thesis - IRS, The Infrared Spectrograph

141• PNe with He/H > 0.14. The stellar progenitors must have been stars with sub-solarmetallicity (i.e. LMC) with masses of 4 − 5 M ⊙ experiencing both the third dredge-upand hot bottom burning. A sub-solar metallicity has to be assumed to match the oxygenabundance. The combination of intermediate mass stars and low metallicity is peculiarand should be further investigated. In this sense, a study of PNe in other metallicityenvironments (such as the LMC or the galactic center) could be very illuminating. Wehave solved the problem of the observed N/O–He/H correlation and the C/O–N/O anticorrelation(Becker & Iben 1980) by assuming a very efficient third dredge-up thatbrings material to the surface containing a small amount of primary carbon synthesisedduring thermal pulses. Good agreement with the observations are found assumingλ ∼ 0.9 and X csh ( 12 C) ∼ 0.02−0.03. These stars must have experienced a significantproduction of 22 Ne via α-captures.Understanding the last stages of evolutionIn chapter 6 we have investigated the infrared fine-structure lines of [C II], [O I] and [Si II]in the photo-dissociation regions associated with PNe. This is of great importance for aproper understanding of the evolution of the ejected material; especially the excitationconditions under the influence of UV photon from the hot central nucleus and the actionof shocks driven by the fast stellar wind into the slower AGB wind. In order to interpretthe observations, we have compared the measured line intensities in nine PNe with thosepredicted by theoretical photo-dissociation and shock models. The comparison with thephoto-dissociation models has been done taking into account the carbon- or oxygen-richnature of the nebula. For NGC 7027, Hb 5 and NGC 6302 rotational lines of H 2 originatingfrom the PDRs, have been used to estimate the rotational temperature and column density ofH 2 . The results indicate that photo-dissociation rather than shocks is the main driving forceresponsible for the atomic lines in the PNe that have been studied. This comparison has beenused to derive the density of these regions. Unfortunately the conditions in the nebulae aresuch that the lines studied have reached the critical density and are no longer sensitive to thisparameter. To reliably derive the density of these regions other indicators (for instance CO)should be used. The rotational temperature derived from the pure rotational lines of H 2 in thePNe, NGC 7027, Hb 5 and NGC 6302 is between 500 and 830 K. The rotational temperatureand H 2 column density of these regions cannot discriminate between a PDR or shocked gas.The atomic, ionized and H 2 masses have been calculated for several PNe. These massestimates have been complemented by molecular masses derived from low J CO observations(Huggins et al. 1996). The results indicate that for the studied nebulae, the PDR is the mainreservoir of gas surrounding these objects. The relative amount of ionic gas increases as thenebula ages at the expense of the atomic and molecular mass. Despite the large uncertainties,there is a clear trend of increasing mass of the envelope with increasing core mass of thestellar remnant. This supports the notion that higher mass progenitors produce more massiveenvelopes and leave more massive stellar cores.There is a tantalizing relationship between the nucleosynthetic evolution of the progenitorstar and the characteristics of the dust emission features. However, in view of the smallsample and the uncertainty in the position of the nebulae in the HR diagram, it is too early todraw firm conclusions on this point.