The giant, asymptotic, and horizontal branches of globular clusters. II - Photographic photometry of the metal-poor clusters M15, M92, and NGC 5466

Details The giant, asymptotic, and horizontal branches of globular clusters. II - Photographic photometry of the metal-poor clusters M15, M92, and NGC 5466 The giant, asymptotic, and horizontal branches of globular clusters. II - Photographic photometry of the metal-poor clusters M15, M92, and NGC 5466

Apr 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985a%26a...145...97b&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 145, no. 1, April 1985, p. 97-117. Research supported by the Consiglio Naziona

Astronomy and Astrophysics

Astrophysics

205

Astronomical Photometry, Color-Magnitude Diagram, Globular Clusters, Hertzsprung-Russell Diagram, Horizontal Branch Stars, Stellar Evolution, Abundance, Helium, Interstellar Extinction, Morphology, Variable Stars

Scientific paper

Photographic photometry of complete samples of stars over well-defined areas down to a limiting magnitude below the horizontal branch blue extreme has been presented in a series of papers on the very metal-poor clusters M15, M92, and NGC 5466. From the analysis and comparison of the color-magnitude diagrams the following conclusions are drawn: (a) The HB's of the three clusters are different: the HB of M15, with stars redder than the instability strip and many variables, presents a gap at (B - V)0 =0.00 and a long blue tail which terminates at about one magnitude brighter than the turn-off; the HB of M92, with almost no variables, has a brighter (by about one magnitude) and redder blue HB extension than M15; the HB of NGC 5466 includes red stars, many variables, and stops at (B - V)0 =0.00 a value which corresponds closely to the position of the gap found in M15. (b) The giant and subgiant branches of the three clusters are narrow. The upper limits of their intrinsic widths in color allow for a permissible spread in the abundance of low ionization potential elements in the three clusters of σ [Fe/H] < 0.1. (c) The ratios of the number of HB, AGB, and RGB stars are in good agreement with the evolutionary models taking into account semiconvection and mass loss. Different estimates of the helium abundances, and in particular the R-method, lead to a common value of Y= 0.23 ± 0.03. (d) The detailed morphology of the HB's in the three clusters is discussed under the assumption that they have the same [Fe/H]- values, the same helium abundance and age. A different value of [O/H] might be responsible for the overall different HB morphologies of M15 and NGC 5466, on the one hand, and M92 on the other. Whilst the strongiy different dynamical configuration could be the cause of the differences found between M15 and NGC 5466. (e) No satisfactory explanation for the presence of the gap in M15 has been found yet. Several alternatives are discussed. (f) The interpretation deduced from the study of these three clusters has been applied to the comparison with other clusters, and the problem of "the second parameter" in the classification of giobular clusters is dealt with. The following conclusions are reached: i) many parameters affect the HB and the restriction "a priori" to the consideration of just a few of them might be misleading since interacting effects may amplify or reduce their individual influence; ii) different parameters may drive the position of the blue and red extremes of the HB of a single cluster; iii) the problem of "finding the second parameter" turns out to be the problem of "defining the appropriate weight for each possible parameter;" iv) [0/H] may play a role in driving the HB morphologies, but further data are urged before drawing any conclusion; v) global structural properties of the clusters may have a strong influence on the HB morphologies. (g) Structure and separation of the main branches and luminosity functions are analysed. (h) Variables and UV-bright stars are discussed.

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