BINSTAR: a new binary stellar evolution code

Tidal interactions

¹ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, ULB CP-226, 1050 Brussels, Belgium
e-mail: siess@astro.ulb.ac.be
² Argelander-institut für Astronomie, University of Bonn, Germany
³ Max-Planck-Institut für Astrophysik, Karl Schwarzschild Str. 1, 85741 Garching, Germany

Received: 3 September 2012
Accepted: 6 December 2012

Abstract

We provide a detailed description of a new stellar evolution code, BINSTAR, which has been developed to study interacting binaries. Based on the stellar evolution code STAREVOL, it is specifically designed to study low- and intermediate-mass binaries. We describe the state-of-the-art input physics, which includes treatments of tidal interactions, mass transfer and angular momentum exchange within the system. A generalised Henyey method is used to solve simultaneously the stellar structure equations of each component as well as the separation and eccentricity of the orbit. Test simulations for cases A and B mass transfer are presented and compared with available models. The results of the evolution of Algol systems are in remarkable agreement with the calculations of the Vrije Universiteit Brussel (VUB) group, thus validating our code. We also computed a large grid of models for various masses (2 ≤ M/M_⊙ ≤ 20) and seven metallicities (Z = 0.0001, 0.001, 0.004, 0.008, 0.01, 0.02, 0.03) to provide a useful analytical parameterisation of the tidal torque constant E₂, which allows the determination of the circularisation and synchronisation timescales for stars with a radiative envelope and convective core. The evolution of E₂ during the main sequence shows noticeable differences compared to available models. In particular, our new calculations indicate that the circularisation timescale is constant during core hydrogen burning. We also show that E₂ weakly depends on core overshooting but is substantially increased when the metallicity becomes lower.