Ice and Dust in the Quiescent Medium of Isolated Dense Cores
  The relation between ices in the envelopes and disks surrounding YSOs and
those in the quiescent interstellar medium is investigated. For a sample of 31
stars behind isolated dense cores, ground-based and Spitzer spectra and
photometry in the 1-25 um wavelength range are combined. The baseline for the
broad and overlapping ice features is modeled, using calculated spectra of
giants, H2O 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 13-25 um is ~50% relative to that at
2.2 um. The strengths of the 6.0 and 6.85 um absorption bands are in line with
those of YSOs. Thus, their carriers, which, besides H2O and CH3OH, may include
NH4+, HCOOH, H2CO and NH3, are readily formed in the dense core phase, before
stars form. The 3.53 um C-H stretching mode of solid CH3OH was discovered. The
CH3OH/H2O 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 um band of silicates relative
to the continuum extinction at 2.2 um is significantly shallower than in the
diffuse interstellar medium. This extends the results of Chiar et al. (2007) to
a larger sample and higher extinctions.
