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Title: The influence of stellar birth environment on protoplanetary disc dispersal
Author: Winter, Andrew
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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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.
Supervisor: Clarke, Cathie ; Booth, Richard Sponsor: Science and Technology Facilities Council (STFC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Astronomy ; Protoplanetary discs ; Star formation ; Planet formation ; Stellar clusters ; Exoplanets ; accretion ; accretion discs ; circumstellar matter ; Cygnus OB2 ; photoevaporation ; tidal encounters ; stellar dynamics ; young stars