SCYON Abstract

Received on June 20 2002

The Formation of Close Binary Systems by Dynamical Interactions and Orbital Decay

AuthorsMatthew R. Bate1,2, Ian A. Bonnell3, and Volker Bromm2,4
Affiliation1School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom,
2Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom,
3School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, United Kingdom,
4Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, U.S.A.
Accepted byMonthly Notices of the Royal Astronomical Society


We present results from the first hydrodynamical star formation calculation to demonstrate that close binary stellar systems (separations ~ 10 AU) need not be formed directly by fragmentation. Instead, a high frequency of close binaries can be produced through a combination of dynamical interactions in unstable multiple systems and the orbital decay of initially wider binaries. Orbital decay may occur due to gas accretion and/or the interaction of a binary with its circumbinary disc. These three mechanisms avoid the problems associated with the fragmentation of optically-thick gas to form close systems directly. They also result in a preference for close binaries to have roughly equal-mass components because dynamical exchange interactions and the accretion of gas with high specific angular momentum drive mass ratios towards unity. Furthermore, due to the importance of dynamical interactions, we find that stars with greater masses ought to have a higher frequency of close companions, and that many close binaries ought to have wide companions. These properties are in good agreement with the results of observational surveys.