Despite the efforts of the past decade , the origin of the bimodal horizontal-branch ( HB ) found in some globular clusters ( GCs ) remains a conundrum .
Inspired by the discovery of multiple stellar populations in the most massive Galactic GC , \omega 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 \omega 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 ( \sim 15 % ) of helium-excess ( Y \simeq 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 .