Mid-infrared imaging and spectroscopy of the southern H II region RCW 38

We present midinfrared images and an 8-13 micron spectrum of the southern H II region RCW 38. We determine the dust color temperature from both our spectrum and images at 10 and 20 microns, and deduce the gas excitation from an image in the forbidden S IV fine-structure line, as well as spectra of the forbidden Ar III, forbidden S IV, and forbidden Ne II fine-structure lines. Our observations are consistent with a complex of sources associated with the RCW 38 IRS1 region, which represent knots of material in a shell, or ridge, surrounding a cavity of about 0.1 pc in radius, which is itself created by the stellar wind of the hot young source IRS2. The dust temperature does not peak closest to IRS2, but rather along the center of the ridge, and is remarkably uniform over the extent of our image. From photoionization models for the observed line ratios at IRS1 we deduce a stellar effective temperature and gas density of about 43,000-48,000 K and 10 exp 4/cu cm respectively. While the star, or star cluster, IRS2 is ultimately responsible for the observed thermal and ionic emission, the relatively uniform dust temperature implies that the bulk of the dust heating in the region is provided by resonantly trapped Lyman-alpha photons, rather than direct stellar photons. This then also implies that the dust is depleted with respect to the gas by a factor of at least 100 from its normal interstellar value. (Author)