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1
Suppression of cooling by strong magnetic fields in white dwarf stars

Valyavin, G.,Shulyak, D.,Wade, G. A.,Antonyuk, K.,Zharikov, S. V.,Galazutdinov, G. A.,Plachinda, S.,Bagnulo, S.,Machado, L. Fox,Alvarez, M.,Clark, D. M.,Lopez, J. M.,Hiriart, D.,Han, Inwoo,Jeon, Young Nature Publishing Group, a division of Macmillan P 2014 Nature Vol.515 No.7525
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Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.
2
THE LORENTZ FORCE IN ATMOSPHERES OF CP STARS: θ AUR

VALYAVIN G.,KOCHUKHOV O.,SHULYAK D.,LEE B.-C.,GALAZUTDINOV G.,KIM K.-M.,HAN I. The Korean Astronomical Society 2005 Journal of The Korean Astronomical Society Vol.38 No.2
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  코리아스칼라
The slow evolution of global magnetic fields and other dynamical processes in atmospheres of CP magnetic stars lead to the development of induced electric currents in all conductive atmospheric layers. The Lorentz force, which results from the interaction between a magnetic field and the induced currents, may modify the atmospheric structure and provide insight into the formation and evolution of stellar magnetic fields. This modification of the pressure-temperature structure influences the formation of absorption spectral features producing characteristic rotational variability of some spectral lines, especially the Balmer lines (Valyavin et al., 2004 and references therein). In order to study these theoretical predictions we began systematic spectroscopic survey of Balmer line variability in spectra of brightest CP magnetic stars. Here we present the first results of the program. A0p star $\Theta$ Aur revealed significant variability of the Balmer profiles during the star's rotation. Character of this variablity corresponds to that classified by Kroll (1989) as a result of an impact of significant Lorentz force. From the obtained data we estimate that amplitudes of the variation at H$\alpha$, H$\beta$, H$\gamma$ and H$\delta$ profiles reach up to $2.4\%$during full rotation cycle of the star. Using computation of our model atmospheres (Valyavin et al., 2004) we interpret these data within the framework of the simplest model of the evolution of global magnetic fields in chemically peculiar stars. Assuming that the field is represented by a dipole, we estimate the characteristic e.m.f. induced by the field decay electric current (and the Lorentz force as the result) on the order of $E {\~} 10^{-11}$ cgs units, which may indicate very fast (< < $10^{10}$ years) evolution rate of the field. This result strongly contradicts the theoretical point of view that global stellar magnetic fields of CP stars are fossil and their the characteristic decay time of about $10^{10}$ yr. Alternatively, we briefly discuss concurring effects (like the ambipolar diffusion) which may also lead to significant atmospheric currents producing the observable Lorentz force.
3
The non force-free magnetic fields in atmospheres of chemically peculiar Ap/Bp stars: a model and observations (BOES)

Gennady G. Valyavin,O. Kochukhov,B-C. Lee,G. Galazutdinov,D. Shulyak,I. Han 한국천문학회 2004 天文學會報 Vol.29 No.2
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  코리아스칼라