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arXiv · 1207.5393

An accurate mass and radius measurement for an ultracool white dwarf

S. G. Parsons·B. T. Gänsicke·T. R. Marsh·P. Bergeron·C. M. Copperwheat·V. S. Dhillon·J. Bento·S. P. Littlefair·M. R. Schreiber
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Abstract

Studies of cool white dwarfs in the solar neighbourhood have placed a limit on the age of the Galactic disk of 8-9 billion years. However, determining their cooling ages requires the knowledge of their effective temperatures, masses, radii, and atmospheric composition. So far, these parameters could only be inferred for a small number of ultracool white dwarfs for which an accurate distance is known, by fitting their spectral energy distributions (SEDs) in conjunction with a theoretical mass-radius relation. However, the mass-radius relation remains largely untested, and the derived cooling ages are hence model-dependent. Here we report direct measurements of the mass and radius of an ultracool white dwarf in the double-lined eclipsing binary SDSS J013851.54-001621.6. We find M(WD)=0.529+/-0.010Msol and R(WD)=0.0131+/-0.0003Rsol. Our measurements are consistent with the mass-radius relation and we determine a robust cooling age of 9.5 billion years for the 3570K white dwarf. We find that the mass and radius of the low mass companion star, M(sec)=0.132+/-0.003Msol and R(sec)=0.165+/-0.001Rsol, are in agreement with evolutionary models. We also find evidence that this >9.5 Gyr old M5 star is still active, far beyond the activity lifetime for a star of its spectral type. This is likely caused by the high tidally-enforced rotation rate of the star. The companion star is close to filling its Roche lobe and the system will evolve into a cataclysmic variable in only 70 Myr. Our direct measurements demonstrate that this system can be used to calibrate ultracool white dwarf atmospheric models.

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S. G. Parsons, B. T. Gänsicke, T. R. Marsh, P. Bergeron, C. M. Copperwheat, V. S. Dhillon, J. Bento, S. P. Littlefair, M. R. Schreiber. 2012-07-23. An accurate mass and radius measurement for an ultracool white dwarf. https://doi.org/10.1111/j.1365-2966.2012.21773.x

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