Hidden Companions to Intermediate-mass Stars. XIX. Spin–Orbit Misalignment and a 0.26M ⊙, 1.8 au Companion in the Astrometric Binary Gamma Trianguli Australis*

Abstract

Gamma Trianguli Australis is a nearby A-type star that was reported to be a 488 days astrometric binary in Gaia DR3. Here we report on a VLTI/GRAVITY observation of γ TrA in which we did not detect the companion down to a K band flux ratio of 0.5%. From the isochrone mass M A  = 3.04M ⊙ of the subgiant primary, this implies a semimajor axis a orb = 1.8 au and that the companion is a M B  = 0.26M ⊙ M dwarf. The primary disk is partially resolved with an angular diameter θ 1 ≈ 1 mas ↔ 13.0R ⊙. The differential visibility phases across the Brγ absorption line allowed us to measure the longitude of the ascending node of the primary’s rotational plane Ωrot = 212° ± 3°. This implies a spin–orbit misalignment of at least 27° ± 3° based on the Gaia astrometric solution.