Ross 19

Ross 19 is a red dwarf of spectral type M3.5. In 2021 it was discovered that Ross 19 has a companion, which is a cold brown dwarf. Ross 19B is likely the coldest brown dwarf found around a main-sequence star, as of July 2024.

Ross 19A
Ross 19A was discovered in 1925 by Frank E. Ross as a proper-motion star with the help of the Yerkes Observatory and archived plates by E. E. Barnard. In 1985 the first spectral information was published by W. P. Bidelman, based on observations by G. P. Kuiper, which reported the star to have a V magnitude of 12.70 and a spectral type of M3.5. In 2020 the star was observed at the Lick Observatory and with IRTF. Ross 19A has a mass of about 0.36 and has an age between 3.6 and 11 billion years. It has a sub-solar metallicity and has a temperature of about 3500 Kelvin. In TESS and ZTF light curves it does not show any variability and does not show any flares.

Ross 19B
Ross 19B (also called CWISE J021948.68+351845.3) was initially found in the Backyard Worlds project by the citizen scientists Samuel Goodman, Léopold Gramaize, Austin Rothermich, and Hunter Brooks. It was then observed by the professional astronomers of the paper lead by Adam C. Schneider with the Keck Observatory in 2020, measuring a J-band magnitude of 21.14 ± 0.02. The researchers calculated that Ross 19B has a 100% probability to be bound to Ross 19A. Ross 19B has a very low temperature of about 400–615 K, making it either a late T-dwarf or a Y-dwarf. It has a mass between 15 and 40, making it a brown dwarf. Its wide separation results in an extremely low gravitational binding energy. It is suggested that Ross 19B has a sub-solar metallicity similar to the red dwarf. Later observation with Gemini North showed a Y-band magnitude of 21.86 ± 0.06. This showed that the spectral type is likely between T9 and Y0. A JWST program is dedicated to observe Ross 19B in Cycle 3.