Journal Article

Modelling fully convective stars in eclipsing binaries: KOI-126 and CM Draconis

F. Spada and P. Demarque

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 422, issue 3, pages 2255-2261
Published in print May 2012 | ISSN: 0035-8711
Published online May 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.20769.x
Modelling fully convective stars in eclipsing binaries: KOI-126 and CM Draconis

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We present models of the components of the systems KOI-126 and CM Draconis, the two eclipsing binary systems known to date to contain stars with masses low enough to have fully convective interiors. We are able to model satisfactorily the system KOI-126, finding consistent solutions for the radii and surface temperatures of all three components, using a solar-like value of the mixing-length parameter α in the convection zone and phoenix NextGen 1D model atmospheres for the surface boundary conditions. Depending on the chemical composition, we estimate the age of the system to be in the range 3–5 Gyr. For CM Draconis, on the other hand, we cannot reconcile our models with the observed radii and Teff using the current metal-poor composition estimate based on kinematics. Higher metallicities lessen but do not remove the discrepancy. We then explore the effect of varying the mixing-length parameter α. As previously noted in the literature, a reduced α can be used as a simple measure of the lower convective efficiency due to rotation and induced magnetic fields. Our models show a sensitivity to α (for α < 1.0) sufficient to partially account for the radius discrepancies. It is, however, impossible to reconcile the models with the observations on the basis of the effect of the reduced α alone. We therefore suggest that the combined effects of high metallicity and α reduction could explain the observations of CM Draconis. For example, increasing the metallicity of the system towards super-solar values (i.e. Z= 2 Z) yields an agreement within 2σ with α= 1.0.

Keywords: convection; methods: numerical; binaries: eclipsing; stars: interiors; stars: low-mass

Journal Article.  4749 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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