YSES 2b is a background star: Differential astrometric M-dwarf measurements in time
Matthew Kenworthy, Tomas Stolker, Jens Kammerer, William Balmer, Arthur Vigan, Sylvestre Lacour, Gilles Otten, Eric Mamajek, Christian Ginski, Mathias Nowak, Steven Martos, Jason Wang, Emily Rickman, Markus Janson, Alexander Bohn, Mariangela Bonavita
Published: 2025/9/11
Abstract
We wish to confirm the nature of YSES 2b, a purportedly faint companion of the young star YSES 2. We used on-sky observations from SPHERE and GRAVITY to measure the astrometric position of 2b with respect to the star YSES 2, and examined the competing hypotheses of (i) a bound substellar companion versus (ii) a distant unrelated background source with a non-zero proper motion. YSES 2b appears to be a late-type M-dwarf star over 2 kiloparsecs behind the star YSES 2. It has a transverse velocity of about 300 km/s and is located within one of the spiral arms of the Galaxy. The main discriminant was multiple epochs of GRAVITY astrometry that identified the sub-milliarcsecond parallactic motion of the star.