Near-Infrared Spectra of the Extended Young Objects GL 2591 and NGC 7538/IRS9

Abstract

The 3.08 µm ice absorption feature is usually found in the line of sight of very young objects deeply embedded in dense molecular clouds. Attempts to fit the feature by means of different kinds of water ices, however, have been unsuccessful, and additional absorptions at both short and long wavelength wings are in fact observed. Some kinds of hydrocarbons seem to be a plausible explanation for the long wavelength wing. The extra absorption at wavelengths shorter than 3.08 µm has been attributed to ammonia ices and, as an alternative explanation, to scattering by large water ice particles. Many protostars are associated with infrared reflection nebulae. This could be considered an indirect support for the scattering origin of the short wavelength wing of the ice feature, although it does not exclude the ammonia ice explanation at all.