Metal-rich globular clusters of the Galaxy: morphology of the helium-burning "clump," and the determination of relative ages through the "$\Delta V$" method

Abstract

Morphological aspects of the $V, \bv$ diagram of metal-rich globular clusters are analyzed, on the basis of stellar evolution models for horizontal-branch (HB) and red giant branch stars. These have been incorporated into detailed synthetic HB models, the influence of differential reddening upon which is also discussed. The synthetic HB models are found to be very clumpy in the colour-magnitude diagram, though more extended structures occasionally result due to the effect of evolution away from the zero-age HB. In particular, "sloped" clumps may be naturally expected for sufficiently high helium abundances. Differential reddening is found to have a smaller influence than evolution itself upon the morphology of the clump. The synthetic HB models are also used to study the age difference between metal-rich clusters on the basis of the "$\Delta V$" method. We emphasize the importance of knowing in advance the relative abundances of helium and metals in order to estimate this age difference quantitatively. We present chemical evolution models that serve to illustrate the effect, and show that, for two clusters of identical [Fe/H] and $\Delta V$, the one enriched predominantly by ejecta from supernovae type II explosions will appear younger than the other which also suffered enrichment from supernovae type Ia ejecta. The implied locations of the red giant branch feature known as the "bump" are also discussed on the basis of recent observational and theoretical results. Our corresponding predictions should be checked against accurate observational data obtained with the Hubble Space Telescope.

18 pages, LaTeX file (requires AAS LaTeX macro v4.0). Two tables and five figures available upon request. To appear in PASP (February 1996 issue)

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• Hot Stars in Globular Cluster - A Spectroscopist's View
• Effects of Helium Enrichment in Globular Clusters I.Theoretical Plane with PGPUC stellar evolution code
• The Age Dependent Luminosities of the Red Giant Branch Bump, Asymptotic Giant Branch Bump, and Horizontal Branch Red Clump
• The helium spread in the Globular cluster 47 Tuc
• On the Origin of Bimodal Horizontal-Branches in Massive Globular Clusters: The Case of NGC 6388 and NGC 6441
• On the red giant branch mass loss in 47 Tucanae: Constraints from the horizontal branch morphology
• A New Giant Branch Clump Structure In the Large Magellanic Cloud
• Double Horizontal Branches in NGC 6440 and NGC 6569 unveiled by the VVV Survey
• The horizontal branch luminosity vs metallicity in M31 globular clusters
• Fundamental properties of nearby stars and the consequences on DeltaY/DeltaZ
• The ages of (the oldest) stars
• Horizontal Branch Stars and the Ultraviolet Universe