A COMBINED FOURIER-BESSEL TRANSFORMATION METHOD TO DERIVE ACCURATE ROTATIONAL VELOCITIES

We describe in some detail the characteristics of a combined Fourier-B essel transformation technique to derive projected equatorial rotation al velocities from spectral line profiles. This technique shares with the Fourier-transformation method, developed by Gray, that it distingu ishes rotational broadening of a spectral line from broadening by othe r mechanisms. The range of rotational velocity values that can be deri ved with this method is limited mainly by the spectral resolution (low velocities) and by line blending and the signal-to-noise ratio (high velocities). We discuss the uncertainty on the outcoming rotational ve locity as a result of various effects, such as limb-darkening, spectra l resolution, noise, data-preparation, and intrinsic broadening. We co nclude that the Fourier-Bessel transformation method can provide rotat ional velocities, with a typical uncertainty down to a few percent. It does not include any modelling of individual stars with effects as an isotropic macroturbulence included and therefore is less suited for a detailed analysis of individual stars. It is suited for statistical in vestigation of a large sample of stars.