Absolute dimensions of solar-type eclipsing binaries

NY Hya: A test for magnetic stellar evolution models^⋆

¹ University of Southern Denmark, Department of Physics, Chemistry and Pharmacy, SDU-Galaxy, Campusvej 55, 5230 Odense M, Denmark
e-mail: tchinse@gmail.com
² Ankara University, Faculty of Science, Department of Astronomy and Space Sciences, Tandoğan, 06100 Ankara, Türkiye
³ Ankara University, Astronomy and Space Sciences Research and Application Center (Kreiken Observatory), İncek Blvd., 06837 Ahlatlıbel, Ankara, Türkiye
⁴ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
⁵ Department of Physics & Astronomy, University of North Georgia, Dahlonega, GA 30597, USA
⁶ Instituto de Investigación en Astronomia y Ciencias Planetarias, Universidad de Atacama, Avenida Copayapu 485, Copiapó, Atacama, Chile
⁷ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
⁸ SETI Institute, 189 Bernardo Ave, Suite 200, Mountain View, CA 94043, USA
⁹ Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ National Astronomical Observatory of Japan, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹¹ Astronomical Science Program, Graduate University for Advanced Studies, SOKENDAI, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
¹² Ankara University, Graduate School of Natural and Applied Sciences, Department of Astronomy and Space Sciences, Tandoğan, 06100 Ankara, Türkiye
¹³ Astronomical Institute of the Czech Academy of Sciences, 251 65 Ondřejov, Czech Republic
¹⁴ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/ Can Magrans s/n, 08193 Bellaterra, (Barcelona), Spain
¹⁵ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels, (Barcelona), Spain
¹⁶ Niels Bohr Institute, University of Copenhagen, Jagtvej 155, 2200 Copenhagen, Denmark
¹⁷ Chungnam National University, Department of Astronomy, Space Science and Geology, Daejeon, Republic of Korea
¹⁸ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, 69117 Heidelberg, Germany
¹⁹ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
²⁰ McGill Space Institute, McGill University, 3550 University Street, Montreal, QC H3A 2A7, Canada
²¹ Department of Astronomy, Arthur C. Clarke Institute for Modern Technologies, 0272 Moratuwa, Sri Lanka
²² Centre for ExoLife Sciences, Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5, 1350 Copenhagen, Denmark
²³ University of St Andrews, Centre for Exoplanet Science, SUPA School of Physics & Astronomy, North Haugh, St Andrews KY16 9SS, UK
²⁴ Centre for Electronic Imaging, School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
²⁵ Instituto de Astrofísica e Ciências do Espaço, Departamento de Física, Universidade de Coimbra, 3040-004 Coimbra, Portugal
²⁶ Centro de Astronomía, Universidad de Antofagasta, Avenida Angamos 601, Antofagasta 1270300, Chile
²⁷ Chungbuk National University Observatory, Chungbuk National University, 28644 Cheongju, South Korea
²⁸ Department of Astronomy and Atmospheric Sciences, Kyungpook National University, 41566 Daegu, South Korea
²⁹ Department of Physics, College of Science, Princess Nourah bint Abdulrahman University PO Box 84428 Riyadh 11671, Saudi Arabia
³⁰ Department of Physics, School of Science, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece
³¹ S.M. Nikolskii Mathematical Institute of the Peoples’ Friendship University of Russia (RUDN University), Moscow 117198, Russia

Received: 20 May 2022
Accepted: 21 March 2024

Abstract

Context. 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.

Aims. 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.

Methods. 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 Strömgren photometry and related colour indices.

Results. 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 ± 0.0090 M_⊙ (0.78%), R_A = 1.407 ± 0.015 R_⊙ (1.1%), T_eff, A = 5595 ± 61 K (1.09%), M_B = 1.1678 ± 0.0096 M_⊙ (0.82%), R_B = 1.406 ± 0.017 R_⊙ (1.2%), and T_eff, B = 5607 ± 61 K (1.09%). The atmospheric properties from Strömgren 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 cos ω) 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.

Conclusions. 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.