Formation and dynamical evolution of young star clusters: Uncovering the mystery of multiple stellar populations

Khalaj, Pouria (2017). Formation and dynamical evolution of young star clusters: Uncovering the mystery of multiple stellar populations PhD Thesis, School of Mathematics and Physics, The University of Queensland.

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Author Khalaj, Pouria
Thesis Title Formation and dynamical evolution of young star clusters: Uncovering the mystery of multiple stellar populations
School, Centre or Institute School of Mathematics and Physics
Institution The University of Queensland
Publication date 2017-02-10
Thesis type PhD Thesis
Supervisor Holger Baumgardt
Michael Drinkwater
Total pages 141
Language eng
Subjects 020103 Cosmology and Extragalactic Astronomy
020104 Galactic Astronomy
Formatted abstract
Many observations and theoretical models suggest that stars form in groups or clusters and then scatter into their host Galaxy. Star clusters provide us with rich samples of stars of known distance, metallicity and age. In addition, stars in a cluster cover a wide spectrum of masses and are consequently in different phases of their evolution. Therefore, star clusters can be viewed as the Rosetta Stone of star formation and evolution. In this thesis, I study the formation and dynamical evolution of open and globular clusters (OCs and GCs) through the use of N-body and Monte Carlo simulations.

The thesis starts by devising a novel approach to find the mass function and binary fraction of OCs (Chapter 2). My method to find the mass function is based on a maximum likelihood approach combined with a Kolmogorov Smirnov test and is more accurate than typical least squares methods. I then apply my method to Praesepe as an example of a young OC in the Milky Way, using photometric and astrometric data from the SDSS and PPMXL catalogues. I find the possible cluster members of Praesepe using the proper motions of stars in the field of the cluster followed by comparing the position of possible members in a colour-magnitude (CMD) diagram with theoretical isochrones. To find the binary fraction, I perform a series of Monte Carlo simulations to compare the observed CMD of Praesepe with synthetic isochrones. My method can be readily used with Gaia data to robustly correct the mass function of OCs for the effect of unresolved binaries.

The remainder of the thesis is devoted to the study of globular clusters (GCs), which are older and more massive than OCs. In particular I focus on the origin of multiple stellar populations (MSPs) whose origin has remained a mystery despite almost a decade of intense research.

Chapter 3 discusses the so-called mass budget problem in GCs with MSPs. First, I study the effect of a top heavy initial mass function (IMF) on the fraction of chemically peculiar stars as well as the mass-to-light ratio of GCs with MSPs. I consider two different cases for the origin of MSPs, i.e. asymptotic giant branch (AGB) stars and fast-rotating massive stars (FRMS). In addition, I consider different retention fractions for compact remnants as well as different mass-loss levels. For both the AGB and FRMS scenario, I derive the range of IMF high-mass slopes and mass-loss levels which produce the observed fraction of chemically peculiar stars and mass-to-light ratios of Galactic GCs. I then show that the higher helium abundance and shorter lifetimes of chemically peculiar stars in conjunction with the fact that stellar mass functions strongly increase towards lower masses, boosts the fraction of chemically peculiar stars which have left the main sequence. This implies that one needs to correct the observed number ratio of evolved chemically peculiar stars to infer the actual number ratio.

I then study the effect of primordial gas expulsion and its link to significant mass loss in globular clusters with multiple stellar populations in Chapter 4. By comparing the outcome of my N-body and Monte Carlo simulations with the observational data of Galactic GCs, I determine the initial mass and size of GCs as well as the gas expulsion time scales required to explain the large fraction of chemically peculiar stars in GCs with MSPs. Moreover, I show that primordial gas expulsion leads to an anti-correlation between the final mass of GCs and the observed number fraction of such stars which is not supported by observations.

In Chapter 5, I address the question of whether significant mass-loss induced by primordial gas expulsion could have happened in the GCs of the Fornax dwarf spheroidal galaxy. Recently, Larsen et al. (2012) found a large ratio of metal poor GCs to field stars in Fornax. This large ratio limits the maximum amount of mass-loss that could have happened in Fornax GCs to 80%. This is considered as evidence against significant mass-loss in GCs that require mass-loss levels of ~90% or more to explain the observed large fraction of chemically peculiar stars in GCs with MSPs. Using a series of numerical simulations (orbit integration) I provide a solution to this problem. I follow the orbit of unbound stars of Fornax GCs in the potential field of Fornax and the Milky Way over a Hubble time. I demonstrate that stars can leave the Fornax galaxy if they have become unbound as a result of gas expulsion but not if they have become unbound due to stellar evolution.

Finally I conclude the thesis in Chapter 6 by stating the major outcomes of my study and their implications and also suggest a number of possible research directions for future work.
Keyword Dynamical evolution
Fornax dsph
Gas expulsion
Globular clusters
Mass function
Multiple stellar populations
Numerical simulations
Open clusters

Document type: Thesis
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Created: Wed, 25 Jan 2017, 08:34:49 EST by Pouria Khalaj on behalf of University of Queensland Graduate School