thesis - IRS, The Infrared Spectrograph

6Physical Conditions in PDRs aroundPlanetary NebulaeBased on:J. Bernard-Salas, & A.G.G.M. TielensTO BE SUBMITTED IN ASTRONOMY & ASTROPHYSICSWE present observations of the infrared fine-structure lines of [Si II] (34.8 µm), [O I](63.2 and 145.5 µm) and [C II] (157.7 µm) obtained with the ISO SWS and LWSspectrographs in nine Planetary Nebulae. These lines originate in the Photo-DissociationRegions (PDRs) associated with the nebulae and provide useful information on the evolutionand exciting conditions of the ejected material in these regions. In order to interpretthe observations, the measured line intensities have been compared with those predicted byphoto-dissociation models. This comparison has been done taking into account the C/O contentin the nebulae. The densities derived with this comparison show a large scatter for somenebulae, probably because the density is higher than the critical density. Therefore, they areno longer sensitive to this parameter implying that transitions from other species with highercritical density should be used. The possible contribution of shocks to the observed emissioncharacteristics of these PNe is briefly discussed and it is shown that the radiation field is themain driving force responsible for the atomic lines in the PNe that have been studied. Inaddition, data on the pure rotational lines of H 2 in three nebulae (NGC 7027, NGC 6302 andHb 5) are also presented. Assuming local thermal equilibrium the rotational temperature anddensities have been derived. We have derived the mass of atomic gas in the PDR associatedwith these PNe and compared those to ionic masses derived from Hβ and molecular massesderived from low J CO observations. This comparison shows that for these nebulae, the PDRis the main reservoir of gas surrounding these objects. Analysis of the results suggests thatas the nebula ages the relative amount of ionic gas increases at the expense of the atomic andmolecular mass. Finally, while the uncertainties are large, there is a clear trend of increasingmass of the envelope with increasing core mass of the stellar remnant, supporting the notionthat higher mass progenitors produce more massive envelopes and leave more massive stellarcores.