English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

The recently-discovered lack of close binaries, among extreme horizontalbranch (EHB) stars in Galactic globular clusters, has thus far constituted amajor puzzle, in view of the fact that blue subdwarf stars - the fieldcounterparts of cluster EHB stars - are well-known to present a high binaryfraction. In this Letter, we provide new results that confirm the lack of closeEHB binaries in globular clusters, and present a first scenario to explain thedifference between field and cluster EHB stars. First, in order to confirm thatthe lack of EHB binaries in globular clusters is a statistically robust result,we undertook a new analysis of 145 horizontal branch stars in NGC6752, out ofwhich forty-one belong to the EHB. To search for radial-velocity variations asa function of time, we repeated high-resolution (R=18500) spectroscopy of allstars, four times during a single night of observations. We detected a single,hot (25000 K), radial-velocity variable star as a close-binary candidate. Fromthese 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), withthe most probable value being f=4%. Thus our results clearly confirm the lackof close binaries among the hot HB stars in this cluster. We suggest that theconfirmed discrepancy between the binary fractions for field and cluster EHBstars is the consequence of an f-age relation, with close binaries being morelikely in the case of younger systems. We analyze theoretical and observationalresults available in the literature, which support this scenario. If so, an agedifference between the EHB progenitors in the field and in clusters, the formerbeing younger (on average) by up to several Gyr, would naturally account forthe startling differences in binary fraction between the two populations.

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