Hidden magnetic fields of young suns. (arXiv:2002.10469v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kochukhov_O/0/1/0/all/0/1">O. Kochukhov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hackman_T/0/1/0/all/0/1">T. Hackman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lehtinen_J/0/1/0/all/0/1">J.J. Lehtinen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wehrhahn_A/0/1/0/all/0/1">A. Wehrhahn</a>
Global magnetic fields of active solar-like stars are nowadays routinely
detected with spectropolarimetric measurements and are mapped with
Zeeman-Doppler imaging (ZDI). However, due to the cancellation of opposite
field polarities, polarimetry captures only a tiny fraction of the magnetic
flux and cannot assess the overall stellar surface magnetic field if it is
dominated by a small-scale component. Analysis of Zeeman broadening in
high-resolution intensity spectra can reveal these hidden complex magnetic
fields. Historically, there were very few attempts to obtain such measurements
for G dwarf stars due to the difficulty of disentangling Zeeman effect from
other broadening mechanisms affecting spectral lines. Here we developed a new
magnetic field diagnostic method based on relative Zeeman intensification of
optical atomic lines with different magnetic sensitivity. Using this technique
we obtained 78 field strength measurements for 15 Sun-like stars, including
some of the best-studied young solar twins. We find that the average magnetic
field strength $Bf$ drops from 1.3-2.0 kG in stars younger than about 120 Myr
to 0.2-0.8 kG in older stars. The mean field strength shows a clear correlation
with the Rossby number and with the coronal and chromospheric emission
indicators. Our results suggest that magnetic regions have roughly the same
local field strength $Bapprox3.2$ kG in all stars, with the filling factor $f$
of these regions systematically increasing with stellar activity. Comparing our
results with the spectropolarimetric analyses of global magnetic fields in the
same stars, we find that ZDI recovers about 1% of the total magnetic field
energy in the most active stars. This figure drops to just 0.01% for the least
active targets.
Global magnetic fields of active solar-like stars are nowadays routinely
detected with spectropolarimetric measurements and are mapped with
Zeeman-Doppler imaging (ZDI). However, due to the cancellation of opposite
field polarities, polarimetry captures only a tiny fraction of the magnetic
flux and cannot assess the overall stellar surface magnetic field if it is
dominated by a small-scale component. Analysis of Zeeman broadening in
high-resolution intensity spectra can reveal these hidden complex magnetic
fields. Historically, there were very few attempts to obtain such measurements
for G dwarf stars due to the difficulty of disentangling Zeeman effect from
other broadening mechanisms affecting spectral lines. Here we developed a new
magnetic field diagnostic method based on relative Zeeman intensification of
optical atomic lines with different magnetic sensitivity. Using this technique
we obtained 78 field strength measurements for 15 Sun-like stars, including
some of the best-studied young solar twins. We find that the average magnetic
field strength $Bf$ drops from 1.3-2.0 kG in stars younger than about 120 Myr
to 0.2-0.8 kG in older stars. The mean field strength shows a clear correlation
with the Rossby number and with the coronal and chromospheric emission
indicators. Our results suggest that magnetic regions have roughly the same
local field strength $Bapprox3.2$ kG in all stars, with the filling factor $f$
of these regions systematically increasing with stellar activity. Comparing our
results with the spectropolarimetric analyses of global magnetic fields in the
same stars, we find that ZDI recovers about 1% of the total magnetic field
energy in the most active stars. This figure drops to just 0.01% for the least
active targets.
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