Ices in the Quiescent IC 5146 Dense Cloud
  This paper presents spectra in the 2 to 20 micron range of quiescent cloud
material located in the IC 5146 cloud complex. The spectra were obtained with
NASA's Infrared Telescope Facility (IRTF) SpeX instrument and the Spitzer Space
Telescope's Infrared Spectrometer. We use these spectra to investigate dust and
ice absorption features in pristine regions of the cloud that are unaltered by
embedded stars. We find that the H2O-ice threshold extinction is 4.03+/-0.05
mag. Once foreground extinction is taken into account, however, the threshold
drops to 3.2 mag, equivalent to that found for the Taurus dark cloud, generally
assumed to be the touchstone quiescent cloud against which all other dense
cloud and embedded young stellar object observations are compared. Substructure
in the trough of the silicate band for two sources is attributed to CH3OH and
NH3 in the ices, present at the ~2% and ~5% levels, respectively, relative to
H2O-ice. The correlation of the silicate feature with the E(J-K) color excess
is found to follow a much shallower slope relative to lines of sight that probe
diffuse clouds, supporting the previous results by Chiar et al. (2007).
