Evolution of low-mass star and brown dwarf eclipsing binaries.pdf

Evolution of low-mass star and brown dwarf eclipsing binaries.pdf

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Evolution of low-mass star and brown dwarf eclipsing binaries

a r X i v : 0 7 0 7 .1 7 9 2 v 1 [ a s t r o - p h ] 1 2 J u l 2 0 0 7 Astronomy Astrophysics manuscript no. ms c? ESO 2008 February 1, 2008 Evolution of low-mass star and brown dwarf eclipsing binaries Gilles Chabrier, Jose? Gallardo, Isabelle Baraffe Ecole Normale Supe?rieure de Lyon, CRAL (UMR CNRS 5574), Universite? de Lyon, France Received/Accepted 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 ~ 0.35M⊙ limit. 1. Introduction Low-mass stars (LMS), i.e., M-type stars in the present context, represent an overwhelming fraction of the Galactic stellar population (Chabrier 2003). Observational determi- nation of their mass-radius relationship

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