The Effect of Tides and Stellar Dynamics on Binary and Cluster Populations

Abstract

We present a rapid binary star evolution (BSE) algorithm that enables modelling of even the most complex binary systems. In addition to all aspects of single star evolution, features such as mass transfer and accretion, angular-momentum loss mechanisms, common-envelope evolution, collisions and supernova kicks are included. In particular, circularization and synchronization of the orbit by tidal interactions are calculated in detail. We investigate the effect that tidal friction has on the outcome of binary evolution and show that modelling of tidal evolution in binary systems is necessary in order to draw accurate conclusions from population synthesis work.

The BSE algorithm is included into a state-of-the-art N-body code which allows direct modelling of stellar and binary evolution together with stellar dynamics. This code is ideal for investigating the evolution of open and globular star clusters and their stellar populations. We use the N-body code to model the blue straggler population of the old open cluster M67. Calculations with our binary population synthesis code show that binary evolution alone cannot explain the observed numbers or properties of the blue stragglers. On the other hand, our N-body model of M67 generates the required number of blue stragglers and provides formation paths for all the various types found in M67. This demonstrates the effectiveness of the cluster environment in modifying the nature of the stars it contains and highlights the importance of combining dynamics with stellar evolution.