The double blue straggler sequence in NGC 2173: a field contamination artefact?. (arXiv:1812.04807v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Li_C/0/1/0/all/0/1">Chengyuan Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Deng_L/0/1/0/all/0/1">Licai Deng</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Grijs_R/0/1/0/all/0/1">Richard de Grijs</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jiang_D/0/1/0/all/0/1">Dengkai Jiang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Xin_Y/0/1/0/all/0/1">Yu Xin</a>
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.
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.
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