[Paper Review] The case of the disappearing CN-strong AGB stars in Galactic globular clusters -- Preliminary Results

S. W. Campbell, David Yong+6 more|arXiv (Cornell University)|Apr 13, 2010

Stellar, planetary, and galactic studies3 citations

TL;DR

This study investigates the unexpected depletion of CN-strong AGB stars in Galactic globular clusters using medium-resolution spectroscopy of 250 AGB stars across nine clusters. It finds that many clusters lack CN-strong AGB stars entirely or show significant reductions compared to their RGB populations, challenging standard stellar evolution models that predict enhanced nitrogen on the AGB.

ABSTRACT

A previously reported literature search suggested that the AGB stars in Galactic globular clusters may be showing different distributions of CN-strong and CN-weak stars as compared to their RGB stars. In most cases the second giant branches of GCs appeared to be deficient in stars with strong CN bands. However the sample sizes of AGB stars at that time were too small to give a definitive picture. Thus an observing campaign targeting GC AGB stars was proposed. We now have medium resolution spectral observations of about 250 GC AGB stars across 9 globular clusters, obtained with the 2dF/AAOmega instrument on the Anglo-Australian Telescope. In this paper we report some preliminary results regarding the distributions of CN-strong and CN-weak stars on the two giant branches of a selection of globular clusters. We find that some GCs show a total lack of CN-strong stars on the AGB, whilst some show a reduction in CN-strong stars as compared to the RGB. Standard stellar evolution does not predict this change in surface abundance between the two giant branches. We discuss some possible causes of this unexpected phenomenon.

Motivation & Objective

• To determine whether CN-strong AGB stars are systematically underabundant in Galactic globular clusters compared to their red giant branch (RGB) populations.
• To resolve inconsistencies in the literature due to small sample sizes in previous AGB studies.
• To test whether the observed CN abundance differences between RGB and AGB stars are real and not due to observational biases or spectral analysis systematics.
• To explore possible evolutionary mechanisms—such as helium or mass-dependent mass loss—that could explain the apparent disappearance of CN-strong stars on the AGB.
• To assess the implications of this phenomenon for the formation and chemical evolution of globular clusters, particularly regarding multiple populations and self-pollution scenarios.

Proposed method

• Acquired medium-resolution spectroscopic data for 250 AGB stars across nine Galactic globular clusters using the 2dF/AAOmega instrument on the Anglo-Australian Telescope.
• Used high-precision photometry and astrometry from existing surveys (e.g., Grundahl et al. 1999; Sandquist & Bolte 2004) to identify AGB stars in color-magnitude diagrams (CMDs), distinguishing them from RGB stars.
• Performed spectral analysis to measure CN band strengths and classify stars as CN-strong or CN-weak, using consistent calibration across all clusters.
• Compared the relative fractions of CN-strong and CN-weak stars on the AGB with those on the RGB within the same clusters to detect systematic differences.
• Assessed potential observational biases by comparing AGB and RGB stars at similar temperatures and luminosities, confirming that spectral resolution and gravity effects do not explain the observed abundance differences.
• Planned spectral synthesis calculations to validate abundance measurements and rule out systematic errors in the analysis pipeline.

Experimental results

Research questions

• RQ1 Are CN-strong AGB stars systematically underabundant in Galactic globular clusters compared to their RGB counterparts?
• RQ2 Does the observed deficit of CN-strong stars on the AGB represent a real astrophysical phenomenon or an artifact of small sample sizes and observational biases?
• RQ3 Can standard stellar evolution models explain the reduction in surface nitrogen (as traced by CN bands) during the transition from the RGB to the AGB?
• RQ4 What physical mechanisms—such as helium abundance differences or enhanced mass loss—could account for the selective absence of CN-strong stars on the AGB?
• RQ5 How do the observed AGB-to-RGB CN abundance differences vary across clusters, and what do these variations imply for the formation history and self-pollution mechanisms in globular clusters?

Key findings

• In several globular clusters, no CN-strong AGB stars were detected, indicating a complete absence of this population on the AGB despite their presence on the RGB.
• In other clusters, CN-strong AGB stars are significantly underrepresented compared to the RGB, with the AGB dominated by CN-weak stars.
• The observed CN abundance differences are not due to spectral resolution, gravity effects, or temperature-dependent band strength variations, as confirmed by comparisons at similar stellar parameters.
• The discrepancy between RGB and AGB CN abundances contradicts standard stellar evolution theory, which predicts that CN (and nitrogen) should be enhanced on the AGB due to deep mixing on the RGB.
• The phenomenon is not uniform across clusters, with some showing total absence of CN-strong AGB stars and others showing partial depletion, suggesting cluster-dependent physical mechanisms.
• The results imply that mechanisms such as helium abundance differences (ΔY ~ 0.05) or mass-dependent mass loss may preferentially remove CN-strong stars from the AGB, though further modeling is required to confirm this.