thesis - IRS, The Infrared Spectrograph

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2 CHAPTER 1: IntroductionO5B2B5 A0F0 K0M010 4PNpost-AGBAGBLuminosity [L sun ]100White dwarfMain sequenceHBRGB3 rd Dredge-up& HBB2 nd Dredge-up1 st Dredge-up110 5 10 4 10 3Effective temperature [K]Figure 1.1–. Hertzsprung-Russel diagram showing the evolution of a low mass star. Different phasesof stellar evolution are indicated together with the main mixing episodes (within squares). The spectraltypes (Cox 2000) are given on top. The acronyms correspond to: MS→Main Sequence, RGB→RedGiant Branch, AGB→Asymptotic Giant Branch, HBB→Hot Bottom Burning, PN→Planetary Nebula.This plot is courtesy of Sacha Hony.1.1 Evolution of low and intermediate mass starsA discussion on the evolution of low and intermediate mass stars can be found in Osterbrock(1989). Descriptions of the mixing episodes occurring during the evolution of these stars aregiven by Renzini & Voli (1981); Iben & Renzini (1983), and more recently by Lattanzio &Forestini (1999).1.1.1 From the cradle to the graveIn Fig. 1.1 the evolution of a low mass star in the Hertzsprung-Russel diagram is shown. Starsspend most of their lives on the Main Sequence (MS) burning hydrogen in their core. Whenthe hydrogen in the nucleus is exhausted, the helium core contracts and, for stars with masses
1.1. Evolution of low and intermediate mass stars 32.5M ⊙ , this core becomes degenerate. Hydrogen fusion starts in a shell around the coreand the star moves to the Red Giant Branch (RGB). The temperature in the core increasesand at some moment becomes high enough to begin helium burning in the core (hydrogenburning continues in the shell), placing the star on the Horizontal Branch (HB). In stars withmasses 2.5M ⊙ helium burning is degenerate (helium-flashes). The helium burning resultsin a degenerate carbon and oxygen core. When all the helium in the core is burned the starmoves to the Asymptotic Giant Branch (AGB). This phase is characterised by two burningshells, one of helium around the nucleus and one of hydrogen further out 1 . The structure of astar in the AGB phase is illustrated in Fig. 1.2. During the AGB the star suffers mass loss viaa slow wind (∼10 km/s). At some – not well understood – moment, the star experiences amuch severe mass loss (accompanied by a super-wind) which results in the ejection of mostof the envelope of the star. As a consequence of the mass loss inner parts of the central star areexposed. These are very hot and eventually the far ultraviolet photons emitted by the centralstar are able to ionize the previously ejected envelope. The star becomes a Planetary Nebula(PN). When the thermonuclear reactions stop, the central star cools down (is no longer ableto ionize the ejected material) and continues to cool as a White Dwarf (WD). The remnants ofPN are returned to the interstellar medium, mix with other material present, waiting to forma new star.1.1.2 NucleosynthesisWe shall briefly explain here the reactions by which stars burn hydrogen and helium, but fora detailed description on these reactions we recommend the reader to look in Clayton (1983).Stars can burn hydrogen in their cores either via the so-called pp-chain (proton-protonchain) and CNO-cycle. The result of both reactions is the conversion of hydrogen into helium.In the CNO-cycle, carbon, oxygen and nitrogen act as catalysts for hydrogen to producehelium. The CNO-cycle has a strong dependence on temperature. Since the temperature inthe core of stars increases with mass, for stars with masses 1.2M ⊙ the hydrogen fusion isdominated by the pp-chain, and for stars with masses higher than that by the CNO-cycle.The helium burning occurs via the triple-alpha reaction where 12 C is produced. Some 12 Cand 4 He can fuse and produce 16 O. During the asymptotic giant branch some reactions inthe helium burning shell may occur in which neutrons are released. These neutrons may becaptured by heavy species and in this way the so-called s-process elements are formed.1.1.3 Mixing episodes during the evolutionIn the course of evolution, stars experience a series of mixing episodes in which nucleosynthesizedmaterial from the inner regions of the star is mixed to the surface. These processesare indicated in Fig. 1.1 and are summarized here:• First dredge-up. At the base of the Red Giant Branch, the convective envelope extendsdeep to regions where partial hydrogen burning via the CN cycle has taken place. Thesurface abundance of 4 He is enhanced and that of H depleted. Since 12 C has beenconverted partly into 13 C and 14 N, the surface abundance of 12 C is depleted and thatof 13 C and 14 N increased. 16 O remains almost unaffected.1 During this phase the helium and hydrogen shell are not always active.
Page 1 and 2: RIJKSUNIVERSITEIT GRONINGENPhysics
Page 3 and 4: To my parents
Page 5 and 6: Contents1 Introduction 11.1 Evoluti
Page 7 and 8: CONTENTSvii5 Probing AGB nucleosynt
Page 9: 1IntroductionPLANETARY NEBULAE are
Page 13 and 14: 1.2. Planetary Nebulae 5Flash−Dri
Page 15 and 16: 1.3. Physics in PNe 7their whole li
Page 17 and 18: 1.3. Physics in PNe 95.35 eVO IIIλ
Page 19 and 20: 1.4. Studying PNe in the infrared 1
Page 21 and 22: 2The ISO-SWS Spectrum ofPlanetary N
Page 23 and 24: 2.3. Data reduction 15Table 2.1-. C
Page 25 and 26: 2.4. Line flux discussion 17Table 2
Page 27 and 28: 2.5. The visual and the ultraviolet
Page 29: 2.6. Analysis 21Table 2.5-. Electro
Page 32 and 33: 24 CHAPTER 2: The ISO-SWS Spectrum
Page 39 and 40: 3An ISO and IUE study of PlanetaryN
Page 41 and 42: 3.3. Corrections to line fluxes 33[
Page 43 and 44: 3.4. Line fluxes 35Table 3.1-. Comp
Page 45 and 46: Table 3.3-. IUE intensities, fluxes
Page 47 and 48: 3.5. Physical parameters 393.4.3 Op
Page 49 and 50: 3.5. Physical parameters 41Table 3.
Page 51 and 52: 3.6. Chemical abundances 43Table 3.
Page 53 and 54: 3.6. Chemical abundances 45Table 3.
Page 55: 3.7. Conclusions 47Acknowledgements
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52 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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5.7. Summary and conclusions 103Fig
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5.7. Summary and conclusions 105A s
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6Physical Conditions in PDRs around
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6.2. Observations 109are not promin
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6.4. General parameters of the PNe
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6.4. General parameters of the PNe
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Table 6.3-. Radius, Zanstra tempera
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6.6. Lines fluxes 117Figure 6.2-. S
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6.7. Physical conditions of the PDR
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6.8. PDRs versus Shocks 127Figure 6
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6.10. Atomic, ionized and molecular
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6.10. Atomic, ionized and molecular
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6.11. General discussion 133Table 6
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6.11. General discussion 135Figure
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6.12. Summary and Conclusions 137su
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7Conclusions and future workTHE res
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141• PNe with He/H > 0.14. The st
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143HERSCHEL The launch of this sate
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Nederlandse SamenvattingSTERREN wor
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Nederlandse Samenvatting 147Figuur
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Nederlandse Samenvatting 149PHOTOIO
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Nederlandse Samenvatting 151Figuur
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Nederlandse Samenvatting 153infraro
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English SummarySTARS are born, live
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English Summary 157Figure 2-. Two e
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English Summary 159PHOTOIONIZATIONR
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English Summary 161Figure 5-. Spect
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English Summary 163all) stages of i
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BibliographyAcker A., Marcout J., O
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BIBLIOGRAPHY 167Henry R.B.C., Kwitt
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BIBLIOGRAPHY 169Pottasch S.R., Bern
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List of PublicationsPublications in
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AcknowledgementsI could easily writ
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Acknowledgements 175sense of humor,
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