AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 47. Between the Stars I: The ISM, Galactic and Extragalactic
Display, Tuesday, June 1, 1999, 10:00am-7:00pm, Southwest Exhibit Hall

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[47.05] ISO Mid-Infrared Observations of Giant HII Regions in M33

B. P. Skelton (University of Washington), W. H. Waller (Starstuff Incorporated), P. W. Hodge (University of Washington), F. Boulanger (Institut D'Astrophysique Spatiale, Orsay), R. H. Cornett (Raytheon STX, NASA/GSFC), M. N. Fanelli (University of North Texas), J. Lequeux (Ecole Normale Superieure), T. P. Stecher (NASA/GSFC), F. Viallefond (Observatoire de Paris), Y. Hui (University of Minnesota)

We present Infrared Space Observatory Camera (ISOCAM) Circular Variable Filter scans of three giant HII regions in M33. IC 133, NGC 595, and CC 93 span a wide range of metallicity, luminosity, nebular excitation, and infrared excess; three other emission regions (CC 43, CC 99, and a region to the northeast of the core of NGC 595) are luminous enough in the mid-infrared to be detected in the observed fields. ISOCAM CVF observations provide spatially resolved observations (5'') of 151 wavelengths between 5.1 and 16.5 microns with a spectral resolution R = 35 to 50.

We observe atomic emission lines ([Ne II], [Ne III], and [S IV]), several ``unidentified infrared bands'' (UIBs; 6.2, 7.7, 8.6, 11.3, 12.0, and 12.7 microns), and in some cases a continuum which rises steeply at longer wavelengths. We conclude that the spectra of these three GHRs are well explained by combinations of ionized gas, PAHs, and very small grains in various proportions and with different spatial distributions. Comparisons between observed ratios of the various UIBs with model ratios indicate that the PAHs in all three of the GHRs are dehydrogenated and that the small PAHs have been destroyed in IC 133 but have survived in NGC 595 and CC 93. The [Ne III]/[Ne II] ratios observed in IC 133 and NGC 595 are consistent with their ages of 5 and 4.5 Myr, respectively; the deduced ionization parameter is higher in IC 133, consistent with its more compact region of emission.

This work is based on observations with the Infrared Space Observatory, an ESA project with instruments funded by the ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA, and was performed under contracts 961583 and 961637 for the Jet Propulsion Laboratory.

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