The Spitzer Ice Legacy: Ice Evolution from Cores to Protostars

Creators: Öberg, Karin I.; Boogert, A. C. Adwin; Pontoppidan, Klaus M.; van den Broek, Saskia; van Dishoeck, Ewine F.; Bottinelli, Sandrine; Blake, Geoffrey A.; Evans, Neal J., II

Abstract

Ices regulate much of the chemistry during star formation and account for up to 80% of the available oxygen and carbon. In this paper, we use the Spitzer c2d Legacy ice survey, complimented with data sets on ices in cloud cores and high-mass protostars, to determine standard ice abundances and to present a coherent picture of the evolution of ices during low- and high-mass star formation. The median ice composition H_(2)O:CO:CO_2:CH_(3)OH:NH_3:CH_4:XCN is 100:29:29:3:5:5:0.3 and 100:13:13:4:5:2:0.6 toward low- and high-mass protostars, respectively, and 100:31:38:4:-:-:- in cloud cores. In the low-mass sample, the ice abundances with respect to H_(2)O of CH_4, NH_3, and the component of CO_2 mixed with H_(2)O typically vary by <25%, indicative of co-formation with H_(2)O. In contrast, some CO and CO_2 ice components, XCN, and CH3OH vary by factors 2-10 between the lower and upper quartile. The XCN band correlates with CO, consistent with its OCN– identification. The origin(s) of the different levels of ice abundance variations are constrained by comparing ice inventories toward different types of protostars and background stars, through ice mapping, analysis of cloud-to-cloud variations, and ice (anti-)correlations. Based on the analysis, the first ice formation phase is driven by hydrogenation of atoms, which results in an H_(2)O-dominated ice. At later prestellar times, CO freezes out and variations in CO freezeout levels and the subsequent CO-based chemistry can explain most of the observed ice abundance variations. The last important ice evolution stage is thermal and UV processing around protostars, resulting in CO desorption, ice segregation, and the formation of complex organic molecules. The distribution of cometary ice abundances is consistent with the idea that most cometary ices have a protostellar origin.

Additional Information

© 2011 American Astronomical Society. Received 2011 April 14; accepted 2011 July 27; published 2011 October 4. Support for K.I.O. is provided by NASA through a Hubble Fellowship grant awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. Astrochemistry in Leiden is supported by a SPINOZA grant of the Netherlands Organization for Scientific Research (NWO). Support for this work, part of the Spitzer Space Telescope Legacy Science Program, was provided by NASA through contracts 1224608 and 1230779 issued by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize the cultural role and reverence that the summit of Mauna Kea has within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We acknowledge helpful comments from an anonymous referee.

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