6 Discussion

Indeed the disk hypothesis is not surprising in the light of the recent analysis of the H₂ and CO  $J=6 \rightarrow 5$ emission in T Tauri and Herbig AeBe stars by Thi et al. (2001). They found that the surveyed stars (about a dozen) have relatively large amounts, $\sim$ $10^{-2}{-}10^{-3}~M_\odot$, of warm gas, whose temperature varies between 100 K and 220 K. The T Tauri star sample have gas at $\sim$100 K, whereas Herbig AeBe stars have higher temperatures, $\geq$150 K. Figure 6 plots the masses and temperatures measured by the H₂ observations of the Thi et al. sample together with the values that we derived for EL 29. This figure shows that EL 29 has a "normal'' disk, when compared to the Herbig AeBe stars of the sample. Since EL 29 is a probable precursor of Herbig AeBe stars (e.g. BHC02), in this respect and on a purely observational basis, EL 29 does not seem to possess a particularly peculiar disk, a fact that strengthens further the thesis that FIR CO emission originates in its disk.

Figure 6: Masses and temperatures measured by the H2 observations of the Thi et al. (2001) sample (diamonds) and by our observations in EL 29 (asterisk). Some Herbig AeBe stars of the Thi et al. (2001) sample are marked for reference.

More specifically, AB Aur (in the sample of Thi et al. 2001) shares many similarities with EL 29. AB Aur and EL 29 have a bolometric luminosity of $\sim$50 and $\sim$ $40~L_\odot$ respectively, similar flat SEDs, and relatively face-on disks. The AB Aur H₂ observations yield a gas temperature of ( $185 \pm 15$) K, against the 170-250 K we find for EL 29, and a warm gas mass of ( $1.3 \pm 0.7$) $\times 10^{-3}~M_\odot$, against 8-20  $\times 10^{-4}~M_\odot$ in EL 29. As a matter of fact, the ¹²CO  $J=6 \rightarrow 5$ intensity is very similar to that of EL 29 (50 against 100 K km s^-1) and the S(1) H₂ line flux is $3 \times 10^{-13}$ erg s^-1 cm^-2 in AB Aur, consistent with the upper limit for EL 29 of $10 \times 10^{-13}$ erg s^-1 cm^-2. The ¹²CO  $J=6 \rightarrow 5$ linewidth is 2.1 km s^-1 in AB Aur against the 3.6 km s^-1 in EL 29, perhaps because of a slightly different inclination angle. As already remarked, it is likely that the ¹²CO  $J=6 \rightarrow 5$ in EL 29 is strongly "contaminated'' by the envelope and some of the ridge in which EL 29 is embedded, more than AB Aur is, and that would explain the stronger and relatively extended ¹²CO  $J=6 \rightarrow 5$ emission in EL 29. Class I (embedded) sources are indeed known to possess envelopes that emit copiously in the CO  $J=6 \rightarrow 5$ transition (Hogerheijde et al. 1998). For example, L1489 IRS has a ¹²CO  $J=6 \rightarrow 5$ intensity very similar to that in EL 29.

We mentioned above the possibility that the gas temperature of the EL 29 super-heated layer is somewhat larger than the dust temperature. There are reasons to think that gas and dust are thermally decoupled in the disk super-heated layer.

Figure 7: Gas and dust temperatures measured by the H2 observations of the Thi et al. (2001) sample (diamonds) and by our observations in EL 29 (asterisk). Some Herbig AeBe stars of the Thi et al. (2001) sample are marked for reference. The dashed line marks the gas temperature equal to the dust temperature.

First and foremost, from an observational point of view this is exactly what has been observed in the disks of T Tauri and Herbig AeBe stars by Thi et al. (2001). They found that the gas temperature derived by the observed H₂ lines is systematically 20 K or more larger than the dust temperature derived by the continuum at the frequency of the H₂ lines. In this respect, once again, EL 29 does not constitute a peculiar case, as shown in Fig. 7. As Thi et al. (1999, 2001) discuss in their articles, the gas may be warmer because of UV photon heating. Here we wish to address the possibility that X-rays from the central source can also play a role in this extra heating (e.g. Glassgold et al. 1997), as Pre-Main Sequence stars are known to be prodigious X-rays emitters (Feigelson & Montemerle 1999). In particular EL 29 emits about $5 \times 10^{-4}~L_\odot$ in the X-ray and it is known to be a X-ray flaring source too (Imanishi et al. 2001). We postpone, however, a full discussion of the heating mechanisms to a forthcoming paper (Ceccarelli et al. in prep.). Whether and how much the gas and dust are decoupled in EL 29 is difficult to establish on the basis of the available observations. Further observations should be able to settle the question.

Finally, we wish to comment the presumed problem of water underabundance (see Introduction). Our LWS observations show that the water abundance is less than $5 \times 10^{-7}$ in the super-heated layer disk of EL 29. This value does not present any "problem'' and it is consistent with theoretical expectations of chemical models (e.g. Lee et al. 1996). Indeed the water abundance in the gas around low mass protostellar envelopes is few times 10^-7 (e.g. Ceccarelli et al. 2000; Maret et al. 2002) and in the molecular clouds is even lower (Caux et al. 1999; Snell et al. 2000). Since the gas temperature does not exceed the fatal threshold of $\sim$250 K, needed to open the route to water formation by endothermic reactions (e.g. Wagner & Graff 1987), there is no reason to expect an enhanced water abundance in the super-heated layer, even if a large fraction of gaseous water is injected from the evaporated grain mantles. In fact the super-heated layer is by definition exposed to the UV photons from the central source and most of water would hence be photo-dissociated (also in the region where CO is re-formed, because of the self-shielding). The upper limit on the water abundance that we find is, in this respect, in good agreement with the value found in the PDR of NGC 133 ($\sim$10^-7 by Bergin et al. 2002). Incidently, this implies that H₂O line emission is not the main gas cooling mechanism as CG97 assumed in their study of the disk characteristics. The EL 29 LWS spectrum shows that CO is indeed the main coolant of the gas in the disk super-heated layer. Since CO is a relatively low efficiency gas cooler, the gas temperature can stay higher than they presumed in their article.