Discovery of a T dwarf + white dwarf binary system

Author： Day-Jones A. C. Pinfield D. J. Ruiz M. T. Beaumont H. Burningham B. Gallardo J. Gianninas A. Bergeron P. Napiwotzki R. Jenkins J. S. Zhang Z. H. Murray D. N. Catalán S. Gomes J.

Publisher： Oxford University Press

ISSN： 0035-8711

Source： Monthly Notices of the Royal Astronomical Society, Vol.410, Iss.2, 2011-01, pp. : 705-716

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Abstract

ABSTRACTWe 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 ( T_eff= 5535 ± 45 K) and magnetic ( B∼ 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 T_eff= 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.