Binaries among Extreme Horizontal Branch Stars in Globular Clusters

TL;DR

This paper reviews the binary star fraction among extreme horizontal branch stars in globular clusters, confirming a low close binary fraction in NGC 6752 and suggesting variability among different clusters, contrasting with higher fractions in the field.

Contribution

It provides new observational data on binary fractions in globular clusters, especially NGC 6752 and NGC 2808, highlighting differences from field populations and discussing implications for star formation theories.

Findings

Low close binary fraction (~4%) in NGC 6752.

Potential higher binary fraction in NGC 2808.

Globular cluster EHB binary fractions differ from field populations.

Abstract

Following a review of our present knowledge about blue subdwarf stars in globular clusters, we present an overview of the results of searches for close binaries among these stars, including results previously published in the literature and reporting recent and preliminary results of new data. Previous investigations revealed a lack of close systems in NGC 6752, which we confirm with new, more extensive observations. Our estimate of the close binary fraction in this cluster is only 4%. From a review of the relevant literature, there are indications that a low close binary fraction among extreme horizontal branch (EHB) stars is a common feature in globular star clusters. On the other hand, the field EHB population shows evidence of a remarkably high binary fraction. Such a difference among globular cluster and field populations, although not yet explained in detail on the basis of theoretical models, must necessarily be related to different formation histories for EHB stars in the field and in clusters. In this framework, preliminary results indicate that close systems could be relatively common in the peculiar globular cluster NGC 2808, although the sample of studied stars is still small. This would imply that not all clusters share the same behavior, as far as EHB star formation is concerned. We briefly explore possible explanations for these results.