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Title: INFRARED SPECTROSCOPIC SURVEY OF THE QUIESCENT MEDIUM OF NEARBY CLOUDS. I. ICE FORMATION AND GRAIN GROWTH IN LUPUS

Infrared photometry and spectroscopy (1-25 μm) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of grains and the composition of ices before they are incorporated into circumstellar envelopes and disks. H{sub 2}O ices form at extinctions of A{sub K} = 0.25 ± 0.07 mag (A{sub 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{sub 2}O ice (2.3 ± 0.1 × 10{sup –5} relative to N{sub H}) is typical for quiescent regions, but lower by a factor of three to four compared to dense envelopes of young stellar objects. The low solid CH{sub 3}OH abundance (<3%-8% relative to H{sub 2}O) 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{sub K} increases as a function of A{sub K}. Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to R{sub V} ∼ 3.5 (A{sub K}more » = 0.71) and R{sub V} ∼ 5.0 (A{sub K} = 1.47). For lines of sight with A{sub K} > 1.0 mag, the τ{sub 9.7}/A{sub K} ratio is a factor of two 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{sub 9.7}/A{sub K} ratio. This process is likely related to grain growth by coagulation, as traced by the A{sub 7.4}/A{sub K} continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts.« less
Authors:
 [1] ;  [2] ; ;  [3] ;  [4] ;  [5] ; ;  [6]
  1. IPAC, NASA Herschel Science Center, Mail Code 100-22, California Institute of Technology, Pasadena, CA 91125 (United States)
  2. SETI Institute, Carl Sagan Center, 189 Bernardo Avenue, Mountain View, CA 94043 (United States)
  3. Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
  4. Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904 (United States)
  5. Solar System Exploration Research Virtual Institute, NASA Ames Research Center, Moffett Field, CA 94035 (United States)
  6. Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden (Netherlands)
Publication Date:
OSTI Identifier:
22270642
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 777; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTRONOMY; ASTROPHYSICS; CARBON MONOXIDE; COMPARATIVE EVALUATIONS; COSMIC DUST; FREEZING OUT; GRAIN GROWTH; HYDROGEN; ICE; INFRARED SPECTRA; METHANOL; MOLECULES; PHOTOMETRY; STAR EVOLUTION; STARS; WATER