Low-excitation atomic gas around evolved stars II. ISO observations of O-rich nebulae

We have observed atomic fine-structure lines in the far-infrared (FIR) from 12 oxygen-rich evolved stars. The sample is composed of mostly proto-plane tary nebulae (PPNe) and some planetary nebulae (PNe) and asymptotic giant b ranch (AGB) stars. ISO LWS and SWS observations of [O I]. [C II], [N II], [ Si I]: [Si II], [S I], [Fe I], and [Fe II] lines were obtained. Taking into account also the sample presented by Fong et al. (Paper I) of carbon-rich evolved stars, we find that PPNe emit in these low-excitation atomic transi tions only when the central star is hotter than similar to 10 000 K. This r esult suggests that such lines predominantly arise from photodissociation r egions (PDRs), and not from shocked regions. The line widths determined fro m our Fabry-Perot data also suggest that the FIR lines arise from relativel y quiescent PDR gas, as opposed to shocked gas. Our results are in reasonab le agreement with predictions from PDR emission models, allowing the estima tion of the density of the emitting layers by comparison with the model res ults. On the other hand, the comparison with predictions of the emission fr om J-type and C-type shocked regions suggests that detected lines do not co me from shocks. The [C II] line flux has been used to measure the mass of t he low-excitation atomic component in PPNe, since this transition has been found to be a useful model-independent probe to estimate the total mass of these PDRs. The derivation of the mass formula and assumptions made are als o discussed.