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Binary Stars

Observations indicate that as many as 50% of the stars in the solar neighbourhood are members of binaries or higher-order sub-systems. The binary frequency in open clusters is similar while in globular clusters it is typically 5-20%.

The evolution of binary stars is no different from that of single stars unless they get in each other's way. If the binary orbit is wide enough the individual stars will not be affected by the presence of a companion so that standard stellar evolution is all that is required to describe their evolution. However, if the stars become close they can interact with consequences for the evolution and appearance of the stars as well as the nature of the orbit.

The effects of close binary evolution are observed in many systems, such as cataclysmic variables, X-ray binaries and Algols, and in the presence of stars such as blue stragglers which cannot be explained by single star evolution. While many of the processes involved are not understood in detail we do have a qualitative picture of how binaries evolve and can hope to construct a model that correctly follows them through the various phases of evolution. Initial conditions are the mass and composition of the stars, the period (or separation) and eccentricity of the orbit. In order to conduct statistical studies of complete binary populations, i.e. population synthesis, such a model must be able to produce any type of binary that is observed in enough detail without being computationally inefficient.

At Swinburne we have a rapid binary evolution algorithm that enables the entire evolution of even the most complex binary systems to be modelled in less than a second of CPU time. This BSE code was developed by Jarrod Hurley during his PhD at Cambridge and is an extension of a rapid single star evolution (SSE) algorithm. It uses a prescription-based approach which undergoes frequent updating to incorporate the latest results from detailed modelling and observations. In addition to all aspects of the SSE algorithm, features such as mass transfer, mass accretion, common-envelope evolution, collisions and supernova kicks are included. Angular momentum loss mechanisms such as gravitational radiation and magnetic braking are also included. Circularization and synchronization of the orbit owing to tidal interactions are calculated for convective, radiative and degenerate damping mechanisms. Paul Kiel has recently added algorithms to treat magnetic field decay and spin-down during pulsar evolution.

As part of his PhD Jarrod used the BSE code to evolve a series of large binary populations and calculate the Galactic formation rates of various interesting individual species, such as X-ray binaries, double-degenerates and symbiotic stars, and events such as type Ia SNe. By comparing the results for populations with and without tidal friction the hitherto ignored systematic effect of tides was quantified. This showed that modelling of tidal evolution in binary systems is necessary in order to draw accurate conclusions from population synthesis work.

Paul Kiel is currently extending the binary population synthesis code to include kinematics and an observational survey component. This will give the capability to evolve millions of binaries while following their orbits within the Galactic potential in order to model the Galactic distribution of stellar populations. Objects of interest, such as pulsars and neutron star binaries, can then be selected and observed according to the parameters and selection effects of a chosen real survey. This will alleviate some of the major uncertainty involved in comparing population synthesis results with survey data and improve our understanding of binary evolution, while also helping to guide future surveys.

References:

* Populating the Galaxy with low-mass X-ray binaries, Kiel P.D., Hurley J.R. (2006), MNRAS, 369, 1152
* Evolution of Binary Stars and the Effect of Tides on Binary Populations, Hurley J.R., Tout C.A., Pols O.R. (2002), MNRAS, 329, 897