The double Blue Straggler sequence in NGC2173: a field contamination artefact

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 by Li et al (2018). In order to investigate this feature we made use of two 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 BSS population selected by Li et al. turns out to be composed by field stars interlopers. This contamination mainly affects one of the two sequences, which therefore disappears in the decontaminated colour-magnitude diagram. We analyse the result of tens different decontamination realisations: in none of them we find evidence of a double BSS sequence. Hence we conclude that NGC 2173 harbours 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.