The Nature of Carbon Dioxide Bearing Ices in Quiescent Molecular Clouds

TL;DR

This study investigates the properties of CO2 ices in molecular clouds using spectral analysis, revealing consistent ice formation processes and similar spectral profiles across different clouds, regardless of compositional differences.

Contribution

The paper provides the first comparison of 15 micron CO2 ice profiles across multiple dark clouds, demonstrating uniformity in ice formation despite compositional variations.

Findings

15 micron CO2 profiles are consistent across Taurus, Serpens, and IC5146.

The polar component constitutes 85% of CO2 in the line of sight.

Ice formation processes are robust and uniform across different molecular clouds.

Abstract

The properties of the ices that form in dense molecular clouds represent an important set of initial conditions in the evolution of interstellar and preplanetary matter in regions of active star formation. Of the various spectral features available for study, the bending mode of solid CO2 near 15 microns has proven to be a particularly sensitive probe of physical conditions, especially temperature. We present new observations of this absorption feature in the spectrum of Q21-1, a background field star located behind a dark filament in the Cocoon Nebula (IC5146). We show the profile of the feature be consistent with a two-component (polar + nonpolar) model for the ices, based on spectra of laboratory analogs with temperatures in the range 10-20K. The polar component accounts for 85% of the CO2 in the line of sight. We compare for the first time 15 micron profiles in three widely separated dark clouds (Taurus, Serpens and IC5146), and show that they are indistinguishable to within observational scatter. Systematic differences in the observed CO2/H2O ratio in the three clouds have little or no effect on the 15 micron profile. The abundance of elemental oxygen in the ices appears to be a unifying factor, displaying consistent behavior in the three clouds. We conclude that the ice formation process is robust and uniformly efficient, notwithstanding compositional variations arising from differences in how the O is distributed between the primary species (H2O, CO2 and CO) in the ices.