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 proposedto be measured by the so-called dynamical clock, A + , a measure of the area enclosed between the cumulativeradial distribution of blue straggler stars and that of a reference population. Since star clusters spend their lifetimeimmersed in the gravitational potential of their host galaxy, they are also subject to the effects of galactic tides. Inthis work, I show that the A + index of a star cluster depends on both, its internal dynamics as it were in isolationand on the effects of galactic tides. Particularly, I focused on the largest sample of open clusters harboring bluestraggler stars with robust cluster membership. I found that these open clusters exhibit an overall dispersion ofthe A + index in diagnostic diagrams where Milky Way globular clusters show a clear linear trend. However, asalso experienced by globular clusters, A + values of open clusters show some dependence on their galactocentricdistances, in the sense that clusters located closer or farther that ∼ 11 kpc from the Galactic center have largerand smaller A + values, respectively. This different response to two-body relaxation and galactic tides in globularand open clusters, which happen concurrently, can be due to their different masses. More massive clusters cansomehow protect their innermost regions from galactic tides more effectively.