Far-infared cooling in pre-MS sources

Paolo Saraceno ¹, B. Nisini ¹, M. Benedettini ¹, A. Di Giorgio ¹, T. Giannini ¹, D. Lorenzetti ², S. Pezzuto ¹, L. Spinoglio ¹, E. Tommasi ², E. Caux ³, C. Ceccarelli ⁴, S.J. Leeks ⁵, R. Liseau ⁶, H. Smith ⁷, & G. White ⁵

¹ Istituto di Fisica dello Spazio Interplanetario - CNR - Roma, Italy

² Osservatorio di Roma, Monteporzio, Italy

³ CESR, BP4346, F-31028 Toulouse Cedex 04, France

⁴ Observatoire de Grenoble, BP 53 F-38041, Grenoble, France

⁵ Queen Mary & Westfield College, London, UK

⁶ Harvard Smithsonian Center for Astrophysics, MA 02138 Cambridge

⁷ Stockholm Observatory, S-133 36 - Saltsjobaden, Sweden

We will present the results of a 45-190 m spectroscopic study, carried out with the Long Wavelength Spectrometer on board of the ISO satellite, of a sample of Class 0,  Class I and Class II (Herbig Ae/Be and TTau stars) sources. We will discuss the following findings:

• Molecular transitions are the dominant FIR coolants in the observed Class 0 and Class I sources, as expected in presence of non dissociative C shocks. In contrast with model predictions we find that most of our sources have L_H2O significantly lower than L_CO, and that sources of similar type and luminosity have a variation of the L_H2O/L_CO ratio of more than a factor 10

• In the Ae/Be stars and in the high luminosity Class I sources, the molecular line luminosity, when observed, is less intense (in few cases comparable) than the [OI] 63 line. In these objects the [OI] 63 emission is generally higher than the one predicted by shocks models and could be explained by the presence of photodissociation regions

• J shocks excitation seems to account for the [OI] 63 intensity, observed in the low luminosity Class 0, Class I sources, in TTau stars and and HH objects

• In most of the observed sources we find a ratio R = [OI]63/[OI]145 substantially lower than the one predicted for a collisionally excited gas. The lowest value (R = 2) is found in the optically visible ClassII objects MWC 297. We will discuss the possible implications.