[Paper Review] Very Massive Star Models: I. Impact of Rotation and Metallicity and Comparisons with Observations

S. Martinet, G. Meynet+3 more|arXiv (Cornell University)|Aug 31, 2023

Stellar, planetary, and galactic studies3 references4 citations

TL;DR

This study presents a comprehensive grid of very massive star (VMS) models from Population III to solar metallicity, incorporating rotation and metallicity effects using an updated equation of state in the GENEC code. It finds that low-metallicity VMS are highly sensitive to rotation, while high-metallicity models are dominated by mass loss; only non- or slowly rotating VMS at R136-like metallicity can produce pair-instability supernovae, limiting black hole masses to <60 M⊙.

ABSTRACT

In addition to being spectacular objects, Very Massive Stars (VMS) are suspected to have a tremendous impact on their environment and on the whole cosmic evolution. The nucleosynthesis both during their advanced stages and their final explosion may contribute greatly to the overall enrichment of the Universe. Their resulting supernovae are candidates for the most superluminous events and their extreme conditions also lead to very important radiative and mechanical feedback effects, from local to cosmic scale. We explore the impact of rotation and metallicity on the evolution of very massive stars across cosmic times. With the recent implementation of an equation of state in the GENEC stellar evolution code, appropriate for describing the conditions in the central regions of very massive stars in the advanced phases, we present new results on VMS evolution from Population III to solar metallicity. Low metallicity VMS models are highly sensitive to rotation, while the evolution of higher metallicity models is dominated by mass loss effects. The mass loss affects strongly their surface velocity evolution, breaking quickly at high metallicity while reaching the critical velocity for low metallicity models. The comparison to observed VMS in the LMC shows that the mass loss prescriptions used for these models are compatible with observed mass loss rates. In our framework for modelling rotation, our models of VMS need a high initial velocity to reproduce the observed surface velocities. The surface enrichment of these VMS is difficult to explain with only one initial composition, and could suggest multiple populations in the R136 cluster. At a metallicity typical of R136, only our non- or slowly rotating VMS models may produce Pair Instability supernovae. The most massive black holes that can be formed are less massive than about 60 M$_\odot$.

Motivation & Objective

• Investigate the impact of rotation and metallicity on the evolution of very massive stars (VMS) from Population III to solar metallicity.
• Assess how mass loss and rotational mixing affect surface velocity evolution and surface enrichment in VMS.
• Compare model predictions with observed VMS in the R136 cluster in the Large Magellanic Cloud.
• Determine the conditions under which VMS can produce pair-instability supernovae and the maximum black hole mass formed.
• Evaluate the reliability of current mass loss prescriptions in reproducing observed VMS properties across metallicities.

Proposed method

• Employed the GENEC stellar evolution code with a newly implemented equation of state to model VMS in advanced evolutionary phases.
• Generated a grid of VMS models spanning initial masses from 100 to 500 M⊙ across metallicities from Z=0 (Population III) to Z=0.02 (solar).
• Incorporated rotation via rotational mixing and angular momentum transport, with initial rotational velocities up to 400 km/s.
• Applied updated mass loss prescriptions, including line-driven winds and effects near the Eddington limit, for different metallicities.
• Tracked core mass evolution, particularly carbon-oxygen core mass at helium burning end, to assess final fate (e.g., pair-instability supernovae).
• Used the equation of state to model pair production and instability conditions within the stellar core.

Figure 1: HRD of the non-rotating (solid lines) and rotating at V/V c =0.4 (dashed lines) models at Z=0.014 (solar) and Z=0.006 (LMC). The top/middle/bottom row shows model with initial masses of 180/250/300M ☉ . The tracks are color-coded according to their mass loss rates. The beginning and the en

Experimental results

Research questions

• RQ1 How does rotation influence the evolution and surface properties of very massive stars at low metallicities?
• RQ2 To what extent do mass loss rates dominate over rotational effects in high-metallicity VMS?
• RQ3 Can current mass loss prescriptions reproduce observed mass loss rates in VMS like those in the R136 cluster?
• RQ4 What initial rotational velocities are required for models to match observed surface velocities in VMS?
• RQ5 Under which conditions can VMS produce pair-instability supernovae, and what is the maximum black hole mass formed?

Key findings

• Low-metallicity VMS models are highly sensitive to rotation, with surface velocities reaching critical values due to weak mass loss.
• High-metallicity VMS evolution is dominated by mass loss, which rapidly reduces surface velocity and limits rotational effects.
• Observed VMS in the R136 cluster require high initial rotational velocities (up to 400 km/s) to match their observed surface velocities in models.
• Surface enrichment in R136 VMS is difficult to explain with a single initial composition, suggesting multiple stellar populations or complex interactions.
• Only non- or slowly rotating VMS at R136-like metallicity (Z ≈ 0.006) can produce pair-instability supernovae.
• The most massive black holes formed from VMS are limited to less than 60 M⊙, primarily due to pair-instability and mass loss effects.

Figure 2: HRD for 180M ☉ non-rotating models at Z=0.014, Z=0.006, Z=10 -5 and Z=0. The tracks are color-coded according to their current total mass. The MS of the models at Z=0.014, Z=0.006, Z=10 -5 are plotted in dashed grey in the lower right panel for comparison.