Ice absorption features in the 5-8 mu m region toward embedded protostars

We have obtained 5-8 mum spectra towards 10 embedded protostars using the Short Wavelength Spectrometer on board the Infrared Space Observatory (ISO-SWS) with the aim of studying the composition of interstellar ices. The spectra are dominated by absorption bands at 6.0 mum and 6.85 mum. The observed peak positions, widths and relative intensities of these bands vary dramatically along the different lines of sight. On the basis of comparison with laboratory spectra, the bulk of the 6.0 mum absorption band is assigned to amorphous H2O ice. Additional absorption, in this band, is seen toward 5 sources on the short wavelength wing, near 5.8 mum, and the long wavelength side near 6.2 mum. We attribute the short wavelength absorption to a combination of formic acid (HCOOH) and formaldehyde (H2CO), while the long wavelength absorption has been assigned to the C-C stretching mode of aromatic structures. From an analysis of the 6.85 mum band, we conclude that this band is composed of two components: a volatile component centered near 6.75 mum and a more refractory component at 6.95 mum. From a comparison with various temperature tracers of the thermal history of interstellar ices, we conclude that the two 6.85 mum components are related through thermal processing. We explore several possible carriers of the 6.85 mum absorption band, but no satisfactory identification can be made at present. Finally, we discuss the possible implications for the origin and evolution of interstellar ices that arise from these new results.