7 Conclusions and perspectives

We presented observations of the CO emission from J=6 to J=19 and H₂ emission, observed towards EL 29 with different instruments, ISO and JCMT. The simultaneous analysis of all these data suggests that the submm/FIR CO emission, J=6 and between J=15 and J=19 is emitted by a $\sim$4'' (i.e. $\sim$250 AU from the center) region of gas, whose temperature is between 170 K and 250 K, whose density is larger than $\sim$10⁶ cm^-3 and whose CO column density is $5 \times 10^{18}$ cm^-2. The mass of the warm gas is 8-24  $\times 10^{-4}~M_\odot$ and its CO abundance is $\geq$ $1 \times 10^{-4}$, implying no substantial CO depletion and/or photodissociation. We propose that this warm gas resides in a super-heated surface disk layer. We applied the disk model previously developed to reproduce the SED of EL 29 (BHC02) and could also reproduce the observed submm/FIR CO lines, assuming a relatively large density in the heated layer ($\geq$10⁶ cm^-3). This density is expected when the dust has started to settle to the midplane. Alternatively, dust and gas may be largely thermally decoupled, with the gas temperature about 250 K.

The derived temperature and mass are similar to the values found in the disks of other Herbig AeBe stars (Thi et al. 2001), supporting the case that EL 29 is rather a precursor or very embedded Herbig AeBe star. Finally, the lack of water emission is fully consistent with the interpretation of the FIR CO emission originating in the disk of EL 29, and does not necessitate any mechanism to lower the water abundance.

The question arises whether the FIR CO emission observed in several other young protostars is also probing their disks, rather than the so-far claimed shocks (e.g. Giannini et al. 2001). Although a full discussion is out of the scope of this paper and we postpone it to a forthcoming paper, we wish to remark that this theory - FIR CO emitted in the super-heated surface layer of disks - would naturally explain the lack of water emission in Class I protostars, as water abundance is expected to be low in the disks. On the contrary, the water emission observed in Class 0 sources is naturally explained in terms of thermal emission from the massive envelopes that surround these sources (Ceccarelli et al. 1996; Ceccarelli et al. 2000; Maret et al. 2002), and has hence a different origin than the CO emission.

Acknowledgements

We wish to thank P. André, S. Molinari, H. Smith and G. White for their contribution in the first stage of this project. CC thanks Wing-Fai Thi for making her aware of his thesis work. We finally thank the referee, G. J. van Zadelhoff, for carefully reading the manuscript.