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

Abstract

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 αin 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 T_eff 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 αcan 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 αalone. We therefore suggest that the combined effects of high metallicity and αreduction could explain the observations of CM Draconis. For example, increasing the metallicity of the system towards super-solar values (i.e. Z = 2 Z_sun) yields an agreement within 2 σwith α= 1.0.

7 pages, 4 figures, accepted for publication in MNRAS

References in corpus (6)

• Evolution of low-mass star and brown dwarf eclipsing binaries
• KOI-126: A Triply-Eclipsing Hierarchical Triple with Two Low-Mass Stars
• YREC: The Yale Rotating Stellar Evolution Code
• Absolute properties of the low-mass eclipsing binary CM Draconis
• Constraints on the structure of the core of subgiants via mixed modes: the case of HD49385
• Accurate Low-Mass Stellar Models of KOI-126

Cited by in corpus (9)

• The radius discrepancy in low mass stars: single vs. binaries
• Magnetic Inhibition of Convection and the Fundamental Properties of Low-Mass Stars. II. Fully Convective Main Sequence Stars
• Self-Consistent Magnetic Stellar Evolution Models of the Detached, Solar-Type Eclipsing Binary EF Aquarii
• Fundamental properties of lower main-sequence stars
• The Metallicity of the CM Draconis System
• A low-mass eclipsing binary within the fully convective zone from the NGTS
• Improved calibration of the radii of cool stars based on 3D simulations of convection: implications for the solar model
• The Solar Neighborhood XLVII: Comparing M Dwarf Models with Hubble Space Telescope Dynamical Masses and Spectroscopy
• Photodynamical Modeling of the Fascinating Eclipses in the Triple-Star System KOI-126