A star under multiple influences. Magnetic activity in V815 Her, a compact 2+2 hierarchical system

Abstract

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. 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. 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. Photometric analysis of archived data revealed multiple cycles on timescales between sim 6.5 and sim 26 years, 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 approx 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. 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 years on average is interpreted as a spot cycle of V815\,Her Aa, which is probably modulated by the eccentric wide orbit.