[Paper Review] Effects of Helium Enrichment in Globular Clusters I.Theoretical Plane with PGPUC stellar evolution code

A. A. R. Valcarce, M. Catelan+1 more|Aug 25, 2012

Stellar, planetary, and galactic studies143 references33 citations

TL;DR

This paper presents theoretical evolutionary tracks, isochrones, and zero-age horizontal branch (ZAHB) loci for globular clusters using the PGPUC stellar evolution code, systematically varying helium abundance (Y = 0.230–0.370) and metallicity (Z = 1.60×10⁻⁴ to 1.57×10⁻²). It demonstrates that helium enrichment induces observable splits in the main sequence, subgiant branch, and red giant branch, with distinct effects on luminosity and temperature at key evolutionary phases, providing a framework to infer initial helium abundance from color-magnitude diagrams.

ABSTRACT

Recently, the study of globular cluster (GC) CMDs has shown that some of them harbor multiple populations with different chemical compositions and/or ages. In the first case, the most common candidate is a spread in the initial helium abundance, but this quantity is difficult to determine spectroscopically due to the fact that helium absorption lines are not present in cooler stars, whereas for hotter GC stars gravitational settling of helium becomes important. As a consequence, indirect methods to determine the initial Y among populations are necessary. For that reason, in this series of papers, we investigate the effects of a Y enrichment in populations covering the range of GC metallicities. In this first paper, we present the theoretical evolutionary tracks, isochrones, and ZAHB loci calculated with the Princeton-Goddard-PUC (PGPUC) stellar evolutionary code, which has been updated with the most recent input physics and compared with other theoretical databases. The chemical composition grid covers 9 Z ranging from Z=1.60x10^-4 to 1.57x10^-2, 7 Y from Y=0.230 to 0.370, and an alpha-element enhancement of [alpha/Fe]=0.3. The effects of different helium abundances that can be observed in isochrones are: splits in the MS, differences in the L and Teff of the turn off point, splits in the SGB being more prominent for lower ages or higher metallicities, splits in the lower red giant branch being more prominent for higher ages or higher metallicities, differences in L of the RGB bump (with small changes in Teff), and differences in L at the RGB tip. At the ZAHB, when Y is increased there is an increase of L for low Teff, which is affected in different degrees depending on the age of the GC being studied. Finally, the ZAHB morphology distribution depending on the age explains how for higher GC metallicities a population with higher helium abundance could be hidden at the red ZAHB locus.

Motivation & Objective

• To model the effects of helium enrichment on stellar evolution in globular clusters across a wide range of metallicities and helium abundances.
• To provide a theoretical basis for interpreting observed color-magnitude diagrams (CMDs) of globular clusters with multiple populations.
• To address the challenge of indirectly inferring initial helium abundance due to spectroscopic limitations in cooler stars.
• To develop a comprehensive grid of evolutionary tracks and isochrones for use in population synthesis and cluster age/metallicity analysis.
• To explain how helium abundance variations can account for observed morphological differences in the horizontal branch and other evolutionary phases.

Proposed method

• Employed the PGPUC stellar evolution code with updated input physics, including opacity tables, equation of state, and energy generation rates.
• Generated a grid of evolutionary tracks for 7 masses (0.5–1.1 M☉), 9 metallicities (Z = 1.60×10⁻⁴ to 1.57×10⁻²), and 7 helium abundances (Y = 0.230 to 0.370), with [α/Fe] = 0.3.
• Used linear interpolation between evolutionary tracks at specific evolutionary phases (EEP) to construct continuous tracks for arbitrary mass, metallicity, and helium abundance.
• Applied equivalent ZAHB points (EZAHBPs) to interpolate ZAHB loci, distinguishing between stars with and without convective envelopes.
• Calculated isochrones by combining interpolated tracks at a given age, enabling comparison with observed CMDs.
• Validated the interpolation method by comparing interpolated tracks and ZAHB loci with original models, confirming accuracy across the parameter space.

Experimental results

Research questions

• RQ1 How does increasing initial helium abundance affect the morphology of the main sequence in globular cluster color-magnitude diagrams?
• RQ2 What are the observable effects of helium enrichment on the luminosity and effective temperature of the turnoff point in isochrones?
• RQ3 How do splits in the subgiant branch and red giant branch depend on helium abundance, metallicity, and cluster age?
• RQ4 In what way does helium abundance variation influence the location and luminosity of the RGB bump and RGB tip?
• RQ5 How does helium enrichment alter the zero-age horizontal branch (ZAHB) morphology, particularly in relation to cluster metallicity and age?

Key findings

• Helium enrichment produces clear splits in the main sequence, especially prominent at higher metallicities and for younger clusters.
• The turnoff point luminosity increases with helium abundance, while its effective temperature shows a moderate shift, depending on metallicity and age.
• Subgiant branch splits become more pronounced for lower ages or higher metallicities, with the effect amplified at higher helium abundances.
• Red giant branch splits are more evident at higher metallicities or older ages, with luminosity differences increasing with helium content.
• The luminosity of the RGB bump increases with helium abundance, while its effective temperature remains nearly constant, indicating a shift in the bump's position along the RGB.
• At the RGB tip, the luminosity increases with helium abundance, and the ZAHB morphology shifts such that higher helium populations are preferentially found at the red end of the ZAHB, especially in metal-rich clusters.