Age as the Second Parameter in NGC 288/NGC 362? II. The Horizontal Branch Revisited

Abstract

We revisit the "" second-parameter ÏÏ pair of globular clusters NGC 288/362 on the basis of theoretical models for red giant branch (RGB) and horizontal branch (HB) stars. The results of the most extensive set of RGB/HB simulations computed so far for these clusters are presented for two di †erent metallicities. Using several di †erent analytical mass-loss formulae for RGB stars, we derive relative "" HB morphology ages.ÏÏ We compare them with the relative main-sequence turno † ages derived by application of the "" bridge test ÏÏ in Paper I, where it was found that NGC 288 is 2^1 Gyr older than NGC 362. We Ðnd that adoption of a higher metallicity ([Fe/H] B [1.2), as favored by the Carretta & Gratton metallicity scale, makes age a much more plausible second-parameter candidate for this pair than is the case when a lower metallicity ([Fe/H] B [1.5), closer to the Zinn & West scale, is adopted. However, while the di †erent HB morphology of these two clusters can be reproduced by canonical HB models with [Fe/H] B [1.2 and an age di †erence of 2 Gyr, this explanation is not without difficulty. In particular, we conÐrm previous suggestions that canonical models are unable to reproduce the detailed HB morphology of NGC 288 at its red end, for as yet unknown reasons. Finally, we show that the mass dispersion on the HB of NGC 362 is substantially larger than for NGC 288 and suggest that there is a correlation between the mass dispersion on the HB phase and the central density of globular clusters. This is presumably related to the way environmental e †ects a †ect RGB mass lossÈanother major second-parameter candidate. We argue that, if conÐrmed, this central densityÈHB mass dispersion correlation will have to be taken into account in order to conclusively determine whether age may be considered the (sole) second parameter of HB morphology for this (and other) second-parameter pair(s).

NGC 362: another globular cluster with a split red giant branch

“…Catelan et al (2001) compared the relative age provided by the HB morphology with that obtained from the main sequence for the pair NGC 362-NGC 288. They used the bimodal HB of NGC 1851 as a "bridge" (see also Bellazzini et al 2001).…”

Fast Winds and Mass Loss From Metal-Poor Field Giants

“…This assumption remains one of the major concerns in the evolution of low-mass stars and may be related to the second-parameter problem (Sandage & Wildey 1967): differing horizontal branch morphology between globular clusters of the same metallicity and age. Various explanations have been offered for differences in the horizontal branch morphology including intrinsic dispersions in the amount of stellar mass loss, rotation, or deep mixing, environmental effects possibly correlated with cluster mass or central density, heterogeneities in He abundance possibly as a result of cluster pollution by intermediate-mass asymptotic giant branch (AGB) stars, or of the infall of planets onto cluster stars (Buonanno et al 1993;Buonanno et al 1998;Catelan et al 2001;Sills & Pinsonneault 2000;Recio-Blanco et al 2006;Sandquist & Martel 2007;Soker et al 2001b;Sneden et al 2004;Sweigart 1997;Peterson et al 1995;Ventura & D'Antona 2005). Most recently with the advent of infrared photometry discussed below, and the identification of multiple populations on the main sequence of globular clusters with the Hubble Space Telescope (Anderson 2002;Bedin et al 2004;Piotto et al 2007), renewed attention is focussing on the mass loss process.…”

Mass Outflow and Chromospheric Activity of Red Giant Stars in Globular Clusters. I. M15

“…Metallicity, as first noted by Sandage & Wallerstein (1960), remains the principal parameter, but pairs of clusters, with the same metallicity, display quite different horizontal branch morphologies thus challenging the canonical models of stellar evolution and leading to the need for a "second parameter." Cluster ages have been examined in many studies (Searle & Zinn 1978;Lee et al 1994;Stetson et al 1996;Lee & Carney 1999;Sarajedini 1997;Sarajedini et al 1997) and, in addition, many other suggestions for the "second parameter(s)" have been proposed, including total cluster mass, stellar environment (and possibly free-floating planets), primordial He abundance, post-mixing surface helium abundance, CNO abundance, stellar rotation, and mass loss (Catelan 2000;Catelan et al 2001;Sills & Pinsonneault 2000;Soker et al 2001;Sweigart 1997;Buonanno et al 1993;Peterson et al 1995;Buonanno et al 1998;Recio-Blanco et al 2006). Many authors (Vandenberg et al 1990;Lee et al 1994;Catelan 2000) have proposed that more than one second parameter may exist in addition to age.…”