The Influence of Stellar Birth Environment on Protoplanetary Disc Dispersal
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The influence of stellar birth environment on protoplanetary disc dispersal Andrew Winter Supervisor: Prof. Cathie Clarke Dr. Richard Booth Institute of Astronomy University of Cambridge This dissertation is submitted for the degree of Doctor of Philosophy Darwin College May 2019 Declaration I hereby declare that except where specific reference is made to the work of others, the contents of this dissertation are original and have not been submitted in whole or in part for consideration for any other degree or qualification in this, or any other university. This dissertation is my own work and contains nothing which is the outcome of work done in collaboration with others, except as specified in the text. This dissertation contains fewer than 60,000 words including appendices, footnotes, tables and equations and has fewer than 150 figures. Andrew Winter May 2019 Abstract The influence of stellar birth environment on protoplanetary disc dispersal Andrew Winter Protoplanetary discs (PPDs) are the progenitors of planets and represent the material available for their formation. Recent surveys indicate that exoplanetary architectures are diverse and the processes that govern the evolution of PPDs contribute significantly to the properties of these architectures. Most studies of PPDs consider their secular evolution, disregarding the influence of environment on the evolution and eventual dispersal of the disc. However,a growing body of empirical studies have found evidence that they are in fact influenced by their stellar neighbours. In this dissertation I focus primarily on two mechanisms by which discs evolving in close proximity to other stars might be truncated and dispersed by their neighbours. These are tidal truncation by star-disc encounters and external photoevaporation due to irradiation by massive stars. I make the distinction between encounters that occur in multiple systems and those that occur between individual stars (type I and type II encounters). I model the specific case of HV and DO Tau, apparently isolated stellar systems connected by an extended dust ‘bridge’, as a historic type I encounter within a quadruple system. I then theoretically quantify the influence of type II tidal encounters on PPDs in the distant and close regimes. Coupling recent developments in the theory of photoevaporating discs with a viscous evolution model, I similarly quantify the dispersal timescale of PPDs due to irradiation by massive stars. Comparing these mechanisms in local environments, I find that external photoevaporation dominates over type II encounters as a dispersal mechanism in local star forming regions. Applying photoevaporation models to the OB association Cygnus OB2, I successfully reproduce the observed disc survival fractions, implying that external photoevaporation is having a significant influence on PPDs in that region. Finally, I link the dispersal timescales to star formation physics, illustrating that outside of the solar neighbourhood a much larger fraction of stars may be exposed to environments that disperse discs rapidly. Tentatively, this indicates that the sun may lie in a special region for planet formation, where the number of stars for which PPDs remain mostly uninfluenced by stellar neighbours is maximised. Acknowledgements I would not have been able to complete this dissertation without the influence of a huge number of people, both during my time in Cambridge and before. First and foremost, I thank my parents, John and Wendy, for their endless support throughout my life. The many people that I have met during the last three years have undoubtedly made my time what it was. Special mentions go to Pablo, Cameron and Aneesh at the Institute of Astronomy, whose shared interest in wine has proved an efficient tool for maintaining sanity during even the most challenging of periods. Further indulgence in vices has been kindly facilitated by Ollie Batey, without whom the last three and a half years would have been far less interesting. I am extremely thankful for the enduring patience and thoughtful suggestions of Richard Booth throughout my Ph.D., but especially during the early stages. Having always made himself available for discussion and freely offered advice, he is in no small way responsible for many of the skills that I have learned during this time. Finally, this work would clearly not have been achievable without the guidance of Cathie Clarke. I have been fortunate to have a supervisor whose advice has proven invaluable for every aspect of my academic life at Cambridge and I have no doubt will continue to be so in the future. Table of contents List of figures xv List of tables xxix 1 Introduction1 1.1 Formation and viscous evolution of PPDs . .3 1.2 Secular dispersal mechanisms . .5 1.2.1 Internal photoevaporation . .6 1.2.2 Magnetohydrodynamic winds . .8 1.2.3 Giant planet formation . 12 1.3 Environment and PPDs . 14 1.3.1 Star-disc encounters . 15 1.3.2 External photoevaporation . 18 1.3.3 Star formation environments . 23 1.4 Dissertation overview . 26 2 A historic type Ib encounter: HV and DO Tau 29 2.1 Introduction . 29 2.2 Observational constraints . 30 2.2.1 Stellar components . 30 2.2.2 Disc properties . 31 2.2.3 Herschel/PACS data . 31 2.2.4 Cloud temperature and mass . 33 2.2.5 Kinematics . 36 2.2.6 Summary of observational constraints . 36 2.3 Numerical method . 37 2.3.1 Kinematic modelling . 38 2.3.2 Hydrodynamics model . 40 x Table of contents 2.3.3 Disc interaction initial conditions . 40 2.4 Modelling results . 42 2.4.1 Kinematic properties . 42 2.4.2 Disc properties . 45 2.4.3 External structure . 46 2.4.4 Gas velocity . 48 2.5 Conclusions . 49 3 Linearised theory of distant type II star-disc encounters 51 3.1 Introduction . 51 3.2 Theory and method . 53 3.2.1 Linearised equations . 53 3.2.2 Ring of test particles . 55 3.2.3 Hydrodynamic modelling . 55 3.3 Numerical results . 57 3.3.1 Perturbed ring . 57 3.3.2 Perturbed disc . 58 3.4 Discussion . 63 3.5 Conclusions . 67 4 Tidally truncated PPD radii in clusters 69 4.1 Introduction . 69 4.2 Numerical method . 70 4.3 Outer radius definition . 71 4.4 Angle-averaged model . 71 4.5 Post-encounter disc radius . 74 4.6 Encounter rate . 76 4.7 Numerical method . 81 4.8 Cluster evolution results . 84 4.8.1 Uniform density cluster . 84 4.8.2 Structured cluster . 84 4.8.3 Cluster with stellar mass distribution . 85 4.9 Mass dependent truncation . 85 4.10 Probability averaging . 85 4.11 Conclusions . 88 Table of contents xi 5 Type II encounters vs. external photoevaporation in star forming regions 91 5.1 Introduction . 91 5.2 Cluster environments . 92 5.2.1 Properties of stellar clusters . 92 5.2.2 UV luminosity and stellar mass . 96 5.2.3 Local environment distribution . 98 5.3 Photoevaporation . 98 5.3.1 EUV vs. FUV induced mass loss . 99 5.3.2 Viscous disc evolution model . 101 5.3.3 PPD destruction timescale . 101 5.4 Type II tidal truncation vs. photoevaporation . 104 5.5 Conclusions . 107 6 External photoevaporation of PPDs in Cygnus OB2 109 6.1 Introduction . 109 6.2 Properties of Cygnus OB2 . 110 6.2.1 Stellar population . 110 6.2.2 Velocity dispersion . 111 6.2.3 Sub-structure . 112 6.2.4 PPD population . 113 6.2.5 Observational summary & modelling challenges . 113 6.3 Numerical Method . 114 6.3.1 Kinematic modelling . 114 6.3.2 Disc evolution model . 119 6.4 Results and discussion . 121 6.4.1 Modelling approach . 121 6.4.2 Disc fractions and gas expulsion . 122 6.4.3 Stellar population properties . 124 6.4.4 Disc properties . 133 6.5 Conclusions . 142 7 Future prospects and context: links to galactic-scale environment 145 7.1 Introduction . 145 7.2 Stellar density distribution . 146 7.2.1 ISM properties . 147 7.2.2 Star formation efficiency . 149 7.3 Cluster properties . 151 xii Table of contents 7.3.1 Cluster mass spectrum . 152 7.3.2 Maximum FUV luminosity of cluster members . 157 7.4 FUV flux distribution . 157 7.4.1 High mass clusters . 157 7.4.2 Flux in the field . 159 7.4.3 Low mass clusters . 160 7.4.4 Dispersion from mean FUV flux . ..