Blue Stragglers as tracers of the dynamical state of two clusters in the Small Magellanic Cloud: NGC 339 and NGC 419

Abstract

The level of central segregation of Blue Straggler stars proved to be an excellent tracer of the dynamical evolution of old star clusters (the so-called "dynamical clock"), both in the Milky Way and in the Large Magellanic Cloud. The $A^{+}$ parameter, used to measure the Blue Stragglers degree of segregation, has in fact been found to strongly correlate with the parent cluster central relaxation time. Here we studied the Blue-Straggler population of two young stellar systems in the Small Magellanic Cloud, namely NGC 339 (which is 6 Gyr old) and NGC 419 (with an age of only 1.5 Gyr), in order to study their dynamical state. Thanks to multi-epoch, high angular resolution Hubble Space Telescope observations available for both clusters, we took advantage of the stellar proper motions measured in the regions of the two systems and we selected a population of likely cluster members, removing the strong contamination from Small Magellanic Cloud stars. This enabled us to study, with unprecedented accuracy, the radial distribution of Blue Stragglers in these two extragalactic clusters and to measure their dynamical age. As expected for such young clusters, we found that both systems are poorly evolved from the dynamical point of view, also fully confirming that the $A^{+}$ parameter is a sensitive "clock hand" even in the dynamically-young regime.

arXiv comment12 pages,11 figures, Accepted for publication by ApJ

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• Discovery of a double sequence of blue straggler stars in the core-collapsed globular cluster NGC 6256
• Empirical measurement of the dynamical ages of three globular clusters and some considerations on the use of the dynamical clock
• The dynamical age of the LMC globular cluster NGC 1835 using the "dynamical clock"
• Extended Main-Sequence Turnoff and Red Clump in intermediate-age star clusters: A study of NGC 419
• Structural parameters, chronological age and dynamical age of the LMC globular cluster NGC 1754