ISO-LWS observations of Herbig Ae Be stars II. Molecular lines

We present the first ISO-LWS observations of the molecular FIR lines in 3 o ut of a sample of 11 Herbig Ae/Be stars (HAEBE), namely IRAS12496-7650, RCr A and LkH alpha 234. High-J rotational CO lines (from J(up),, = 14 to J(up) ,, = 19) have been observed in all the spectra, while two (at 79 mu m and 8 4 mu m) and three OH lines (at 71 mu m, 79 mu m and 84 mu m) were detected in LkH alpha 234 and RCrA respectively. For all sources the molecular emission has been consistently fitted with a Large Velocity Gradient (LVG) model and it results originated in a warm (T greater than or similar to 200 K) and dense (n(H2), greater than or similar to 10(5) cm(-3)) gas located in very compact regions having diameters of f ew hundreds of AU. These three sources are those with the highest density among the stars of t he sample; this suggests that the molecular emission arises in regions show ing density peaks. By comparing the observed cooling ratios with model predictions, we find th at the FUV radiation from the central source (or from a more embedded compa nion) is the most likely responsible for the line excitation. At least for the sources where OH has been observed, the contribution of shocks to the l ine emission can be reasonably ruled out because of the absence in the spec tra of any water vapour lints, in contrast with the predictions for molecul ar emission coming from warm shocked environments.