2002A&A...394L..27T

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≃10000. The spectrum shows strong solid CO absorption, with a lower limit on the column density of 2.2x10¹⁸cm^–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 C¹⁸O J=2->1 emission 30" south. Gas-phase ro-vibrational CO absorption lines are also detected with a mean temperature of 50±10K. The average gas/solid CO ratio is ∼1 along the line of sight. For an estimated inclination of 20°±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.