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

## TL;DR

This study investigates the rotational velocities and macroturbulence variations in RR Lyrae and red horizontal branch stars, revealing phase-dependent behaviors and challenging the use of surface angular momentum as a total stellar angular momentum indicator.

## Contribution

The paper provides new relations between spectral line widths and velocities, and offers insights into the surface angular momentum evolution of horizontal branch stars.

## Key findings

- RRab stars have an upper limit on V_macrot of 5 +/- 1 km/s.
- RRc stars show V_macrot ranging from 2 to 12 km/s.
- Surface angular momentum is not a reliable indicator of total stellar angular momentum.

## Abstract

We have derived relations between full-width-half-maxima 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 stars (RHB). We also have 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 with weak evidence of an anti-correlation with T_eff. The RRc minima range from 2 to 12 km/s. 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 time scales short compared to HB lifetimes. 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. (2008); they conform qualitatively to the expectations of Tanner et al. (2013). A general conclusion of our investigation is that surface angular momentum as measured by V_rot*sini is not a reliable indicator of total stellar angular momentum anywhere along the HB.