Differences in the rotational properties of multiple stellar populations in M 13: a faster rotation for the "extreme" chemical subpopulation

TL;DR

This study reveals that the 'extreme' chemical subpopulation in globular cluster M13 exhibits significantly faster rotation than other populations, suggesting early formation differences rather than dynamical evolution effects.

Contribution

The paper introduces a Bayesian MCMC method to analyze kinematic differences among stellar populations in M13, highlighting a novel rotational disparity linked to chemical composition.

Findings

Extreme population shows ~4 km/s faster rotation than intermediate population.

Rotational axes of different populations are aligned within uncertainties.

Results imply early formation processes influenced the rotational properties.

Abstract

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