Ross 19B: An Extremely Cold Companion Discovered via the Backyard\n Worlds: Planet 9 Citizen Science Project

Abstract

Through the Backyard Worlds: Planet 9 citizen science project, we have\nidentified a wide-separation ($\\sim$10', $\\sim$9900 au projected) substellar\ncompanion to the nearby ($\\sim$17.5 pc), mid-M dwarf Ross 19. We have developed\na new formalism for determining chance alignment probabilities based on the\nBANYAN $\\Sigma$ tool, and find a 100% probability that this is a physically\nassociated pair. Through a detailed examination of Ross 19A, we find that the\nsystem is metal-poor ([Fe/H]=$-$0.40$\\pm$0.12) with an age of\n7.2$^{+3.8}_{-3.6}$ Gyr. Combining new and existing photometry and astrometry,\nwe find that Ross 19B is one of the coldest known wide-separation companions,\nwith a spectral type on the T/Y boundary, an effective temperature of\n500$^{+115}_{-100}$ K, and a mass in the range 15-40 $M_{\\rm Jup}$. This new,\nextremely cold benchmark companion is a compelling target for detailed\ncharacterization with future spectroscopic observations using facilities such\nas the Hubble Space Telescope or James Webb Space Telescope.\n