Evidence for Enrichment by Supernovae in the Globular Cluster NGC 6273

Abstract

In our recent investigation (Lim et al. 2015), we have shown that narrow-band photometry can be combined with low-resolution spectroscopy to effectively search for globular clusters (GCs) with supernovae (SNe) enrichments. Here we apply this technique to the metal-poor bulge GC NGC 6273, and find that the red giant branch stars in this GC are clearly divided into two distinct subpopulations having different calcium abundances. The Ca rich subpopulation in this GC is also enhanced in CN and CH, showing a positive correlation between them. This trend is identical to the result we found in M22, suggesting that this might be a ubiquitous nature of GCs more strongly affected by SNe in their chemical evolution. Our results suggest that NGC 6273 was massive enough to retain SNe ejecta which would place this cluster in the growing group of GCs with Galactic building block characteristics, such as ω Centauri and Terzan 5.

A Chemical Composition Survey of the Iron-complex Globular Cluster NGC 6273 (M19)*

“…NGC 6273 is known to have a broad RGB and a peculiar HB morphology that is similar to ω Cen (Piotto et al 1999;Momany et al 2004;Brown et al 2010Brown et al , 2016Han et al 2015). Therefore, in support of our spectroscopic observations, we have obtained new HST Wide Field Camera 3 UVIS channel (WFC3/UVIS) data centered on NGC 6273 that includes the F336W, F438W, F555W, and F814W filters.…”

“…In fact, the few AGB stars in our sample appear to belong to both the metal-poor and metalintermediate populations, and the two metal-rich stars could also belong to a more metal-rich AGB sequence. Therefore, we regard the combined evidence of separate sub-giant and RGB sequences observed by Han et al (2015) with the hk filter, the RGB color dispersions seen in Figures 2 and 11 here, the large metallicity spreads detected previously by Johnson et al (2015b) and Yong et al (2016), and the spectroscopic data presented here as strong evidence that NGC 6273 possesses an intrinsic metallicity spread.…”

“…Specifically, Figure 8 suggests that at least three major components may exist: (1) a "metal-poor" population ([Fe/H]−1.65); (2) Johnson et al (2015b) and Han et al (2015), and the presence of a metal-rich tail extending up to at least [Fe/H] ≈ −1 matches the findings of Yong et al (2016). Figure 9 indicates that a radial metallicity gradient may exist in the cluster such that the metal-intermediate stars are more centrally concentrated than the metal-poor stars.…”

“…In order to investigate this phenomenon further, we have obtained high-resolution spectra of >800 RGB and asymptotic giant branch (AGB) stars located near the massive bulge cluster NGC 6273. Following Johnson et al (2015b), Han et al (2015), and Yong et al (2016), we aim to investigate the cluster's metallicity distribution function and trace the cluster's detailed chemical composition across its various stellar populations.…”

The CN–CH Positive Correlation in the Globular Cluster NGC 5286

“…The spectroscopic and photometric data are taken from Han et al (2015) and Lim et al (2015Lim et al ( , 2016. The slope of the CN-CH with the s-rich stars more enhanced in both [Na/Fe] and δCN.…”