Chemical evolution from cores to disks Visser, R.

Chemical evolution from cores to disks

Visser, R.

Citation

Visser, R. (2009, October 21). Chemical evolution from cores to disks. Retrieved from https://hdl.handle.net/1887/14225

Version: Corrected Publisher’s Version

License: Licence agreement concerning inclusion of doctoral thesis in the Institutional Repository of the University of Leiden

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Chemical evolution from cores to disks

Chemical evolution from cores to disks

PROEFSCHRIFT

ter verkrijging van

de graad van Doctor aan de Universiteit Leiden,

op gezag van de Rector Magnificus prof. mr. P. F. van der Heijden, volgens besluit van het College voor Promoties

te verdedigen op woensdag 21 oktober 2009 klokke 15.00 uur

door

Ruud Visser

geboren te Amersfoort in 1983

Promotiecommisie

Promotor: Prof. dr. E. F. van Dishoeck

Overige leden: Prof. dr. H. V. J. Linnartz Prof. dr. K. Kuijken Dr. M. R. Hogerheijde

Prof. dr. G. A. Blake California Institute of Technology Prof. dr. S. D. Doty Denison University

Prof. dr. E. A. Bergin University of Michigan

Dr. C. P. Dullemond Max-Plank-Institut für Astronomie

voor papa & mama

“Welcome the task that makes you go beyond yourself.”

– Frank McGee

Inhoudsopgave

1 Introduction 1

1.1 From ancient astronomy to modern astrochemistry . . . 2

1.2 Low-mass star formation and the role of chemistry . . . 4

1.3 Planets, comets and meteorites . . . 8

1.4 Chemical models . . . 10

1.4.1 Historical development and reaction types . . . 10

1.4.2 Solution methods . . . 12

1.5 This thesis . . . 14

2 The chemical history of molecules in circumstellar disks I: Ices 19 2.1 Introduction . . . 21

2.2 Model . . . 22

2.2.1 Envelope . . . 23

2.2.2 Disk . . . 25

2.2.3 Star . . . 28

2.2.4 Temperature . . . 30

2.2.5 Accretion shock . . . 30

2.2.6 Model parameters . . . 31

2.2.7 Adsorption and desorption . . . 32

2.3 Results . . . 33

2.3.1 Density profiles and infall trajectories . . . 34

2.3.2 Temperature profiles . . . 36

2.3.3 Gas and ice abundances . . . 37

2.3.4 Temperature histories . . . 44

2.4 Discussion . . . 45

2.4.1 Model parameters . . . 45

2.4.2 Complex organic molecules . . . 49

2.4.3 Mixed CO-H2O ices . . . 52

2.4.4 Implications for comets . . . 53

2.4.5 Limitations of the model . . . 54

2.5 Conclusions . . . 55

Appendix: Disk formation efficiency . . . 56

3 The chemical history of molecules in circumstellar disks II: Gas-phase species 59 3.1 Introduction . . . 61

3.2 Collapse model . . . 63 ix

Inhoudsopgave

3.2.1 Step-wise summary . . . 63

3.2.2 Differences with Chapter 2 . . . 64

3.2.3 Radiation field . . . 65

3.3 Chemical network . . . 66

3.3.1 Photodissociation and photoionisation . . . 67

3.3.2 Gas-grain interactions . . . 67

3.4 Results from the pre-collapse phase . . . 70

3.5 Results from the collapse phase . . . 72

3.5.1 One single parcel . . . 72

3.5.2 Other parcels . . . 79

3.6 Chemical history versus local chemistry . . . 88

3.7 Discussion . . . 97

3.7.1 Caveats . . . 97

3.7.2 Comets . . . 98

3.7.3 Collapse models: 1D versus 2D . . . 102

3.8 Conclusions . . . 102

4 Sub-Keplerian accretion onto circumstellar disks 105 4.1 Introduction . . . 107

4.2 Equations . . . 109

4.3 Size and mass of the disk . . . 111

4.4 Gas-ice ratios . . . 112

4.5 Crystalline silicates . . . 114

4.5.1 Observations and previous model results . . . 114

4.5.2 New model results . . . 115

4.5.3 Discussion and future work . . . 120

4.6 Conclusions . . . 122

5 The photodissociation and chemistry of CO isotopologues: applications to interstellar clouds and circumstellar disks 123 5.1 Introduction . . . 125

5.2 Molecular data . . . 126

5.2.1 Band positions and identifications . . . 127

5.2.2 Rotational constants . . . 130

5.2.3 Oscillator strengths . . . 131

5.2.4 Lifetimes and predissociation probabilities . . . 132

5.2.5 Atomic and molecular hydrogen . . . 134

5.3 Depth-dependent photodissociation . . . 134

5.3.1 Default model parameters . . . 134

5.3.2 Unshielded photodissociation rates . . . 135

5.3.3 Shielding by CO, H2and H . . . 135

5.3.4 Continuum shielding by dust . . . 137

5.3.5 Uncertainties . . . 138

5.4 Excitation temperature and Doppler width . . . 139 x

Inhoudsopgave

5.4.1 Increasing T_ex(CO) . . . 139

5.4.2 Increasing b(CO) . . . 142

5.4.3 Increasing Tex(H2) or b(H2) . . . 142

5.4.4 Grid of Texand b . . . 143

5.5 Shielding function approximations . . . 146

5.5.1 Shielding functions on a grid of N(CO) and N(H2) . . . 146

5.5.2 Comparison between the full model and the approximations . . . 147

5.6 Chemistry of CO: astrophysical implications . . . 151

5.6.1 Translucent clouds . . . 151

5.6.2 Photon-dominated regions . . . 159

5.6.3 Circumstellar disks . . . 162

5.7 Conclusions . . . 166

6 PAH chemistry and IR emission from circumstellar disks 169 6.1 Introduction . . . 171

6.2 PAH model . . . 172

6.2.1 Characterisation of PAHs . . . 172

6.2.2 Photoprocesses . . . 173

6.2.3 Absorption cross sections . . . 176

6.2.4 Electron recombination and attachment . . . 177

6.2.5 Hydrogen addition . . . 179

6.2.6 PAH growth and destruction . . . 179

6.2.7 Other chemical processes . . . 180

6.3 Disk model . . . 182

6.3.1 Computational code . . . 182

6.3.2 Template disk with PAHs . . . 183

6.4 Results . . . 183

6.4.1 PAH chemistry . . . 183

6.4.2 PAH emission . . . 186

6.4.3 Other PAHs . . . 190

6.4.4 Spatial extent of the PAH emission . . . 190

6.4.5 Sensitivity analysis . . . 191

6.4.6 T Tauri stars . . . 193

6.4.7 Comparison with observations . . . 194

6.5 Conclusions . . . 194

Nederlandse samenvatting 197

Literatuur 204

Publicaties 219

Curriculum vitae 221

Nawoord 223

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