Zero-metallicity stars I. Evolution at constant mass

TL;DR

This paper presents comprehensive models of zero-metallicity stars across a wide mass range, exploring their evolution, nucleosynthesis, surface pollution, and pulsational properties, with implications for early universe stellar populations.

Contribution

It provides detailed evolutionary models of zero-metallicity stars with updated physics, including nuclear reactions, overshooting, and pulsation analysis, extending previous work to cover a broad mass spectrum.

Findings

First synthesis of 12C in zero-metallicity stars triggers CNO-cycle activation.

Surface pollution occurs only in specific mass ranges due to dredge-up events.

Identified mass limits for different stellar classes and super-Eddington luminosity thresholds.

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 .