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+5-14 MJup for its T5.5 companion. For 2M1059-21 we find a 1.9 year orbital period and a mass of 67+4-5 MJup 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.