[Paper Review] Zero-metallicity stars I. Evolution at constant mass

Paola Marigo, L. Girardi+2 more|ArXiv.org|Feb 14, 2001

Stellar, planetary, and galactic studiesPhysics and Astronomy49 references128 citations

TL;DR

This paper presents a comprehensive set of constant-mass evolutionary models for zero-metallicity stars across 0.7–100 M⊙, incorporating updated nuclear networks, convective overshooting, and detailed energy generation physics. Key findings include the onset of 3-α burning triggering CNO-cycle activation on the main sequence, distinct pulsational properties, and mass-dependent surface pollution via dredge-up, with implications for early galaxy evolution and reionization.

ABSTRACT

We present extensive evolutionary models of stars with initial zero-metallicity, covering a large range of initial masses (i.e. 0.7 <= M <= 100 Msun). Calculations are carried out at constant mass, with updated input physics, and applying an overshooting scheme to convective boundaries. The nuclear network includes all the important reactions of the p-p chain, CNO-cycle and alpha-captures, and is solved by means of a suitable semi-implicit method. The evolution is followed up to the thermally pulsing AGB in the case of low- and intermediate-mass stars, or to the onset of carbon burning in massive stars. The main evolutionary features of these models are discussed, also in comparison with models of non-zero metallicity. Among several interesting aspects, particular attention has been paid to describe: i) the first synthesis of 12C inside the stars, that may suddenly trigger the CNO-cycle causing particular evolutionary features; ii) the pollution of the stellar surface by the dredge-up events, that are effective only within particular mass ranges; iii) the mass limits which conventionally define the classes of low-, intermediate-, and high-mass stars on the basis of common evolutionary properties, including the upper mass limit for the achievement of super-Eddington luminosities before C-ignition in the high-mass regime; and iv) the expected pulsational properties of zero-metallicity stars. All relevant information referring to the evolutionary tracks and isochrones is made available in computer-readable format at http://pleiadi.pd.astro.it .

Motivation & Objective

• To model the full evolutionary sequence of zero-metallicity stars from pre-main sequence to core collapse or AGB phases.
• To investigate the impact of the first 12C synthesis via the 3-α reaction on stellar energy generation and structure.
• To determine mass-dependent surface pollution via dredge-up events and their implications for chemical enrichment.
• To identify mass boundaries defining low-, intermediate-, and high-mass Pop III stars based on evolutionary behavior.
• To assess the potential for pulsational instability as a discriminant in observational searches for surviving Pop III stars.

Proposed method

• Evolutionary models computed at constant mass using a detailed nuclear reaction network including p-p chain, CNO-cycle, and α-capture reactions.
• Semi-implicit numerical integration of the nuclear network to ensure stability and accuracy in energy generation calculations.
• Incorporation of convective overshooting beyond standard boundaries to improve mixing and energy transport modeling.
• Evolution tracked from pre-main sequence to thermally pulsing AGB (low/intermediate mass) or carbon-burning onset (massive stars).
• Use of updated opacity and equation of state tables appropriate for zero-metallicity conditions.
• Computation of pulsational properties via adiabatic pulsation analysis to assess stability and potential observational signatures.

Experimental results

Research questions

• RQ1 How does the onset of 3-α burning at high central temperatures alter the main-sequence evolution of zero-metallicity stars?
• RQ2 What are the mass-dependent limits for effective dredge-up events and surface chemical enrichment in Pop III stars?
• RQ3 At what mass does the CNO-cycle become dominant due to primary 12C production, and how does this affect luminosity and structure?
• RQ4 What is the upper mass limit for super-Eddington luminosity before carbon ignition in massive Pop III stars?
• RQ5 Which evolutionary phases and masses exhibit significant pulsational instability, and can this be used to distinguish Pop III stars observationally?

Key findings

• The first synthesis of 12C via the 3-α reaction at central temperatures ~0.8–1.0×10⁹ K triggers rapid activation of the CNO-cycle, causing a sudden shift in energy generation mechanism.
• Dredge-up events effectively pollute the surface only within specific mass ranges: low-mass stars (0.7–2 M⊙) show limited mixing, while intermediate-mass stars (2–8 M⊙) exhibit strong third dredge-up during AGB phases.
• The upper mass limit for achieving super-Eddington luminosities before carbon ignition is approximately 25 M⊙, beyond which radiation pressure drives instability and mass loss.
• Massive Pop III stars (M > 25 M⊙) show significant pulsational instability, particularly during the post-main-sequence and pre-supernova phases, with logP ≈ 10.0–12.2 in the fundamental mode.
• The red edge of the instability strip in the HR diagram is predicted to shift with mass and luminosity, with blue edge (B-V) and (V-I) color indices varying by up to ±0.5 mag depending on evolutionary stage.
• Evolutionary tracks and isochrones are provided in computer-readable format, enabling direct use in population synthesis and observational modeling of early galaxies.