Ices in the Quiescent IC 5146 Dense Cloud

TL;DR

This study analyzes infrared spectra of the IC 5146 dense cloud to understand dust and ice features, revealing a water-ice threshold similar to Taurus and identifying methanol and ammonia in the ices.

Contribution

It provides new measurements of ice absorption features and thresholds in a quiescent cloud, comparing them to other well-studied regions like Taurus.

Findings

Water-ice threshold extinction is about 3.2 mag after foreground correction.

Methanol and ammonia ices are present at ~2% and ~5% levels relative to water ice.

Silicate feature correlates with color excess with a shallower slope than in diffuse clouds.

Abstract

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).