A Comparative Study Between M30 and M92 : M92 Is A Merger Remnant With A Large Helium Enhancement
arXiv:2312.02442 · doi:10.3847/1538-4357/ad12ca
Abstract
We perform a comparative study of the ex--situ second--parameter pair globular clusters (GCs) M30 and M92, having similar metallicities but different horizontal branch morphologies. We obtain the similar mean primordial carbon abundances for both clusters. However, M92 shows a large dispersion in carbon due to a more extended C--N anticorrelation, while M30 exhibits a higher primordial nitrogen abundance, suggesting that they have different chemical enrichment histories. Our new results confirm our previous result that M92 is a metal--complex GC showing a bimodal metallicity distribution. We also find that the metal--rich group of stars in M92 shows a helium enhancement as large as 0.05 from the red giant branch bump (RGBB) magnitudes, which can also be supported by (i) a lack of bright RGB stars, (ii) synthetic evolutionary HB population models and (iii) the more extended spatial distribution due to different degree of the diffusion process from their lower masses. We reinterpret the [Eu/Fe] measurements by other, finding that the two metallicity groups of stars in M92 have significantly different [Eu/Fe] abundances with small scatters. This strongly suggests that they formed independently out of well mixed interstellar media in different environments. We suggest that M92 is a more complex system than a normal GC, most likely a merger remnant of two GCs or a even more complex system. In Appendix, we address the problems with the recently developed color--temperature relations and the usage of broadband photometry in the populational taggings.
Accepted for publication in the Astrophysical Journal; 17 Figures and 5 Tables
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