Sahlmann, J., Burgasser, A. J., Gagliuffi, D. C. B., Lazorenko, P. F., Segransan, D., Osorio, M. R. Z., Blake, C. H., Gelino, C. R., Martín, E. L., Bouy, H. (2020). Astrometric orbits of spectral binary brown dwarfs – I. Massive T dwarf companions to 2M1059-21 and 2M0805+48. Monthly Notices of the Royal Astronomical Society 495, 1, 1136-1147 DOI: 10.1093/mnras/staa1235

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 multiyear 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-yr radial velocity orbit determined previously and we find a mass of 66(-14)(+5) M-Jup for its T5.5 companion. For 2M1059-21, we find a 1.9-yr orbital period and a mass of 67(-5)(+4) M-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 characterizing ultracool binary systems.