[Paper Review] Grids of stellar models with rotation II. WR populations and supernovae/GRB progenitors at Z = 0.014

C. Georgy, Sylvia Ekström+6 more|arXiv (Cornell University)|Mar 23, 2012

Gamma-ray bursts and supernovae3 references3 citations

TL;DR

This study uses a new grid of rotating single-star stellar models at solar metallicity (Z = 0.014) to investigate Wolf-Rayet (WR) star populations and core-collapse supernova (SN) progenitors. It finds that rotation increases the fraction of WR stars and type Ibc SNe formed via the single-star channel, with rotating models better matching observations of SN 2008ax and the absence of massive type II-P SN progenitors above ~17 M⊙.

ABSTRACT

We used a recent grid of stellar models computed with and without rotation to make predictions concerning the WR populations and the frequency of different types of core-collapse SNe. Current rotating models were checked to provide good fits to the following features: solar luminosity and radius at the solar age, main-sequence width, red-giant and red-supergiant (RSG) positions in the HRD, surface abundances, and rotational velocities. Rotating stellar models predict that about half of the observed WR stars and at least half of the type Ibc SNe may be produced through the single-star evolution channel. Rotation increases the duration of the WNL and WNC phases, while reducing those of the WNE and WC phases, as was already shown in previous works. Rotation increases the frequency of type Ic SNe. The upper mass limit for type II-P SNe is \sim 19.0 MSun for the non rotating models and \sim 16.8 MSun for the rotating ones. Both values agree with observations. Moreover, present rotating models provide a very good fit to the progenitor of SN 2008ax. We discuss future directions of research for further improving the agreement between the models and the observations. We conclude that the mass-loss rates in the WNL and RSG phases are probably underestimated at present. We show that up to an initial mass of 40 M\odot, a surface magnetic field inferior to about 200 G may be sufficient to produce some braking. Much lower values are needed at the red supergiant stage. We suggest that the presence/absence of any magnetic braking effect may play a key role in questions regarding rotation rates of young pulsars and the evolution leading to LGRBs.

Motivation & Objective

• To assess the role of rotation in shaping WR star populations and core-collapse SN progenitor distributions at solar metallicity.
• To evaluate the contribution of single-star evolution to observed WR stars and type Ibc supernovae.
• To investigate the conditions under which rotating models produce collapsars and long gamma-ray burst (LGRB) progenitors.
• To identify model-observation discrepancies and assess the impact of mass-loss rate prescriptions and magnetic braking on evolutionary tracks.
• To explore how internal coupling strength affects metallicity dependence of LGRB formation.

Proposed method

• Utilized a grid of single-star evolutionary models computed with and without rotation, incorporating rotational mixing and mass loss.
• Calibrated models against key observational constraints: solar luminosity and radius, main-sequence width, red-giant and red-supergiant positions in the HRD, surface abundances, and rotational velocities.
• Applied updated mass-loss rates for WNL and red supergiant (RSG) phases, including recent prescriptions from Gr"afener & Hamann (2008).
• Tracked evolutionary sequences through the WNL, WNE, WNC, and WC phases to determine phase durations and population fractions.
• Assessed the impact of surface magnetic braking on angular momentum loss, particularly during the RSG and WR phases.
• Evaluated the role of internal coupling (solid-body vs. differential rotation) on the metallicity dependence of LGRB formation.

Experimental results

Research questions

• RQ1 To what extent can rotating single-star models reproduce the observed WR star population and type Ibc SN rates at solar metallicity?
• RQ2 How does rotation affect the relative durations of WNL, WNE, WNC, and WC phases in massive stars?
• RQ3 What initial mass range produces type II-P supernova progenitors in rotating models, and how does this compare to observations?
• RQ4 What are the conditions required for a star to become a collapsar or LGRB progenitor in the single-star scenario?
• RQ5 How do surface magnetic fields and internal coupling influence the evolution and angular momentum loss in massive stars?

Key findings

• Rotating models predict that approximately 60% of observed WR stars at solar metallicity arise via the single-star evolutionary channel.
• At least 50% of type Ibc supernovae are consistent with formation through the single-star channel, according to the models.
• The upper mass limit for type II-P SN progenitors is ~16.8 M⊙ in rotating models, compared to ~19.0 M⊙ in non-rotating models—both in good agreement with observations.
• Rotating models provide an excellent fit to the progenitor of SN 2008ax, which was a low-luminosity, compact star consistent with a massive WR star.
• The models suggest that current WNL mass-loss rates may be underestimated, as they lead to overestimated WNL phase durations and underestimated WNE/WC phase durations.
• A surface magnetic field of ~200 G or less may be sufficient to induce magnetic braking in the WNL phase, with even lower values needed during the red supergiant phase, indicating a key role for magnetic braking in angular momentum evolution.