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Issue A&A
Volume 394, Number 2, November I 2002
Page(s) L27 - L30
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20021353



A&A 394, L27-L30 (2002)
DOI: 10.1051/0004-6361:20021353

Letter

Detection of abundant solid CO in the disk around CRBR 2422.8-3423

W. F. Thi1, 2, K. M. Pontoppidan2, E. F. van Dishoeck2, E. Dartois3 and L. d'Hendecourt3

1  Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
2  Leiden Observatory, PO Box 9513, 2300 RA, Leiden, The Netherlands
3  Astrochimie Expérimentale, Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay, France

(Received 15 August 2002 / Accepted 17 September 2002)

Abstract
We present direct evidence for CO freeze-out in a circumstellar disk around the edge-on class I object CRBR 2422.8-3423 , observed in the M band with VLT-ISAAC at a resolving power $R\approx 10\,000$. The spectrum shows strong solid CO absorption, with a lower limit on the column density of $2.2 \times 10^{18}$  cm -2. The solid CO column is the highest observed so far, including high-mass protostars and background field stars. Absorption by foreground cloud material likely accounts for only a small fraction of the total solid CO, based on the weakness of solid CO absorption toward nearby sources and the absence of gaseous C 18O $J=2\rightarrow 1$ emission 30´´ south. Gas-phase ro-vibrational CO absorption lines are also detected with a mean temperature of $50 \pm 10$ K. The average gas/solid CO ratio is ~1 along the line of sight. For an estimated inclination of 20 $\degr \pm 5$°, the solid CO absorption originates mostly in the cold, shielded outer part of the flaring disk, consistent with the predominance of apolar solid CO in the spectrum and the non-detection of solid OCN -, an indicator of thermal/ultraviolet processing of the ice mantle. By contrast, the warm gaseous CO likely originates closer to the star.


Key words: star formation -- ISM: dust, extinction -- molecules -- abundances -- infrared: ISM: lines and bands

Offprint request: W. F. Thi, wfdt@star.ucl.ac.uk



© ESO 2002


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