Physical conditions in Photo-Dissociation Regions around Planetary Nebulae

We present observations of the infrared fine-structure lines of [Si II] (34.8 mum), [O I] (63.2 and 145.5 mum) and [C II] (157.7 mum) obtained with the ISO SWS and LWS spectrographs of nine Planetary Nebulae (PNe). These lines originate in the Photo-Dissociation Regions (PDRs) associated with the nebulae and provide useful information on the evolution and excitation conditions of the ejected material in these regions. In order to interpret the observations, the measured line intensities have been compared with those predicted by photo-dissociation models. This comparison has been done taking into account the C/O content in the nebulae. The densities derived with this comparison show a large scatter for some nebulae, probably because the density is higher than the critical density. Therefore, they are no longer sensitive to this parameter implying that transitions from other species with higher critical density should be used. The possible contribution of shocks to the observed emission characteristics of these PNe is briefly discussed and it is shown that the radiation field is the main driving force responsible for the atomic lines in the PNe that have been studied. In addition, data on the pure rotational lines of H-2 in three nebulae (NGC7027, NGC6302 and Hb 5) are also presented. Assuming local thermal equilibrium the rotational temperature and densities have been derived. We have derived the mass of atomic gas in the PDR associated with these PNe and compared those to ionic masses derived from H and molecular masses derived from low J CO observations. This comparison shows that for these nebulae, the PDR is the main reservoir of gas surrounding these objects. A comparison of the results of these evolved PNe with very young PNe from the literature suggests that as the nebula ages the relative amount of ionic gas increases at the expense of the atomic and molecular mass.