SPITZER MID-INFRARED SPECTROSCOPY OF ICES TOWARD EXTINCTED BACKGROUND STARS

摘要

A powerful way to directly observe the solid-state inventory of dense molecular clouds is by infrared spectros-copy of background stars. We present Spitzer IRS 5-20 μm 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 μm bands. In contrast to some young stellar objects (YSOs), most (～75%) of the 6.0 μm band is explained by the bending mode of pure H_2O ice. In realistic mixtures this number increases to 85%, because the peak strength of the H_2O bending mode is very sensitive to the molecular environment. The strength of the 6.85 μm 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 CO_2 (20%-40% with respect to H_2O ice). Compared with YSOs, the band profile of the 15 μm CO_2 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 NH_3. Clearly, models of star formation should begin with dust models already coated with a fairly complex mixture of ices.