Rotation and Macroturbulence in Metal-poor Field Red Giant and Red Horizontal Branch Stars

TL;DR

This study measures rotational velocities and macroturbulence in metal-poor red giant and horizontal branch stars, revealing low rotation in most but increased rotation in the most luminous stars, with implications for stellar evolution.

Contribution

It provides new high-resolution spectroscopic measurements of rotation and macroturbulence in metal-poor giants and horizontal branch stars, and introduces empirical methods to interpret line broadening data.

Findings

Most metal-poor RGB stars show no detectable rotation.

Luminous RGB stars (M(V) <= -1.5) exhibit net rotation of 2-4 km/sec.

Macroturbulence values are similar to those in metal-rich giants.

Abstract

We report the results for rotational velocities, Vrot sin i, and macroturbulence dispersion, zeta(RT), for 12 metal-poor field red giant branch stars and 7 metal-poor field red horizontal branch 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) spectra obtained with the Gecko spectrograph at CFHT. We find that the zeta(RT) 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/sec, while five show significant rotation, over 3 km/sec. The fraction of rapidly rotating RHB stars is somewhat lower than found among BHB stars. We devise two empirical methods to translate the line-broadening results obtained by Carney et al. (2003, 2008) 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. However, the most luminous stars, with M(V) <= -1.5, do show net rotation, with mean values of 2 to 4 km/sec, depending on the algorithm employed, and these stars also show signs of radial velocity jitter and mass loss.