Hidden Companions to Intermediate-mass Stars. XXII. Inferring a 0.43M _⊙, 1.65 au Companion in the Astrometric Binary within the Triple System Lambda Muscae*

Abstract Lambda Muscae is a nearby A-type star reported as a P = 474 days astrometric binary in Gaia DR3 (and previously in Hipparcos) and which also has a 0.16 M ⊙ M dwarf companion at a projected separation 6.4 au discovered by VLT/SPHERE. Based on a K band flux ratio upper limit of 1% from a VLTI/GRAVITY interferometric observation, we estimate a primary mass M Aa = 2.28 M ⊙ and infer that the closer companion is a M Ab = 0.43 M ⊙ star (almost certainly an M dwarf) orbiting at a = 1.65 au. The triple system might be dynamically interesting given the low hierarchy and the ratio of angular momenta of the inner and outer orbits of about one.