Submm/FIR CO line emission from the disk of the Class I protostar EL 29

Abstract

We present observations towards the Class I protostar EL 29 of the CO J = 6 → 5 and J ≥ 15 transitions obtained with JCMT and ISO LWS respectively, and of five H_2 rotational lines obtained with ISO SWS. The simultaneous analysis of these observations reveals the presence of a warm gas component at about 170–250 K, ~550 AU in size, and whose density is ≥ 10^6 cm^(-3). The mass of the warm gas is ~8-24 x 10^(-4) M_⊙. These values compare extremely well with the predictions of the temperature and mass of the flaring disk surrounding EL 29, probed by the dust continuum. We propose that the observed FIR CO emission originates in the super-heated surface layer of the disk of EL 29 and discuss its characteristics. We find that the CO abundance in the disk is large, ≥10^(-4), implying no depletion or photodissociation and we present arguments in favor of the idea that the dust has settled on the midplane disk of EL 29 and that it is thermally decoupled from the gas. We compare the characteristics of the El 29 disk with those of the disks observed in other Herbig AeBe stars using recent studies of H_2 rotational line emission. The gas temperature and mass derived for the disk of EL 29 are similar to the disks of the previously studied sample. In EL 29, as in Herbig AeBe stars, the gas and dust are probably thermally decoupled. Finally, the upper limit on the H_2O emission that we obtain suggests that water is not an important coolant of the disk gas, in agreement with theoretical water abundance predictions. The present study challenges previous claims that the FIR CO line emission observed in sources similar to EL 29 originates in shocks.

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