Fourier analysis of He 4471/Mg 4481 line profiles for separating rotational velocity and axial inclination in rapidly-rotating B-type stars

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 (ve) and the inclination angle (i) of rotational axis. Although line-widths 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 (sigma1) in the Fourier transform of line profile. Equipped with this technique, we analyzed the profiles of HeI 4471 and MgII 4481 lines of six rapidly-rotating (vesini~150-300km/s) late B-type stars, while comparing them with the theoretical profiles simulated on a grid of models computed for various combination of (ve, i). According to our calculation, sigma1 tends to be larger than the classical value for given vesini. This excess progressively grows with an increase in ve, and is larger for the He line than the Mg line, which leads to sigma1He > sigma1Mg. It was shown that ve and i are separately determinable from the intersection of two loci (sets of solutions reproducing the observed sigma1 for each line) on the ve vs. i plane. Yet, line profiles alone are not sufficient for their unique discrimination, for which photometric information (such as colors) needs to be simultaneously employed.