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

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 in isolation and the effects of galactic tides. Mainly, 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, whereas 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 than \(\sim \)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 protect their innermost regions from galactic tides more effectively.