Far infrared spectroscopy of FU Ori objects - ISO-LWS observations

We present the results of the first spectrophotometric observations of a sa mple of FU Ori objects obtained with the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). The [OI] (63 mu m) and the [CII] (158 mu m) lines are commonly observed in all spectra (both ON and OF F source). The observational novelty is the presence in most of the sources of the transition of ionised nitrogen [NII] (122 mu m), which is not detec ted in other objects in a similar evolutionary phase. This line probes low ionisation and low density material not easily traced by other lines. Line intensities and intensity ratios are used along with model predictions to i nfer the prevailing mechanisms for line excitation. To reconcile our far-in frared spectroscopy with previous knowledge of these objects, the simultane ous presence of two components is required: well localised J-shocks, respon sible for the [OI] emission, and an extended low density ionised medium pro duced by UV photons from the disc boundary layer, responsible for the [NII] and [CII] emission. A few molecular lines.(CO, OH, H2O) associated with re latively cold and dense peaks are revealed and their intensities are in goo d agreement with the proposed scenario. Other ionic lines ([OIII] and [NIII ]) are detected in two sources in the Cyg OB7 region and likely trace the p resence of nearby HII regions.