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The Multiple Star η Muscae

Abstract High-resolution spectroscopy of η Mus is combined with literature and new photometry to provide a comprehensive analysis of its components. Our main absolute parameters for the close binary system are M 1 and M 2 3.34 ± 0.04 M⊙, R 1 2.13 ± 0.07 R⊙, R 2 2.34 ± 0.10 R⊙; T 1 13 000 ± 300, T 2 12 600 ± 300, K; and distance 125 ± 10 pc. Our findings update earlier results in a number of respects. We thus confirm that η Mus B is a gravitationally bound companion of the close binary. This relates to the variable γ velocity of the radial velocities of η Mus A. We connect this to the recently discovered member η Mus D, whose orbit we link to new data. We also provide a spectroscopic examination of the Ap star η Mus B, listing over 450 identified lines. We argue that the system is still young, and the apparently anomalous rotation of the close binary’s secondary can be reconciled with its being a physically larger star, still condensing to the zero-age main sequence. Models of young condensing stars permit such expanded states, particularly during the deuterium-burning stage, and our results are in agreement with appropriate low-age models. This possible configuration may make η Mus an important example for testing young star models, formation, and evolution scenarios. This multiple star can be compared with V831 Cen and the general properties related to its membership of the Sco–Cen OB2 association.