2007/12/24 by Suk-Jin Yoon, Suk‐Jin Yoon, Seok-Joo Joo +8
Physics and Astronomy · #Astronomy and Astrophysical Research #Ejecta #Galaxy #Globular cluster #Hertzsprung–Russell diagram #History and Developments in Astronomy #Horizontal branch #Metallicity #Population #RR Lyrae variable #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/533510
Accepted in ApJ, 33 pages, 5 figures, 4 tables
arxiv created 2007/12/24 · openalex publication_date 2008/04/11 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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, 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.