The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC6388 and NGC6441
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I present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC6388 and NGC6441, in order to investigate the nature of the physical mechanism(s) responsible for the existence of an extended blue tail and of a slope in the horizontal branch. Further evidence that the horizontal branch tilt cannot be interpreted as an effect of differential reddening is provided, while I show that a possible solution of the puzzle is to assume that a small fraction of the stellar population in the two clusters is strongly helium enriched.Keywords:
Horizontal branch
Stellar population
Horizontal branch
Blue straggler
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We present a photometric (U band) investigation of the globular clusters in the central region of M87 from deep calibrated photographic and electronographic material, obtained with the CFH telescope. The aim of this work is to complete previous photometric studies at longer wavelengths (Grillmair et al. , 1986; Lauer and Kormendy, 1986).
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We present theoretical predictions concerning horizontal branch stars in globular clusters, including RR Lyrae variables, as derived from synthetic procedures collating evolutionary and pulsational constraints. On this basis, we explore the predicted behavior of the pulsators as a function of the horizontal branch morphology and over the metallicity range 0.0001 to 0.006, revealing an encouraging concordance with the observed distribution of fundamentalised periods with metallicity. Theoretical relations connecting periods to K magnitudes and BV or VI Wesenheit functions are presented, both appearing quite independent of the horizontal branch morphology only with 0.001. Predictions concerning the parameter R are also discussed and compared under various assumptions about the horizontal branch reference luminosity level.
RR Lyrae variable
Horizontal branch
Concordance
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We present theoretical predictions concerning horizontal branch stars in globular clusters, including RR Lyrae variables, as derived from synthetic procedures collating evolutionary and pulsational constraints. On this basis, we explore the predicted behavior of the pulsators as a function of the horizontal branch morphology and over the metallicity range Z=0.0001 to 0.006, revealing an encouraging concordance with the observed distribution of fundamentalised periods with metallicity. Theoretical relations connecting periods to K magnitudes and BV or VI Wesenheit functions are presented, both appearing quite independent of the horizontal branch morphology only with Z greater or equal than 0.001. Predictions concerning the parameter R are also discussed and compared under various assumptions about the horizontal branch reference luminosity level.
RR Lyrae variable
Horizontal branch
Concordance
Instability strip
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There is a growing evidence that several globular clusters must contain multiple stellar generations, differing in helium content. This hypothesis has helped to interpret peculiar unexplained features in their horizontal branches. In this framework we model the peaked distribution of the RR Lyr periods in M3, that has defied explanation until now. At the same time, we try to reproduce the colour distribution of M3 horizontal branch stars. We find that only a very small dispersion in mass loss along the red giant branch reproduces with good accuracy the observational data. The enhanced and variable helium content among cluster stars is at the origin of the extension in colour of the horizontal branch, while the sharply peaked mass loss is necessary to reproduce the sharply peaked period distribution of RR Lyr variables. The dispersion in mass loss has to be <~ 0.003 Msun, to be compared with the usually assumed values of ~0.02 Msun. This requirement represents a substantial change in the interpretation of the physical mechanisms regulating the evolution of globular cluster stars.
Horizontal branch
Red-giant branch
RR Lyrae variable
Red giant
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Supra Horizontal Branch stars are rare objects found in globular clusters to lie above and to the blue of the Horizontal branch (HB). They are believed to be descendants from the HB.
Horizontal branch
Blue straggler
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Horizontal branch
Blue straggler
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Photometry in the UBV system for five recognized globular clusters in NGC 185 shows that their colors are normal and the luminosities are low. A brief rediscussion of the use of globular clusters as extragalactic distance indicators is based on the recent photometry of globular clusters in the M 31 group. Key words: globular clusters - extragalactic distance scale
Blue straggler
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A major uncertainty in the spectroscopic dating of extragalactic globular clusters concerns the degenerate effect that age and horizontal-branch morphology have on the strength of Balmer lines. In this Letter we show that the ratio between the equivalent widths of HδF and Hβ is far more sensitive to horizontal-branch morphology than to age, thus making it possible to break the degeneracy. We show that it is possible to distinguish intermediate-age globular clusters from those whose Balmer lines are strengthened by the presence of blue horizontal-branch stars purely on the basis of the clusters' integrated spectra. The degeneracy between age and horizontal-branch morphology can be lifted with Hβ and HδF line strengths from spectra with S/N ≳ 30 Å-1, which is typical of current studies of integrated spectroscopy of extragalactic globular clusters.
Horizontal branch
Balmer series
Degeneracy (biology)
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Horizontal branch
Blue straggler
Identification
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