A lack of close binaries among hot horizontal branch stars in globular clusters

Abstract

Context. Recent investigations have revealed a surprising lack of close binaries among extreme horizontal branch (EHB) stars in the globular cluster NGC 6752, at variance with the analogous sdB field stars. Another puzzling result concerns the derived spectroscopic masses for some EHB stars. Aims. The present paper extends our study of NGC 6752 to M 80 and NGC 5986, to establish whether the unexpected properties of EHB stars in NGC 6752 are also present in other clusters. Methods. Twenty-one horizontal branch stars (out of which 5 EHBs) in NGC 5986 and 31 in M 80 (11 EHBs) were observed during four consecutive nights. We measured radial velocity variations and evaluated statistical and systematic errors. Temperatures, gravities, and helium abundances were also measured. Results. By means of a statistical analysis of the observed radial velocity variations, we detected one EHB close binary candidate per cluster. In M 80, 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 NGC 5986, although our best estimate is rather low ( f = 25%). For the discrepancy in spectroscopic derived masses with theoretical models observed in NGC 6752, our analysis of M 80 EHB stars shows a similar trend. For the first time, we report a clear trend in surface helium abundance with temperature, although the trend for the hottest stars is still unclear. Conclusions. 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 NGC 6752. 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.

Spectroscopy of horizontal branch stars inω Centauri

“…Very interestingly, Moni Bidin et al (2009) suggested that their helium abundances could be systematically higher, by a quantity similar to what was found here, compared to the collection of literature measurements of field stars presented by O'Toole (2008), which is based on spectra of higher resolution (Edelmann et al 2003;Lisker et al 2005;Ströer et al 2005;Hirsch et al 2008). Moni Bidin et al (2009) used the same instrument and resolution as in the present work. There should therefore be a systematic difference between measurements based on data with different resolution, with the lower resolution returning higher helium abundances …”

“…A similar turnoff of the surface iron abundance at ≈15 000 K can be observed in Pace et al (2006). Hence, the trend is not a systematic error, but it indicates that the efficiency of the atmospheric diffusion varies with temperature along the HB, as discussed in detail by Moni Bidin et al (2009). The results for stars hotter than 20 000 K are scattered in a wide range of nearly two orders of magnitudes, much larger than the observational errors (0.1−0.2 dex at these temperatures).…”

“…As well known, the atmospheric diffusion erases the chemical differences between stars in clusters of different metallicity (Behr 2003;Pace et al 2006). The surface helium abundance of the targets between 11 500 and 20 000 K follows the trend discovered by Moni Bidin et al (2009). This is seen clearly in the lower panel of Fig.…”

“…In this paper, we will present the results on the surface helium abundance for all the stars, focusing on the BH candidates hotter than 32 000 K, which were not analyzed in Paper I. Whenever applicable, we will compare our results to those of Moni Bidin et al (2007Bidin et al ( , 2009, who measured the parameters of HB stars in NGC 6752, M 80, and NGC 5986. At variance with ω Cen, these three clusters do not show main sequence (MS) splitting, although the MS of NGC 6752 is broadened (Milone et al 2010).…”

“…At variance with ω Cen, these three clusters do not show main sequence (MS) splitting, although the MS of NGC 6752 is broadened (Milone et al 2010). Moni Bidin et al (2007Bidin et al ( , 2009) used the same instrument, software, and models as we did, and the comparison can thus easily reveal intrinsic differences between the clusters.…”