THE<i>SPITZER</i>ICE LEGACY: ICE EVOLUTION FROM CORES TO PROTOSTARS

Abstract

Ices regulate much of the chemistry during star formation and account for up\nto 80% of the available oxygen and carbon. In this paper, we use the Spitzer\nc2d ice survey, complimented with data sets on ices in cloud cores and\nhigh-mass protostars, to determine standard ice abundances and to present a\ncoherent picture of the evolution of ices during low- and high-mass star\nformation. The median ice composition H2O:CO:CO2:CH3OH:NH3:CH4:XCN is\n100:29:29:3:5:5:0.3 and 100:13:13:4:5:2:0.6 toward low- and high-mass\nprotostars, respectively, and 100:31:38:4:-:-:- in cloud cores. In the low-mass\nsample, the ice abundances with respect to H2O of CH4, NH3, and the component\nof CO2 mixed with H2O typically vary by &lt;25%, indicative of co-formation with\nH2O. In contrast, some CO and CO2 ice components, XCN and CH3OH vary by factors\n2-10 between the lower and upper quartile. The XCN band correlates with CO,\nconsistent with its OCN- identification. The origin(s) of the different levels\nof ice abundance variations are constrained by comparing ice inventories toward\ndifferent types of protostars and background stars, through ice mapping,\nanalysis of cloud-to-cloud variations, and ice (anti-)correlations. Based on\nthe analysis, the first ice formation phase is driven by hydrogenation of\natoms, which results in a H2O-dominated ice. At later prestellar times, CO\nfreezes out and variations in CO freeze-out levels and the subsequent CO-based\nchemistry can explain most of the observed ice abundance variations. The last\nimportant ice evolution stage is thermal and UV processing around protostars,\nresulting in CO desorption, ice segregation and formation of complex organic\nmolecules. The distribution of cometary ice abundances are consistent with with\nthe idea that most cometary ices have a protostellar origin.\n