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|Title:||Global and Nonglobal Parameters of Horizontal-branch Morphology of Globular Clusters||Authors:||Milone, A. P.
Marino, A. F.
Norris, J. E.
Bedin, L. R.
Recio Blanco, A.
|Issue Date:||2014||Journal:||THE ASTROPHYSICAL JOURNAL||Number:||785||Issue:||1||First Page:||21||Abstract:||The horizontal-branch (HB) morphology of globular clusters (GCs) is mainly determined by metallicity. However, the fact that GCs with almost the same metallicity exhibit different HB morphologies demonstrates that at least one more parameter is needed to explain the HB morphology. It has been suggested that one of these should be a global parameter that varies from GC to GC and the other a nonglobal parameter that varies within the GC. In this study we provide empirical evidence corroborating this idea. We used the photometric catalogs obtained with the Advanced Camera for Surveys of the Hubble Space Telescope and analyze the color-magnitude diagrams of 74 GCs. The HB morphology of our sample of GCs has been investigated on the basis of the two new parameters L1 and L2 that measure the distance between the red giant branch and the coolest part of the HB and the color extension of the HB, respectively. We find that L1 correlates with both metallicity and age, whereas L2 most strongly correlates with the mass of the hosting GC. The range of helium abundance among the stars in a GC, characterized by ∆Y and associated with the presence of multiple stellar populations, has been estimated in a few GCs to date. In these GCs we find a close relationship among ∆Y, GC mass, and L2. We conclude that age and metallicity are the main global parameters, while the range of helium abundance within a GC is the main nonglobal parameter defining the HB morphology of Galactic GCs. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.||Description:||We thank F. D’Antona and J. Lattanzio for useful discussion and suggestion. We are grateful to P. Stetson for providing unpublished light curves of RR Lyrae. A.P.M., J.E.N., and H.J. acknowledge the financial support from the Australian Research Council through Discovery Project grant DP120100475. J.E.N. is also supported by the Australian Research Council through Discovery Project grant DP0984924. A.D. and M.A. acknowledge support from the Australian Research Council (grant FL110100012). G.P. and S.C. acknowledge financial support from PRIN MIUR 2010–2011, project “The Chemical and Dynamical Evolution of the Milky Way and Local Group Galaxies,” prot. 2010LY5N2T, and from PRIN-INAF 2011 “Multiple Populations in Globular Clusters: their role in the Galaxy assembly.” Support for this work has been provided by the IAC (grant 310394) and the Education and Science Ministry of Spain (grants AYA2007-3E3506, and AYA2010-16717).||URI:||http://hdl.handle.net/20.500.12386/12654||ISSN:||0004-637X||DOI:||10.1088/0004-637X/785/1/21||Bibcode ADS:||2014ApJ...785...21M|
|Appears in Collections:||1.01 Articoli in rivista|
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