The solar-type eclipsing binary system LL Aquarii

Abstract

The eclipsing binary LL Aqr consists of two late-type stars in an eccentric orbit with a period of 20.17 d. We use an extensive light curve from the SuperWASP survey augmented by published radial velocities and UBV light curves to measure the physical properties of the system. The primary star has a mass of 1.167 ± 0.009 M and a radius of 1.305 ± 0.007 R . The secondary star is an analogue of the Sun, with a mass and radius of 1.014 ± 0.006 M and 0.990 ± 0.008 R respectively. The system shows no signs of stellar activity: the upper limit on spot-induced rotational modulation is 3 mmag, it is slowly rotating, has not been detected at X-ray wavelengths, and the calcium H and K lines exhibit no emission. Theoretical stellar models provide a good match to its properties for a sub-solar metal abundance of Z = 0.008 and an age of 2.5 Gyr. Most low-mass eclipsing binary systems are found to have radii larger than expected from theoretical predictions, blamed on tidally-enhanced magnetic fields in these short-period systems. The properties of LL Aqr support this scenario: it exhibits negligible tidal effects, shows no signs of magnetic activity, and matches theoretical models well.

A solar twin in the eclipsing binary LL Aquarii

“…There is a small difference in the systemic velocities of the two components, where the primary component is blueshifted by 195 ± 12 m s −1 with respect to the secondary. A very similar difference of 270 ± 140 m s −1 was reported by Southworth (2013) based on velocimetry from Griffin (2013). However our absolute systemic velocity of the system is differ-D. Graczyk et al: The eclipsing binary LL Aqr (Henden et al 2016), 6 -URAT1 (Zacharias et al 2015) ent by as much as 1.4 km s −1 , which is a value that is much larger than the measurements errors.…”

“…The above reddening is much smaller than the value derived byİbanoǧlu et al (2008) and used in subsequent analysis of the system by Southworth (2013). The likely reason for the strong overestimate of extinction byİbanoǧlu et al (2008) is that they used the Q method, which for solar-type stars gives large uncertainties because of heavy line blanketing in the wavelength region covered by the U, B, and V bands (e.g.…”

“…The light curve was cleaned of outliers using 3σ clipping and details are given in Southworth (2013). Because the cleaned light curve contains 21 362 datapoints and it is much larger then the combined Johnson photometry, we decided to reduce the number of datapoints as follows.…”

“…We used the previously measured radial velocities and we repeated the iterations twice. In order to renormalise the spectra we used the photometric parameters from Southworth (2013) but with temperatures lower by about 500 K (see Sect. 3.5), and we calculated appropriate light ratios with the Wilson-Devinney code (hereafter WD; see Sec.…”

“…We assumed synchronous rotation for both components. The epoch of the primary minimum was set according to the ephemeris given by Southworth (2013). Both the logarithmic limb-darkening law (Klinglesmith & Sobieski 1970), with coefficients tabulated by van Hamme (1993), and the linear law, with adjusted linear coefficients, were used during analysis.…”