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

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 18 O $J=2\rightarrow 1$ 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.