Warm and cool starspots with opposite polarities. A high-resolution Zeeman-Doppler-Imaging study of II Pegasi with PEPSI. (arXiv:1902.11201v1 [astro-ph.SR])
<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:+Carroll_T/0/1/0/all/0/1">T. A. Carroll</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ilyin_I/0/1/0/all/0/1">I. V. Ilyin</a>
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.
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.
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