A solar twin in the eclipsing binary LL Aquarii

¹ Millenium Institute of Astrophysics, Santiago, Chile
e-mail: darek@astro-udec.cl
² Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
³ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland
⁴ Department of Physics, University of Zagreb, Bijenićka cesta 32, 10000 Zagreb, Croatia
⁵ Astrophysics Group, Keele University, Keele, Staffordshire, ST5 5BG, UK
⁶ Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
⁷ Physics and Astronomy Department, The Johns Hopkins University, 3400 North Charles St, Baltimore, MD 21202, USA
⁸ Département d’Astronomie, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland

Received: 13 May 2016
Accepted: 23 June 2016

Abstract

Aims. In the course of a project to study eclipsing binary stars in vinicity of the Sun, we found that the cooler component of LL Aqr is a solar twin candidate. This is the first known star with properties of a solar twin existing in a non-interacting eclipsing binary, offering an excellent opportunity to fully characterise its physical properties with very high precision.

Methods. We used extensive multi-band, archival photometry and the Super-WASP project and high-resolution spectroscopy obtained from the HARPS and CORALIE spectrographs. The spectra of both components were decomposed and a detailed LTE abundance analysis was performed. The light and radial velocity curves were simultanously analysed with the Wilson-Devinney code. The resulting highly precise stellar parameters were used for a detailed comparison with PARSEC, MESA, and GARSTEC stellar evolution models.

Results. LL Aqr consists of two main-sequence stars (F9 V + G3 V) with masses of M₁ = 1.1949 ± 0.0007 and M₂ = 1.0337 ± 0.0007 M_⊙, radii R₁ = 1.321 ± 0.006 and R₂ = 1.002 ± 0.005 R_⊙, temperatures T₁ = 6080 ± 45 and T₂ = 5703 ± 50 K and solar chemical composition [M/H] = 0.02 ± 0.05. The absolute dimensions, radiative and photometric properties, and atmospheric abundances of the secondary are all fully consistent with being a solar twin. Both stars are cooler by about 3.5σ or less metal abundant by 5σ than predicted by standard sets of stellar evolution models. When advanced modelling was performed, we found that full agreement with observations can only be obtained for values of the mixing length and envelope overshooting parameters that are hard to accept. The most reasonable and physically justified model fits found with MESA and GARSTEC codes still have discrepancies with observations but only at the level of 1σ. The system is significantly younger that the Sun, with an age between 2.3 Gyr and 2.7 Gyr, which agrees well with the relatively high lithium abundance of the secondary, A(Li) = 1.65 ± 0.10 dex.