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Title: ON THE EVOLUTION OF MAGNETIC WHITE DWARFS

We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-β parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1–50 kG, which is much smaller than the typical 1–1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B ≳ 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (T{sub eff}) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection due to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, the effects of magnetism on cooling rates are not expected to be observed. This resultmore » contrasts with a recent suggestion that magnetic white dwarfs with T{sub eff} ≲ 10,000 K cool significantly slower than non-magnetic degenerates.« less
Authors:
 [1] ; ;  [2] ;  [3] ;  [4] ;  [5] ;  [6] ;  [7]
  1. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
  2. Département de Physique, Université de Montréal, C. P. 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7 (Canada)
  3. Department of Physics and Astronomy at Uppsala University, Regementsvägen 1, Box 516, SE-75120 Uppsala (Sweden)
  4. Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, D-79104 Freiburg (Germany)
  5. Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, D-69117 Heidelberg (Germany)
  6. Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, D-14482 Potsdam (Germany)
  7. Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo (Norway)
Publication Date:
OSTI Identifier:
22518865
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 812; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPUTERIZED SIMULATION; CONVECTION; CRITICAL FIELD; LAYERS; MAGNETIC STARS; MAGNETISM; MAGNETOHYDRODYNAMICS; PHOTOSPHERE; PLASMA; STAR EVOLUTION; WHITE DWARF STARS