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Dust and Gas in Protoplanetary Discs

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Dust and Gas in Protoplanetary Discs 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-Universitat¨ 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¨ffentlichen 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) 4 Contents 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 i ii 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 model ................................ 70 5.4 2D line radiative transfer calculations ......................... 73 5.4.1 Calculated excitation temperatures ....................... 74 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 ................................. 83 5.5.3 Evolutionary status of the AB Aur system ................... 87 5.6 Summary and conclusions ............................... 90 6 Conclusions and prospects for the future 95 7 Zusammenfassung 97 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 xiii List 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 opacities are compared with other studies .. 17 2.4 Thermal disc structure derived with two different 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 different sets of chemical processes responsible for the fractional ionisation ................................... 42 4.2 Fractional ionisation 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 at t = 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 Calculated 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 Comparison of the normalised single-dish and interferometric HCO+(1-0) spectra .. 79 5.10 Determination of the disc inclination angle. ...................... 80 5.11 Determination of the disc positional angle. ....................... 81 5.12 Observed and synthetic single-dish CO(2-1) spectra for three envelope models .... 84 5.13 Observed and synthetic single-dish CO(2-1) spectra for three physical models .... 85 5.14 Comparison of the observed and synthetic single-dish HCO+(1-0), HCO+(3-2), C18O(2- 1), and CS(2-1) spectra ................................. 93 iii iv LIST OF FIGURES List of Tables 2.1 Mass fractions f j and densities ρ j of dust constituents in the opacity model ...... 7 2.2 Dust composition as a function of temperature (ρ ≈ 10−10 g cm−3) .......... 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 Reduced network for dark, cold chemistry in the midplane .............. 46 4.5 Physical conditions in the intermediate layer ...................... 47 4.6 Physical conditions in the surface layer ......................... 50 4.7 Reduced network for X-ray dominated chemistry in the surface layer ......... 51 4.8 Reduced network for UV-dominated chemistry in the surface layer .......... 51 4.9 The SIREN chemical network ............................. 52 4.10 Chemical models of protoplanetary discs ........................ 53 4.11 The observed and calculated column densities (cm−2) for r = 370 AU at t = 1 Myr .. 54 5.1 Parameters of the detected single-dish emission lines ................. 63 5.2 Parameters of the central star .............................. 68 5.3 Parameters of the best-fit disc model .......................... 69 5.4 Parameters of the best-fit envelope model ....................... 70 5.5 Parameters of the 2D LRT calculations ......................... 74 5.6 Disc mass as a function of model parameters ...................... 84 5.7 Comparison of the AB Aur envelope models ...................... 88 A.1 Fit coefficients for the filling factors and percolation strengths of aggregates ..... iii B.1 Desorption energies of surface species ......................... v B.2 Set of the adopted surface reactions .......................... vi v vi LIST OF TABLES Chapter 1 Introduction Nowadays the study of planet formation attracts particular attention in astrophysics since the recent discovery of extrasolar planets (e.g., Marcy and Butler, 2000). Up to now (September 2004) more than 100 exoplanets orbiting solar-like stars have been detected, with masses ranging from one Neptune mass to ten Jupiter masses and average star-planet distances between ∼ 1% and a factor of 3 that of the Sun-Earth separation (e.g., Butler et al., 2004; Jones et al., 2002; McArthur et al., 2004). This is a strong indication that formation of planets
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