# The double blue straggler sequence in NGC 2173: a field contamination artefact?

## TL;DR

This study re-evaluates the apparent double blue straggler sequence in NGC 2173, suggesting it is not solely due to field contamination, based on spatial distribution analysis of the supposed contaminant stars.

## Contribution

It provides evidence that the bifurcated BSS pattern in NGC 2173 is likely intrinsic, challenging previous claims that it was caused by field contamination artifacts.

## Key findings

- Field stars show central concentration inconsistent with random distribution.
- Overestimation of field contamination by previous study.
- The double BSS sequence may be an intrinsic feature of NGC 2173.

## Abstract

Li et al. (2018) (hereafter L18) detected two apparently distinct populations of blue straggler stars (BSSs) in the young globular cluster NGC 2173, a similar feature as observed in numerous Galactic globular clusters (GCs). Recently, Dalessandro et al. (2018) (D18) compared the observed color--magnitude diagrams (CMDs) of both the cluster and a nearby reference field (although observed with the ACS/WFC instrument). They conclude that the bifurcated pattern of BSSs in NGC 2173 observed by L18 is a field contamination artefact. In this note, we explore the central concentration properties of the removed `field stars' identified by D18. Our purpose is to examine if these `field stars' are spatially homogeneously distributed. Employing a Monte Carlo-based approach, we have carefully studied the probability that any such central concentration may be caused by small number statistics. We find that, in most cases of, the `field stars' removed by D18 exhibit a clear central concentration, which cannot be explained on the basis of small number statistics alone. Therefore, we suggest that D18 may well have overestimated the field contamination level, implying that the bifurcated BSS pattern in NGC 2173 cannot, in fact, be explained by field contamination.