On the rotational velocity of Sirius A

Abstract

ABSTRACT With an aim of getting information on the equatorial rotation velocity (ve) of Sirius A separated from the inclination effect (sin i), a detailed profile analysis based on the Fourier transform technique was carried out for a large number of spectral lines, while explicitly taking into account the line-by-line differences in the centre–limb behaviours and the gravity darkening effect (which depend on the physical properties of each line) based on model calculations. The simulations showed that how the first-zero frequencies (q1) of Fourier transform amplitudes depends on ve is essentially determined by the temperature-sensitivity parameter (K) differing from line to line, and that Fe i lines (especially those of very weak ones) are more sensitive to ve than Fe ii lines. The following conclusions were drawn by comparing the theoretical and observed q1 values for many Fe i and Fe ii lines: (1) The projected rotational velocity (vesin i) for Sirius A is fairly well established at 16.3(±0.1) km s−1 by requiring that both Fe i and Fe ii lines yield consistent results. (2) Although precise separation of ve and i is difficult, ve is concluded to be in the range of $16 \le v_{\rm e} \lesssim$ 30–40 km s−1, which corresponds to $25^{\circ } \lesssim i \le 90^{\circ }$. Accordingly, Sirius A is an intrinsically slow rotator for an A-type star, being consistent with its surface chemical peculiarity.