Abstract
Aims. We investigate magnetic tracers in the photosphere and the chromosphere of the ultra-rapid rotator (P ~ 0.32 d) V530 Per, a cool member of the open cluster α Persei, to characterize the short-term variability of the magnetic activity and large-scale magnetic field of this prototypical young, rapidly rotating solar-like star. Methods. With time-resolved spectropolarimetric observations spread over four close-by nights, we reconstructed the brightness distribution and large-scale magnetic field geometry of V530 Per through Zeeman-Doppler imaging. Simultaneously, we estimated the short-term variability of the surface through latitudinal differential rotation. Using the same data set, we also mapped the spatial distribution of prominences through tomography of Hα emission. Results. As in our previous study, a large dark spot occupies the polar region of V530 Per with smaller, dark, and bright spots at lower latitudes. The large-scale magnetic field is dominated by a toroidal, mostly axisymmetric component. The maximal radial field strength is equal to ~1 kG. The surface differential rotation is consistent with a smooth Sun-like shear dΩ = 0.053 ± 0.004 rad d−1, close to the solar shear level. The prominence pattern displays a stable component that is confined close to the corotation radius. We also observe rapidly evolving Hα emitting structures, over timescales ranging from minutes to days. The fast Hα evolution was not linked to any detected photospheric changes in the spot or magnetic coverage.
Highlights
Stellar activity is an important aspect of the early evolution of solar-type stars
From the time-series of Least squares deconvolution (LSD) profiles, we reconstructed the surface brightness and large-scale magnetic field geometries of V530 Per with a Zeeman-Doppler imaging (ZDI) code developed in Python and described in Folsom et al (2018)
We first calculated the equivalent width (EW) of the emission component of Hα by subtracting from the measured EW a reference EW estimated from a PolarBase spectrum of HD 225261, a quiet star chosen for its low S-index and effective temperature close to the one of V530 Per (Marsden et al 2014)
Summary
Stellar activity is an important aspect of the early evolution of solar-type stars. A large fraction of young Suns rotate rapidly (Gallet & Bouvier 2015), and the sustained rotation is the root of strong activity, as empirically illustrated by the well-known relationship between X-ray emission and the rotation rate (e.g., Wright et al 2011). ZDI investigations of young, active stars reveal a more diverse situation, where temporal changes of differential rotation were observed, and shear values were reported to depend on the adopted surface tracer (brightness or magnetic field, e.g., Donati et al 2003b; Yu et al 2019). The rotationally modulated Hα emission was consistent with an extended prominence system, characterized by an accumulation of hydrogen clouds near the corotation radius (in agreement with the previous work of Barnes et al 2001), and variations of the prominence system within a few days Since this first data set was not optimized to highlight fast changes, the purpose of the present study is to investigate in greater details the short-term variability of the prominence pattern, as well as the possible links of the coronal variability with the evolution of surface magnetic features, by investigating a denser time-series of observations. The main conclusions of this work are summarized (Sect. 7)
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