Optical/IR spectrointerferometric astrometry: a key diagnostic when understanding complex massive binary systems

Massive stars (those greater than 8 solar masses) are hugely important objects in our Universe. They synthesise heavy elements through and produce huge amounts of ionising radiation and distribute this into the interstellar medium. Their winds shape their local star forming regions and also affect the morphology of galactic superwinds (Leitherer 1994). Additionally, gravitational waves are the end product of massive stellar mergers, with black holes and neutron stars colliding to create ripples that are felt across the galaxy (Abbot et al 2016). The effects of a massive star can be irrevocably changed by their multiplicity. At least 90% of massive stars are expected to be in at least a binary system (e.g. Moe & di Stefano 2017) and 70% of these stars are expected to interact with eachother (e.g. Sana+ 2012). In this talk I will discuss how astrometry, particularly from spectrointerferometry with the VLTI, is now proving invaluable in the study of complicated massive binary systems. Through the presentation of two case studies, I will describe how the precise data of VLTI/GRAVITY is allowing us to break degeneracies when characterising orbits, attribute spectral features to specific stars and in doing so allow us to definitively identify massive stellar systems that have gone through interactions (e.g. Frost+ 2022) and even main sequence merger events (Frost+ 2024).