Nonextensive Behavior of Stellar Rotation in the Galactic Disk Components

Abstract

The solar neighborhood is a unique stellar astrophysical laboratory formed by a variety of stars from different origins. In particular, two of the most notable populations known are the thick and thin disk stars, each characterized by distinct chemical compositions, ages, kinematics, and origins. Based on Tsallis nonextensive statistics, we investigate the observed distribution of the projected rotational velocity of the thin and thick disk component stars. Through Bayesian inference, our results show that the distributions of the Galactic disk populations selected from both kinematic and chemical criteria follow a nonextensive behavior, where non-Gaussian statistics provide a more accurate representation. We also observed an anticorrelation between the entropic index q and the age of disk components confirming the interpretation of initial angular momentum memory loss scaled by the parameter q. In contrast, a subextensivity case with q > 1 was found for the old high-α metal-rich subgroup hαmr, and due to their distinguished rotational behavior and atypical subextensive regime, we infer that thick disk and hαmr stars are, in fact, distinct objects. Our results also suggest that the rotational velocities of stars are defined not only by their evolutionary spin-down processes but also by their birth sites.