[Paper Review] Helium variation due to self-pollution among Globular Cluster stars: consequences on the horizontal branch morphology

F. D’Antona, V. Caloi+3 more|ArXiv.org|Sep 17, 2002

Stellar, planetary, and galactic studiesPhysics and Astronomy23 references199 citations

TL;DR

This paper investigates how self-pollution by intermediate-mass AGB stars—which enrich the interstellar medium with helium and CNO elements—can explain the morphology of the horizontal branch (HB) in globular clusters. It shows that helium enrichment (Y ≈ 0.28) from such pollution leads to lower-mass turnoff stars, enabling the formation of extremely blue horizontal branch stars, particularly in clusters like M13 and NGC 6752 with strong CNO abundance spreads.

ABSTRACT

It is becoming clear that `self--pollution' by the ejecta of massive asymptotic giant branch stars has an important role in the early chemical evolution of globular cluster stars, producing CNO abundance spreads which are observed also at the surface of unevolved stars. Considering that the ejecta which are CNO processed must also be helium enriched, we have modelled stellar evolution of globular cluster stars by taking into account this possible helium enhancement with respect to the primordial value. We show that the differences between the main evolutionary phases (main sequence, turn--off and red giants) are small enough that it would be very difficult to detect them observationally. However, the difference in the evolving mass may play a role in the morphology of the horizontal branch, and in particular in the formation of blue tails, in those globular clusters which show strong CNO abundance variations, such as M13 and NGC 6752.

Motivation & Objective

• To assess the impact of helium enrichment due to self-pollution by AGB stars on the evolutionary tracks of globular cluster stars.
• To determine whether helium variations can explain the observed morphology of the horizontal branch, particularly the presence of blue tails in clusters like M13 and NGC 6752.
• To evaluate whether helium-enhanced populations can be observationally distinguished from standard populations via main sequence, turnoff, and red giant branch evolution.
• To investigate the role of helium spread as a potential solution to the 'second parameter problem' in horizontal branch morphology.

Proposed method

• Comparison of stellar evolution isochrones with primordial helium (Y = 0.23–0.24) and enhanced helium (Y = 0.28) at metallicities Z = 2×10⁻⁴ and Z = 10⁻³.
• Modeling of evolutionary phases including main sequence, turnoff, red giant branch, and horizontal branch for varying helium abundances.
• Use of ZAHB (Zero Age Horizontal Branch) models extended down to 0.491 M⊙ for Y = 0.28 to simulate extreme blue HB stars.
• Analysis of luminosity and mass differences in turnoff stars due to helium enrichment, focusing on mass loss and evolutionary timescales.
• Incorporation of observed CNO abundance spreads and helium enrichment from AGB ejecta into stellar evolution models.
• Assessment of observational detectability of helium variations through color-magnitude diagram features, especially HB morphology.

Experimental results

Research questions

• RQ1 Can helium enrichment from self-pollution by AGB stars explain the formation of blue tails in the horizontal branch of globular clusters?
• RQ2 To what extent do helium variations affect the location and morphology of the main sequence, turnoff, and red giant branch?
• RQ3 How does a helium abundance increase (Y = 0.28) influence the mass and luminosity of stars evolving along the horizontal branch?
• RQ4 Is the observed morphology of the horizontal branch in clusters like M13 and NGC 6752 consistent with a spread in helium abundance due to self-pollution?
• RQ5 Can helium enrichment serve as a solution to the 'second parameter problem' in horizontal branch morphology?

Key findings

• Helium enrichment from AGB ejecta (Y ≈ 0.28) leads to a significant reduction in turnoff mass, with 0.6 M⊙ models being more than 0.4 mag brighter than standard Y = 0.24 models.
• The differences in main sequence, turnoff, and red giant branch evolution between standard and helium-enriched populations are too small to be observationally detectable.
• The horizontal branch morphology is significantly affected by helium variation, particularly in enabling the formation of extremely blue HB stars with very low envelope masses.
• Only stars with the maximum helium content (Y = 0.28) would appear significantly more luminous on the RR Lyrae region, but this effect is masked by standard mass loss (ΔM ≥ 0.2 M⊙).
• The presence of blue tails in clusters like M13 and NGC 6752 is best explained by a spread in helium abundance, as it allows for lower-mass turnoff stars that evolve into low-envelope-mass blue HB stars.
• The observed bimodal CN abundance distribution and extended blue HBs in some clusters suggest that self-pollution must have affected a large fraction of the cluster's interstellar medium, implying efficient reprocessing of AGB ejecta.