Open cluster BSS dynamical clock dependence on the Milly Way gravitational field

TL;DR

This paper investigates how the dynamical clock index $A^+$, measuring mass segregation in star clusters, depends on internal dynamics and galactic tides, revealing different behaviors in open and globular clusters related to their mass and galactic position.

Contribution

It demonstrates that the $A^+$ index in open clusters is influenced by both internal relaxation and galactic tides, highlighting differences from globular clusters due to mass effects.

Findings

$A^+$ index varies with galactocentric distance in open clusters.

Open clusters show a dispersion in $A^+$ unlike the linear trend in globular clusters.

Cluster mass influences the response to galactic tides and internal dynamics.

Abstract

Since recent years, mass segregation driven by two-body relaxation in star clusters has been proposed to be measured by the so-called dynamical clock, $A^+$, a measure of the area enclosed between the cumulative radial distribution of blue straggler stars and that of a reference population. Since star clusters spend their lifetime immersed in the gravitational potential of their host galaxy, they are also subject to the effects of galactic tides. In this work, I show that the $A^+$ index of a star cluster depends on both, its internal dynamics as it were in isolation and on the effects of galactic tides. Particularly, I focused on the largest sample of open clusters harboring blue straggler stars with robust cluster membership. I found that these open clusters exhibit an overall dispersion of the $A^+$ index in diagnostic diagrams where Milky Way globular clusters show a clear linear trend. However, as also experienced by globular clusters, $A^+$ values of open clusters show some dependence on their galactocentric distances, in the sense that clusters located closer or farther that $\sim$ 11 kpc from the Galactic center have larger and smaller $A^+$ values, respectively. This different response to two-body relaxation and galactic tides in globular and open clusters, which happen concurrently, can be due to their different masses. More massive clusters can somehow protect their innermost regions from galactic tides more effectively.