The EBLM Project

V. Physical properties of ten fully convective, very-low-mass stars^⋆

¹ Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
e-mail: alexander.vonboetticher@physics.ox.ac.uk
² Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, UK
³ School of Physics & Astronomy, University of Birmingham, Edgabston, Birmingham B15 2TT, UK
⁴ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁵ Observatoire Astronomique de l’Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
⁶ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
⁷ King Abdullah Centre for Crescent Observations and Astronomy, Makkah Clock, Mecca 24231, Saudi Arabia
⁸ Université de Liège, Allée du 6 août 17, Sart Tilman, 4000 Liège 1, Belgium
⁹ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, KY16 9SS St Andrews, Fife, UK
¹⁰ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹¹ Instituto de Astronomía, Universidad Nacional Autónoma de Mexico, Ciudad Universitaria, Ciudad de Mexico 04510, Mexico
¹² Hobart and William Smith Colleges, Department of Physics, Geneva, NY 14456, USA
¹³ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
¹⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁵ University of Chicago, 5640 S Ellis Ave, Chicago, IL 60637, USA

Received: 29 October 2018
Accepted: 9 February 2019

Abstract

Measurements of the physical properties of stars at the lower end of the main sequence are scarce. In this context we report masses, radii and surface gravities of ten very-low-mass stars in eclipsing binary systems, with orbital periods of the order of several days. The objects probe the stellar mass-radius relation in the fully convective regime, M_⋆ ≲ 0.35 M_⊙, down to the hydrogen burning mass-limit, M_HB ∼ 0.07 M_⊙. The stars were detected by the WASP survey for transiting extra-solar planets, as low-mass, eclipsing companions orbiting more massive, F- and G-type host stars. We use eclipse observations of the host stars, performed with the TRAPPIST, Leonhard Euler and SPECULOOS telescopes, and radial velocities of the host stars obtained with the CORALIE spectrograph, to determine the physical properties of the low-mass companions. Surface gravities of the low-mass companions are derived from the eclipse and orbital parameters of each system. Spectroscopic measurements of the host star effective temperature and metallicity are used to infer the host star mass and age from stellar evolution models for solar-type stars. Masses and radii of the low-mass companions are then derived from the eclipse and orbital parameters of the binary systems. The objects are compared to stellar evolution models for low-mass stars, to test for an effect of the stellar metallicity and orbital period on the radius of low-mass stars in close binary systems. Measurements are found to be in good agreement with stellar evolution models; a systematic inflation of the radius of low-mass stars with respect to model predictions is limited to 1.6 ± 1.2%, in the fully convective low-mass regime. The sample of ten objects indicates a scaling of the radius of low-mass stars with the host star metallicity. No correlation between stellar radii and the orbital periods of the binary systems is determined. A combined analysis with thirteen comparable objects from the literature is consistent with this result.