[Paper Review] Stellar evolution with rotation VIII: Models at Z = 10^{-5} and CNO yields for early galactic evolution

G. Meynet, A. Maeder|ArXiv.org|May 22, 2002

Stellar, planetary, and galactic studiesPhysics and Astronomy44 references244 citations

TL;DR

This paper presents a grid of stellar evolution models at Z = 10⁻⁵, including rotation effects, to investigate CNO nucleosynthesis and yields in early galactic environments. It demonstrates that rotational mixing in intermediate-mass and massive stars at very low metallicity produces significant primary ¹⁴N, explaining the observed N/O plateau in metal-poor stars, with rotation being the dominant factor in N/O ratios and mass range determining C/O ratios in early galaxy chemical evolution.

ABSTRACT

We calculate a grid of star models with and without the effects of axial rotation for stars in the mass range between 2 and 60 M$_{\odot}$ for the metallicity $Z = 10^{-5}$. Star models with initial masses superior or equal to 9 M$_\odot$ were computed up to the end of the carbon--burning phase. Star models with masses between 2 and 7 M$_\odot$ were evolved beyond the end of the He--burning phase through a few thermal pulses during the AGB phase. Compared to models at $Z=0.02$, the low $Z$ models show faster rotating cores and stronger internal $Ω$--gradients, which favour an important mixing of the chemical elements. In very low $Z$ models, primary nitrogen is produced during the He--burning phase by rotational diffusion of $^{12}$C into the H--burning shell. The intermediate mass stars of very low $Z$ are the main producers of primary $^{14}$N, but massive stars also contribute to this production; no significant primary nitrogen is made in models at metallicity $Z$=0.004 or above. We calculate the chemical yields in He, C, N, O and heavy elements and discuss the chemical evolution of the CNO elements at very low Z. Remarkably, the C/O vs O/H diagram is mainly sensitive to the interval of stellar masses, while the N/O vs O/H diagram is mainly sensitive to the average rotation of the stars contributing to the element synthesis. The presently available observations in these diagrams seem to favour contributions either from stars down to about 2 M$_{\odot}$ with normal rotation velocities or from stars above 8 M$_{\odot}$ but with very fast rotation.

Motivation & Objective

• To understand the role of rotation in chemical yields at extremely low metallicity (Z = 10⁻⁵), relevant to early galactic evolution.
• To resolve the puzzle of primary nitrogen production in metal-poor stars, where standard models fail to explain observed N/O ratios.
• To calculate detailed CNO and heavy-element yields from rotating stars across 2–60 M⊙, including evolution through carbon-burning and AGB phases.
• To assess how rotation and initial mass function shape the C/O vs. O/H and N/O vs. O/H diagrams in early galaxies.
• To provide a theoretical basis for interpreting observations of metal-poor halo stars and low-metallicity galaxies.

Proposed method

• Stellar evolution models were computed for masses between 2 and 60 M⊙ at Z = 10⁻⁵, including both non-rotating and rotating cases (initial v_rot = 300 km/s, with some at 400 km/s).
• Rotation-induced mixing was modeled via meridional circulation and shear diffusion, with effects on chemical element transport and internal angular momentum gradients.
• Nuclear reaction rates were based on the NACRE compilation, and opacities included low-temperature molecular contributions.
• Evolution was tracked through core hydrogen and helium burning, carbon burning (for ≥9 M⊙), and thermal pulses in the AGB phase (2–7 M⊙).
• Chemical yields of He, C, N, O, and heavy elements were calculated and analyzed in the context of galactic chemical evolution.
• Theoretical C/O and N/O ratios were compared to observational data in metal-poor stars and galaxies to constrain stellar population properties.

Experimental results

Research questions

• RQ1 How does rotational mixing affect CNO surface abundances and yields in very low-metallicity stars (Z = 10⁻⁵)?
• RQ2 Can rotational effects explain the observed plateau in N/O ratios at low metallicity, particularly the presence of primary ¹⁴N?
• RQ3 What is the relative contribution of intermediate-mass stars (2–8 M⊙) versus massive stars (≥8 M⊙) to early galactic nitrogen enrichment?
• RQ4 How do the C/O and N/O ratios in the C/O vs. O/H and N/O vs. O/H diagrams depend on the initial mass function and stellar rotation?
• RQ5 To what extent do stellar wind mass loss and rotation influence the final yields of C, N, and O in low-metallicity environments?

Key findings

• Rotational mixing in low-metallicity models enhances surface nitrogen by 2–3 orders of magnitude, especially in stars with initial rotation ≥300 km/s.
• Primary ¹⁴N is efficiently produced during helium burning via rotational diffusion of ¹²C into the H-burning shell, with a large fraction surviving to the AGB phase.
• Intermediate-mass stars (2–8 M⊙) are the dominant producers of primary ¹⁴N at Z = 10⁻⁵, though massive stars (≥8 M⊙) also contribute significantly.
• The N/O vs. O/H diagram is highly sensitive to average stellar rotation, with rotating models showing up to a 2-order-of-magnitude increase in N/O, while C/O is primarily governed by the initial mass function.
• The observed N/O plateau at log(N/O) ≈ -1.7 is best reproduced by models including intermediate-mass stars (down to 2–8 M⊙) with moderate to fast rotation (≥230 km/s average), not by non-rotating or only massive-star models.
• The C/O vs. O/H diagram is insensitive to rotation but strongly dependent on the mass range of contributing stars, with lower mass limits yielding higher C/O ratios.