Blue Hook Stars in Globular Clusters

TL;DR

This study investigates the occurrence of blue hook stars in globular clusters, finding a correlation with cluster mass but suggesting the pattern might be due to statistical effects rather than a direct causal relationship.

Contribution

It provides a comparative analysis of globular clusters with and without blue hook stars, highlighting the potential link to cluster mass and discussing the statistical nature of their distribution.

Findings

All known BHk clusters are unusually massive.

BHk clusters are consistent with a uniform distribution in mass.

The apparent mass threshold may be a statistical effect.

Abstract

Blue hook (BHk) stars are a rare class of horizontal branch stars that so far have been found in only very few Galactic globular clusters (GCs). The dominant mechanism for producing these objects is currently still unclear. In order to test if the presence of BHk populations in a given GC is linked to specific physical or structural cluster properties, we have constructed a parent sample of GCs for which existing data is sufficient to establish the presence or absence of BHk populations with confidence. We then compare the properties of those clusters in our parent sample that do contain a BHk population to those that do not. We find that there is only one compelling difference between BHk and non-BHk clusters: all known BHk clusters are unusually massive. However, we also find that the BHk clusters are consistent with being uniformly distributed within the cumulative mass distribution of the parent sample. Thus, while it is attractive to suggest there is is a lower mass cut-off for clusters capable of forming BHk stars, the data do not require this. Instead, the apparent preference for massive clusters could still be a purely statistical effect: intrinsically rare objects can only be found by searching a sufficiently large number of stars.