Discovery of AN similar to 23 M-Jup brown dwarf orbiting similar to 700 AU from the massive star HIP 78530 in Upper Scorpius

We present the discovery of a substellar companion on a wide orbit around the similar to 2.5 M-circle dot star HIP 78530, which is a member of the 5 Myr old Upper Scorpius association. We have obtained follow-up imaging over two years and show that the companion and primary share common proper motion. We have also obtained JHK spectroscopy of the companion and confirm its low surface gravity, in accordance with the young age of the system. A comparison with DRIFT-PHOENIX synthetic spectra indicates an effective temperature of 2800 +/- 200 K and a comparison with template spectra of young and old dwarfs indicates a spectral type of M8 +/- 1. The mass of the companion is estimated to be 19-26 M-Jup based on its bolometric luminosity and the predictions of evolutionary models. The angular separation of the companion is 4 ''.15, which at the distance of the primary star, 156.7 pc, corresponds to a projected separation of similar to 710 AU. This companion features one of the lowest mass ratios (similar to 0.009) of any known companion at separations greater than 100 AU.