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 $\sigma$[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${}_{\mathrm{I}}$, being the major CN-s component, with the intermediate carbon and nitrogen abundance and the CN-s${}_{\mathrm{E}}$ with the most carbon-poor and nitrogen-rich abundance. We find that the CN-s${}_{\mathrm{E}}$ is significantly more centrally concentrated than the others, while CN-w and CN-s${}_{\mathrm{I}}$ 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${}_{\mathrm{E}}$ 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${}_{\mathrm{I}}$ 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.

arXiv commentThe Astrophysical Journal Letters (in press)

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