Freeze-out of gas phase species in interstellar and circumstellar environments is a key process in the formation and evolution of molecules. Some fraction of these ices may end up in planet-forming disks as building blocks of, for example, comets. Telescope observations of the infrared ice bands toward young stars and quiescent dense clouds show that much carbon accretes in the form of CO. Due to its high volatility, the solid CO abundance depends strongly on the physical conditions. In regions with strong CO freeze-out, a CO-based ice chemistry results in enhancements in CO2, H2CO, CH3OH, and probably OCN- and other minor species [1, 2, 3]. Some carbon accretes in atomic form, leading to CH4. The accreted carbon is thus distributed over a suite of species that are precursors to larger molecules, relevant to astrobiology. We will also review the observational evidence for other sources of carbon. The carbon content of the ices could double if PAH species were embedded in them [4, 5]. Another source could come from reactions between H2O ice and an underlying refractory carbon surface (e.g., graphite), forming CO2 and carbon-chain oxides, in the presence of energetic particles (cosmic rays) [6 and references therein]. References: [1] Cuppen et al. 2009. A&A 508, 275 [2] Boogert et al. 2011. ApJ 729, 92 [3] Öberg et al. 2011. ApJ 740, 109 [4] Bouwman et al. 2011. A&A 525, 93 [5] Hardegree-Ullman et al. 2014. ApJ 784, 172 [6] Ioppolo et al. 2013. A&A 554, 34

Observations of carbon in interstellar and circumstellar ices

249th National Meeting (2015)

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