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We present the discovery of the first T dwarf + white dwarf binary system LSPM 1459+0857 AB, confirmed through common proper motion and spectroscopy. The white dwarf is a high proper motion object from the LSPM catalogue that we confirm spectroscopically to be a relatively cool Teff = 5535 +- 45 K and magnetic B similar to 2 MG hydrogen-rich white dwarf, with an age of at least 4.8 Gyr. The T dwarf is a recent discovery from the UKIRT Infrared Deep Sky Survey ULAS 1459+0857 and has a spectral type of T4.5 +- 0.5 and a distance in the range 43-69 pc. With an age constraint inferred from the white dwarf of > 4.8 Gyr, we estimate Teff = 1200-1500 K and log g = 5.4-5.5 for ULAS 1459+0857, making it a benchmark T dwarf with well-constrained surface gravity. We also compare the T dwarf spectra with the latest LYON group atmospheric model predictions, which, despite some shortcomings, are in general agreement with the observed properties of ULAS 1459+0857. The separation of the binary components 16 500-26 500 au or 365 arcsec on the sky is consistent with an evolved version of the more common brown dwarf + main-sequence MS binary systems now known, and although the system has a wide separation, it is shown to be statistically robust as a non-spurious association. The observed colours of the T dwarf show that it is relatively bright in the z band compared to other T dwarfs of similar type, and further investigation is warranted to explore the possibility that this could be a more generic indicator of older T dwarfs. Future observations of this binary system will provide even stronger constraints on the T dwarf properties, and additional systems will combine to give a more comprehensively robust test of the model atmospheres in this temperature regime.Nota general

Artículo de publicación ISI

Author: Day-Jones, A. C.; - Pinfield, D. J.; - Ruiz González, María Teresa; - Beaumont, H.; - Burningham, B.; - Gallardo, J.; - Giannin



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