On the observational properties of He-burning stars: some clues on the tilt of the HB in metal rich clusters

Abstract

We investigate the predicted Color-Magnitude distribution of metal-rich Horizontal Branch (HB) stars, discussing selected theoretical models computed under various assumptions about the star metallicity and the efficiency of super-adiabatic convection. We find that canonical Zero Age Horizontal Branches with metallicity larger or of the order of Z=0.002 should be all affected by a tilt, by an amount which increases when the metallicity is increased and/or the mixing length is decreased, reaching a tilt of $\Delta V\sim$0.2 mag in the case of solar metallicity when a mixing length value $\alpha$=1.6 is assumed ($\Delta V$ is the magnitude difference between the top of the blue HB and the fainter magnitude reached by the red HB). Uncertainties in the luminosity of the red HB due to uncertainty in the mixing length value are discussed. We finally discuss the much larger tilt observed in the clusters NGC 6441 and NGC 6388, reporting additional evidence against suggested non-canonical evolutionary scenarios. Numerical experiments show that differential reddening could produce such sloped HBs. Further, HST-PC imaging of NGC 6441 gives clear indications about the occurrence of differential reddening across the cluster. However, the same imaging shows that the observed slope of the red HB {\em is not} an artifact of differential reddening. We finally show that sloping red HBs in metal rich clusters are a common occurrence not necessarily correlated with the appearance of extended blue HB.

arXiv comment14 pages, 9 figures, Accepted by ApJ

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• The intriguing stellar populations in the globular clusters NGC 6388 and NGC 6441
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• Joint Analysis of near-infrared properties and surface brightness fluctuations of LMC star clusters
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• Metal abundances of RR Lyrae stars in the metal rich globular cluster NGC 6441
• 2MASS Studies of Differential Reddening Across Three Massive Globular Clusters
• Deep HST Photometry of NGC 6388: Age and Horizontal Branch Luminosity
• On the absolute age of the metal-rich globular M71 (NGC 6838): I. optical photometry
• On the nature of the brightest globular cluster in M81
• Overluminous Blue Horizontal Branch Stars Formed by Low Mass Companions
• A stellar population synthesis approach to the Oosterhoff dichotomy
• Stellar-Encounter Driven Red-Giant Star Mass-Loss in Globular Clusters
• Variable Stars in Metal-Rich Globular Clusters. II. NGC 6316
• Metallicity of the globular cluster NGC 6388 from high resolution spectra of more than 160 giant stars
• Core collapse and horizontal-branch morphology in galactic globular clusters
• Multipopulation aftereffects on the color-magnitude diagram and Cepheid variables of young stellar systems
• Observations of the Hot Horizontal-Branch Stars in the Metal-Rich Bulge Globular Cluster NGC 6388 - Indications of Helium Enrichment and a Lesson in Crowded Field Spectroscopy
• Multiple stellar populations in Globular Clusters: collection of information from the Horizontal Branch
• The Curious Radial Distributions of Horizontal Branch Stars in NGC 6441