Vincke, Kirsten
(2019).
How star cluster evolution shapes protoplanetary disc sizes.
PhD thesis, Universität zu Köln.
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Abstract
The majority of stars form from cold, collapsing Giant Molecular Clouds (GMCs), which not only yield single stars, but groups of a few up to many hundreds of thousands of stars. The gas and dust which was not transformed into stars is expelled at the end of the star formation process. Stellar groups react very differently to this mass removal, depending on their virial state and on the fraction of gas which is transformed into stars, called star-formation efficiency (SFE). In one type of group, called associations, the SFE is rather low (=< 0.3$) and the gas removal leaves the stellar members (largely) unbound. On the other hand, if the SFE is higher, observations and theory find that the stellar accumulations largely remain bound and can survive many billions of years in this state, which makes them stellar clusters. These two types of stellar groups evolve on very distinct tracks concerning their density, size, and mass. It is probable that most - if not all - stars are initially surrounded by a protoplanetary disc, the formation site of planets. In the last decades - and especially since the launch of Kepler in 2009 - observations were able to find more than 3700 planets orbiting other stars. Many of these extrasolar planets (exoplanets) are part of planetary systems, which differ significantly from our own solar system. External processes in the stellar birth environments like gravitational interactions between the cluster members (fly-bys) and external photoevaporation are possible reasons for these differences. The strength of such processes is directly connected to the dynamical and density evolution of the environments. Simulations of different associations and clusters were performed and the influence of fly-bys on protoplanetary discs was investigated. In associations, the most fly-bys happen in the phase, where they are still embedded in their natal gas. After gas expulsion, most members of the associations become unbound and thus the effect of stellar fly-bys becomes less important. In systems comparable to the Orion Nebular Cluster (ONC), the discs in the simulations are cut down to a few hundreds of AU, which fits observational findings very well. By contrast, stellar clusters, like for example the Arches, retain their high stellar density even after gas expulsion. In such dense clusters, fly-bys play an important role in shaping disc properties at later evolutionary stages as well, cutting down discs to much smaller sizes of ~20AU. For a long time, such very dense systems were considered to be too hostile to yield, for example, a planetary system like our own solar system. However, under the assumption that the steep drop in mass density at 30AU in our solar system was caused by a fly-by, the results presented in this thesis show that the solar system was most probably part of a very massive association, like for example NGC6611, or a stellar cluster, like Arches.
| Item Type: | Thesis (PhD thesis) | ||||||||
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| URN: | urn:nbn:de:hbz:38-98367 | ||||||||
| Date: | 28 May 2019 | ||||||||
| Language: | English | ||||||||
| Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
| Divisions: | Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I | ||||||||
| Subjects: | Physics | ||||||||
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| Date of oral exam: | 23 July 2018 | ||||||||
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| Refereed: | Yes | ||||||||
| URI: | http://kups.ub.uni-koeln.de/id/eprint/9836 |
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