Accurate stellar rotational velocities using the Fourier transform of the cross correlation maximum

Abstract

We propose a method for measuring the projected rotational velocity $v sin i$ with high-precision even in spectra with blended lines. Though not automatic, our method is designed to be applied systematically to large numbers of objects without excessive computational requirement. We calculate the cross correlation function (CCF) of the object spectrum against a zero-rotation template and use the Fourier transform (FT) of the CCF central maximum to measure the parameter $v sin i$ taking the limb darkening effect and its wavelength dependence into account. The procedure also improves the definition of the CCF base line resulting in errors related to the continuum position under 1 % even for $vsin i$ = 280 km/s. Tests with high-resolution spectra of F-type stars indicate that an accuracy well below 1 % can be attained even for spectra where most lines are blended. We have applied the method to measure $v sin i$ in 251 A-type stars. For stars with $vsin i$ over 30 km/s (2-3 times our spectra resolution) our measurement errors are below 2.5 % with a typical value of 1%. We compare our results with Royer et al. (2002a) using 155 stars in common, finding systematic differences of about 5 % for rapidly rotating stars.