Properties and nature of Be stars

31. The binary nature, light variability, physical elements, and emission-line changes of HD 81357^⋆

¹ Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 298, 251 65 Ondřejov, Czech Republic
e-mail: pavel.koubsky@asu.cas.cz
² Astronomical Institute of Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 180 00 Praha 8, Troja, Czech Republic
³ Physics & Astronomy Department, University of Victoria, PO Box 3055 STN CSC, Victoria, BC V8W 3P6, Canada
⁴ Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kačićeva 26, 10000 Zagreb, Croatia
⁵ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussel, Belgium
⁶ Department of Theoretical Physics and Astrophysics, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic

Received: 7 November 2018
Accepted: 1 August 2019

Abstract

Reliable determination of the basic physical properties of hot emission-line binaries with Roche-lobe filling secondaries is important for developing the theory of mass exchange in binaries. It is not easy, however, due to the presence of circumstellar matter. Here, we report the first detailed investigation of a new representative of this class of binaries, HD 81357, based on the analysis of spectra and photometry from several observatories. HD 81357 was found to be a double-lined spectroscopic binary and an ellipsoidal variable seen under an intermediate orbital inclination of ∼(63 ± 5)°, having an orbital period of 33_.^d77445(41) and a circular orbit. From an automated comparison of the observed and synthetic spectra, we estimate the component’s effective temperatures to be 12930(540) K and 4260(24) K. The combined light-curve and orbital solutions, also constrained by a very accurate Gaia Data Release 2 parallax, give the following values of the basic physical properties: masses 3.36 ± 0.15 and 0.34 ± 0.04M_⊙^N, radii 3.9 ± 0.2 and 13.97 ± 0.05R_⊙^N, and a mass ratio 10.0 ± 0.5. Evolutionary modelling of the system including the phase of mass transfer between the components indicated that HD 81357 is a system observed in the final slow phase of the mass exchange after the mass-ratio reversal. Contrary to what has been seen for similar binaries like AU Mon, no cyclic light variations were found on a time scale an order of magnitude longer than the orbital period.