YSES 2b is a background star

Differential astrometric M-dwarf measurements in time

¹ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
³ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁴ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁵ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
⁶ Academia Sinica, Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, NTU/AS campus, No. 1, Section 4, Roosevelt Rd., Taipei 10617, Taiwan
⁷ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, M/S 321-162, Pasadena, CA, 91109, USA
⁸ School of Natural Sciences, Center for Astronomy, University of Galway, Galway, H91 CF50, Ireland
⁹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
¹⁰ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹¹ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
¹² European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
¹⁴ Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, Scotland, UK

^★ Corresponding author.

Received: 30 June 2025
Accepted: 4 August 2025

Abstract

Aims. We wish to confirm the nature of YSES 2b, a purportedly faint companion of the young star YSES 2.

Methods. 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.

Results. 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 ∼300 km s⁻¹ 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.