The Nature of Carbon Dioxide Bearing Ices in Quiescent 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 CO 2 near 15 μm 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 (IC 5146). We show the profile of the feature to be consistent with a two-component (polar + nonpolar) model for the ices, based on spectra of laboratory analogs with temperatures in the range 10-20 K. The polar component accounts for ∼85% of the CO 2 in the line of sight. We compare for the first time 15 μm profiles in three widely separated dark clouds (Taurus, Serpens, and IC 5146), and show that they are indistinguishable to within observational scatter. Systematic differences in the observed CO 2 /H 2 O ratio in the three clouds have little or no effect on the 15 μm 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 (H 2 O, CO 2 , and CO) in the ices.

Highly resolved infrared spectra of pure CO2ice (15–75 K)

“…It constitutes a significant part of interstellar ice with abundances relative to solid H 2 O varying from ∼15 to 40% in quiescent dark clouds (d 'Hendecourt & Jourdain de Muizon 1989;Whittet et al 1998Whittet et al , 2007Whittet et al , 2009Bergin et al 2005;Knez et al 2005) and circumstellar envelopes of low-and high-mass protostars (Gerakines et al 1999;Nummelin et al 2001;Boogert et al 2004;Pontoppidan et al 2008;Zasowski et al 2009;Cook et al 2011). Also in planetary environments CO 2 ice has been detected (see e.g., Cruikshank et al 2010).…”

Ices in the Quiescent Ic 5146 Dense Cloud

“…The observations show similarities but also significant differences in ice properties and the conditions needed for ice to form. These differences may be parameterized in terms of the ice "extinction threshold," i.e., the minimum observed continuum extinction necessary for the detection of the ices (e.g., Whittet 2003), and in the relative abundances of H 2 O, CO, and CO 2 (Whittet et al 2007(Whittet et al , 2009). To establish a reliable benchmark for studies of ice evolution in regions of active star formation, it is vital to understand the initial conditions in quiescent clouds and the extent to which they vary from cloud to cloud.…”

Photosynthesis of Carbon Dioxide From Carbon Surfaces Coated With Oxygen: Implications for Interstellar Molecular Clouds and the Outer Solar System

“…Condensed carbon dioxide has been detected in the interstellar medium, where it represents an important fraction of the icy mantles of molecular cloud grains (8%-35% with respect to water ice; e.g., Gerakines et al 1999;Boogert et al 2004;Whittet et al 2009). …”