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Title: The impact of star cluster environments on planet formation
Author: Nicholson, R.
ISNI:       0000 0004 7970 531X
Awarding Body: Liverpool John Moores University
Current Institution: Liverpool John Moores University
Date of Award: 2019
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It is thought that most stars, including our Sun, form within clusters alongside many other stars. Planet formation and star formation occur simultaneously, and therefore the birth environment of stars will shape the formation of planets. Properties of the present day Solar System hint to a past in which the Sun formed in the presence of many other stars. There is an intrinsic interest in knowing the birthplace of the only known planetary system hosting life within the Universe and whether conditions of the birth environment are common or atypical. Understanding the origin of the Solar System could provide important constraints on star and planet formation. Observations of the present day Solar System have revealed a conflict between the need for a large cluster containing massive stars so that disc enrichment can occur and to explain the orbits and enrichment levels of some objects within the Solar System, and the need for a low-mass, quiescent cluster where dynamical and radiative effects will not disrupt or disperse planet forming discs. Low-mass clusters containing massive stars may present a solution to this problem. The presence of massive stars within star-forming regions will affect planet forming discs to varying degrees, depending on the initial conditions of the region. How the initial conditions of star-forming regions change the relationship between massive stars and protoplanetary discs is unknown. This thesis focuses on how the birth environment of stars can shape planet formation. The processes behind star formation, planet formation, and the impacts that star-forming regions can have on the formation of planets are reviewed. This thesis reviews what is currently known about the birth environment of the Sun based on evidence found within the Solar System and what this tells us. In this thesis, I show that low-mass star-forming regions containing one or two massive stars are viable environments for creating enriched planet forming discs that resemble the levels of enrichment found in the Solar System. These unusual clusters enrich planet forming discs at a similar rate to more massive (>1000 Mₒ) clusters. Based on the percentage of stars enriched by Supernovae within low-mass clusters, and the percentage of low-mass clusters containing massive stars in comparison to more massive clusters, it is possible that a significant number of unperturbed, enriched stars have been produced within these star-forming environments. However, the UV radiation produced by massive stars within low-mass clusters is still strong enough to cause protoplanetary discs to disperse on short timescales, potentially inhibiting planet formation. I find that the rate at which protoplanetary discs are dispersed depends on the initial conditions of the cluster and the location of the massive stars. The mass of the cluster, if massive stars are present, does not significantly alter the rate at which planet forming discs are dispersed. The initial density of the cluster is the most important aspect to consider. The background UV field produced by star-forming regions also varies depending on the initial spatial stellar profile. Planet forming discs within simulations that represent the initial conditions of nearby star-forming regions show that planet forming discs are dispersed on short (~1-3 Myr) time-scales. This means on of three things; either the majority of planets form in low-mass star-forming regions, giant planet formation must occur on very short timescales, or the current calculations of mass-loss in discs due to external photoevaporation severely overestimate the detrimental effects of EUV and FUV radiation. By calculating the UV background field in star-forming regions, I find that the initial spatial distribution of stars greatly affects the amount of UV radiation that stars receive. Delaying the effects of UV radiation by 0.5 Myr, to replicate the effects of delaying massive star formation, still results in vastly different UV field strengths. The types of stars that produce UV radiation are not limited to massive (≥15 Mₒ) stars, and lower mass (3 < M < 15 Mₒ) stars contribute large enough UV fields to potentially affect protoplanetary discs. This thesis has shown that the environment in which protoplanetary discs are born can dictate and strongly shape their evolution. The constraints on the birth environment of the Solar System have been relaxed, and the importance of considering the initial conditions of the star-forming region have been highlighted.
Supervisor: Parker, R. ; Habergham-Mawson, S. ; James, P. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: QB Astronomy ; QC Physics