Revisiting the Evidence for Double Sequences of Blue Straggler Stars in Globular Clusters

TL;DR

This study reevaluates the evidence for double blue straggler star sequences in globular clusters, finding no strong statistical support for bimodality and suggesting previous claims may be coincidental or due to skewed distributions.

Contribution

The paper challenges prior claims of bimodal BSS sequences by applying rigorous statistical tests to a large sample of globular clusters, showing most are better described by skewed unimodal distributions.

Findings

No strong evidence of bimodality in 56 GCs using Hartigan Dip Test.

Akaike model comparison favors skewed unimodal models in 94 out of 112 cases.

Reanalysis of NGC 7099 (M30) shows bimodality detection is less significant than previously reported.

Abstract

Blue straggler stars (BSSs) are believed to form through mass transfer in binary systems or stellar collisions. The reported presence of double BSS sequences in some globular clusters (GCs) has been interpreted as evidence that these two formation channels produce distinct sequences in color-magnitude diagram (CMD). We reassess this claim using HST UV Globular Cluster Survey (HUGS) photometry of 56 Galactic GCs. We used the Hartigan Dip Test to test bimodality, and Akaike model comparison to test whether BSS distance distributions are better described by a mixture of two unskewed Gaussians or a skewed unimodal Gaussian model. We find no strong statistical evidence for bimodality; no cluster yields a dip test p-value below 0.15, and Akaike model comparison favors the skewed unimodal model in 94 out of 112 cases. We re-examine NGC 7099 (M30), the prototypical case of a double BSS sequence, using three reductions of HST data. We find bimodality is detected at p = 4 x 10-3 , vs. the originally reported p ~ 10-5 , in the original photometry. The observed uncertainties derived from the subgiant branch widths are comparable to the suggested separation between the proposed BSS sequences, making the detection of statistically significant bimodality challenging. Our results suggest that the dip between two BSS sequences in M30 photometry is a coincidence, and that later bifurcation claims can be explained as skew in the BSS color distribution, rather than two separate distributions.