Dust and gas in protoplanetary discs [Elektronische Ressource] / von Dmitry Semenov

Sujets

• Astronomie

Informations

• Publié par: friedrich-schiller-universitat_jena
• Publié le: 01 janvier 2005
• Nombre de lectures: 32
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Dust and Gas in Protoplanetary Discs
Dissertation
zur Erlangung des akademischen Grades
doctor rerum naturalium (Dr. rer. nat.)
vorgelegt dem Rat der Physikalisch Astronomischen Fakultat¨
der Friedrich Schiller Universit at¨ Jena
von Diplom Astronom Dmitry Semenov
geboren am 20 Februar 1978 in Sankt Petersburg (Rußland)Gutachter
1. Prof. Dr. Thomas Henning (Jena/Heidelberg)
2. Prof. Dr. Tom Millar (Manchester, UK)
3. Prof. Dr. Ewine van Dishoeck (Leiden, NL)
Tag des Rigorosums: 2005
Tag der o¨entlichen Verteidigung: 2005”Do not keep saying to yourself, if you can possibly avoid it, ’But how can it be like that?’ because
you will get ’down the drain’ into a blind alley from which nobody has yet escaped. Nobody knows
how it can be like that.”
Richard Feynman
”Astronomy”, Alain of Lille (XII century)4Contents
1 Introduction 1
2 Opacities for protoplanetary discs 3
2.1 A link between disc hydro models and opacities . . . . . . . . . . . . . . . . . . . . 3
2.2 The model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.1 Dust opacities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.2 Gas opacities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.3 Opacity table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3 Computed monochromatic and mean opacities . . . . . . . . . . . . . . . . . . . . . 14
2.3.1 Opacities and dust models . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.2 Comparison to other studies . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.3 Opacities and disc structure . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.4 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Chemical evolution of protoplanetary discs 23
3.1 Gas phase reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1.1 Bond formation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1.2 Ionisation and bond destruction processes . . . . . . . . . . . . . . . . . . . 26
3.1.3 Bond rearrangement processes . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 Gas grain interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.1 Accretion and sticking on dust grains . . . . . . . . . . . . . . . . . . . . . 31
3.2.2 Desorption processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2.3 Grain charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.3 Surface reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.4 Deuterium fractionation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.5 Initial conditions for chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.6 Chemical modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4 Reduction of chemical networks 39
4.1 Need for the reduction of chemical networks . . . . . . . . . . . . . . . . . . . . . . 39
4.2 Reduction method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.3 Ionisation state of a protoplanetary disc . . . . . . . . . . . . . . . . . . . . . . . . 41
4.3.1 Importance of the ionisation fraction for disc evolution . . . . . . . . . . . . 41
4.3.2 Disc model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.3.3 Chemical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.3.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.4 Column densities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.6 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
iii CONTENTS
5 Millimetre observations and modelling of AB Aur 59
5.1 Why AB Aur? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2 Observations of AB Aur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.2.1 IRAM 30 m data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.2.2 Plateau de Bure interferometric data . . . . . . . . . . . . . . . . . . . . . . 64
5.3 Model of the AB Aur system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.3.1 Disc model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.3.2 Envelope model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.3.3 Chemical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.4 2D line radiative transfer calculations . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.4.1 Calculated excitation temperatures . . . . . . . . . . . . . . . . . . . . . . . 73
5.5 Results of the line radiative transfer modelling . . . . . . . . . . . . . . . . . . . . . 76
+5.5.1 Interferometric HCO (1 0) map . . . . . . . . . . . . . . . . . . . . . . . . 77
5.5.2 Single dish data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.5.3 Evolutionary status of the AB Aur system . . . . . . . . . . . . . . . . . . . 88
5.6 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
6 Conclusions and prospects for the future 93
7 Zusammenfassung 95
A Scheme to compute the optical constants of aggregate particles i
B Surface species and reactions adopted in the disc chemical model v
C Acknowledgements xi
D Cirriculum vitae xiiiList of Figures
2.1 Topology of aggregate, composite, and multishell particles . . . . . . . . . . . . . . 9
2.2 Monochromatic and Rosseland mean dust opacities calculated for two silicate models 13
2.3 Calculated Rosseland and Planck mean are compared with other studies . . 17
2.4 Thermal disc structure derived with two dierent opacity models . . . . . . . . . . . 19
2.5 Midplane temperature of the disc obtained with the same opacity models . . . . . . . 19
3.1 Percentage agreement between calculated and observed gas phase abundances. . . . 36
3.2 The computational time needed to simulate the chemical evolution in a disc location
with chemical networks of various sizes. . . . . . . . . . . . . . . . . . . . . . . . . 37
4.1 Three layers of a disc with dierent sets of chemical processes responsible for the
fractional ionisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.2 Fractional as a function of height above the disc plane . . . . . . . . . . . 43
4.3 Evolution of the fractional ionisation in the intermediate layer at R = 3 AU . . . . . . 48
4.4 Main routes governing evolution of long carbon chains at R = 3 AU . . . . . . . . . 49
4.5 Evolution of the fractional ionisation in the surface layer at R = 1 AU. . . . . . . . . 50
4.6 Sizes of the reduced networks governing the disc fractional ionisation . . . . . . . . 56
4.7 Magnetic Reynolds numbers in the disc computed for t = 1 Myr. . . . . . . . . . . . 57
4.8 Comparison of the equilibrium and time dependent fractional ionisations att = 1 Myr 58
5.1 Single dish emission lines observed toward AB Aur with the IRAM 30 m antenna. . 62
+5.2 Velocity map of the AB Aur disc observed with the PbBI in HCO (1 0) . . . . . . . 64
5.3 Scheme of the AB Aur system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.4 The thermal and density structure of the disc model . . . . . . . . . . . . . . . . . . 67
5.5 Calculated column densities in the disc and envelope. . . . . . . . . . . . . . . . . . 71
+5.6 excitation temperatures in the disc for CO(2 1), CS(2 1), HCO (1 0), and
+HCO (3 2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.7 Algorithm of the applied modelling approach . . . . . . . . . . . . . . . . . . . . . 77
+5.8 Comparison of the synthetic and observed HCO (1 0) interferometric maps . . . . . 78
+5.9 of the normalised single dish and interferometric HCO (1 0) spectra . . 79
5.10 Determination of the disc inclination angle. . . . . . . . . . . . . . . . . . . . . . . 80
5.11 of the disc positional angle. . . . . . . . . . . . . . . . . . . . . . . . 81
5.12 Observed and synthetic single dish CO(2 1) spectra for three envelope models. . . . 84
5.13ed and for three physical . . . . 84
+ + 185.14 Comparison of the observed and synthetic single dish HCO (1 0), HCO (3 2), C O(2
1), and CS(2 1) spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
iiiiv LIST OF FIGURESList of Tables
2.1 Mass fractions f and densities of dust constituents in the opacity model . . . . . . 7j j
10 32.2 Dust composition as a function of temperature ( 10 g cm ) . . . . . . . . . . 8
3.1 Types of chemical reactions in space . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.2 Cosmic elemental abundances in respect to hydrogen . . . . . . . . . . . . . . . . . 36
4.1 Dominant ions in the midplane, intermediate layer, and surface layer at t = 1 Myr . . 43
4.2 Physical conditions in the midplane . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.3 Reduced network for dark, hot chemistry in the disc midplane . . . . . . . . . . . . 45
4.4 network for dark, cold in the midplane . . . .