Warm and cool starspots with opposite polarities. A high-resolution Zeeman-Doppler-Imaging study of II Pegasi with PEPSI

Abstract

We present a temperature and a magnetic-field surface map of the K2 subgiant of the active binary II Peg. Employed are high resolution Stokes IV spectra obtained with the new Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT). Our main result is that the temperature features on II Peg closely correlate with its magnetic field topology. We find a warm spot (350K warmer with respect to the effective temperature) of positive polarity and radial field density of 1.1 kG coexisting with a cool spot (780K cooler) of negative polarity of 2 kG. Several other cool features are reconstructed containing both polarities and with (radial) field densities of up to 2 kG. The largest cool spot is reconstructed with a temperature contrast of 550 K, an area of almost 10% of the visible hemisphere, and with a multipolar magnetic morphology. A meridional and an azimuthal component of the field of up to +/-500G is detected in two surface regions between spots with strong radial fields but different polarities. A force-free magnetic-field extrapolation suggests that the different polarities of cool spots and the positive polarity of warm spots are physically related through a system of coronal loops of typical height of approx. 2 Rstar. While the H-alpha line core and its red-side wing exhibit variations throughout all rotational phases, a major increase of blue-shifted H-alpha emission was seen for the phases when the warm spot is approaching the stellar central meridian indicating high-velocity mass motion within its loop. We explain the warm spots due to photospheric heating by a shock front from a siphon-type flow between regions of different polarities while the majority of the cool spots is likely formed due to the expected convective suppression like on the Sun.

arXiv comment12 pages, 8 figures

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