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

Abstract

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. 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. 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 NGC6752, 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=18500) spectroscopy of all stars, four times during a single night of observations. We detected a single, hot (25000 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. 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.

Accepted for publication in A&A Lettersto the Editor

References in corpus (19)

• The Origin of Subdwarf B Star (I): the Formation Channels
• The origin of sdB stars (II)
• The binary fraction of extreme horizontal branch stars
• An extensive library of 2500-10500 Ang synthetic spectra
• A helium spread among the main sequence stars in NGC 2808
• A binary model for the UV-upturn of elliptical galaxies (MNRAS version)
• Can Planets Influence the Horizontal Branch Morphology?
• Close binary EHB stars from SPY
• Orbital periods of twenty two subdwarf B stars
• Hot subdwarfs from the ESO Supernova Ia Progenitor Survey - I. Atmospheric parameters and cool companions of sdB stars
• Close Binary White Dwarf Systems: Numerous New Detections and Their Interpretation
• HST Observations of New Horizontal Branch Structures in the Globular Cluster omega Centauri
• Resolving subdwarf B stars in binaries by HST imaging
• The presence of intermediate-mass black holes in globular clusters and their connection with extreme horizontal branch stars
• A New Feature Along the Extended Blue Horizontal Branch of NGC 6752
• The lack of close binaries among hot horizontal branch stars in NGC6752
• Spectroscopy of horizontal branch stars in NGC6752 - Anomalous results on atmospheric parameters and masses
• Highly He-Rich Matter Dredged Up by Extra Mixing through Stellar Encounters in Globular Clusters
• The orbital period distribution of subdwarf B binaries

Cited by in corpus (22)

• Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
• Post-common-envelope binaries from SDSS. IX: Constraining the common-envelope efficiency
• A possible solution for the lack of EHB binaries in globular clusters
• How to Make a Singleton sdB Star via Accelerated Stellar Evolution
• A lack of close binaries among hot horizontal branch stars in globular clusters - M80 and NGC5986
• A lack of close binaries among hot horizontal branch stars in globular clusters. II. NGC\,2808
• SHOTGLAS I: The ultimate spectroscopic census of extreme horizontal branch stars in $\omega$ Centauri
• A possible formation channel for blue hook stars in globular cluster
• Pulsating hot O subdwarfs in Omega Cen: mapping a unique instability strip on the Extreme Horizontal Branch
• Young stars in the periphery of the Large Magellanic Cloud
• Blue Hook Stars in Globular Clusters
• The Spectral Types of White Dwarfs in Messier 4
• SHOTGLAS II. MUSE spectroscopy of blue horizontal branch stars in the core of $\omega$ Centauri and NGC 6752
• Discovery of a rapidly pulsating subdwarf B star candidate in omega Cen
• Spectroscopy of blue horizontal branch stars in NGC 6656 (M22)
• The ultraviolet colour of globular clusters in M31: a core density effect?
• A hot horizontal branch star with a close K-type main-sequence companion
• The HYPERMUCHFUSS Campaign -- an undiscovered high velocity population
• Fewer Companions in the Crowd: The Low Close Binary Fraction in Globular Clusters from Gaia RVS
• Binarity as the solution to the stellar evolution enigma posed by NGC 6791
• Binary Evolutionary Models
• A search for dormant binaries with degenerate components in omega Centauri and NGC 6397