δ Circini: A massive hierarchical triple system with an eclipsing binary

¹ Astronomical Institute of Charles University, Faculty of Mathematics and Physics V Holešovičkách 2 CZ-180 00 Praha 8, Czech Republic
² European Organisation for Astronomical Research in the Southern Hemisphere (ESO) Casilla 19001 Santiago 19, Chile
³ Department of Physics and Astronomy, University of North Carolina at Chapel Hill Chapel Hill NC 27599, USA
⁴ FZU – Institute of Physics of the Czech Academy of Sciences, Na Slovance 1999/2, Prague 182 21, Czech Republic
⁵ Variable Star and Exoplanet Section of the Czech Astronomical Society Fričova 298 251 65 Ondřejov, Czech Republic

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Received: 26 August 2025
Accepted: 15 November 2025

Abstract

δ Circini is known to be a massive multiple system containing a 3.9 d inner eclipsing binary in a slightly elliptical orbit exhibiting slow apsidal motion and a distant tertiary with a probable period of 1644 d. All three components of the system are O- or B-type stars. We carried out a comprehensive study of the system, based on light curves from TESS and other instruments, a new series of echelle spectra, older spectra from the ESO archive, and several VLTI interferometric observations. Due to the large amount of different types of data covering both orbits in the system, we obtained a more precise value of the long orbital period (1603.24 ± 0.19 d) and fully determined all other orbital parameters. Although both orbits are eccentric, their period ratio is large enough for the system to be dynamically stable. The inner and outer orbits are in the same plane, which means that no Kozai-Lidov mechanism is acting in the system. Assuming solar metallicity in our MESA models, we found ages of (4.4 ± 0.1), (4.7 ± 0.2), and (3.8 ± 1.3) Myr for the primary, the secondary, and the tertiary, respectively. Our evolutionary scenario predicts that the inner eclipsing binary will merge within approximately 1.7 Myr and eventually evolve into a black hole. The distance to the system, estimated from the angular size of the outer orbit is (809.9 ± 1.8) pc, which implies that δ Cir might be located close to the centre of a stellar population ASCC 79, a subgroup of the young Circinus complex. With a total mass of (53.04 ± 0.29) M_⊙, δ Cir can contribute a significant fraction of the total mass of the population.