ROTATION AND MACROTURBULENCE IN METAL-POOR FIELD RED GIANT AND RED HORIZONTAL BRANCH STARS

Abstract

We report the results for rotational velocities, Vrot sin i, and macroturbulence dispersions, ζRT, for 12 metal-poor field red giant branch (RGB) stars and 7 metal-poor field red horizontal branch (RHB) stars. The results are based on Fourier transform analyses of absorption line profiles from high-resolution (R ≈ 120,000), high-S/N (≈215 per pixel; ≈345 per resolution element) spectra obtained with the Gecko spectrograph at the Canada-France-Hawaii Telescope (CFHT). The stars were selected from the authors' previous studies of 20 RHB and 116 RGB stars, based primarily on larger-than-average line-broadening values. We find that ζRT values for the metal-poor RGB stars are very similar to those for metal-rich disk giants studied earlier by Gray and his collaborators. Six of the RGB stars have small rotational values, less than 2.0 km s−1, while five show significant rotation/enhanced line broadening, over 3 km s−1. We confirm the rapid rate for RHB star HD 195636, found earlier by Preston. This star's is comparable to that of the fastest known rotating blue horizontal branch (BHB) stars, when allowance is made for differences in radii and moments of inertia. The other six RHB stars have somewhat lower but show a trend to higher values at higher temperatures (lower radii). Comparing our results with those for BHB stars from Kinman et al., we find that the fraction of rapidly rotating RHB stars is somewhat lower than is found among BHB stars. The number of rapidly rotating RHB stars is also smaller than we would have expected from the observed of the RGB stars. We devise two empirical methods to translate our earlier line-broadening results into Vrot sin i for all the RGB and RHB stars they studied. Binning the RGB stars by luminosity, we find that most metal-poor field RGB stars show no detectable sign, on average, of rotation, which is not surprising given the stars' large radii. However, the most luminous stars, with MV ≤ −1.5, do show net rotation, with mean values of 2-4 km s−1, depending on the algorithm employed, and also show signs of radial velocity jitter and mass loss. This rotation may in fact prove to be due to other line-broadening effects, such as shock waves or pulsation.