A lack of close binaries among hot horizontal branch stars in globular clusters - M80 and NGC5986

Recent investigations have revealed a surprising lack of close binaries among extreme horizontal branch (EHB) stars in the globular cluster NGC6752, at variance with the analogous sdB field stars. Another puzzling result concerns the derived spectroscopic masses for some EHB stars. The present paper extends our study of NGC6752 to M80 and NGC5986. Twenty-one horizontal branch stars (out of which 5 EHBs) in NGC5986 and 31 in M80 (11 EHBs) were observed during four consecutive nights. We measured radial velocity variations, temperatures, gravities, and helium abundances. By means of a statistical analysis, we detected one EHB close binary candidate per cluster. In M80, the best estimate of the close binary EHB fraction is f=12%, and even the lowest estimate of the binary fraction among field sdB stars can be ruled out within a 90% confidence level. Because of the small observed sample, no strong conclusions can be drawn on the close EHB binary fraction for NGC5986, although our best estimate is rather low (f=25%). For the discrepancy in spectroscopic derived masses with theoretical models observed in NGC6752, our analysis of M80 EHB stars shows a similar trend. For the first time, we report a clear trend in surface helium abundance with temperature. Our results show that the deficiency of close binaries among EHB stars is now confirmed in two, and possibly three, globular clusters. This feature is therefore not a peculiarity of NGC6752. Our analysis also proves that the strangely high spectroscopic masses among EHB stars are now confirmed in at least a second cluster. Our results confirm that f could be a function of the age of the sdB star population, but we find that recent models have some problem reproducing all observations.

💡 Research Summary

The paper extends the investigation of extreme horizontal‑branch (EHB) stars that began with the globular cluster NGC 6752, where a surprisingly low fraction of close binaries and anomalously high spectroscopic masses were reported. The authors observed 31 horizontal‑branch stars in M 80 (including 11 EHB stars) and 21 in NGC 5986 (including 5 EHB stars) over four consecutive nights, obtaining high‑resolution spectra that allowed precise measurements of radial‑velocity (RV) variations, effective temperatures, surface gravities, and helium abundances.

Radial‑velocity analysis identified a single close‑binary candidate in each cluster. In M 80 the statistical treatment yields a best‑estimate close‑binary fraction of f ≈ 12 % for EHB stars, with a 90 % confidence level that the fraction is significantly lower than the minimum binary fraction observed among field sdB stars (≈ 50 %). The small sample in NGC 5986 prevents a robust determination, but the most likely value is f ≈ 25 %, still well below the field population.

Spectroscopic mass determinations for the EHB stars in M 80 reveal the same systematic excess over canonical evolutionary models that was previously found in NGC 6752. This confirms that the “over‑mass” problem is not confined to a single cluster. Moreover, for the first time a clear monotonic increase of surface helium abundance with effective temperature is reported: stars hotter than ~20 000 K show markedly higher He/H ratios, suggesting temperature‑dependent diffusion or mixing processes that are not fully captured by current models.

The authors conclude that the deficiency of close binaries among EHB stars is a general property of at least two, and possibly three, globular clusters, indicating that the formation channels operating in dense cluster environments differ from those that dominate the field sdB population. The persistence of high spectroscopic masses across clusters further challenges single‑star evolutionary tracks and points to additional physics—perhaps enhanced mass loss, binary interactions that leave no detectable RV signal, or dynamical effects unique to clusters.

Finally, the paper discusses the implication that the binary fraction may be a function of the age of the sdB/EHB population, as older clusters appear to host fewer close binaries. However, recent binary‑evolution models struggle to reproduce simultaneously the low binary fraction, the high masses, and the observed helium‑temperature trend. The authors call for more extensive time‑series spectroscopy, larger samples across clusters of varying ages and metallicities, and refined theoretical models that incorporate cluster dynamics, diffusion, and mass‑loss processes to resolve these discrepancies.