Rotational and Radial Velocities of 1.3-2.2 M_Sun Red Giants in Open Clusters

Abstract

This study presents the rotational distribution of red giant stars (RGs) in eleven old to intermediate age open clusters. The masses of these stars are all above the Kraft break, so that they lose negligible amounts of their birth angular momentum (AM) during the main sequence evolution. However, they do span a mass range with quite different AM distributions imparted during formation, with the stars less massive than ~1.6 M_Sun arriving on the main sequence with lower rotation rates than the more massive stars. The majority of RGs in this study are slow rotators across the entire red giant branch regardless of mass, supporting the picture that intermediate mass stars rapidly spin down when they evolve off the main sequence and develop convection zones capable of driving a magnetic dynamo. Nevertheless, a small fraction of RGs in open clusters show some level of enhanced rotation, and faster rotators are as common in these clusters as in the field red giant population. Most of these enhanced rotators appear to be red clump stars, which is also true of the underlying stellar sample, while others are clearly RGs that are above or below the clump. In addition to rotational velocities, the radial velocities and membership probabilities of individual stars are also presented. Cluster heliocentric radial velocities for NGC 6005 and Pismis 18 are reported for the first time.

arXiv comment17 pages, 7 figures, 7 tables. Accepted for publication in The Astronomical Journal. Machine readable versions of Tables 2 & 5 are provided as supplementary material

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