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Title: A SIGNIFICANTLY LOW CO ABUNDANCE TOWARD THE TW Hya PROTOPLANETARY DISK: A PATH TO ACTIVE CARBON CHEMISTRY?

In this Letter we report the CO abundance relative to H{sub 2} derived toward the circumstellar disk of the T-Tauri star TW Hya from the HD (1 – 0) and C{sup 18}O (2 – 1) emission lines. The HD (1 – 0) line was observed by the Herschel Space Observatory Photodetector Array Camera and Spectrometer whereas C{sup 18}O (2 – 1) observations were carried out with the Submillimeter Array at a spatial resolution of 2.''8 × 1.''9 (corresponding to ∼151 × 103 AU). In the disk's warm molecular layer (T > 20 K) we measure a disk-averaged gas-phase CO abundance relative to H{sub 2} of χ(CO) = (0.1-3) × 10{sup –5}, substantially lower than the canonical value of χ(CO) = 10{sup –4}. We infer that the best explanation of this low χ(CO) is the chemical destruction of CO followed by rapid formation of carbon chains, or perhaps CO{sub 2}, that can subsequently freeze-out, resulting in the bulk mass of carbon locked up in ice grain mantles and oxygen in water. As a consequence of this likely time-dependent carbon sink mechanism, CO may be an unreliable tracer of H{sub 2} gas mass.
Authors:
; ;  [1] ;  [2] ;  [3]
  1. Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109 (United States)
  2. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
  3. California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125 (United States)
Publication Date:
OSTI Identifier:
22215432
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 776; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; CARBON; CARBON DIOXIDE; CARBON MONOXIDE; CARBON SINKS; FREEZING OUT; HYDROGEN; ICE; MASS; OXYGEN; PROTOPLANETS; SPATIAL RESOLUTION; SPECTROMETERS; T TAURI STARS; TIME DEPENDENCE; WATER