Title: Single or multiple O stars within Galactic UCHII regions from 
Gemini/Michelle imaging and spectroscopy?

Author(s): Paul Crowther (Sheffield UK), James Furness (Sheffield UK)

Abstract: We present mid-infrared imaging and spectroscopy of Galactic 
ultracompact HII (UCHII) regions with Gemini/Michelle. The formation of 
high mass stars remains an unresolved problem in astrophysics. 
However, the UCHII phase -  characterised by high Lyman continuum ionizing 
fluxes from dense (10^4  cm^-3), compact (0.1 pc) regions - is recognised 
as the first phase when high mass  O-type stars are known to have 
formed, as derived from radio surveys of the Milky Way disk. 
Unfortunately, young O stars remain veiled at optical and near-infrared 
wavelengths due to circumstellar dust extinction in such regions. 
Consequently, their stellar content are generally inferred from radio 
continuum observations, on the assumption that a single O star provides 
the  ionization. The advent of efficient mid-infrared instruments mounted 
at  8-10m telescopes allows diffraction limited imaging and spectroscopy 
of  such regions (0.4 arcsec = 0.01 pc at D=5kpc), pioneered by 
Subaru/COMICS  studies of UCHII regions by Okamoto  et al. (2001, 2003). 
Here, we present  Gemini/Michelle imaging and spectroscopy of 24 UCHII 
regions, enabling a test of whether single or multiple ionizing 
sources lie within. Mid-infrared fine structure lines of [NeII], [SIV] 
and [ArIII] allow estimates of the spectral type of the constituent O 
stars, based on a Spitzer-IRS and ISO-SWS derived calibration for inner 
Milky Way O stars.