The binarity of Massive Young Stellar Objects

Multiplicity is known to be a common occurrence in massive stars (>8M⊙), and is intrinsically linked to their formation. However the specific formation processes of multiple star systems are still in question. In order to study this phenomenon, massive stars must be observed and studied at an early a stage as possible, and Massive Young Stellar Objects (MYSOs) provide an ideal opportunity for this. Very few studies on MYSO multiplicity currently exist, and so our knowledge of the phenomenon is limited. This thesis presents three studies into the multiplicity of MYSOs, covering all scales and multiple wavebands. Firstly, K−band imaging was used to search for companions around hundreds of MYSOs between 1000-100,000 au, comprising the largest MYSO multiplicity study to date. ∼50% of MYSOs have at least one companion, and a significant number of companions have mass ratios greater than 0.5. Monte Carlo simulations show that triple systems must be a common occurrence in order to explain the observed separation distribution. Secondly, high- and medium-resolution K−band spectra of MYSOs were used to search for radial velocity variations (an indication of a close-in companion), and multiplicity fractions of 0% and ∼8% were found respectively. Masses and separations of these companions were also estimated. Finally, Gaia DR3 data was used to search for MYSO companions in the optical regime for the first time, using parallaxes and proper motions to find companions comoving with MYSOs. A multiplicity fraction of ∼60% was found for this sample. After factoring in observational limitations of these studies, it can be asserted that up to 100% of MYSOs form in multiple systems, with triple systems possibly forming a significant fraction of those multiples. Additionally, the relatively high mass ratios indicate that the capture scenario for binary formation is unlikely for MYSOs.