Starspots on eclipsing giant stars

I. The sample and eclipse mapping examples

¹ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary
² HUN-REN CSFK, MTA Centre of Excellence, Budapest, Konkoly Thege Miklós út 15–17, 1121 Budapest, Hungary
³ Eötvös University, Department of Astronomy, Pf. 32, 1518 Budapest, Hungary
⁴ Department of Physics, PO Box 64, 00014 University of Helsinki, Finland
⁵ Department of Physics, Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁶ Amateur Astronomer, Glendale, AZ 85308, USA
⁷ Amateur Astronomer, Missouri City, TX 77459, USA
⁸ Brorfelde Observatory, Observator Gyldenkernes Vej 7, 4340 Tølløse, Denmark
⁹ Citizen Scientist, c/o Zooniverse, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁰ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹¹ SETI Institute, 189 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA

^★ Corresponding author: olah@konkoly.hu

Received: 15 January 2025
Accepted: 17 April 2025

Abstract

Context. Spotted stars in eclipsing binary systems allow us to gather significant information about the stellar surface inhomogeneities that is otherwise unobtainable from only photometric data. Starspots can be scanned using the eclipse (or transit) mapping technique, which takes advantage of the passage of a companion star (or planet) in front of a spotted giant star in a binary system.

Aims. Based on the characteristics of their ultraprecise space photometric light curves, we compile a list of eclipsing binaries whose primary component is a spotted subgiant or giant star, with the aim of applying the eclipse mapping technique to them.

Methods. Eclipsing binaries with giant primaries were selected from Transiting Exoplanet Survey Satellite (TESS) light curves by visual inspection. Spots showing up as bumps during eclipses were modeled with an eclipse mapping technique specialized for two stars, and the number of spots was found with the help of Bayes factors. The full light curves themselves were analyzed with time series spot modeling, and the results of the two approaches were compared.

Results. We present a catalog of 29 eclipsing close binaries with active giant components and analyze TIC 235934420, TIC 271892852, and TIC 326257590 from the continuous viewing zones (CVZs) of TESS. Remarkable agreement is found between the starspot temperatures, sizes, and longitudes from the eclipse mapping results and the corresponding full light curve solutions. Spots are always present at the substellar points of the tidally locked binaries. Data from the TESS CVZs allow us to follow the changes of spot patterns on yearly timescales.