Carbon Suboxide in Astrophysical Ice Analogs

Abstract

As suggested by W. T. Huntress et al. (1991, Nature 352, 316–318), carbon suboxide (C 3O 2) is a potential extended source of both the CO and atomic carbon emission observed in cometary comae. However, laboratory experiments on the formation and stability of C 3O 2 in space environments have not been published. In this work, we study solid C 3O 2 in ices representative of cometary nuclei as well as interstellar icy grain mantles, specifically addressing the issues of C 3O 2 formation and stability under exposure to energetic processing in the forms of proton irradiation and UV photolysis. The formation rate of C 3O 2 is measured in laboratory ices of pure CO and CO 2 and mixtures of these molecules with H 2O at 18 K. Destruction rates in H 2O-dominated mixtures appropriate to a cometary nucleus or interstellar icy grain mantle are also measured. Differences in rates between photolysis and irradiation experiments are observed and quantified. Mid-infrared spectra of C 3O 2-containing mixtures are presented together with measurements of carbon suboxide's infrared band strengths and vapor pressures from 110 to 125 K. Implications are discussed for the existence of C 3O 2 under the energetic conditions found in astrophysical environments as well as the possibility for its detection in cometary and/or interstellar ices.