Variations in the Na-O anticorrelation in globular clusters: Evidence for a deep mixing episode in red giant branch stars

TL;DR

This paper presents evidence that internal deep mixing episodes in red giant branch stars influence the Na-O anticorrelation in globular clusters, challenging the sole reliance on primordial pollution models and affecting stellar population classification.

Contribution

It demonstrates that the shape of the Na-O anticorrelation varies with luminosity, indicating deep mixing episodes, thus questioning current classification schemes based solely on abundance ratios.

Findings

Na-O anticorrelation shape differs above and below the red giant branch bump.

Deep mixing episodes occur during the ascent of low-mass red giants.

Deep mixing may help resolve sodium under-production issues in primordial pollution models.

Abstract

The Na-O anticorrelation seen in almost all globular clusters ever studied using high-resolution spectroscopy is now generally explained by the primordial pollution from the first generation of the intermediate-mass asymptotic giant branch stars to the proto-stellar clouds of the second generation of stars. However, the primordial pollution scenario may not tell the whole story for the observed Na-O anticorrelations in globular clusters. Using the recent data by Carretta and his collaborators, the different shapes of the Na-O anticorrelations for red giant branch stars brighter than and fainter than the red giant branch bump can be clearly seen. If the elemental abundance measurements by Carretta and his collaborators are not greatly in error, this variation in the Na-O anticorrelation against luminosity indicates an internal deep mixing episode during the ascent of the low-mass red giant branch in globular clusters. Our result implies that the multiple stellar population division scheme solely based on [O/Fe] and [Na/Fe] ratios of a globular cluster, which is becoming popular, is not reliable for stars brighter than the red giant branch bump. Our result also suggests that sodium supplied by the deep mixing may alleviate the sodium under-production problem within the primordial asymptotic giant branch pollution scenario.