Evolution of low-mass star and brown dwarf eclipsing binaries (original) (raw)

A&A 472, L17-L20 (2007)

Letter to the Editor

École Normale Supérieure de Lyon, CRAL (UMR CNRS 5574), Université de Lyon, France e-mail: chabrier@ens-lyon.fr

Received: 23 April 2007
Accepted: 26 June 2007

Abstract

_Context._We examine the evolution of low-mass star and brown dwarf eclipsing binaries. These objects are rapid rotators and are believed to shelter large magnetic fields.

_Aims._We suggest that reduced convective efficiency, due to fast rotation and large field strengths, and/or to magnetic spot coverage of the radiating surface significantly affect their evolution, leading to a reduced heat flux and thus larger radii and cooler effective temperatures than for regular objects.

_Methods._We have considered such processes in our evolutionary calculations, using a phenomenological approach.

_Results._This yields mass–radius and effective temperature–radius relationships in agreement with the observations. We also reproduce the effective temperature ratio and the radii of the two components of the recently discovered puzzling eclipsing brown dwarf system.

_Conclusions._These calculations show that fast rotation and/or magnetic activity may significantly affect the evolution of eclipsing binaries and that the mechanical and thermal properties of these objects depart from the ones of non-active low-mass objects. We find that, for internal field strengths compatible with the observed surface value of a few kiloGauss, convection can be severely inhibited. The onset of a central radiative zone for rapidly rotating active low-mass stars might thus occur below the usual ~ $0.35~M_\odot$ limit.

Key words: convection / stars: activity / stars: binaries: eclipsing / stars: evolution / stars: low-mass, brown dwarfs / stars: rotation