Determination of rotational velocities in B and Be stars: physical processes and methods

Abstract

In the present work we have used the method suggested by Carrol (1933) for the determination of the projected equatorial rotation velocity: v sin(i) by means of the Fourier Transform of a sample of B8 to B9.5 spectral type stars and luminosity classes III and V with Be and Bn type stars among them, which were observed by Baade (1989) with the Coudé Echelle spectrometer (C.E.S.) on the 1.4 m telescope (C.A.T.) at the E.S.O. The measured lines were MgII λ 4481 and HeI λ 4471 (this one was blended with [HeI] λ 4470 in some cases) and the signal to noise ratio was 1500. This method was tested with convolution of the rotational profile with the non-L.T.E. theoretical line profile of HeI λ 4471 (Teff = 20000 K, log(g) = 3.0) computed by Mihalas (1974) and then the Fourier transform was applied to the red wing of the profile and from it's zeroes the imposed values of v sin(i) were recovered. From the observed stars a very good correlation was obtained the rotation velocity from MgII λ 4481 measured in this work and Baade's determinations -which were obtained from the MgII λ 4481 full half width calibrated with the rotational velocities of the B.S.C. (Hoffleit, 1982)-. We obtained a linear relation between rotational velocities MgII λ 4481 and HeI λ 4471 up to v sin(i) < 140 km/s and beyond this value HeI yields greater velocities than MgII. We concluded that the formation region of HeI λ4471 is affected by non rigid rotational laws, in special for high rotators where this region could be quite large.