Multiple Stellar Populations of Globular Clusters From Homogeneous Ca-CN-CH-NH Photometry. VII. Metal-Poor Populations in 47 Tucanae (NGC 104)

TL;DR

This study uses Ca-CN photometry to analyze multiple stellar populations in 47 Tucanae, revealing bimodal metallicity distributions, population concentration differences, and implications for cluster formation history.

Contribution

It provides the first detailed photometric metallicity bimodality analysis of 47 Tuc, suggesting a merger origin for its multiple populations.

Findings

Bimodal metallicity distribution with peaks at [Fe/H] ~ -0.72 and -0.92 dex.

Metal-poor populations are more centrally concentrated.

No helium abundance difference between the two metal-poor populations.

Abstract

We present new large field-of-view ($\sim$1\deg$\times$1\deg) Ca-CN photometry of the prototypical metal-rich globular cluster 47 Tucanae (NGC 104). Our results are the following. (1) The populational number ratios of the red giant branch (RGB) and red horizontal branch (RHB) are in excellent agreement: n(CN-w):n(CN-s) = 30:70 ($\pm$1--2), where the CN-w and CN-s stand for the CN-weak and CN-strong populations, respectively. Both the CN-s RGB and RHB populations are more centrally concentrated than those of CN-w populations are. (2) Our photometric metallicities of individual RGB stars in each population can be well described by bimodal distributions with two metallicity peaks, [Fe/H] $\sim$ $-$0.72 and $-$0.92 dex, where the metal-poor components occupy $\sim$ 13% of the total RGB stars. The metal-poor populations are more significantly centrally concentrated than the metal-rich populations, showing a similar result that we found in M3. (3) The RGB bump $V$ magnitudes of individual populations indicate that there is no difference in the helium abundance between the two metal-poor populations, while the helium enhancement of $\Delta Y$ $\sim$ 0.02--0.03 is required between the the two metal-rich populations. (4) The RHB morphology of 47 Tuc appears to support our idea of the bimodal metallicity distribution of the cluster. We suggest that 47 Tuc could be another example of merger remnants of two globular clusters, similar to M3 and M22.