Investigating the formation and properties of multiple star systems using Monte Carlo models

Stellar multiplicity has been extensively studied over the past few decades. It is believed that most stars form in multiple systems, meaning that understanding multiplicity is crucial for understanding star formation as a whole. Studying binary systems can tell us about their formation histories and orbital properties. In this thesis, I investigate the properties of multiple systems using three different methods. Firstly, I tested various simple rules for `universal' star formation using a toy Monte Carlo model. These rules define how protostellar cores fragment into multiple systems and how these systems then decay (through secular processes or encounters). I found that the number of stars formed in each core must be a weak function of core mass in order to produce the canonical IMF and multiplicity fractions that increase with primary mass. However, it is difficult to match the multiplicity fractions exactly without a more detailed model for dynamical and secular decay. I then present a summary of FOBOS - an orbit fitting algorithm designed to estimate the orbital parameters of binaries, triple systems, or directly imaged exoplanets from as few as two astrometric observations. FOBOS uses a brute force Monte Carlo method to sample parameter space using uniform priors. I tested the code on a large sample of fake systems, as well as two real systems with brown dwarf companions, showing that it is a reliable method of estimating the semi-major axis, inclination, and eccentricity of a system. Finally, I created a binary population synthesis model. I used this model to estimate the selection effects present in samples of visual binaries, by first performing some general tests and then applying it to a sample of binary YSOs, taken from archival data. I found that the majority of the YSOs in the sample had a significantly larger semi-major axis than was first expected, implying that the sample included only (a) ultrawide binaries or (b) tertiary companions that were too close for the survey to resolve.