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

We present mid-infrared images and an 8-13 mu m spectrum of the southern HI T region RCW 38. We determine the dust colour temperature from both our spe ctrum and images at 10 and 20 mu m, and deduce the gas excitation from an i mage in the [S IV] fine-structure line, as well as spectra of the [Ar III], [S IV] and [Ne II] fine-structure lines. Our observations are consistent w ith a complex of sources associated with the RCW 38 IRS1 region, which repr esent 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 y oung source IRS2, The dust temperature does not peak closest to IRS2, but r ather along the centre of the ridge, and is remarkably uniform over the ext ent of our image. From photoionization models for the observed line ratios at IRS I we deduce a stellar effective temperature and gas density of about 43 000-48 000 K and 10(4) cm(-3) respectively. Whilst the star, or star cl uster, IRS2 is ultimately responsible for the observed thermal and ionic em ission, the relatively uniform dust temperature implies that the bulk of th e dust heating in the region is provided by resonantly trapped Lyman alpha photons, rather than direct stellar photons. This then also implies that th e dust is depleted with respect to the gas by a factor of at least 100 from its normal interstellar value. The small-scale spatial variations in the c ontinuum emission and temperature can be explained by changes in the densit y and/or gas-to-dust mass ratio.