thesis - IRS, The Infrared Spectrograph

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126 CHAPTER 6: Physical Conditions in PDRs around Planetary Nebulae6.8 PDRs versus ShocksIn the introduction we mentioned that shocks are also able to photo-dissociate and heatthe gas. This mechanism is thought not be very important in PNe (Hollenbach & Tielens1999). Indeed, the results by Fong et al. (2001) and Castro-Carrizo et al. (2001) show thatphoto-dissociation is the dominant process in the circumstellar envelope in the PNe that theystudied. A discussion on the relevance of shocks will follow but first it is important to spenda few words on how shocks are produced.At the end of the AGB, the mass loss rate increases considerably. This so-calledsuperwind phase separates the AGB phase from the post AGB phase. The star then starts tomove to higher effective temperature in the HR diagram and will start to ionize the previouslyejected AGB wind material. At the same time, the star may develop a high velocity (1000km/s) but tenuous wind. This high velocity wind will drive strong shock waves into theAGB ejecta which can dissociate and heat the gas. Some objects also show evidence forpowerful jets interacting with the AGB wind material. It is important to remark that thereare two kind of shocks, C-shocks (continuous or magnetic) and J-shocks. The former onespropagate with velocities up to 40 km/s, the gas properties change more gradually, andmolecules are not dissociated or species ionized. J-shocks on the other hand are more violent.The shock front is very thin and causes a discontinuity in the gas properties before andafter the front. Molecules are readily dissociated and – for high velocities – the gas is ionized.The intensity of the cooling lines is different in photo-dissociated gas than in shockedgas. Comparison of observations with models can be used to distinguish between both mechanisms.Van den Ancker (1999) deduced some useful tips to distinguish the presence of aPDR or a shock (C or J type) by comparing a large sample of objects to PDR and shock models.Among all of them, the criteria relevant for this study are: 1) The detection of [Si II] or[Fe II] implies the presence of a PDR or a J-shock respectively. 2) The presence of strong [S I]emission unambiguously indicates that a shock is present. 3) The ratio [O I] (63 µm)/[C II]is a good indicator between PDRs of average or high densities and J-shocks. The readercould check van den Ancker et al. (2000) for more details. In assessing these discriminatorsit should be kept in mind that the penetrating FUV photons keep the sulfur ionized to verydeep in the region and hence the [S I] lines is only very weak. By the same token, silicon issingly ionized throughout most of the PDR and hence PDRs are copious emitters of [Si II]line radiation. C-shocks, on the other hand are by necessity, characterized by low degrees ofionization and hence not bright in ionic lines such as [Si II] and [C II]. J-shocks can be brightin both [S II] and [C II] as well as [S I] since the gas recombines as it cools down. More extensiveexplanations can be found in Hollenbach & Tielens (1999) and Hollenbach & McKee(1989).In six PNe, [Si II] line has been detected (Table 6.5). This rules out the presence of C-shocks in these nebulae. The ratio [O I] (63 µm)/[C II] is high in all PNe with measured [C II](seven in total, including the three that have no [Si II]) and this points to the presence of PDRsor J-shocks. We can therefore rule out the presence of C-shocks in these nebulae. We haveplotted the intensities of [Si II] and [C II] with those predicted by J-shock models at differentdensities and velocities of the shocked gas in Fig. 6.7. It can be seen that the J-shock modelscannot reproduce the observed intensities except for NGC 3918. This comparison together
6.8. PDRs versus Shocks 127Figure 6.7–. Comparison of the observed intensities with J-shock model predictions (van den Ancker1999). Solid lines are values for constant density and dashed lines of constant velocity. On the x-axis,ΣI is the sum of all measured lines.
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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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Page 87 and 88: 5.4. Nucleosynthesis in low- and in
Page 89 and 90: 5.5. Synthetic TP-AGB calculations
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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
Page 121 and 122: 6.4. General parameters of the PNe
Page 123 and 124: Table 6.3-. Radius, Zanstra tempera
Page 125 and 126: 6.6. Lines fluxes 117Figure 6.2-. S
Page 127 and 128: 6.7. Physical conditions of the PDR
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Page 137 and 138: 6.10. Atomic, ionized and molecular
Page 139 and 140: 6.10. Atomic, ionized and molecular
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 and 148: 7Conclusions and future workTHE res
Page 149 and 150: 141• PNe with He/H > 0.14. The st
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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