Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter

¹ Dpt. Cassiopée, UMR 6202, Observatoire de la Cote d'Azur, BP 4229, 06304 Nice Cedex 04, France e-mail: arecio@obs-nice.fr
² Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: [recio;piotto;deangeli]@pd.astro.it
³ Departamento de Astrofísica, Universidad de La Laguna, Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain e-mail: antapaj@iac.es
⁴ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁵ Institute of Astronomy, Madingley Rd, CB3 0HA Cambridge, UK e-mail: fda@ast.cam.ac.uk
⁶ Astronomy Department, California Institute of Technology, MC 105-24, Pasadena, CA 91125, USA e-mail: george@astro.caltech.edu

Received: 7 March 2005
Accepted: 16 November 2005

Abstract

Aims.The interpretation of globular cluster horizontal branch (HB) morphology is a classical problem that can significantly blur our understanding of stellar populations.

Methods.We present a new multivariate analysis connecting the effective temperature extent of the HB with other cluster parameters. The work is based on Hubble Space Telescope photometry of 54 Galactic globular clusters.

Results. The present study reveals the important role of the total mass of the globular cluster on its HB morphology. More massive clusters tend to have HBs more extended to higher temperatures. For a set of three input variables including the temperature extension of the HB, [Fe/H] and M_V, the first two eigenvectors account for 90% of the total sample variance.

Conclusions. Possible effects of cluster self-pollution on HB morphology, stronger in more massive clusters, could explain the results derived here.