Hidden Companions to Intermediate-mass Stars. XXI. Discovery of a 0.5 M ⊙, 1.5 au Companion to Sadachbia = Gamma Aquarii*

Abstract

Sadachbia = Gamma Aquarii is a nearby A0V star that was suspected to be a spectroscopic binary. Here we report on the direct detection of a companion with a K band flux ratio of 1.2% at a projected separation of 38.2 mas based on a VLTI/GRAVITY near-infrared interferometric observation. The implied mass and separation of the companion are 0.46 M⊙ and 1.5 au based on the Gaia DR3 distance (or 0.55 M⊙ and 1.9 au if the discrepant Hipparcos distance is used instead). The companion explains the X-ray emission of γ Aqr and the orbital motion with a period of about one year likely explains the discrepant Gaia and Hipparcos parallaxes. Further observations are necessary to measure the binary semimajor axis but it is likely that the system will undergo common envelope evolution. The primary is partially resolved with an angular diameter of 0.52 mas.