Th Dynamics of Star Cluster Formation

Licentiatavhandling, 2018

A major question in astrophysics is how star clusters form. These objects are important, since they are the birth sites of most stars, perhaps including our own Sun.

There are different theoretical models of cluster formation and our main goal is to examine how they may affect the dynamical evolution of the stars in the system, including those stars that are ejected from the cluster. In particular, we set up cluster formation models with global initial conditions of the Turbulent Clump Model, which has been proposed as a description of gas structures within molecular clouds. We then investigate how global star formation efficiency from such a natal gas clump, overall clump density, degree of primordial mass segregation, degree of primordial binarity and binary population properties affect the subsequent dynamical evolution. In a second paper, after a major code development that allows modeling of gradual star formation, we investigate how the rate of star cluster formation affects its dynamical evolution, which is the first time such a study has been conducted for realistic clusters that have realistic binary properties.

We show througth this thesis that star clusters that formed fast, i.e., during about one free-fall time, show quite different properties than star clusters that forms in a slow quasi-equilibirum fashion. Quickly-formed clusters tend to expand much faster compared to slow-formed clusters, thus requiring higher initial densities to agree with observations.
Creation of the runaway stellar population is also sensitive to the rate of cluster formation. Future directions of this work, adding greater degrees of realism are also discussed.

Finally, we carry out an example study of how the observed properties of a particular set of runaway stars can constrain properties of the dynamical ejection event, with implications for the closest region of massive star formation in the Orion Nebula Cluster.