The connection between missing AGB stars and extended horizontal branches

TL;DR

This study links the absence of CN-strong AGB stars in globular clusters to the minimum mass of stars on the horizontal branch, showing that stars with the smallest mass do not evolve through the full AGB phase, especially those with high He and N content.

Contribution

It establishes a correlation between horizontal branch star mass and AGB star frequency, explaining the missing CN-strong AGB stars in terms of stellar evolution and composition.

Findings

AGB star frequency varies significantly between clusters.

A strong correlation exists between HB star mass and AGB star presence.

Stars with the smallest HB mass do not evolve into full AGB stars.

Abstract

Recent surveys confirm early results about a deficiency or even absence of CN-strong stars on the asymptotic giant branch (AGB) of globular clusters (GCs), although with quite large cluster-to-cluster variations. In general, this is at odds with the distribution of CN band strengths among first ascent red giant branch (RGB) stars. Norris et al. proposed that the lack of CN-strong stars in some clusters is a consequence of a smaller mass of these stars that cannot evolve through the full AGB phase. In this short paper we found that the relative frequency of AGB stars can change by a factor of two between different clusters. We also find a very good correlation between the minimum mass of stars along the horizontal branch (Gratton et al. 2010) and the relative frequency of AGB stars, with a further dependence on metallicity. We conclude that indeed the stars with the smallest mass on the HB cannot evolve through the full AGB phase, being AGB-manque'. These stars likely had large He and N content, and large O-depletion. We then argue that there should not be AGB stars with extreme O depletion, and few of them with a moderate one.