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

TL;DR

A new extremely cold substellar companion to Ross 19 was discovered via citizen science, with detailed analysis confirming its properties and potential for future spectroscopic study.

Contribution

This paper reports the discovery and characterization of a wide-separation, extremely cold substellar companion using citizen science and introduces a new formalism for chance alignment probability.

Findings

Companion has a spectral type on the T/Y boundary.

Effective temperature of approximately 500 K.

Mass estimated between 15 and 40 Jupiter masses.

Abstract

Through the Backyard Worlds: Planet 9 citizen science project, we have identified a wide-separation ($\sim$10', $\sim$9900 au projected) substellar companion to the nearby ($\sim$17.5 pc), mid-M dwarf Ross 19. We have developed a new formalism for determining chance alignment probabilities based on the BANYAN $\Sigma$ tool, and find a 100% probability that this is a physically associated pair. Through a detailed examination of Ross 19A, we find that the system is metal-poor ([Fe/H]=$-$0.40$\pm$0.12) with an age of 7.2$^{+3.8}_{-3.6}$ Gyr. Combining new and existing photometry and astrometry, we find that Ross 19B is one of the coldest known wide-separation companions, with a spectral type on the T/Y boundary, an effective temperature of 500$^{+115}_{-100}$ K, and a mass in the range 15-40 $M_{\rm Jup}$. This new, extremely cold benchmark companion is a compelling target for detailed characterization with future spectroscopic observations using facilities such as the Hubble Space Telescope or James Webb Space Telescope.