Robotic observations of the most eccentric spectroscopic binary in the sky ^⋆,⋆⋆

Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: KStrassmeier@aip.de; MWeber@aip.de; TGranzer@aip.de

Received: 27 May 2013
Accepted: 2 September 2013

Abstract

Context. The visual A component of the Gliese 586AB system is a double-lined spectroscopic binary consisting of two cool stars with the exceptional orbital eccentricity of 0.976. Such an extremely eccentric system may be important for our understanding of low-mass binary formation.

Aims. Precise stellar masses, ages, orbital elements, and rotational periods are a prerequisite for comparing stellar observations to angular-momentum evolution models.

Methods. We present a total of 598 high-resolution échelle spectra from our robotic facility STELLA from 2006–2012, which we used to compute orbital elements of unprecedented accuracy. New Johnson VI photometry for the two visual components is also presented.

Results. Our double-lined orbital solution for the A system has average velocity residuals for a measure of unit weight of 41 m s^-1 for the G9V primary and 258 m s^-1 for the M0V secondary, better by a factor ≈10 than the discovery orbit. The orbit constrains the eccentricity to 0.97608 ± 0.00004 and the orbital period to 889.8195 ± 0.0003 d. The masses of the two components are 0.87 ± 0.05  M_⊙ and 0.58 ± 0.03  M_⊙ if the inclination is 55 ± 1.5° as determined from adaptive-optics images, that is good to only 6% due to the error of the inclination, although the minimum masses reached a precision of 0.3%. The flux ratio Aa:Ab in the optical is between 30:1 in Johnson-B and 11:1 in I. Radial velocities of the visual B-component (K0-1V) appear constant to within 130 m s^-1 over six years. Sinusoidal modulations of T_eff of Aa with an amplitude of ≈55 K are seen with the orbital period. Component Aa appears warmest at periastron and coolest at apastron, indicating atmospheric changes induced by the high orbital eccentricity. No light variations larger than approximately 4 mmag are detected for A, while a photometric period of 8.5 ± 0.2 d with an amplitude of 7 mmag is discovered for the active star B, which we interpret to be its rotation period. We estimate an orbital period of ≈50 000 yr for the AB system. The most likely age of the AB system is ≥2 Gyr, while the activity of the B component, if it were a single star, would imply 0.5 Gyr. Both Aa and B are matched with single-star evolutionary tracks of their respective mass.