BRITE-Constellation photometry of Ï5 Orionis, an ellipsoidal SPB variable

Results of an analysis of the BRITE-Constellation photometry of the SB1 system and ellipsoidal variable Ï⁵Ori (B2 III) are presented. In addition to the orbital light-variation, which can be represented as a five-term Fourier cosineseries with the frequencies f_orb, 2f_orb, 3f_orb, 4f_orb, and 6f_orb, where f_orbis the systemâ™s orbital frequency, the star shows five lowamplitude but highly significant sinusoidal variations with frequencies f_i(i = 2..5, 7) in the range from 0.16 to 0.92 d^â'1. With an accuracy better than 1Ï, the latter frequencies obey the following relations: F₂â' f₄= 2f_orb, f₇â' f₃= 2f_orb, f₅= f₃â' f₄= f₇â' f₂. We interpret the first two relations as evidence that two high-order ℓ = 1, m = 0 gravity modes are selfexcited in the systemâ™s tidally distorted primary component. The star is thus an ellipsoidal SPB variable. The last relations arise from the existence of the first-order differential combination term between the two modes. Fundamental parameters, derived from photometric data in the literature and the Hipparcos parallax, indicate that the primary component is close to the terminal stages of its main-sequence (MS) evolution. Extensive Wilsona-Devinney modelling leads to the conclusion that best fits of the theoretical to observed light curves are obtained for the effective temperature and mass consistent with the primaryâ™s position in the HR diagram and suggests that the secondary is in an early MS evolutionary stage.