Formation of Multiple Populations of M5 (NGC 5904)

Abstract

With our new Ca-CN-CH-NH photometry, we revisit the globular cluster (GC) M5. We find that M5 is a mono-metallic GC with a small metallicity dispersion. Our carbon abundances show that the σ[C/Fe] of the M5 CN-s population, with depleted carbon and enhanced nitrogen abundances, is significantly large for a single stellar population. Our new analysis reveals that the M5 CN-s population is well described by the two stellar populations: the CN-s I , being the major CN-s component, with the intermediate carbon and nitrogen abundance and the CN-s E with the most carbon-poor and nitrogen-rich abundance. We find that the CN-s E is significantly more centrally concentrated than the others, while CN-w and CN-s I have similar cumulative radial distributions. The red giant branch bump V magnitude, the helium abundance barometer in mono-metallic populations, of individual populations appears to be correlated with their mean carbon abundance, indicating that carbon abundances are anticorrelated with helium abundances. We propose that the CN-s E formed out of gas that experienced proton-capture processes at high temperatures in the innermost region of the proto-GC of M5 that resided in a dense ambient density environment. Shortly after, the CN-s I formed out of gas diluted from the pristine gas in the more spatially extended region, consistent with the current development of numerical simulations by others.