Properties and nature of Be stars. 31. The binary nature, light variability, physical elements, and emission-line changes of HD 81357

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.

Based on new spectroscopic and photometric observations from the following instruments: CCD coudé spectrograph of the 2.0 m reflector of the Astronomical Institute AS ČR, Ondřejov, Czech Republic; CCD coudé spectrograph of the 1.22 m reflector of the Dominion Astrophysical Observatory, Victoria, Canada; photoelectric photometer of the 0.65 m Cassegrain reflector of the Hvar Observatory, Croatia, the H_p photometry from the ESA HIPPARCOS mission, and the ASAS-SN all-sky survey V photometry.