Horizontal branch morphology: A new photometric parametrization

¹ Department of Physics, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
² INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
e-mail: marianna.torelli@inaf.it
³ NRC-Herzberg, Dominion Astrophysical Observatory, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁴ Astrophysics Research Institute, Liverpool John Moores University, IC2 Building, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁵ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁶ IAC – Instituto de Astrofisica de Canarias, Calle Via Lactea s/n, 38205 La Laguna, Spain
⁷ Departamento de Astrofisica, Universidad de La Laguna, Avenida Astrofísico Francisco Sánchez s/n, 38200 Tenerife, Spain
⁸ INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini snc, Loc. Collurania, 64100 Teramo, Italy

Received: 31 May 2019
Accepted: 19 July 2019

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, τ_HB, 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 τ_HB and absolute cluster ages. We also found a quadratic anti-correlation with [Fe/H], becoming linear when we eliminate the age effect on τ_HB values. Moreover, we identified a subsample of eight clusters that are peculiar according to their τ_HB values. 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.