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Subsections

  
3 Results and analysis

3.1 VLT-ISAAC spectrum

A saturated absorption feature centered at $\sim$4.67 $\mu $m and assigned to solid CO is seen in the M-band spectrum displayed in the upper panel of Fig. 1. No broad absorption at 4.62 $\mu $m, usually attributed to solid OCN- and a signpost of thermal and/or ultraviolet processing, is present, although the limit of $\tau
\leq 0.05$ is not strong. Unresolved gas-phase CO lines are also detected. A continuum which takes into account absorption by silicate is fitted to the data. The continuum substracted spectrum is shown in the lower panel of Fig. 1 in transmission scale.


  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{Eh151_f1.eps}\end{figure} Figure 1: VLT-ISAAC M band spectrum toward CRBR 2422.8-3423 (upper panel). The insert shows the VLT Ks image taken by Brandner et al. (2000). The upper curve of the lower panel shows the gas-phase model, convolved to the observed spectral resolution and shifted in wavelength to account for a source heliocentric velocity of 10 km s-1. The middle curve displays laboratorium absorption spectra of apolar (dotted) and polar (dashed) CO ice mixtures. The sum of the two models including Pfund $\beta $ emission at 4.65 $\mu $m is overlaid on the observed spectrum in the bottom curve.

The best fit to the saturated solid CO band is $\tau \approx 6.2$, corresponding to N(CO $_{\rm ice}) \approx 2.2 \times 10^{18}$ cm-2 using the integrated band strength of Gerakines et al. (1995). This is the highest solid CO column density found to date, even compared with ice-rich high-mass sources such as NGC 7538 IRS9 (e.g. Chiar et al. 1998, 1995). The solid CO profile toward CRBR 2422.8-3423 can be decomposed into a narrow saturated component, usually ascribed to apolar CO in a H2O-poor matrix, and a broad red wing at 4.685 $\mu $m, indicative of polar CO in an H2O-rich matrix. Using optical constants from the Leiden database including grain shape effects (Ehrenfreund et al. 1997), a $\chi ^2$ search for the best-fitting laboratory mixtures was performed. The saturation of the line prevents any unique fit, but the apolar CO is best matched by a mixture CO:N2:CO2=100:50:20 at 10 K with $N=2.0 \times 10^{18}$ cm-2. The red-wing is fitted by H2O:CO = 4:1 at 40 K with $2.6 \times 10^{17}$ cm-2. The latter column density is comparable to that found toward L 1489, which has a luminosity of 3.7 $L_{\odot}$. The large amount of apolar CO indicates low temperatures along the line of sight, since it evaporates around $\sim$20 K.

The VLT spectrum shows the presence of narrow gas-phase 12CO and 13CO ro-vibrational lines originating from levels up to J=9(250 K above ground). Synthetic LTE model spectra were fitted to the observed spectrum using data from the HITRAN database (Rothman et al. 1992). The fit parameters are the gas temperature $T_{\rm
ex}$, the column density $N_{\rm gas}$(CO) and the velocity broadening $\Delta V$, assumed to be smaller than 2 km s-1 from the JCMT data. The limited number and the high optical depth of the 12CO lines prevent a unique fit, but the best result (see Fig. 1) is obtained for $T_{\rm ex}=40{-}60$ K, which is probably a mean between the cold and warm components along the line of sight. The fit includes the 13CO lines. The optical depth of the 12CO lines is likely underestimated so that the gas-phase CO column density can range from 1 to $6 \times 10^{18}$ cm-2. Adopting a mean value of $3 \times 10^{18}$ cm-2, the line of sight average gas/solid CO ratio is $\sim$1. Comparing the column of gas-phase CO with that of polar solid CO only, the ratio drops to $\sim$0.1. The latter value is comparable to that found for L 1489 (0.07, see Boogert et al. 2002a) but still higher than for Elias 18 (0.01, Shuping et al. 2001). If the lines were significantly wider than 2 km s-1 as found for L 1489 ($\sim$20 km s-1), the gaseous CO column density would drop by an order of magnitude.

3.2 JCMT spectra

The C18O $J=2\rightarrow 1$ emission observed with the JCMT is shown in the upper panel of Fig. 2. Strong 12CO $J=3\rightarrow 2$ ($\sim$12 K peak temperature) and 13CO $J=3\rightarrow 2$ ($\sim$7 K peak temperature) lines are also detected, but are not displayed here since their profiles are similar to that of C18O. C18O $J=2\rightarrow 1$ is detected on source only. Compared with single-dish CO 3-2 and 2-1 lines from disks around older isolated pre-main-sequence stars in Taurus (e.g., Thi et al. 2001), the lines toward CRBR 2422.8-3423 are more than an order of magnitude stronger and do not show the double-peak profile resulting from the projection of a disk in Keplerian rotation. The CRBR 2422.8-3423 spectrum shows three peaks, two of which have velocities similar to those seen toward Elias 29 and interpreted as arising from the cloud ridge in which the source is embedded ( $N({\rm CO_{gas}})_{\rm
cloud}\simeq 2.9 \times 10^{18}$ cm-2, Boogert et al. 2002b). The total integrated C18O 2-1 intensity on source is $13.6 \pm 4$ K km s-1. The corresponding 12CO column density is (0.5-1 $) \times 10^{19}$ cm-2 assuming $T_{\rm ex}=15$ K (cf. Motte et al. 1998 for $\rho$ Oph clump F) and 18O/16O = 560 (Wilson & Rood 1994). For $T_{\rm ex}=50$ K, the column densities are lowered by 30%. The non-detection of C18O $J=2\rightarrow 1$ at the offset position suggests a 12CO column density less than $1 \times 10^{17}$ cm-2 for $T_{\rm ex}=15$ K, around 20 times lower.


  \begin{figure}
\par\includegraphics[angle=90,width=8.8cm,clip]{Eh151_f2.eps}\end{figure} Figure 2: C18O $J=2\rightarrow 1$ obtained using the JCMT toward CRBR 2422.8-3423 on source (upper spectrum) and at an offset position 30'' south (lower spectrum).


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