Differences in the rotational properties of multiple stellar populations in M13: a faster rotation for the ‘extreme’ chemical subpopulation

Abstract

We use radial velocities from spectra of giants obtained with the WIYN\ntelescope, coupled with existing chemical abundance measurements of Na and O\nfor the same stars, to probe the presence of kinematic differences among the\nmultiple populations of the globular cluster (GC) M13. To characterise the\nkinematics of various chemical subsamples, we introduce a method using Bayesian\ninference along with an MCMC algorithm to fit a six-parameter kinematic model\n(including rotation) to these subsamples. We find that the so-called "extreme"\npopulation (Na-enhanced and extremely O-depleted) exhibits faster rotation\naround the centre of the cluster than the other cluster stars, in particular\nwhen compared to the dominant "intermediate" population (moderately Na-enhanced\nand O-depleted). The most likely difference between the rotational amplitude of\nthis extreme population and that of the intermediate population is found to be\n$\\sim$4 km s$^{-1}$, with a 98.4% probability that the rotational amplitude of\nthe extreme population is larger than that of the intermediate population. We\nargue that the observed difference in rotational amplitudes, obtained when\nsplitting subsamples according to their chemistry, is not a product of the\nlong-term dynamical evolution of the cluster, but more likely a surviving\nfeature imprinted early in the formation history of this GC and its multiple\npopulations. We also find an agreement (within uncertainties) in the inferred\nposition angle of the rotation axis of the different subpopulations considered.\nWe discuss the constraints that these results may place on various formation\nscenarios.\n