Rotation and Macroturbulence of RR Lyrae and Red Horizontal Branch Stars
Abstract
We have derived new upper limits on the axial equatorial rotational velocities of RR Lyrae (RRL) and metal-poor red horizontal branch stars (RHB), using the very large set of Las Campanas du Pont echelle spectra gathered over the last 15 years. We also have determined the variations of macroturbulent velocities in RRL atmospheres during pulsation cycles. During RRab pulsation cycles line widths are dominated by phase-dependent convolutions of axial rotation and macroturbulence (called here Vmacrot). Variations in Vmacrot are essentially uniform among the RRab stars, but the behavior of Vmacrot among RRc stars varies strongly from star to star. For RRab stars we find Vmacrot ≤ 5 km/s, while the minima for RRc stars range from 2 to 10 km/s. The Vmacrot minimum for RRab stars provides an upper limit for the rotational velocity Vsini. The abrupt decline in large Vmacrot values with decreasing effective temperature at the blue edge 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 that are short compared to HB lifetimes. We suggest that surface angular momentum as measured by Vsini is not a reliable indicator of total stellar angular momentum anywhere along the HB. This work has been supported in part by NSF grant AST1616040 to C.S. and by the University of Texas Rex G. Baker, Jr. Centennial Research Endowment.
- Publication:
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American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23336420S