The double blue-straggler sequence in NGC 2173: an artifact of field contamination

¹ INAF - Astrophysics and Space Science Observatory Bologna, Via Gobetti 93/3, 40129, Bologna, Italy
e-mail: emanuele.dalessandro@oabo.inaf.it
² Dipartimento di Fisica e Astronomia, Via Gobetti 93/2, 40129, Bologna, Italy
³ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK

Received: 3 August 2018
Accepted: 29 October 2018

Abstract

Here we discuss the case of the double blue straggler star (BSS) sequence recently detected in the young stellar cluster NGC 2173 in the Large Magellanic Cloud (LMC) by Li et al. (2018, ApJ, 856, 25). In order to investigate this feature we made use of two Hubble Space Telescope (HST) sets of observations, one (the same one used by Li et al.) probing the cluster central regions, and the other sampling the surrounding field. We demonstrate that when field star decontamination is applied, ∼40% of the BSS population selected by Li et al. turns out to be composed of field stars interlopers. This contamination mainly affects one of the two sequences, which therefore disappears in the decontaminated color-magnitude diagram. We analyze the result of tens different decontamination realizations: we find no evidence of a double BSS sequence in any of them. We therefore conclude that NGC 2173 harbors a normal single (poorly populated) BSS sequence and that particular care needs to be devoted to the field decontamination process in any study aimed at probing stellar population features or star counts in the LMC clusters.