Spectrum of the secondary component and new orbital elements of the massive triple star δ Ori A^⋆,⋆⋆

¹ Charles University, Faculty of Mathematics and Physics, Astronomical Institute, V Holešovičkách 2, 180 00 Praha 8-Trója, Czech Republic
e-mail: betsimsim@seznam.cz
² Uniwersytet Wrocławski, Instytut Astronomiczny, Kopernika 11, 51-622 Wrocław, Poland
³ Hvar Observatory, Faculty of Geodesy, Zagreb University, Kačicéva 26, 10000 Zagreb, Croatia
⁴ Czech Academy of Sciences, Astronomical Institute, 25165 Ondřejov, Czech Republic
⁵ AAVSO, 185 Alewife Brook Pkwy, Cambridge, MA, USA
⁶ Department of Physics, Mount Allison University, Sackville, NB E4L1E6, Canada
⁷ Department of Physics and Space Science, Royal Military College of Canada, Kingston, Ontario K7K 7B4, Canada
⁸ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria
⁹ Department of Electronics, Electrical Engineering and Microelectronics, Silesian University of Technology, Akademicka 16, 44-100 Gliwice, Poland
¹⁰ University of Vienna, Institute for Astrophysics, Türkenschanzstraße 17, 1180 Vienna, Austria

Received: 23 October 2022
Accepted: 9 January 2023

Abstract

δ Orionis is the closest massive multiple stellar system and one of the brightest members of the Orion OB association. The primary (Aa1) is a unique evolved O star. In this work, we applied a two-step disentangling method to a series of spectra in the blue region (430–450 nm), and we detected spectral lines of the secondary (Aa2). For the first time, we were able to constrain the orbit of the tertiary (Ab) – to 55 450 d or 152 yr – using variable γ velocities and new speckle interferometric measurements, which have been published in the Washington Double Star Catalogue. In addition, the Gaia DR3 parallax of the faint component (Ca+Cb) constrains the distance of the system to (381 ± 8) pc, which is just in the centre of the Orion OB1b association, at (382 ± 1) pc. Consequently, we found that the component masses according to the three-body model are 17.8, 8.5, and 8.7 M_⊙, for Aa1, Aa2, and Ab, respectively, with the uncertainties of the order of 1 M_⊙. We used new photometry from the BRITE satellites together with astrometry, radial velocities, eclipse timings, eclipse duration, spectral line profiles, and spectral energy distribution to refine radiative properties. The components, classified as O9.5 II + B2 V + B0 IV, have radii of 13.1, 4.1, and 12.0 R_⊙, which means that δ Ori A is a pre-mass-transfer object. The frequency of 0.478 cycles per day, known from the Fourier analysis of the residual light curve and X-ray observations, was identified as the rotation frequency of the tertiary. δ Ori could be related to other bright stars in Orion, in particular, ζ Ori, which has a similar architecture, or ε Ori, which is a single supergiant, and possibly a post-mass-transfer object.