4 Contamination by foreground material?

Absorption studies toward young stellar objects located in the $\rho$ Oph cloud complex can be dominated by foreground cloud(s) (Boogert et al. 2002b). The SCUBA 850 $\mu$m map of $\rho$ Oph by Johnstone et al. (2000) shows that CRBR 2422.8-3423 is indeed at the edge of the ridge which contains both Elias 29 and the nearby class I object IRS 43. The lack of CO emission at the offset position indicates however that the bulk of the gas-phase CO is located within 3000 AU from CRBR 2422.8-3423. The similarity of the gaseous CO column densities derived from the on-source C¹⁸O 2-1 emission and the VLT-ISAAC infrared absorption may be fortuitous since the VLT-ISAAC data do not probe very cold CO with small ($\la$1 km s^-1) line widths. We cannot exclude that part of the gas-phase CO seen in the infrared arises in a more extended envelope or cloud, but the fact that the CO excitation temperature is significantly above 10 K indicates that at least some fraction must originate close to the young star in a warm part of the disk.

For solid CO, there are strong arguments that most of the absorption must arise in the disk. The bright nearby ($\sim$30 $^{\prime\prime}$ east) source IRS 43 was observed simultaneously with CRBR 2422.8-3423 with VLT-ISAAC and has a CO ice optical depth of only $1.87 \pm 0.02$, corresponding to N(CO $_{\rm ice})=7 \times 10^{17}$ cm^-2 (Pontoppidan et al., in prep.). This is at least a factor of 3 lower than toward CRBR 2422.8-3423 even though its 850 $\mu$m flux is a factor of 1.5 higher. Toward Elias 29, solid CO has an optical depth of only 0.25, most of which is believed to be located in the foreground clouds. Compared to the optical depth found toward CRBR 2422.8-3423 ( $\tau_{\rm ice}$(CO$) \ga6$), this amount of foreground material can account for only an insignificant fraction of the observed solid CO. Finally, if clouds happen to lie in front of CRBR 2422.8-3423, the moderate extinction ( $A_{\rm V}< 10$) of those clouds probably prevents them to harbour significant amounts of solid CO (Shuping et al. 2000). Indeed, in a M-band survey of more than 30 young stellar objects, this is the deepest CO ice band observed.