Radial velocities in the globular NGC 2808

In the past decade, models based on their binary origin have been very successful in reproducing the properties of field subdwarf-B stars, but the observations of their analogues in globular clusters has posed new problems. In addition, the discovery of multiple populations offered an appealing alternative scenario for the formation of these stars.

We search for binaries of period P<=200days among a sample of 83 blue horizontal branch stars (Teff=12000-22000K) in NGC 2808, a cluster known to host three distinct stellar populations and a multimodal horizontal branch. The final sample, after the rejection of stars with incomplete observations or poor quality data, consists of 64 targets. The radial velocity of the targets was measured in fourteen epochs, spanning a temporal interval of ~75days.