Spitzer mid-infrared spectroscopy of ices toward extincted background stars

A powerful way to directly observe the solid-state inventory of dense molecular clouds is by infrared spectroscopy of background stars. We present Spitzer IRS 5-20 mm spectra of ices toward stars behind the Serpens and Taurus molecular clouds, probing visual extinctions of 10-34 mag. These data provide the first complete inventory of solid-state material in dense clouds before star formation begins. The spectra show prominent 6.0 and 6.85 mm bands. In contrast to some young stellar objects (YSOs), most (similar to 75%) of the 6.0 mm band is explained by the bending mode of pure H2O ice. In realistic mixtures this number increases to 85%, because the peak strength of the H2O bending mode is very sensitive to the molecular environment. The strength of the 6.85 mm band is comparable to what is observed toward YSOs. Thus, the production of the carrier of this band does not depend on the energetic input of a nearby source. The spectra show large abundances of CO and CO2 (20%-40% with respect to H2O ice). Compared with YSOs, the band profile of the 15 mm CO2 bending mode lacks the signatures of crystallization, confirming the cold, pristine nature of these lines of sight. After the dominant species are removed, there are residuals that suggest the presence of minor species such as HCOOH and possibly NH3. Clearly, models of star formation should begin with dust models already coated with a fairly complex mixture of ices.