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