Ice and Dust in the Quiescent Medium of Isolated Dense Cores

The relation between ices in the envelopes and disks surrounding young stellar objects (YSOs) and those in the quiescent interstellar medium (ISM) is investigated. For a sample of 31 stars behind isolated dense cores, ground-based and Spitzer spectra and photometry in the 1–25 μm wavelength range are combined. The baseline for the broad and overlapping ice features is modeled, using calculated spectra of giants, H 2 O ice and silicates. The adopted extinction curve is derived empirically. Its high resolution allows for the separation of continuum and feature extinction. The extinction between 13 and 25 μm is ∼50% relative to that at 2.2 μm. The strengths of the 6.0 and 6.85 μm absorption bands are in line with those of YSOs. Thus, their carriers, which, besides H 2 O and CH 3 OH, may include NH + 4 , HCOOH, H 2 CO, and NH 3 , are readily formed in the dense core phase, before stars form. The 3.53 μm C-H stretching mode of solid CH 3 OH was discovered. The CH 3 OH/H 2 O abundance ratios of 5%–12% are larger than upper limits in the Taurus molecular cloud. The initial ice composition, before star formation occurs, therefore depends on the environment. Signs of thermal and energetic processing that were found toward some YSOs are absent in the ices toward background stars. Finally, the peak optical depth of the 9.7 μm band of silicates relative to the continuum extinction at 2.2 μm is significantly shallower than in the diffuse ISM. This extends the results of Chiar et al. to a larger sample and higher extinctions.