Hidden Companions to Intermediate-mass Stars. Upgraded Multiplicity 5 → 6. XXVII. Discovery of a 1.45<i>M</i> _⊙, 0.6 au Companion to Beta2 Tucanae C*

Abstract Beta2 Tucanae ( β 2 Tuc) is a nearby a = 22 au visual binary (C+D) and one of the brighter members of the young Tucana–Horologium association. Here we report on a VLTI/GRAVITY interferometric observation which revealed that β 2 Tuc is actually a rather compact triple system: the primary is itself a close binary (Ca+Cb) with a projected separation ρ = 13.56 mas ↔ 0.59 au and a K band flux ratio of 56.65%. We estimate isochrone masses M Ca ≃ 2.0 M ⊙ , M Cb ≃ 1.45 M ⊙ and M D ≃ 1.45 M ⊙ . Together with its wide (∼1200 au) companion β 1 Tuc (itself a binary) and very wide (∼23,000 au) companion β 3 Tuc, this makes the β Tuc system a sextuple. The compactness and rather low effective hierarchy of β 2 Tuc <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfrac> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">out</mml:mi> </mml:mrow> </mml:msub> <mml:mo>(</mml:mo> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:msub> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">out</mml:mi> </mml:mrow> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">in</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mfrac> <mml:mo>≲</mml:mo> <mml:mn>7</mml:mn> </mml:math> make it a particularly interesting system for triple dynamics and stability studies.