3 Results

3.1 CO $\mathsfsl{J=6\rightarrow5}$ emission

CO $J=6 \rightarrow 5$ emission is elongated in the NW-SE direction. Detailed information about the morphology of the region can be found in Boogert et al. (2002; hereinafter BHC02), who report single dish and interferometric observations of several molecular transitions. Here we give a short summary to help understand the CO  $J=6 \rightarrow 5$ observed emission. An envelope+disk system centered on EL 29 was resolved by interferometric observations of the ¹³CO  $1\rightarrow 0$ transition and has a rest velocity of $v_{\rm LRS} \sim$ 5 km s^-1. It is embedded in a dense ridge which extends NW-SE. The ridge has a velocity similar to the EL 29 envelope+disk system, namely $\sim$5 km s^-1, and it is probed by the emission of the HCO⁺  $J=3\rightarrow2$ transition (single dish data) and HCO⁺  $J=1\rightarrow 0$ (interferometric data). The EL 29 envelope+disk and the ridge are behind two molecular clouds whose rest velocity is $\sim$2.7 and $\sim$3.8 km s^-1 respectively. The same situation is visible in the observed CO  $J=6 \rightarrow 5$ spectra. In all mapped positions the line is heavily absorbed around $\sim$3.8 km s^-1 because of the cloud foreground material. Towards EL 29 (at 0'',0'') the line is absorbed at $\sim$5 km s^-1 (Fig. 2), by material belonging to the ridge and/or envelope+disk.

Notably, we detected ¹³CO  $J=6 \rightarrow 5$ emission with $T^*_R=(10 \pm 1)$ K towards the central position, whereas we obtained a 3$\sigma$ upper limit of $T^*_R \leq 2.6$ K at (-14'',+14'') (Fig. 2), i.e. in a positions still inside the ridge. The on-source ¹³CO  $J=6 \rightarrow 5$ line peaks at $\sim$5 km s^-1, suggesting that the bulk of the ¹²CO  $J=6 \rightarrow 5$ emission is due to the envelope+disk and/or ridge associated with EL 29. However, since the on-source ¹³CO  $J=6 \rightarrow 5$ line peak is about half the ¹²CO line peak and less than 0.15 at -14'',  +14'', i.e. in the ridge, it is likely that most of the absorption (and emission) towards the central position is due to the envelope+disk rather than the ridge.

Figure 2: 12CO (top) and 13CO (bottom) $J=6 \rightarrow 5$ line spectra towards (0'', 0'') (left panel) and ( -14'', +14'') (right panel) respectively. Vertical lines mark the rest velocity of the foreground clouds studied in BHC02.

The ¹²CO spectra of Fig. 1 present wings at high velocity ($\geq$4 km s^-1), which probe the presence of outflowing gas. The map of the wing emission (Fig. 3) shows the characteristic bipolar shape, with the blue lobe on the west side and the red lobe to the east. The outflow traced by our $J=6 \rightarrow 5$ observations is roughly in agreement with that reported by Bontemps et al. (1996) and Sekimoto et al. (1997), obtained in the $2 \rightarrow 1$ transition. However, the morphology of the outflow seems better resolved in the $J=6 \rightarrow 5$ line, probably because of the smaller contamination from the cloud material.

Figure 3: Contour map of the wing CO  $J=6 \rightarrow 5$ emission toward EL 29. The solid lines show the blue shifted emission integrated between -10 and 0 km s-1, while the dotted lines show the red shifted emission in the interval +7 to +17 km s-1. Levels of 5, 10, 15, 20, 25, and 30 K km s-1 are shown.

3.2 FIR line emission

Figure 4 shows the line spectrum observed towards EL 29, after removal of the continuum (for the continuum analysis look at BHC02). The line spectra towards EL 29 and the off-source position are dominated by the [OI]63 $\mu$m and [CII]158 $\mu$m lines, which have comparable fluxes at the two (ON and OFF) positions: $6 \times 10^{-12}$ erg s^-1 cm^-2 ([OI]) and $10 \times 10^{-12}$ erg s^-1 cm^-2 ([CII]). The fact that the fluxes are similar at ON and OFF positions indicates that both lines are (mainly) formed in the "diffuse'' material which belongs to the molecular cloud, and very little of the emission, if any, can be attributed to EL 29 itself (see Liseau et al. 1999 for a discussion of the C⁺ and O$^{\rm o}$ emission in the $\rho$ Oph cloud).

In contrast, the ISO observations detected five CO lines, from J=15 to J=19, only toward EL 29. The line parameters are summarized in Table 1.

   

Table 1: Parameters of the CO lines observed towards EL 29. First column gives the theoretical line center, second column the observed fluxes with their statistical errors, third column the total error on the flux, including baseline uncertainty (see text), and fourth column the transition. The upper limits are 2$\sigma$ of the noise.

$\lambda$	Flux	Error	Transition
($\mu$m)	10-12 erg s-1 cm-2		CO
130.37	$\leq$1.0		$20 \rightarrow 19$
137.20	0.5 $\pm$ 0.3	0.5	$19 \rightarrow 18$
144.78	1.0 $\pm$ 0.3	0.6	$18 \rightarrow 17$
153.27	1.6 $\pm$ 0.3	0.6	$17 \rightarrow 16$
162.81	1.2 $\pm$ 0.3	0.6	$16 \rightarrow 15$
173.63	1.2 $\pm$ 0.3	0.4	$15 \rightarrow 14$
185.95	$\leq$0.8		$14 \rightarrow 13$

The values reported in Table 1 were computed by best fitting a Gaussian whose FWHM is equal to the resolution of the spectrometer and by defining a baseline around each line. The definition of the baseline is also a source of error in the flux estimates (note that the lines are $\sim$1/50 the continuum) and gives around 20% uncertainty. The statistical errors given in Table 1 are those due to the residuals from the Gaussian fit to the fluxes, and the total error also takes into account uncertainty in the baseline determination. An additional 20% uncertainty should be considered when considering absolute fluxes. Note that although the total errors reported in Table 1 are large with respect to the quoted fluxes, they represent only an uncertainty on the flux estimates and not on the line detection, for which one has to consider the statistical errors.

Figure 4: The line spectrum of EL 29 after removal of the continuum. The expected brightest lines are marked from O0, C+, CO, H2O and OH species. Two lines clearly detected at 146.8 $\mu$m and 139.6 $\mu$m are unidentified and marked as UL in the figure.

Figure 4 shows the presence of two unidentified lines at 146.8 $\mu$m and 139.6 $\mu$m respectively. The only correspondence we found is with H₂S lines, but the lack of other detected H₂S lines in the spectrum excludes these identifications. Finally, we did not detect any other line, specifically from H₂O, OH and ¹³CO, at a level of $\sim$ $0.7 \times 10^{-12}$ erg s^-1 cm^-2 ($2\sigma$ rms). An apparent peak close to the CO line at 163 $\mu$m is far too red to be associated with the OH line at 163.4 $\mu$m. There may be some NH₃ emission responsible for the feature visible at 170 $\mu$m, but given the uncertainty in the existence of such a feature we do not explore further this possibility. No H₂ lines were detected in the SWS spectrum and the obtained $2\sigma$ upper limits are 1 and 2  $\times 10^{-12}$ erg s ^-1 cm^-2 for the S(1) and S(2) lines, and 6 $\times 10^{-13}$ erg s^-1 cm^-2 for the other lines.