Torelli, M, Iannicola, G, Stetson, PB, Ferraro, I, Bono, G, Salaris, M, Castellani, M, Dall'Ora, M, Fontana, A, Monelli, M and Pietrinferni, A (2019) Horizontal branch morphology: A new photometric parametrization. Astronomy and Astrophysics, 629. ISSN 0004-6361

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Abstract

Context. Theory and observations indicate that the distribution of stars along the horizontal branch of Galactic globular clusters mainly depends on the metal content. However, the existence of globular clusters with similar metal content and absolute age but different horizontal branch morphologies, suggests the presence of another parameter affecting the star distribution along the branch. Aims. To investigate the variation of the horizontal branch morphology in Galactic globular clusters, we define a new photometric horizontal branch morphology index, overcoming some of the limitations and degeneracies affecting similar indices available in the literature. Methods.We took advantage of a sample of 64 Galactic globular clusters, with both space-based imaging data (Advanced Camera for Surveys survey of Galactic globular clusters) and homogeneous ground-based photometric catalogues in five different bands (U, B, V, R, I). The new index, τHB, is defined as the ratio between the areas subtended by the cumulative number distribution in magnitude (I) and in colour (V - I) of all stars along the horizontal branch. Results. This new index shows a linear trend over the entire range in metallicity (-2.35 ≤ [Fe/H] ≤ -0.12) covered by our Galactic globular cluster sample. We found a linear relation between τHBand absolute cluster ages. We also found a quadratic anti-correlation with [Fe/H], becoming linear when we eliminate the age effect on τHBvalues. Moreover, we identified a subsample of eight clusters that are peculiar according to their τHBvalues. These clusters have bluer horizontal branch morphology when compared to typical ones of similar metallicity. These findings allow us to define them as the second parameter clusters in the sample. A comparison with synthetic horizontal branch models suggests that they cannot be entirely explained with a spread in helium content. © 2019 EDP Sciences. All rights reserved.

Item Type:	Article
Uncontrolled Keywords:	0201 Astronomical and Space Sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	EDP Sciences
Date Deposited:	21 Sep 2020 11:33
Last Modified:	04 Sep 2021 06:40
DOI or ID number:	10.1051/0004-6361/201935995
URI:	https://researchonline.ljmu.ac.uk/id/eprint/13674

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