Ices in the Quiescent IC 5146 Dense Cloud
This paper presents spectra in the 2 to 20 μm range of quiescent cloud material located in the IC 5146 cloud complex. The spectra were obtained with NASA's Infrared Telescope Facility SpeX instrument and the Spitzer Space Telescope's Infrared Spectrometer. We use these spectra to investigate dust and ice absorption features in pristine regions of the cloud that are unaltered by embedded stars. We find that the H_2O-ice threshold extinction is 4.03 ± 0.05 mag. Once foreground extinction is taken into account, however, the threshold drops to 3.2 mag, equivalent to that found for the Taurus dark cloud, generally assumed to be the touchstone quiescent cloud against which all other dense cloud and embedded young stellar object observations are compared. Substructure in the trough of the silicate band for two sources is attributed to CH_3OH and NH_3 in the ices, present at the ~2% and ~5% levels, respectively, relative to H_2O-ice. The correlation of the silicate feature with the E(J – K) color excess is found to follow a much shallower slope relative to lines of sight that probe diffuse clouds, supporting the previous results by Chiar et al.
Additional Information© 2011 American Astronomical Society. Received 2010 December 20; accepted 2011 January 24; published 2011 March 17. This work is based (in part) on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided to J.E.C. by NASA through an award issued by JPL/Caltech and to Y.J.P., L.J.A., K.E., T.P.G., T.L.R., and S.A.S. by NASA. L.J.A. gratefully acknowledges support from NASA's Astrobiology (grant 811073.02.12.03) and Laboratory Astrophysics (grant 09-APRA09-0019) programs. D.C.B.W. is grateful to the NASA Exobiology and Evolutionary Biology Program (grant NNX07AK38G) and the NASA Astrobiology Institute (grant NNA09DA80A) for financial support. R.E.M. and T.R.G. are supported by the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., on behalf of the international Gemini partnership of Argentina, Australia, Brazil, Canada, Chile, the United Kingdom, and the United States of America. The work is also based in part on observations made at the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement no. NCC 5-538 with the National Aeronautics and Space Administration, Science Mission Directorate, and Planetary Astronomy Program. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Facilities: Spitzer(IRS), IRTF(SpeX)
Published - Chiar2011p13751Astrophys_J.pdf