Eclipsing spotted giant star with K2 and historical photometry⋆
1 Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly Thege M. u. 15-17, 1121 Budapest, Hungary
2 Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
3 Baja Astronomical Observatory of Szeged University, Szegediút Kt. 766, 6500 Baja, Hungary
4 Amateur Astronomer, 12812 SE 69th Place, Bellevue, WA, 98006, USA
5 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, 02138, USA
6 Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
7 Department of Astronomy, The University of Texas at Austin, NASA Sagan Fellow, 2515 Speedway, Stop C1400, Austin, TX, 78712, USA
8 Amateur Astronomer, 7507 52nd Place NE, Marysville, WA, 98270, USA
9 Department of Astrophysical Sciences, Princeton University, Princeton, USA
10 MTA Distinguished Guest Fellow, Konkoly Observatory, Budapest, Hungary
11 Citizen Scientist, c/o Zooniverse, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
Accepted: 7 October 2018
Context. Stars can maintain their observable magnetic activity from the pre-main sequence (PMS) to the tip of the red giant branch. However, the number of known active giants is much lower than active stars on the main sequence (MS) since the stars spend only about 10% of their MS lifetime on the giant branch. Due to their rapid evolution it is difficult to estimate the stellar parameters of giant stars. A possibility for obtaining more reliable stellar parameters for an active giant arises when it is a member of an eclipsing binary system.
Aims. We have discovered EPIC 211759736, an active spotted giant star in an eclipsing binary system during the Kepler K2 Campaign 5. The eclipsing nature allows us to much better constrain the stellar parameters than in most cases of active giant stars.
Methods. We have combined the K2 data with archival HATNet, ASAS, and DASCH photometry, new spectroscopic radial velocity measurements, and a set of follow-up ground-based BVRCIC photometric observations, to find the binary system parameters as well as robust spot models for the giant at two different epochs.
Results. We determined the physical parameters of both stellar components and provide a description of the rotational and long-term activity of the primary component. The temperatures and luminosities of both components were examined in the context of the Hertzsprung–Russell diagram. We find that both the primary and the secondary components deviate from the evolutionary tracks corresponding to their masses in the sense that the stars appear in the diagram at lower masses than their true masses.
Conclusions. We further evaluate the proposition that traditional methods generally result in higher masses for active giants than what is indicated by stellar evolution tracks in the HR diagram. A possible reason for this discrepancy could be a strong magnetic field, since we see greater differences in more active stars.
Key words: stars: activity / binaries: eclipsing / stars: evolution / stars: late-type / starspots / stars: fundamental parameters
© ESO 2018