Time-series Doppler images and surface differential rotation of the effectively single, rapidly rotating K-giant KU Pegasi

Abstract

Context. According to most stellar dynamo theories, differential rotation (DR) plays a crucial role in the generation of toroidal magnetic fields. Numerical models predict surface differential rotation to be anti-solar for rapidly rotating giant stars, i.e. their surface angular velocity could increase with stellar latitude. However, surface differential rotation has been derived only for a handful of individual giant stars to date. Aims. The spotted surface of the K-giant KU Pegasi is investigated in order to detect its time evolution and to quantify the surface differential rotation. Methods. We present 11 Doppler images from spectroscopic data collected with the robotic telescope STELLA between 2006 and 2011. All maps are obtained with the surface reconstruction code iMap. Differential rotation is extracted from these images by detecting systematic (latitude-dependent) spot displacements. We apply a cross-correlation technique to find the best differential rotation law. Results. The surface of KU Peg shows cool spots at all latitudes and one persistent warm spot at high latitude. A small cool polar spot exists for most but not all of the epochs. Re-identification of spots in at least two consecutive maps is mostly possible only at middle and high latitudes and thus restricts the differential-rotation determination mainly to these latitudes. Our cross-correlation analysis reveals solar-like differential rotation with a surface shear of α = +0.040 ± 0.006, i.e., approximately five times weaker than on the Sun. We also derive a more accurate and consistent set of stellar parameters for KU Peg including a small Li abundance of ten times less than solar.

Superflares on the late-type giant KIC 2852961

“…Still, other mechanisms could also be con-sidered to spin up the surface of an evolved star on the red giant branch. A certain fraction of red giants have undergone such spin up phases (e.g., Carlberg et al 2011;Ceillier et al 2017), which may involve mixing processes (Simon & Drake e.g., 1989, but see also Kriskovics et al 2014;Kővári et al 2017b), planet engulfment (Siess & Livio 1999;Kővári et al 2016), binary mergers (Webbink 1976;Strassmeier et al 1998) or other, less known mechanisms. However, the lack of systematic radial velocity measurements does not enable to know whether KIC 2852961 is a member of a wide binary system or rather a close binary.…”

Rotation–differential rotation relationships for late‐type single and binary stars from Doppler imaging

“…cB05: Barnes (), BCL05: Barnes et al (), BJC04: Barnes et al (), DC97: Donati & Collier Cameron (), MWC05: Marsden et al (), DCP03: Donati et al (), PDV04 Petit et al (), KSG04: Kővári et al (), KWF09: Kővári et al (), KBH00: Korhonen et al (), PDO04: Petit et al (), VKS07: Vida et al (), HSK16: Harutyunyan et al (), KW04: Kővári & Weber (), KKO14: Kővári et al (), KKS13: Kővári et al (), KBS07: Kővári et al (), KKK12: Kővári et al (), KKV14: Kriskovics et al (), KKK15: Kővári et al (), WSW05: Weber et al (), ÖCK16: Özdarcan et al (), KKS16: Kővári et al (), MBD07: Marsden et al (), SKW03: Strassmeier et al (), KSO17: Kővári et al (), BVW86: Balthasar et al (), BJM16: Boro Saikia et al ().…”

KIC 9451096: Magnetic activity, flares and differential rotation

“…More reliable way of detecting differential rotation with its magnitude and type is Doppler imaging, which is based on high resolution time series spectroscopy. Considering other stars whose k values were determined by Doppler imaging, we see mostly weak differential rotation with a k value of a few percent, either among solar type differential rotators (HD 208472 k = 0.015 ( Özdarcan et al 2016), XX Tri k = 0.016 (Künstler et al 2015), ζ And k = 0.055 (Kővári et al 2012), KU Peg k = 0.04 (Kővári et al 2016)) or anti-solar type differential rotators (UZ Lib k = −0.004 (Vida et al 2007), σ Gem k = −0.04 (Kővári et al 2015), HU Vir k = −0.029 (Harutyunyan et al 2016)). Due to the binary nature of KIC 9451096, considerable effect of tidal forces on redistribution of the angular momentum in the convective envelope of the components can be expected, which would alter the magnitude of differential rotation (Scharlemann 1982).…”