Outflows from young objects observed with the ISO-LWS - I. Fine structure lines [O I] 63 mu m, [O I] 145 mu m and [C II] 157 mu m

Far infrared fine structure line data from the ISO archive have been extracted for several hundred YSOs and their outflows, including molecular ( CO) outflows, optical jets and Herbig-Haro (HH) objects. Given the importance of these lines to astrophysics, their excitation and transfer ought to be investigated in detail and, at this stage, the reliability of the diagnostic power of the fine structure transitions of OI and CII has been examined. Several issues, such as the extremely small intensity ratios of the oxygen 63 mu m to 145 mu m lines, are still awaiting an explanation. It is demonstrated that, in interstellar cloud conditions, the 145 mu m line is prone to masing, but that this effect is likely an insufficient cause of the line ratio anomaly observed from cold dark clouds. Very optically thick emission could in principle also account for this, but would need similar, prohibitively high column densities and must therefore be abondoned as a viable explanation. One is left with [OI] 63 mu m self absorption by cold and tenuous foreground gas, as has been advocated for distant luminous sources. Recent observations with the submillimeter observatory Odin support this scenario also in the case of nearby dark molecular clouds. On the basis of this large statistical material we are led to conclude that in star forming regions, the [OI] and [CII] lines generally have only limited diagnostic value.