[Paper Review] Very low metallicity massive star models: Pre-SN evolution and primary nitrogen production

Raphaël Hirschi|arXiv (Cornell University)|Aug 8, 2006

Gamma-ray bursts and supernovaePhysics and Astronomy61 references137 citations

TL;DR

This paper presents rotating massive star models at extremely low metallicities (Z = 10⁻⁸) to explain the surface abundances of extremely metal-poor stars. Rotation drives CNO mixing and mass loss, leading to primary nitrogen production and the formation of WO-type Wolf-Rayet stars, predicting Type Ic supernovae and potential long soft gamma-ray bursts at very low metallicities.

ABSTRACT

Two series of models were computed. The first series consists of 20 solar mass models with varying initial metallicity (Z=0.02 down to Z=10^{-8}) and rotation (V_{ini}=0-600 km/s). The second one consists of models with an initial metallicity of Z=10^{-8}, masses between 9 and 85 solar masses and fast initial rotation velocities (V_{ini}=600-800 km/s). The most interesting models are the models with Z=10^{-8} ([Fe/H]~-6.6). In the course of helium burning, carbon and oxygen are mixed into the hydrogen burning shell. This boosts the importance of the shell and causes a reduction of the CO core mass. Later in the evolution, the hydrogen shell deepens and produces large amount of primary nitrogen. For the most massive models (M>~60 solar masses), significant mass loss occurs during the red supergiant stage. This mass loss is due to the surface enrichment in CNO elements via rotational and convective mixing. The 85 solar mass model ends up as a WO type Wolf-Rayet star. Therefore the models predict SNe of type Ic and possibly long and soft GRBs at very low metallicities. The rotating 20 solar mass models can best reproduce the observed CNO abundances at the surface of extremely metal poor (EMP) stars and the metallicity trends when their angular momentum content is the same as at solar metallicity (and therefore have an increasing surface velocity with decreasing metallicity). The wind of the massive star models can also reproduce the CNO abundances of the most metal-poor carbon-rich star known to date, HE1327-2326.

Motivation & Objective

• Explain the observed surface abundances of extremely metal-poor (EMP) stars, particularly the overproduction of primary nitrogen.
• Investigate the role of rotation and metallicity in pre-supernova evolution of massive stars.
• Determine whether stellar winds from massive stars can reproduce the CNO abundances of the most metal-poor star known, HE1327-2326.
• Assess the conditions under which massive stars at very low metallicity produce Wolf-Rayet stars and potential long-duration gamma-ray bursts.
• Provide stellar yields for light elements to inform galactic chemical evolution models.

Proposed method

• Computed 20 M☉ models with initial metallicities from Z = 0.02 down to Z = 10⁻⁸ and rotation velocities from 0 to 600 km s⁻¹.
• Computed models with Z = 10⁻⁸ and masses from 9 to 85 M☉ at high initial rotation (600–800 km s⁻¹).
• Used stellar evolution code with diffusive convection and overshooting (α = 0.1 H_P) from oxygen burning onward.
• Applied NACRE reaction rates and a solar-like initial composition with Y_p = 0.24 and ΔY/ΔZ = 2.5.
• Tracked surface abundances through helium burning and hydrogen shell burning, focusing on CNO production and mass loss.
• Compared wind and core yields to observed abundances in EMP stars, including HE1327-2326 and CS22949-037.

Experimental results

Research questions

• RQ1 To what extent can rotation in massive stars at Z = 10⁻⁸ explain the observed primary nitrogen overproduction in EMP stars?
• RQ2 What is the role of rotational and convective mixing in driving mass loss and surface enrichment in low-metallicity massive stars?
• RQ3 Can the wind composition of massive stars at Z = 10⁻⁸ reproduce the CNO abundances of the most metal-poor star HE1327-2326?
• RQ4 At what initial mass and metallicity do massive stars evolve into WO-type Wolf-Rayet stars, and what are the implications for SN Ic and GRB formation?
• RQ5 How do the yields of light elements (C, N, O) from these models compare to observations and galactic chemical evolution constraints?

Key findings

• At Z = 10⁻⁸, rotational mixing during helium burning transports carbon and oxygen into the hydrogen-burning shell, reducing the CO core mass.
• The deepening hydrogen shell in later evolution produces large amounts of primary nitrogen, explaining the observed N overabundance in EMP stars.
• For M ≥ 60 M☉, significant mass loss occurs during the red supergiant phase due to surface enrichment in CNO elements via rotation and mixing.
• The 85 M☉ model evolves into a WO-type Wolf-Rayet star, indicating that such stars can form at very low metallicities.
• The models predict that massive stars at Z = 10⁻⁸ can produce Type Ic supernovae and potentially long, soft gamma-ray bursts.
• The wind composition of the 85 M☉ model matches the C, N, and O abundances of HE1327-2326, supporting a scenario where this star formed from material enriched by a single Pop II.5 star.