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Infrared Spectroscopic Survey of the Quiescent Medium of Nearby Clouds: I. Ice Formation and Grain Growth in Lupus

Boogert, A. C. A. and Chiar, J. E. and Knez, C. and Öberg, K. I. and Mundy, L. G. and Pendleton, Y. J. and Tielens, A. G. G. M. and van Dishoeck, E. F. (2013) Infrared Spectroscopic Survey of the Quiescent Medium of Nearby Clouds: I. Ice Formation and Grain Growth in Lupus. Astrophysical Journal, 777 (1). Art. No. 73. ISSN 0004-637X. doi:10.1088/0004-637X/777/1/73.

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Infrared photometry and spectroscopy (1-25 µm) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of the grains and the composition of the ices before they are incorporated into circumstellar envelopes and disks. H_2O ices form at extinctions of A_k=0.25 ± 0.07 mag (A_v=2.1 ± 0.6). Such a low ice formation threshold is consistent with the absence of nearby hot stars. Overall, the Lupus clouds are in an early chemical phase. The abundance of H_2O ice (2.3 ± 0.1 x 10^(-5) relative to N_h) is typical for quiescent regions, but lower by a factor of 3-4 compared to dense envelopes of YSOs. The low solid CH_3OH abundance (<3-8% relative to H_2O) indicates a low gas phase H/CO ratio, which is consistent with the observed incomplete CO freeze out. Furthermore it is found that the grains in Lupus experienced growth by coagulation. The mid-infrared (>5 µm) continuum extinction relative to A_k increases as a function of A_k. Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to R_v ~ 3.5 (A_k=0.71) and R_v ~ 5.0 (A_k=1.47). For lines of sight with A_k>1.0 mag, the τ_(9.7)/A_k ratio is a factor of 2 lower compared to the diffuse medium. Below 1.0 mag, values scatter between the dense and diffuse medium ratios. The absence of a gradual transition between diffuse and dense medium-type dust indicates that local conditions matter in the process that sets the τ_(9.7)/A_k ratio. This process is likely related to grain growth by coagulation, as traced by the A_(7.4)/A_k continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts.

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Öberg, K. I.0000-0001-8798-1347
van Dishoeck, E. F.0000-0001-7591-1907
Additional Information:© 2013 American Astronomical Society. Received 2013 May 15; accepted 2013 August 29; published 2013 October 17. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program IDs 083.C-0942 and 085.C-0620. We thank the anonymous referee for detailed comments that improved the presentation of the results. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology (Caltech) under a contract with the National Aeronautics and Space Administration (NASA). Support for this work was provided by NASA through awards issued by JPL/Caltech to JEC and CK. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and JPL/Caltech, funded by NASA. 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/Caltech, funded by NASA and the National Science Foundation.
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Subject Keywords: infrared: ISM; infrared: stars; ISM: abundances; ISM: molecules; stars: formation
Issue or Number:1
Record Number:CaltechAUTHORS:20130930-100722637
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Official Citation: Infrared Spectroscopic Survey of the Quiescent Medium of Nearby Clouds. I. Ice Formation and Grain Growth in Lupus A. C. A. Boogert et al. 2013 ApJ 777 73
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41548
Deposited By: Ruth Sustaita
Deposited On:30 Sep 2013 20:21
Last Modified:10 Nov 2021 04:31

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