M92 (NGC~6341) Is a Metal-Complex Globular Cluster with an Atypical Primordial Population

Abstract

We present a multiple stellar population study of the metal-poor globular cluster (GC) M92 (NGC 6341), which is long known for the substantial metallicity dispersion, using our own photometric system. We find two groups with slightly different mean metallicities, the metal-poor (MP) stars with [Fe/H] = $-$2.412$\pm$0.03, while the metal-rich (MR) ones with $-$2.282$\pm$0.002. The MP constitutes about 23\% of the total mass with a more central concentration. Our populational tagging based on the [C/Fe] and [N/Fe] provides the mean n(P):n(I):n(E) = 32.2:31.6:36.2 ($\pm$2.4), where P, I, and E denote the primordial, intermediate, and extreme populations, respectively. Our populational number ratio is consistent with those of others. However, the MP has a significantly different populational number ratio than the mean value, and the domination of the primordial population in the MP is consistent with observations of Galactic GCs that less massive GCs contain larger fractions of the primordial population. Structural and constituent differences between the MP and MR may indicate that M92 is a merger remnant in a dwarf galaxy environment, consistent with recent suggestions that M92 is a GC in a dwarf galaxy or a remnant nucleus of the progenitor galaxy. Discrepancy between our method and those widely used for the HST photometry exists in the primordial population. Significant magnesium and oxygen depletions of $-$0.8 and $-$0.3 dex, respectively, and helium enhancement of $\Delta Y$ $\gtrsim$ 0.03 are required to explain the presence of this abnormal primordial group. No clear explanation is available with limited information of detailed elemental abundances.

Accepted for publication to the Astrophysical Journal Letters

References in corpus (12)

• Formation and Dynamical Evolution of Multiple Stellar Generations in Globular Clusters
• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. IX. The Atlas of Multiple Stellar Populations
• The updated BaSTI stellar evolution models and isochrones: II. alpha-enhanced calculations
• Multiple populations in globular clusters and their parent galaxies
• The eye of Gaia on globular clusters structure: tidal tails
• Multiple Stellar Populations of Globular Clusters from Homogenous Ca by Photometry I. M22 (NGC 6656)
• Multiple stellar populations of globular clusters from homogeneous Ca-Cn photometry. II. M5 (NGC 5904) and a new filter system
• The Pristine survey -- XVI. The metallicity of 26 stellar streams around the Milky Way detected with the STREAMFINDER in Gaia EDR3
• Confirmation of a metallicity spread amongst first population stars in globular clusters
• Multiple stellar populations of globular clusters from homogeneous Ca--CN photometry. III. NGC 6752
• Multiple Stellar Populations of Globular Clusters From Homogeneous Ca-CN-CH-NH Photometry. VII. Metal-Poor Populations in 47 Tucanae (NGC 104)
• Formation of Multiple Populations of M5 (NGC 5904)

Cited by in corpus (6)

• MgAl burning chain in Omega Centauri
• r-process Abundance Patterns in the Globular Cluster M92
• MOCCA: Effects of pristine gas accretion and cluster migration on globular cluster evolution, global parameters, and multiple stellar populations
• A Comparative Study Between M30 and M92 : M92 Is A Merger Remnant With A Large Helium Enhancement
• Benchmarking the effective temperature scale of red giant branch stellar models: the case of the metal-poor halo giant HD 122563
• Carbon Abundance of Globular Cluster M22 (NGC 6656) and the Surface Carbon Depletion Rates of the Milky Way Globular Clusters