thesis - IRS, The Infrared Spectrograph

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4 CHAPTER 1: Introduction• Second dredge-up. This occurs early in the asymptotic giant branch phase for starswith original masses larger than 3-5M ⊙ (the limit depends on the composition). Inthese stars as the helium burning core moves to the shell the hydrogen burning shell isextinguished. This allows the envelope to penetrate into the helium core. Consequently,4 He and 14 N are dredged-up to the surface, their abundances increase at the expense of12 C, 13 C and 16 O.• Third dredge-up (see Fig. 1.2). This takes place during the asymptotic giant branchand consists of several mixing episodes. The helium shell is thermally unstable andeventually produces huge amounts of energy. These helium shell flashes are also knownas thermal pulses (TP). The release of this energy produces a flash-driven convectionzone (see Fig. 1.2) and the 12 C produced in the helium burning shell is mixed with thatof the helium inter-shell. Following the pulse, the star expands and cools. Due to thiscooling the hydrogen burning shell is extinguished. This allows the inward penetrationof the convective envelope into regions where 12 C has been mixed with the ashes of thehydrogen burning shell (helium). Therefore, the surface abundances of 4 He and 12 Cincrease.• Hot bottom burning (see Fig. 1.2). This process is experienced by asymptotic giantbranch stars with masses larger than 4-4.5 M ⊙ . In these stars the convective envelopepenetrates into the hydrogen burning shell, and the CN-cycle actually occurs at thebase of the convective envelope. 12 C is converted into 13 C and then into 14 N. If thetemperatures are high enough the ON-cycle is activated causing the destruction of 16 Oto produce 14 N.1.2 Planetary Nebulae1.2.1 PNe and the chemical enrichment of the ISMStars are formed in molecular clouds of star forming regions such as the one pictured inFig. 1.3 (top). Molecular clouds consist mostly of hydrogen but they also contain moleculesand dust. At some moment, parts of these dense clouds start to collapse. Gravity does therest. As more material is accumulated the force of the gravity increases and therefore morematerial is attracted. This cumulative process involves an increase in temperature in the innerregions. The temperature rises until at some moment it is able to ignite nuclear reactions andit is then that we can say that a star is born. Regardless of the mass, stars spend most of theirlife burning hydrogen in the core to produce helium (like our Sun, Fig. 1.3 bottom-left). Asthe star evolves further thermonuclear reactions take place in the interior of stars. Via thesereactions, stars produce helium and metals. These are brought to the surface in a series ofdredge-up events that take place at different stages in stellar evolution. In the latest stagesof evolution the star experiences mass loss episodes. By the ejection of the outer layers theplanetary nebula returns gas to the interstellar medium that has been enriched with metalsfrom which new generations of stars will be formed (Fig. 1.3, bottom-right). In this way, bythe ejection of the outer layers PNe contribute to the enrichment of the ISM.
1.2. Planetary Nebulae 5Flash−Drivenintershell convectionDredge−UpHot Bottom BurningC,O CoreHe intershellConvective envelopeHe burning shellH burning shellFigure 1.2–. Onion shell-like structure of an AGB star. See text for details. Plot based on Lattanzio& Forestini (1999) (their Fig. 1).1.2.2 Importance of PNeThe planetary nebula phase offers for the first time in the life of a star the possibility to reliablycheck the results of the nuclear processes that have taken place in the interior of the star.The potential of studying the composition of the ejected material is enormous. Quantitativeinformation on the contribution of low and intermediate mass stars to the interstellar mediumcan be obtained. With the aid of evolutionary models the nucleosynthetic history of the starcan be reconstructed to learn the detailed history and characteristics of the progenitor starfrom which it evolved (Fig. 1.3, bottom-left). Furthermore the composition of the molecularcloud from which the progenitor star was formed (Fig. 1.3, top) can be inferred. In thisway, by studying the PN phase we become archeo-astronomers; digging through layers ofaccumulated historical dirt to uncover the full glory of past physical processes. However, allof this requires the derivation of accurate abundances of the ejected material in the PN phase.1.2.3 Neutral material around PNeAs we have seen, the FUV flux emitted by the hot central star will ionize the gas aroundit. The sphere where hydrogen is ionized is called Strömgren sphere. Outside this sphere,neutral hydrogen, molecules and ions of species that requires radiation hν < 13.6 eV to beionized, may exist. These regions are very important because it seems that the amount ofneutral material associated with young PNe is much larger than the ionized material. Neutralregions that are governed by the radiation of the central star are called photo-dissociationregions (PDRs). Emission from these regions includes species such as O 0 , C + , Si + , CO andH 2 . Besides far-ultraviolet radiation, shocks might also dominate the heating of these neu-
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 and 10: 1IntroductionPLANETARY NEBULAE are
Page 11: 1.1. Evolution of low and intermedi
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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54 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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