A hot horizontal branch star with a close K-type main-sequence companion

TL;DR

This paper reports the discovery of a unique close binary system in a globular cluster, involving an extreme horizontal branch star and a main-sequence companion, challenging existing formation theories.

Contribution

It presents the first known example of an EHB with a close K-type main-sequence companion in a globular cluster, indicating different formation mechanisms from the Galactic field.

Findings

Discovery of a unique EHB binary in NGC6752

No similar systems found among known Galactic field EHB binaries

Implications for different formation mechanisms in clusters and field

Abstract

Dynamical interactions in binary systems are thought to play a major role in the formation of extreme horizontal branch stars (EHBs) in the Galactic field. However, it is still unclear if the same mechanisms are at work in globular clusters, where EHBs are predominantly single stars. Here we report on the discovery of a unique close binary system (period ~1.61 days) in the globular cluster NGC6752, comprising an EHB and a main-sequence companion of 0.63+-0.05 Msun. Such a system has no counterpart among nearly two hundred known EHB binaries in the Galactic field. Its discovery suggests that either field studies are incomplete, missing this type of systems possibly because of selection effects, or that a particular EHB formation mechanism is active in clusters but not in the field.