Molecular emission lines in Herbig Ae/Be stars

We present the first ISO-LWS observations of the molecular FIR lines in 3 out of a sample of 11 Herbig Ae/Be stars (HAEBE), namely IRAS12496-7650, RCrA and LkHalpha234. High-J pure rotational CO lines (from Jup = 14 to Jup = 19) have been observed in all the spectra, while two (at 79 mum and 84 mum) and three OH lines (at 71, 79 and 84 mum) have been detected in LkHalpha234 and RCrA, respectively. For all the sources the molecular emission has been consistently fitted with a Large Velocity Gradient (LVG) model and results as originated in a warm (T >= 200 K) and dense (nH2 >= 105 cm-3) gas. By comparing the observed cooling ratios with models predictions, we find that the FUV radiation is the most probable responsible for line excitation. Shock mechanism contributions can be reasonably ruled out, especially because of the absence in the spectra of any water vapour line, in contrast with the predictions on the molecular emission coming from warm shocked