A star under multiple influences. Magnetic activity in V815 Her, a compact 2+2 hierarchical system. (arXiv:2312.08416v1 [astro-ph.SR])

A star under multiple influences. Magnetic activity in V815 Her, a compact 2+2 hierarchical system. (arXiv:2312.08416v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kovari_Z/0/1/0/all/0/1">Zs. Kovari</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Strassmeier_K/0/1/0/all/0/1">K.G. Strassmeier</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kriskovics_L/0/1/0/all/0/1">L. Kriskovics</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Olah_K/0/1/0/all/0/1">K. Olah</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Borkovits_T/0/1/0/all/0/1">T. Borkovits</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Radvanyi_A/0/1/0/all/0/1">A. Radvanyi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Granzer_T/0/1/0/all/0/1">T. Granzer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Seli_B/0/1/0/all/0/1">B. Seli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vida_K/0/1/0/all/0/1">K. Vida</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Weber_M/0/1/0/all/0/1">M. Weber</a>

We are conducting 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 TESS 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 revealed multiple cycles on timescales between ~6.5 and
~26 years. From TESS photometry we obtained orbital solution for the V815 Her B
subsystem. The STELLA spectra covering the 200 day-long observing season
enabled to create 19 time-series Doppler images, which revealed a constantly
changing spotted surface. From the consecutive image pairs we measured a weak
solar-type surface differential rotation of the spotted star. 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, i.e., 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 differential rotation of V815 Her Aa is presumably
confined by tidal forces of the close companion V815 Her Ab. The slowly
increasing photometric cycle of 6.5 years on average is interpreted as a spot
cycle of V815 Her Aa, which is probably modulated by the eccentric wide orbit.

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