

[9]  Yukiko Kamata,et al.  The First Light of the Subaru Telescope: A New Infrared Image of the Orion Nebula , 2000 .

[10]  H. Matsuhara,et al.  [Ne II] 12.8 Micron Images of Four Galactic Ultracompact H II Regions: Ionized Neon Abundance as a Tracer of the Ionizing Stars , 2000, astro-ph/0008229.

[11]  W. Goss,et al.  A bipolar outflow of ionized gas in K3-50A: H76 alpha radio recombination line and continuum observations of K3-50 , 1994 .

[12]  H. Matthews,et al.  Shell structure in ultracompact H II regions. , 1984 .

[13]  P. Roche,et al.  12-μm fine-structure emission line and continuum images of G333.6-0.2 , 1998 .

[14]  P. Storey,et al.  Atomic data from the IRON Project , 1999 .

[15]  Shigeyuki Sako,et al.  COMICS: the cooled mid-infrared camera and spectrometer for the Subaru telescope , 2000, Astronomical Telescopes and Instrumentation.

[16]  E. Howard,et al.  A Near-Infrared Study of the K3-50 Region of High-Mass Star Formation , 1996 .

[17]  W. Welch,et al.  A rotating protocluster in W58: HCO+ aperture synthesis maps , 1983 .

[18]  K. M. Merrill,et al.  The 8-13 micron spectra of compact H II regions , 1975 .

[19]  T. Geballe,et al.  Infrared emission line studies of the structure and excitation of H II regions , 1982 .

[20]  S. L. Scott,et al.  Interferometric measurements of the millimeter-wave dust emission from OMC-1. , 1985 .

[21]  D. Buhl,et al.  Observations of Interstellar Formaldehyde , 1970 .

[22]  Mark R. Kidger,et al.  Spectral Irradiance Calibration in the Infrared. X. A Self-Consistent Radiometric All-Sky Network of Absolutely Calibrated Stellar Spectra , 1999 .

[23]  E. Chaisson,et al.  Low-Energy X-Rays Ruled Out as Interstellar Ionizing Mechanism Toward K3-50 , 1972 .

[24]  Stuart A. Kurtz,et al.  Ultracompact H II Regions. II. New High-Resolution Radio Images , 1994 .

[25]  H. Kataza,et al.  High-Resolution Mid-Infrared Imaging and Spectroscopic Observations of a Massive Star Forming Region W51 IRS 2 , 2001 .

[26]  Hiroshi Terada,et al.  CISCO: cooled infrared spectrograph and camera for OHS , 1998, Astronomical Telescopes and Instrumentation.

[27]  N. Panagia,et al.  Some physical parameters of early-type stars , 1973 .

[28]  H. Thronson,et al.  Compact H II regions in the far-infrared , 1979 .

[29]  E. Churchwell,et al.  Galactic temperature and metallicity gradients from ultracompact H II regions , 1996 .

[30]  B. Draine.  Tabulated optical properties of graphite and silicate grains , 1985 .