Journal Article

Discovery of a T dwarf + white dwarf binary system

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

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 410, issue 2, pages 705-716
Published in print January 2011 | ISSN: 0035-8711
Published online December 2010 | e-ISSN: 1365-2966 | DOI:
Discovery of a T dwarf + white dwarf binary system

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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∼ 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.

Keywords: binaries: general; brown dwarfs; white dwarfs

Journal Article.  7865 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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