The 8-13 micron spectra of compact H II regions

Abstract

Observations are reported of the spectra between 8 and 13 microns of four compact H II regions: K3-50, W51-IRS 1, W51-IRS 2, and W49-OH(2). All of the spectra show a broad absorption feature centered around 9.7 microns. In addition, a 12.8-micron emission line, attributed to the Ne(+) fine-structure transition, has been found in W51-IRS 1 and probably in W49-OH(2). It is marginally present in K3-50 and W51-IRS 2. The broad absorption feature is consistent with absorption due to cold material, similar to that seen in emission from the Trapezium. Limits on the amount of absorption are established by considering two models of the underlying source emission spectrum: blackbody emission and Trapezium-like emission. It is found that the models with Trapezium emission spectra produce consistently better fits to the data. The visual extinction is compared with the optical depth at 9.7 microns. The derived 9.7-micron optical depths are combined with knowledge of the size of the molecular clouds to derive cloud masses that are consistent with molecular determinations. It is found that the emitting regions have larger gas/dust mass ratios than interstellar values by a factor of 20 to 200.