The Axial Rotation and Variable Macroturbulence of RR Lyrae and Red Horizontal Branch Stars

Abstract

Abstract We have derived relations between FWHMs and equivalent widths of metallic absorption lines in the spectra of RR Lyrae stars to estimate new upper limits on the axial equatorial rotational velocities of RR Lyrae and metal-poor red horizontal branch (RHB) stars. We have also derived the variations of RR Lyrae macroturbulent velocities during the pulsation cycles. In RRab cycles, the line widths are dominated by phase-dependent convolutions of axial rotation and macroturbulence, which we designate as V macrot. The behavior of V macrot is remarkably uniform among the RRab stars, but the behavior of V macrot among RRc stars varies strongly from star to star. The RRab stars exhibit an upper limit on V macrot of 5 ± 1 km s−1 with weak evidence of an anticorrelation with T eff. The RRc minima range from 2 to 12 km s−1. The abrupt decline in large rotations with decreasing T eff at the blue boundary of the instability strip and the apparently smooth continuous variation among the RRab and RHB stars suggests that HB stars gain/lose surface angular momentum on timescales that are short compared to HB lifetimes. The V macrot values for our metal-poor RHB stars agree well with those derived by Fourier analysis of an independent but less metal-poor sample of Carney et al.; they conform qualitatively to the expectations of Tanner et al. A general conclusion of our investigation is that surface angular momentum as measured by V rot sin (i) is not a reliable indicator of total stellar angular momentum anywhere along the HB.