A Hot Horizontal Branch Star with a Close K-type Main-sequence Companion

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 (GCs), where EHBs are predominantly single stars. Here, we report on the discovery of a unique close binary system (period ̃1.61 days) in the GC NGC 6752, comprising an EHB and a main-sequence companion of 0.63 ± 0.05 {M}<SUB>☉ </SUB>. Such a system has no counterpart among nearly 200 known EHB binaries in the Galactic field. Its discovery suggests that either field studies are incomplete, missing this type of system possibly because of selection effects, or that a particular EHB formation mechanism is active in clusters but not in the field. <P />Based on observations with the ESO Very Large Telescope at Paranal Observatory, Chile (program IDs 69.D-0682, 075.D-0492, and 079.D-0674), and with the 6.5 m Magellan Telescopes at Las Campanas Observatory, Chile (program IDs 2006B-LC-7 and CHILE-2007B-007).