J/A+A/625/A27 II Pegasi spectra with PEPSI (Strassmeier+, 2019)
Warm and cool starspots with opposite polarities.
A high-resolution Zeeman-Doppler-Imaging study of II Pegasi with PEPSI.
Strassmeier K.G., Carroll T.A., Ilyin I.V.
<Astron. Astrophys. 625, A27 (2019)>
=2019A&A...625A..27S 2019A&A...625A..27S (SIMBAD/NED BibCode)
ADC_Keywords: Stars, variable ; Spectroscopy
Keywords: stars: imaging - stars: activity - starspots -
stars: individual: II Peg
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).
Fourteen average line profiles are inverted using our iMap code. We
have employed an iterative regularization scheme without the need of a
penalty function and incorporate a physical 3D description of the
surface field vector. The spectral resolution of our data is 130000
which converts to 20 resolution elements across the disk of II Peg.
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 1.1kG coexisting with a cool spot (780K
cooler) of negative polarity of 2kG. Several other cool features are
reconstructed containing both polarities and with (radial) field
densities of up to 2kG. The largest cool spot is reconstructed with a
temperature contrast of 550K, 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 ~=2R*. While the Hα line core
and its red-side wing exhibit variations throughout all rotational
phases, a major increase of blue-shifted Hα emission was seen
for the phases when the warm spot is approaching the stellar central
meridian indicating high-velocity mass motion within its loop.
Active stars such as II Peg can show coexisting cool and warm spots on
the surface that we interpret resulting from two different formation
mechanisms. 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.
Description:
The polarimetric observations in this paper were obtained with PEPSI
at the 2x8.4m LBT in Arizona. We employed both polarimeters in the
LBT's two symmetric straight-through Gregorian foci.
Objects:
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RA (2000) DE Designation(s)
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23 55 04.05 +28 38 01.2 II Pegasi = HD 224085
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 111 84 List of spectra
fits/* . 84 Individual spectra
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 30 A30 --- FileName Name of the fits spectrum,
in subdirectory fits (1)
32- 35 A4 --- Stokes [I V Null] Stokes
37- 46 A10 "DD/MM/YYYY" Obs.date Observation date
49- 58 A10 "h:m:s" Obs.time Observation time
61- 72 A12 "h:m:s" ExpTime Exposure time
75- 88 A14 --- CDispt Cross-disperser
91- 94 A4 --- Fiber Fiber
97-111 F15.7 d HJD Heliocentric Julian date of observation
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Note (1): the files are written as FITS binary tables:
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1- 8 1D Angstroms Arg Wavelength in the stellar rest frame with the
heliocentric and orbital velocities removed
8- 16 1D Intensity Fun Continuum normalized intensity
16- 24 1D Variance Var Variance for each wavelength pixel
24- 25 1L Byte Mask A mask column for data processing
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Acknowledgements:
Ilya Ilyin, ilyin(at)aip.de
(End) Ilya Ilyin [AIP, Germany], Patricia Vannier [CDS] 23-Apr-2019