Solid CO₂ in quiescent dense molecular clouds

Comparison between Spitzer and laboratory spectra

¹ Physikalisches Institut, Westfälische Wilhelms Universität, Wilhelm Klemm Straße 10, 48149 Münster, Germany
² INAF–Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy
e-mail: mepalumbo@oact.inaf.it
³ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

Received: 26 January 2017
Accepted: 27 June 2017

Abstract

Context. Carbon dioxide (CO₂) is one of the most abundant species detected in icy grain mantles in star forming regions. Laboratory experiments have shown that CO₂ molecules are efficiently formed in the solid state under interstellar conditions. Specifically, solid CO₂ can be formed through energetic (e.g. UV photolysis, electron and ion bombardment) and non-energetic mechanisms (atom-addition reactions).

Aims. Here we investigate the role of low-energy cosmic-ray bombardment in the formation of solid CO₂ in quiescent dense molecular clouds.

Methods. We performed laboratory experiments to study the formation of CO₂ after ion irradiation with 200 keV H⁺ of astrophysical relevant ice mixtures. Laboratory spectra are used to fit the profile of the CO₂ bending mode band observed at about 15.2 μm (660 cm^-1) by the Spitzer Space Telescope in the line of sight to background sources.

Results. From a qualitative point of view, good fits to observations are obtained by considering either three or four laboratory components. From a quantitative point of view, a better result is obtained with four components, i.e. when a spectrum of CO₂ formed after ion irradiation of CH₃OH ice is added to the fitting procedure.

Conclusions. Our results support the hypothesis that energetic processing of icy grain mantles is an efficient formation mechanism of CO₂ ice also in quiescent dark cloud regions, and indirectly suggest the presence of CH₃OH in icy grain mantles in interstellar cold regions.