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Paper: RR Lyrae Statistics in the Retrograde and Prograde Globular Clusters of the Milky Way
Volume: 529, RR Lyrae/Cepheid 2019: Frontiers of Classical Pulsators
Page: 189
Authors: Farrell, E. M.; Wallerstein, G.
Abstract: In the past, Oosterhoff types derived from RR Lyrae statistics in globular clusters had suggested that Oosterhoff type I clusters, which are less metal-poor than average and commonly found in the Galactic Halo, are more likely to be retrograde when compared to the metal-poor clusters found in the Thick Disk. Given that globulars are unlikely be turned by gravitational interactions, as might individual stars, these clusters are more likely to be captured from mergers with the Milky Way. In an effort to better understand the kinematics and stellar populations of globular clusters, we have assembled a list of clusters with astrometric data from Gaia and RR Lyrae star populations. The focus of this project was to determine whether or not there is a difference between the prograde and retrograde clusters. To do this, we compared their horizontal branch morphologies, via the Lee parameter, and RR Lyrae statistics. In analyzing our results, we find no significant difference in population between the prograde and retrograde clusters with the notable exception that there are prograde clusters of intermediate Oosterhoff type, but no retrograde cluster of intermediate Oosterhoff type. Both groups have horizontal branch morphologies which cover roughly the entire range between –1.00 and 1.00, suggesting that there are young red clusters and old blue clusters in both groups. Given these results, it is unlikely that Oosterhoff type and RR Lyrae statistics alone can be used to identify cluster populations reliably and, instead, should be used in conjunction with other observations.
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