Hidden Companions to Intermediate-mass Stars. III. Discovery of a 1.6M ⊙, 1.3 au Companion to HIP 92041 = ϕ Sagittarii* *Based on observations collected at the European Southern Observatory, Chile, Program ID 099.D-2020(A).

Abstract

B-type stars with masses up to about 8 M ⊙ are the progenitors of the heavier white dwarfs yet our knowledge about their multiplicity is still incomplete. The nearby subgiant HIP 92041 = ϕ Sagittarii has had claims of binarity in the past but they have all been refuted as spurious. In this report we show that ϕ Sgr is a binary after all based on a VLTI/GRAVITY interferometric observation. The companion has a K band flux ratio of 6% and a projected separation ρ = 17.7 mas ↔ 1.3 au. The inferred isochrone masses 3.5M ⊙ + 1.6M ⊙ make ϕ Sgr a potential progenitor for a white dwarf binary with a combined mass exceeding the Chandrasekhar limit.