Integrated colours of Milky Way globular clusters and horizontal branch morphology

Broadband colours are often used as metallicity proxies in the study of extragalactic globular clusters. A common concern is the effect of variations in horizontal branch (HB) morphology - the second-parameter effect - on such colours. We have used UBVI, Washington, and DDO photometry for a compilation of over 80 Milky Way globular clusters to address this question. Our method is to fit linear relations between colour and [Fe/H], and study the correlations between the residuals about these fits and two quantitative measures of HB morphology. While there is a significant HB effect seen in U - B, for the commonly used colours B - V, V - I, and C - T-1, the deviations from the baseline colour-[Fe/H] relations are less strongly related to HB morphology. There may be weak signatures in B - V and C - T-1, but these are at the limit of observational uncertainties. The results may favour the use of B - I in studies of extragalactic globular clusters, especially when its high [Fe/H]-sensitivity is considered. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.