2017MNRAS.472..230T

While the effect of rotation on spectral lines is complicated in rapidly rotating stars because of the appreciable gravity-darkening effect differing from line to line, it is possible to make use of this line-dependent complexity to separately determine the equatorial rotation velocity (v_e) and the inclination angle (i) of rotational axis. Although linewidths of spectral lines were traditionally used for this aim, we tried in this study to apply the Fourier method, which utilizes the unambiguously determinable first-zero frequency (σ₁) in the Fourier transform of line profile. Equipped with this technique, we analysed the profiles of He I 4471 and Mg I 4481 lines of six rapidly rotating (v_e sin i ∼ 150-300 km s^–1) late B-type stars, while comparing them with the theoretical profiles simulated on a grid of models computed for various combination of (v_e, i). According to our calculation, σ₁ tends to be larger than the classical value for given v_e sin i. This excess progressively grows with an increase in v_e, and is larger for the He line than the Mg line, which leads to σ₁^ He^ > σ₁^ Mg^. It was shown that v_e and i are separately determinable from the intersection of two loci (sets of solutions reproducing the observed σ₁ for each line) on the v_e versus i plane. Yet, line profiles alone are not sufficient for their unique discrimination, for which photometric information (such as colours) needs to be simultaneously employed.