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Do magnetic fields inhibit convection in white dwarfs? A case study on WD2105-820 Nicola Pietro Gentile Fusillo, Pier-Emmanuel Tremblay [email protected] 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 eff values. We compared the eff 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 eff 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 B≈43 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 WD2105−820. With eff = 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: WD0839−327, WD1544−374, WD1310+583. For this magnetic white dwarf radiative atmospheric models are necessary to obtain consistent optical and UV 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 eff 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 eff and > 500 K different from the convective spectroscopic eff .
