On the Origin of Bimodal Horizontal Branches in Massive Globular Clusters: The Case of NGC 6388 and NGC 6441

Abstract

Despite the efforts of the past decade, the origin of the bimodal horizontalbranch (HB) found in some globular clusters (GCs) remains a conundrum. Inspired by the discovery of multiple stellar populations in the most massive Galactic GC, ω Centauri, we investigate the possibility that two distinct populations may coexist and are responsible for the bimodal HBs in the third and fifth brightest GCs, NGC 6388 and NGC 6441. Using the population synthesis technique, we examine two different chemical "self-enrichment" hypotheses in which a primordial GC was sufficiently massive to contain two or more distinct populations as suggested by the populations found in ω Cen: (1) the age-metallicity relation scenario in which two populations with different metallicity and age coexist, following an internal age-metallicity relation, and (2) the super-helium-rich scenario in which GCs contain a certain fraction of helium-enhanced stars, for instance, the second generation stars formed from the helium-enriched ejecta of the first. The comparative study indicates that the detailed color-magnitude diagram morphologies and the properties of the RR Lyrae variables in NGC 6388 and NGC 6441 support the latter scenario; i.e., the model which assumes a minor fraction (∼ 15 %) of helium-excess (Y ≃ 0.3) stars. The results suggest that helium content is the main driver behind the HB bimodality found most often in massive GCs. If confirmed, the GC-to-GC variation of helium abundance should be considered a local effect, further supporting the argument that age is the global second parameter of HB morphology.Recent studies suggest that the long-standing puzzle of the HB bimodality may no longer be a complete mystery. It has been discovered that at least four discrete populations coexist in the most massive Galactic GC, ω Cen (Lee et al. 1999b; Standford et al. 2006, and references therein). This finding obviously opposes the conventional "single-population" picture of GCs, and also provides an instructive precedent for the bimodal-HB feature in other GCs. The multiple populations in ω Cen imply an internal age-metallicity relation (AMR), in that stars having a higher metallicity are younger. Furthermore, subsequent

The horizontal branch morphology of M 31 globular clusters

“…This feature suggests a similarity with NGC 6441 and NGC 6388, the metal-rich Galactic GCs displaying a strong red HB clump as well as a significant blue HB tail, that it is currently interpreted as due to a large spread in He abundance (Caloi & D'Antona 2007;Yoon et al 2008). Also several bright GCs in M87 are interpreted as hosting a He abundance spread (see Rey et al 2007, their Fig.…”

“…NGC 6388, NGC 6441), since they show larger SMI values. NGC 6388 and NGC 6441 have a strongly bimodal HB morphology that is difficult to explain without invoking a significant spread in He abundance among cluster stars (Caloi & D'Antona 2007;Yoon et al 2008). Also, many Sample B clusters have (FUV − V) 0 colors suggesting that the presence of extended blue HB tails may be more common in M 31 clusters than in the Milky Way.…”

“…This should lead to strong effects on the HB morphology of presentday clusters (see, e.g. D 'Antona et al 2005;Caloi & D'Antona 2007;Yoon et al 2008;Gratton et al 2010).…”

Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569

“…In general, clusters with higher masses and more extended blue HBs tend to exhibit signatures associated with more extreme light-element processing (e.g., He enrichment; large O and Mg depletions; some Si production) and higher burning temperatures (e.g., Carretta et al 2007b;Yoon et al 2008;Milone et al 2014). However, less advanced nuclear processing is expected as a cluster's metallicity increases owing to effects such as a general decline in the temperatures required to maintain hydrostatic equilibrium, enhanced mass loss, and an overall reduction in the range of light-element yields from polluting stars (e.g., Ventura & D'Antona 2009;Ventura et al 2013).…”

Core collapse and horizontal-branch morphology in Galactic globular clusters

“…More recently, this long-standing issue attracted renewed interest after the discovery of multiple 1 Even though we retain this terminology for historical reasons, the implied assumption that a single second parameter can account for all the characteristics of the distribution of stars on the HB is unjustified in light of the current understanding of the issue. stellar populations in GCs (Bedin et al 2004;Piotto et al 2005Piotto et al , 2007Busso et al 2007;Yoon et al 2008, among several others). However, a comprehensive picture is still lacking, and it is now clear that a single quantity is not capable of determining HB morphology in its whole complexity, so that more than one parameter is likely to play a role.…”