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تسجيل مستخدم جديدThe lack of close binaries among hot horizontal branch stars in NGC6752
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C. Moni Bidin
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We present the results of a spectroscopic search for close binaries among horizontal branch (HB) stars in NGC6752. We used the ESO VLT-FORS2 instrument to obtain medium resolution (R=4100) spectra of 51 hot HB stars with 8000 < Teff < 32000 K during four consecutive nights. Eighteen of our targets are extreme horizontal branch (EHB) stars with Teff > 22000 K. Radial velocity variations were measured with cross-correlation techniques and we carefully evaluated the statistical and systematic errors associated with them. No close binary system has been detected among our 51 targets. The data corrected for instrumental effects indicate that the radial velocity variations are always below 15 km/s (3sigma level). From a statistical analysis of our results, we conclude that (at 95 confidence level) the fraction of binaries with a ~0.5 solar masses companion among EHB stars in NGC6752 is smaller than 20%. This empirical evidence sharply contrasts with what has been found for hot subdwarfs in the field, and opens new questions about the formation of EHB stars in globular clusters (and possibly in the field as well.
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We present preliminary results of a our search for close (period P < 10 days) binary systems among hot (Teff > 20000 K) horizontal branch stars in globular clusters M80 and NGC5986. We measured radial velocity variations of 11 targets in M80 and 5 in
NGC5986, spectroscopically observed at medium resolution (R=4100) during four nights at VLT-FORS2. We found one close binary candidate in M80, showing statistically significative radial velocity variations, and no clear evidence in NGC5986. These results confirm the lack of close binaries among hot horizontal branch stars in globular clusters, as already found in NGC6752, at variance with what observed among hot subdwarfs in the field.
UV observations of some massive globular clusters have revealed a significant population of stars hotter and fainter than the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either by the late hot
flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in omega Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C and N enrichment cannot be verified. We compare the observed effective temperatures, surface gravities, helium abundances, and carbon line strengths (where detectable) of our targets stars with the predictions of the two aforementioned scenarios. Moderately high resolution spectra of hot HB stars in the globular cluster omega Cen were analysed for radial velocity variations, atmospheric parameters, and abundances using LTE and non-LTE model atmospheres. We find no evidence of close binaries among our target stars. All stars below 30,000K are helium-poor and very similar to HB stars observed in that temperature range in other globular clusters. In the temperature range 30,000K to 50,000K, we find that 28% of our stars are helium-poor (log(He/H) < -1.6), while 72% have roughly solar or super-solar helium abundance (log(He/H) >= -1.5). We also find that carbon enrichment is strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass. A strong carbon enrichment in tandem with helium enrichment is predicted by the late hot flasher scenario, but not by the helium-enrichment scenario. We conclude that the helium-rich HB stars in omega Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence.
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