thesis - IRS, The Infrared Spectrograph

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140 CHAPTER 7: Conclusions and future workinfrared emission is hardly affected by extinction. The ultraviolet data is essential to derivethe abundance of carbon.In many cases ionization correction factors were not necessary because all the importantstages of ionization were observed. When needed, ionization correction factors were typicallyless than ∼1.2. This improvement shows that the combination of the infrared, ultraviolet andoptical data is a powerful tool to derive abundances. It is important mentioning that thenebular abundances are often very different from the abundances of surface layers of thecentral stars which excite them. This is especially noticeable in nebulae which are excited bycentral stars of the Wolf-Rayet type. This still remains a puzzle.Temperature gradientInfrared and optical line fluxes combined, provided information on the temperature of thegas for a different number of ions. In NGC 7027 and NGC 2440 a trend of electron temperaturewith ionization potential is found. Ions with higher ionization potential show highertemperatures because they are probably formed closer to the central star. The temperaturein NGC 6543 is constant with ionization potential. This information allows us to constrain(based on the ionization potential) the temperature of ions whose abundances have been derivedusing ultraviolet or optical data that are temperature dependent.Stellar nucleosynthesis in low and intermediate mass starsThe derived abundances of the PNe in our sample, supplemented by comparable datafrom the literature analyzed in a similar way, are compared in chapter 5 with AsymptoticGiant Branch models to give insight in the nuclear reactions and mixing processes thatthe progenitor stars must have experienced. These models put especial emphasis on theprocesses that are expected to produce the most significant changes in CNO and He surfaceabundances, namely the third dredge-up and the hot bottom burning. The former consists ofseveral mixing episodes in which products of the He burning shell are brought to the surface.This process is characterised by its efficiency (λ) and by the elemental abundance of theconvective intershell (X csh ) developed at each thermal pulse. The hot bottom burning takesplace when the convective envelope deeply penetrates into the hydrogen burning shell whereCN-nucleosynthesis is taking place.The sample separates quite naturally in two different groups The first group show He/H ≤0.14. The other group presents higher helium content (He/H > 0.14) together with low carbonand oxygen abundances. This division may point to two different interpretative scenarios.• PNe with He/H ≤ 0.14. The stellar progenitors must have been stars with solar metallicityin the range of masses from 0.9 to 4 M ⊙ . The efficiency of hot bottom burningshould not be very large in order to match the nitrogen abundance. The helium contentcan be reproduced with the first, and possibly second and third dredge-up. The carbonabundances can be reproduced using models with variable opacities and efficiencies ofthe third dredge-up λ ∼ 0.3 − 0.4. The progenitors of some carbon rich PNe in thesample that also show traces of carbon enrichment must have been carbon stars whichhad experienced the third dredge-up.
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.
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RIJKSUNIVERSITEIT GRONINGENPhysics
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To my parents
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Contents1 Introduction 11.1 Evoluti
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CONTENTSvii5 Probing AGB nucleosynt
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1IntroductionPLANETARY NEBULAE are
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1.1. Evolution of low and intermedi
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1.2. Planetary Nebulae 5Flash−Dri
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1.3. Physics in PNe 7their whole li
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1.3. Physics in PNe 95.35 eVO IIIλ
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1.4. Studying PNe in the infrared 1
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2The ISO-SWS Spectrum ofPlanetary N
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2.3. Data reduction 15Table 2.1-. C
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2.4. Line flux discussion 17Table 2
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2.5. The visual and the ultraviolet
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2.6. Analysis 21Table 2.5-. Electro
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24 CHAPTER 2: The ISO-SWS Spectrum
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3An ISO and IUE study of PlanetaryN
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3.3. Corrections to line fluxes 33[
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3.4. Line fluxes 35Table 3.1-. Comp
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Table 3.3-. IUE intensities, fluxes
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3.5. Physical parameters 393.4.3 Op
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3.5. Physical parameters 41Table 3.
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3.6. Chemical abundances 43Table 3.
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3.6. Chemical abundances 45Table 3.
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3.7. Conclusions 47Acknowledgements
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50 CHAPTER 4: Abundances of Planeta
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5Probing AGB nucleosynthesis viaacc
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5.1. Introduction 71about the synth
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Table 5.2-. Radius, Zanstra tempera
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5.3. Accurate abundances of ISO-obs
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5.3. Accurate abundances of ISO-obs
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5.4. Nucleosynthesis in low- and in
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5.5. Synthetic TP-AGB calculations
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5.6. Comparison between models and
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Page 111 and 112: 5.7. Summary and conclusions 103Fig
Page 113 and 114: 5.7. Summary and conclusions 105A s
Page 115 and 116: 6Physical Conditions in PDRs around
Page 117 and 118: 6.2. Observations 109are not promin
Page 119 and 120: 6.4. General parameters of the PNe
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Page 125 and 126: 6.6. Lines fluxes 117Figure 6.2-. S
Page 127 and 128: 6.7. Physical conditions of the PDR
Page 135 and 136: 6.8. PDRs versus Shocks 127Figure 6
Page 137 and 138: 6.10. Atomic, ionized and molecular
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Page 141 and 142: 6.11. General discussion 133Table 6
Page 143 and 144: 6.11. General discussion 135Figure
Page 145 and 146: 6.12. Summary and Conclusions 137su
Page 147: 7Conclusions and future workTHE res
Page 151 and 152: 143HERSCHEL The launch of this sate
Page 153 and 154: Nederlandse SamenvattingSTERREN wor
Page 155 and 156: Nederlandse Samenvatting 147Figuur
Page 157 and 158: Nederlandse Samenvatting 149PHOTOIO
Page 159 and 160: Nederlandse Samenvatting 151Figuur
Page 161 and 162: Nederlandse Samenvatting 153infraro
Page 163 and 164: English SummarySTARS are born, live
Page 165 and 166: English Summary 157Figure 2-. Two e
Page 167 and 168: English Summary 159PHOTOIONIZATIONR
Page 169 and 170: English Summary 161Figure 5-. Spect
Page 171 and 172: English Summary 163all) stages of i
Page 173 and 174: BibliographyAcker A., Marcout J., O
Page 175 and 176: BIBLIOGRAPHY 167Henry R.B.C., Kwitt
Page 177 and 178: BIBLIOGRAPHY 169Pottasch S.R., Bern
Page 179 and 180: List of PublicationsPublications in
Page 181 and 182: AcknowledgementsI could easily writ
Page 183: Acknowledgements 175sense of humor,
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