Do magnetic fields inhibit
convection in white dwarfs?
A case study on WD2105-820
Nicola Pietro Gentile Fusillo, Pier-Emmanuel Tremblay
Comparison with photometry
Around 10% of white dwarfs exhibit global magnetic structures with fields ranging from 1kG to hundreds of MG. Recently the first radiation
magnetohydrodynamics simulations of the atmosphere of white dwarfs showed that convection should be suppressed in their photospheres
for magnetic fields with strengths B=1-50 kG (Tremblay et al. 2015, ApJ, 812,19). These predictions are clearly in agreement with our
knowledge of stellar physics (e.g. sunspots), but have yet to be directly confirmed from white dwarf observations. We obtained COS far-UV
spectroscopy of the weakly magnetic white dwarf WD2105-820 and of three additional non-magnetic, convective white dwarfs. We fitted both
the COS and the optical spectra with convective and radiative atmospheric models, and find that, unlike non-magnetic white dwarfs, for
WD2105-820 only radiative models predict consistent UV and optical đeff values.
We compared the đeff values
derived from model fit of the UV
and optical spectra.
The solid line represents a
perfect match between UV and
optical temperatures, while the
dashed lines represent a ±350K
difference.
The plot clearly shows that
convective atmosphere models
produce consistent UV and
optical đeff for most stars in this
temperature regime, but not for
WD2105-820.
Convective and radiative models WD2105-820
Comparison with other white dwarfs
WD2105-820 was unambiguously
recognized to be magnetic by
Landstreet et al. (2012, A&A,
545, 30) during their small
spectropolarimetric survey of
cool white dwarfs.
They measured a mean field
strength of Bâ43 kG which is too
small to produce any visible
Zeeman splitting in medium
resolution spectroscopy.
Consequently the standard
spectroscopic method to
evaluate atmospheric parameters
by comparing the Balmer line
profiles with model atmospheres can still be reliably used for
WD2105â820.
With đeff = 10389 K and log g = 8.01 (Gianninas et al. 2011,
ApJ, 743, 138; +3D corrections from Tremblay et al. 2013,
A&A, 559, 104) WD2105-820 should have a convective
atmosphere and it is therefore the perfect target to
observationally test the prediction that weak fields (B = 1-50
kG) should inhibit convection (Tremblay et al. 2015, ApJ,
812,19).
We obtained far UV HST Cosmic Origin Spectrograph (COS)
observations of WD2105-820 and, in order to establish a
reliable comparison, of three additional cool, non-magnetic,
apparently single, hydrogen-atmosphere white dwarfs:
WD0839â327, WD1544â374, WD1310+583.
For this magnetic white dwarf radiative atmospheric models are
necessary to obtain consistent optical and UV đ»eff values.
The significantly offset blue point represents WD1310+583 which we
think is an unknown double degenerate where a hotter and a cooler
white dwarf dominate different parts of the spectrum.
Artistic impression of a magnetic white dwarf
We fitted the optical spectrum of
WD2105-820 using a newly
computed grid of purely radiative
model atmospheres. Both the
radiative and the convective models
can reproduce the Balmer line profile
of the white dwarf equally well,
though at significantly different
đeff and log g values (left figure).
However, while the best fitting
radiative model optical solution also
reproduces the COS ultraviolet
spectral profile, the corresponding
convective model solution fails to do
so (bottom figure).
WD2105-820 Balmer line fit with both convective and radiative model
atmospheres
Comparison of best fitting optical models with COS spectrum of WD2105-820
We retrieved optical
and near infrared
photometry of
WD2105-820 (gri
bands from APASS
and JHK from 2MASS)
and proceeded to fit
it using both
convective and
radiative models.
Photometric fits are
relatively insensitive
to the different
models and log g
values.
We obtain photometric temperatures consistent with the
radiative spectroscopic đ»eff and > 500 K different from the
convective spectroscopic đ»eff .
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