Axial Rotation and Turbulence of RR ab Stars: The Peterson Conundrum Revisited

We calibrate and then use the relation between equivalent width ( EW ) and full–width–half–maximum ( FWHM ) of metallic absorption lines in the spectra of RR Lyrae stars to estimate a new upper limit of V rot sini ≤ 6 km.s −1 on their axial equatorial rotational velocities, and to derive the variations of macroturbulent velocities in their atmospheres during pulsation cycles. Finally, we present a simple way to estimate macroturbulent/rotational velocity from FWHM of the cross–correlation function.