Absolute dimensions of solar-type eclipsing binaries. NYHya: A test for magnetic stellar evolution models

Abstract

The binary star NY\,Hya is a bright, detached, double-lined eclipsing system with an orbital period of just under five days with two components each nearly identical to the Sun and located in the solar neighbourhood. The objective of this study is to test and confront various stellar evolution models for solar-type stars based on accurate measurements of stellar mass and radius. We present new ground-based spectroscopic and photometric as well as high-precision space-based photometric and astrometric data from which we derive orbital as well as physical properties of the components via the method of least-squares minimisation based on a standard binary model valid for two detached components. Classic statistical techniques were invoked to test the significance of model parameters. Additional empirical evidence was compiled from the public domain; the derived system properties were compared with archival broad-band photometry data enabling a measurement of the system's spectral energy distribution that allowed an independent estimate of stellar properties. We also utilised semi-empirical calibration methods to derive atmospheric properties from Str\"omgren photometry and related colour indices. We measured (percentages are fractional uncertainties) masses, radii, and effective temperatures of the two stars in NY\,Hya and found them to be $M_A = 1.1605 sun \,(0.78<!PCT!>)$, $R_A sun \,(1.1<!PCT!>)$ eff, A 61$\,K\,(1.09<!PCT!>), $M_B = 1.1678 sun \,(0.82<!PCT!>)$, $R_B = 1.406 sun \,(1.2<!PCT!>)$, and eff, B 61$\,K\,(1.09<!PCT!>). The atmospheric properties from Str\"omgren photometry agree well with spectroscopic results. No evidence was found for nearby companions from high-resolution imaging. A detailed analysis of space-based data revealed a small but significant eccentricity $(e of the orbit. The spectroscopic and frequency analysis on photometric time series data reveal evidence of clear photospheric activity on both components likely in the form of star spots caused by magnetic activity. We confronted the observed physical properties with classic and magnetic stellar evolution models. Classic models yielded both young pre-main-sequence and old main-sequence turn-off solutions with the two components at super-solar metallicities, in disagreement with observations. Based on chromospheric activity and X-ray observations, we invoke magnetic models. While magnetic fields are likely to play an important role, we still encounter problems in explaining adequately the observed properties. To reconcile the observed tensions we also considered the effects of star spots known to mimic magnetic inhibition of convection. Encouraging results were obtained, although unrealistically large spots were required on each component. Overall we conclude that NY\,Hya proves to be complex in nature, and requires additional follow-up work aiming at a more accurate determination of stellar effective temperature and metallicity.