Astrometric orbits of spectral binary brown dwarfs I: Massive T dwarf companions to 2M1059$-$21 and 2M0805$+$48

Abstract

Near-infrared spectroscopic surveys have uncovered a population of short-period, blended-light spectral binaries composed of low-mass stars and brown dwarfs. These systems are amenable to orbit determination and individual mass measurements via astrometric monitoring. Here we present first results of a multi-year campaign to obtain high-precision absolute astrometry for spectral binaries using the Gemini-South and Gemini-North GMOS imagers. We measure the complete astrometric orbits for two systems: 2M0805$+$48 and 2M1059$-$21. Our astrometric orbit of 2M0805$+$48 is consistent with its 2-year radial velocity orbit determined previously and we find a mass of $66_{-14}^{+5}{M}_{\mathrm{Jup}}$ for its T5.5 companion. For 2M1059$-$21 we find a 1.9 year orbital period and a mass of $67_{-5}^{+4}{M}_{\mathrm{Jup}}$ for its T3.5 companion. We demonstrate that sub-milliarcsecond absolute astrometry can be obtained with both GMOS imagers and that this is an efficient avenue for confirming and characterising ultracool binary systems.

arXiv comment14 pages, 13 figures, accepted for publication in MNRAS

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