A star under multiple influences

Magnetic activity in V815 Her, a compact 2+2 hierarchical system^⋆

¹ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
e-mail: kovari@konkoly.hu
² HUN-REN CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
³ Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Institute for Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
⁵ Baja Astronomical Observatory of University of Szeged, Szegedi út, Kt. 766, 6500 Baja, Hungary
⁶ HUN-REN-SZTE Stellar Astrophysics Research Group, Szegedi út, Kt. 766, 6500 Baja, Hungary
⁷ ELTE Gothard Astrophysical Observatory, Szent Imre h. u. 112, 9700 Szombathely, Hungary
⁸ Moholy-Nagy University of Art and Design, Centre for Data Science and Digital Development, Zugligeti út 9, 1121 Budapest, Hungary

Received: 19 October 2023
Accepted: 10 January 2024

Abstract

Context. Close binaries with magnetically active components are astrophysical laboratories for studying the effects of binarity on activity. Of particular interest are binary and multiple star systems that contain a solar-type active component with an internal structure similar to the Sun, allowing us to study how the dynamo of a solar-type star would work under different conditions.

Aims. We have conducted a comprehensive investigation of V815 Her using photometric and spectroscopic data to understand the origin of the activity and what influences it in the short and long term.

Methods. Using space photometry we performed light curve modeling in order to derive astrophysical and orbital parameters for the eclipsing binary subsystem V815 Her B. Using archival photometric data covering a century we carried out a time frequency analysis. Spectral synthesis was applied to determine the basic astrophysical parameters of the rapidly rotating primary using high-resolution STELLA spectra recorded in 2018.

Results. Photometric analysis of archived data revealed multiple cycles on timescales between ∼6.5 and ∼26 yr, some of which may be harmonic. From TESS photometry we obtained an orbital solution for the V815 Her B subsystem. By placing the primary component on the Hertzsprung–Russell-diagram, we could deduce an age of ≈30 Myr, in line with the high Li-6707 abundance. The STELLA spectra covering the 200 day-long observing season enabled us to create 19 time-series Doppler images, which revealed a constantly changing spotted surface on a timescale of a few weeks. From the consecutive image pairs we built up the average cross-correlation function map to measure the surface differential rotation of the spotted star, from which we derive a weak solar-type surface shear.

Conclusions. We found evidence that the V815 Her B component previously apostrophized as a “third body” is actually an eclipsing close binary subsystem of two M dwarfs with a period of 0.5 d, that is, V815 Her is a 2+2 hierarchical quadruple system. The system is apparently young, only a few times ten million years old, consistent with the spotted primary V815 Her Aa being a zero-age main-sequence star. Spot activity on the primary was found to be vivid. Fast starspot decay suggests that convective-turbulent erosion plays a more significant role in such a rapidly rotating star. The weak surface shear of V815 Her Aa due to differential rotation is presumably confined by tidal forces of the close companion V815 Her Ab. The slowly increasing photometric cycle of about 6.5 yr on average is interpreted as a spot cycle of V815 Her Aa, which is probably modulated by the eccentric wide orbit.