A lack of close binaries among hot horizontal branch stars in globular clusters. II. NGC 2808

Context. The formation mechanism of hot horizontal branch (HB) stars is still one of the most uncertain points of stellar evolution theories. 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. Aims: We search for binaries of period ℘ ≤ 200 days among a sample of 83 blue horizontal branch stars (Teff = 12 000-22 000 K) 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. Methods: The radial velocity of the targets was measured in fourteen epochs, spanning a temporal interval of ~75 days. The significant variations were identified by means of a detailed error analysis and a statistical study. Results: We detect no RV variable object among stars cooler than the photometric G1 gap at ~17 000 K, while two close (℘ ≤ 10 days) and two intermediate-period (℘ = 10-50 days) systems are found among hotter targets. The close and intermediate-period binary fraction for stars cooler than the gap are fc ≤ 5% and fip ≤ 10%, respectively, with 95% confidence. The most probable values among hotter stars are fc ~ 20% and fip ~ 30%, but the 90%-confidence level intervals are still large (6-42% and 11-72%, respectively). Conclusions: The G1 gap appears as a discontinuity in the binary faction along the HB, with a higher incidence of binaries among hotter stars, but a constant increase in f with temperature rather than a discontinuity cannot be excluded from our observations. We also find that intermediate-period binaries, never investigated before among cluster HB stars, could play an important role among hotter stars, being more than ~15-20% of the hottest stars of our sample. Our results, compared with previous estimates for other clusters, indicate that fc among hot HB stars is most probably higher for younger clusters, confirming the recently proposed age-fc relation. However, the large observed difference in binary fraction between clusters (e.g. NGC 2808 and NGC 6752) is still not reproduced by binary population synthesis models. Based on observations with the ESO Very Large Telescope at Paranal Observatory, Chile (proposal ID 078.D-0825).