2001A&A...367..674C

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-planetary nebulae (PPNe) and some planetary nebulae (PNe) and asymptotic giant branch (AGB) stars. ISO LWS and SWS observations of [OI], [CII], [NII], [SiI], [SiII], [SI], [FeI], and [FeII] lines were obtained. Taking into account also the sample presented by Fong et al. (2001A&A...367..652F, Paper I) of carbon-rich evolved stars, we find that PPNe emit in these low-excitation atomic transitions only when the central star is hotter than ∼10000K. This result suggests that such lines predominantly arise from photodissociation regions (PDRs), and not from shocked regions. The line widths determined from our Fabry-Perot data also suggest that the FIR lines arise from relatively quiescent PDR gas, as opposed to shocked gas. Our results are in reasonable agreement with predictions from PDR emission models, allowing the estimation of the density of the emitting layers by comparison with the model results. On the other hand, the comparison with predictions of the emission from J-type and C-type shocked regions suggests that detected lines do not come from shocks. The [CII] line flux has been used to measure the mass of the 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 also discussed.