The Formation of Massive Binary Stars.pdf

a r X i v : a s t r o - p h / 0 5 0 1 2 6 0 v 1 1 3 J a n 2 0 0 5 **FULL TITLE** ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION** **NAMES OF EDITORS** The Formation of Massive Binary Stars Ian A. Bonnell School of Physics and Astronomy, University of St Andrews, St Andrews, Scotland, KY16 9SS, UK Abstract. The formation of massive stars in close binary systems is com- plicated due to their high radiation pressure, the crowded environment and the expected minimum separation for fragmentation being many times greater than the orbital separation. I discuss how massive star formation can be understood as being due to competitive accretion in stellar clusters. Massive binary systems are then formed due to accretion onto wider low-mass systems. The combina- tion of accretion and dynamical interactions with other stars in the cluster then result in producing a close binary system. Tidal and 3-body captures in dense stellar cores can also play a role in forming massive binary systems while stellar mergers of such close binary systems due to interactions may play an important role in overcoming the radiation pressure. 1. Introduction: The Problem Although massive stars are commonly found in close binary systems, their for- mation mechanisms are poorly understood. There are a number of difficulties that need to be overcome in order to explain their formation mechanism. These include the intrinsic difficulty of forming massive stars, and the intrinsic diffi- culty of forming close binary systems, even in amongst low-mass stars. 1.1. Formation of Massive Stars There are several potential difficulties in forming high-mass stars. Firstly, the timescale of less than 106 years to assemble 10 to more than 100 M⊙ implies large accretion rates. Secondly, their crowded location in the centre of young stellar clusters implies that there was not enough room to fit in the precursor fragment. In order to get the Jeans radius to be smaller than the interstellar separatio