The second and third parameters of the horizontal branch in globular clusters

  title={The second and third parameters of the horizontal branch in globular clusters},
  author={Raffaele Gratton and Eugenio Carretta and Angela Bragaglia and Sara Lucatello and Valentina D'Orazi INAF-Osservatorio Astronomico di Padova and INAF-Osservatorio astronomico di Bologna and Excellence Cluster Universe},
  journal={Astronomy and Astrophysics},
Context. The second parameter (the first being metallicity) defining the distribution of stars on the horizontal branch (HB) of globular clusters (GCs) has long been one of the major open issues in our understanding of the evolution of normal stars. Large photometric and spectroscopic databases are now available: they include large and homogeneous sets of colour-magnitude diagrams, cluster ages, and homogeneous data about chemical compositions from our FLAMES survey. Aims. We use these… 
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The results support the hypothesis of Searle & Zinn that the inner halo is more uniform in age and is older in the mean than the outer halo, and it is estimated to be approximately 2 Gyr.
The horizontal branch (HB) morphology of globular clusters (GCs) is most strongly influenced by metallicity. The second parameter phenomenon, first described in the 1960s, acknowledges that
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