Is a binary fraction-age relation responsible for the lack of EHB binaries in globular clusters?

Abstract

Context. The recently-discovered lack of close binaries, among extreme horizontal branch (EHB) stars in Galactic globular clusters, has thus far constituted a major puzzle, in view of the fact that blue subdwarf stars – the field counterparts of cluster EHB stars – are well-known to present a high binary fraction. Aims. In this Letter, we provide new results that confirm the lack of close EHB binaries in globular clusters, and present a first scenario to explain the difference between field and cluster EHB stars. Methods. First, in order to confirm that the lack of EHB binaries in globular clusters is a statistically robust result, we undertook a new analysis of 145 horizontal branch stars in NGC 6752, out of which forty-one belong to the EHB. To search for radial-velocity variations as a function of time, we repeated high-resolution (R = 18 500) spectroscopy of all stars, four times during a single night of observations. Results. We detected a single, hot (25 000 K), radial-velocity variable star as a close-binary candidate. From these results, we estimate an upper-limit for the close (period P ≤ 5 day) binary fraction f among NGC6752 EHB stars of 16% (95% confidence level), with the most probable value being f = 4%. Thus our results clearly confirm the lack of close binaries among the hot HB stars in this cluster. Conclusions. We suggest that the confirmed discrepancy between the binary fractions for field and cluster EHB stars is the consequence of an f -age relation, with close binaries being more likely in the case of younger systems. We analyze theoretical and observational results available in the literature, which support this scenario. If so, an age difference between the EHB progenitors in the field and in clusters, the former being younger (on average) by up to several Gyr, would naturally account for the startling differences in binary fraction between the two populations.