Discovery of a T dwarf + white dwarf binary system

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.
Referencia bibliográfica

Monthly Notices of the Royal Astronomical Society, Volume 410, Issue 2, pp. 705-716.

Fecha de publicación:
1
2011
Número de autores
14
Número de autores del IAC
0
Número de citas
59
Número de citas referidas
45
Descripción
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.