ISO-LWS observations of Herbig Ae Be stars I. Fine structure lines

We present the results of the first spectrophotometric survey of a sample o f eleven Herbig Ae/Be stars (HAEBE) obtained with the Long Wavelength Spect rometer (LWS) on board the Infrared Space Observatory (ISO). The [OI] 63 mu m and the [CII] 158 mu m lines an obsereved in all the investigated source s, while the [OI] 145 mu m transition, due to its relative faintness, somet imes remains undetected. By comparing line intensity ratios with model pred ictions, photodissociation, due to the UV photons from the central star, re sults the dominating excitation mechanism although contributions of C-shock s to the [OI] emission cannot be ruled out. A clear example for the presenc e of a photodissociation region (PDR) illuminated by an HAEBE is shown by L WS spectroscopic mapping of NGC 7129. Some diagnostic probes of the radiati on held and density are provided for the objects in our sample: these subst antially agree with the known characteristics of both the star and its circ umstellar environment, although the observed ratio [OI]63/[OI] 145 tends to be smaller than predicted by PDR models. The most likely explanation for t his behaviour is self-absorption at 63 mu m bq cold atomic oxygen. Fine str ucture lines of the ionised species [OIII], [NII] were detected whenever th e star had a spectral type of B0 or earlier: in particular, around the star CoD-42 degrees 11721, besides a compact HII region, evidence is given for an extended low electron density ionised region. Finally, molecular line em ission is associated with stars powering a CO outflow, and clumpy PDR model s, better than C-shock models, predict for them relative cooling (CO vs OI and CO vs OH) similar to the observed ones.