1 Introduction

Far InfraRed (FIR) CO emission, lines with $J \geq 14$, has been observed by ISO in several low mass protostars, both Class 0 or Class I objects, to which they belong (a recent review is given in Ceccarelli 2000). This FIR CO emission usually has been attributed to the shocked gas at the interface between the outflowing gas and the surroundings (e.g. Giannini et al. 2001 and references therein). The reason for that is the relatively high temperature, $\geq$200 K on a scale of at least a few hundred AUs, derived by the analysis of the CO spectra, which has been taken as a sign of extra heating, other than the radiation from the central source. However, the shock interpretation has some difficulties. First, if the FIR CO emission originates in shocked gas, water emission is expected to be bright as well, as water is expected to be copiously formed under such conditions. The expectations are based on theoretical modeling (e.g. Hollenbach & McKee 1989; Kaufman & Neufeld 1996) and on the actual observation of abundant water in clear-cut shocked regions, like HH54B (Liseau et al. 1996) and Orion (Harwit et al. 1998; Cernicharo et al. 1999). On the contrary, water emission is only observed towards Class 0 sources and it seems to be more correlated with the mass of the envelope surrounding those sources than with their SiO emission - usually considered a shock tracer of (Ceccarelli et al. 1999). Indeed, the water emission observed towards the two Class 0 sources IRAS16293-2422 and NGC 1333-IRAS4 has been successfully explained as due to the thermal emission from their envelopes (Ceccarelli et al. 2000; Maret et al. 2002). In addition, water emission is usually stronger towards the central positions and weaker, when detected, towards the emission peaks of millimeter CO and other shock tracers, like SiO for example (Caselli et al. 1997; Schilke et al. 1997). A notable example is L1448 (Nisini et al. 2000), where the strongest water emission is detected at the two ISO positions encompassing the two Class 0 sources of the region, L1448-mm and L1448-N, rather than at the outflow south lobe, which shows bright millimeter emission from a large variety of molecules (Bachiller et al. 1990; Curiel et al. 1999). Another example is IRAS16293-2422, where very bright water emission is detected only towards the central position and is undetected towards the bright SiO and H₂CO (Castets et al. 2001), methanol (Garay et al. 2002), and SO/SO₂ (Wakelam et al. 2002) emission peaks. Other examples include NGC 1333-IRAS4 (Maret et al. 2002) and IRAS2 (Caux et al. 1999). In summary, if the observed FIR CO emission originates in shocks close to the central object, those shocks do not seem to produce much water, which is a somewhat troubling result requiring additional explanation. Explanation of the water under-abundance is certainly possible and includes depletion of oxygen and/or water into grain mantles, as well as re-condensation on grains (e.g. Bergin et al. 1999).

Figure 1: Map of the 12CO $J=6 \rightarrow 5$ emission. The map is centered on the infrared position (see text). Each panel gives the average of the spectra obtained with respect to the A and B sky background positions, i.e. "(A+B)/2'' (top), as well as the difference between these A and B spectra, "(A-B)/2'', which was shifted by -20 K for clarity (bottom). Any signal in the difference spectrum is due to contamination by the A or B sky background positions. The plotted velocity scale is $V_{\rm lsr}=-10$ to +20 km s-1, and the intensity scale $T_{\rm R}^*=-25$ to +25 K.

In this paper we will not discuss directly the "water problem'', but we challenge the thesis that the FIR CO emission is emitted by shocked gas. For this we analyze the case of EL 29, a well-studied 36 $L_\odot$ (distance 160 pc; Chen et al. 1995) Class I source in the $\rho$ Ophiuchus complex (Wilking et al. 1989), on which we carried out an extensive observational study of the millimeter to near infrared emission (Boogert et al. 2000, 2002). In this paper we present (Sect. 2) ISO observations in the 43-197 $\mu$m spectral range obtained with the Long Wavelength Spectrometer (LWS; Clegg et al. 1996), observations of the H₂ rotational transitions obtained with the Short Wavelength Spectrometer (SWS; de Graauw et al. 1996), and a map of the CO  $J=6 \rightarrow 5$ emission obtained with the James Clerk Maxwell Telescope (JCMT). The goal is to pin down the origin of the FIR CO emission combining the FIR and the $J=6 \rightarrow 5$ observations, and the further constraint provided by the H₂ observations. The two sets of CO line observations are complementary, as LWS ISO observations have a relatively poor spatial (beam FWHM $\Delta \theta \sim 80''$) and spectral ( $\Delta {v} \sim 1500$ km s^-1) resolution, but probe warm and dense gas, whereas $J=6 \rightarrow 5$ observations provide much better access to the spatial extent ( $\Delta \theta \sim 12''$) and kinematics ( $\Delta v \sim 0.6$ km s^-1) of the warm gas. Finally, H₂ observations are a crucial test for the gas column density in the line of sight.

Using all this information we are able to constrain very accurately the temperature and mass of the emitting gas (Sect. 4), and, consequently, to discuss the origin of the observed CO FIR emission, which we propose is the flaring disk of EL 29 (Sect. 5).