Georges Meynet

Stellar evolution with rotation. XIII. Predicted GRB rates at various Z

We present the evolution of rotation in models of massive single stars covering a wide range of masses and metallicities. These models reproduce observations during the early stages of the evolution very well, in particular Wolf-Rayet (WR) populations and ratio between type II and type Ib,c supernovae at different metallicities. Our models predict the production of fast-rotating black holes. Models with large initial masses or high metallicity end their lives with less angular momentum in their central remnant with respect to the break-up limit for the remnant. Many WR star models satisfy the three main criteria (black hole formation, loss of hydrogen-rich envelope, and enough angular momentum to form an accretion disk around the black hole) for gamma-ray bursts (GRB) production via Woosleys collapsar model. If we consider all types of WR stars as GRB progenitors, there would be too many GRBs compared to observations but if we consider only WO stars (type Ic supernovae as is the case for SN2003dh/GRB030329) as GRB progenitors, the GRB production rates are in much better agreement with observations. WO stars are produced only at low metallicities in the present series of models. This prediction can be tested by future observations.

Evolution and fate of very massive stars

There is observational evidence that supports the existence of very massive stars (VMS) in the local universe. First, VMS (Mini ≲ 320 M⊙) have been observed in the Large Magellanic Clouds (LMC). Secondly, there are observed supernovae (SNe) that bear the characteristics of pair creation supernovae (PCSNe, also referred to as pair instability SN) which have VMS as progenitors. The most promising candidate to date is SN 2007bi. In order to investigate the evolution and fate of nearby VMS, we calculated a new grid of models for such objects, for solar, LMC and Small Magellanic Clouds (SMC) metallicities, which covers the initial mass range from 120 to 500 M⊙. Both rotating and non-rotating models were calculated using the GENEVA stellar evolution code and evolved until at least the end of helium burning and for most models until oxygen burning. Since VMS have very large convective cores during the main-sequence phase, their evolution is not so much affected by rotational mixing, but more by mass loss through stellar winds. Their evolution is never far from a homogeneous evolution even without rotational mixing. All the VMS, at all the metallicities studied here, end their life as WC(WO)-type Wolf-Rayet stars. Because of very important mass losses through stellar winds, these stars may have luminosities during the advanced phases of their evolution similar to stars with initial masses between 60 and 120 M⊙. A distinctive feature which may be used to disentangle Wolf-Rayet stars originating from VMS from those originating from lower initial masses would be the enhanced abundances of Ne and Mg at the surface of WC stars. This feature is however not always apparent depending on the history of mass loss. At solar metallicity, none of our models is expected to explode as a PCSN. At the metallicity of the LMC, only stars more massive than 300 M⊙ are expected to explode as PCSNe. At the SMC metallicity, the mass range for the PCSN progenitors is much larger and comprises stars with initial masses between about 100 and 290 M⊙. All VMS in the metallicity range studied here produce either a Type Ib SN or a Type Ic SN but not a Type II SN. We estimate that the progenitor of SN 2007bi, assuming a SMC metallicity, had an initial mass between 160 and 175 M⊙. None of models presented in this grid produces gamma-ray bursts or magnetars. They lose too much angular momentum by mass loss or avoid the formation of a black hole by producing a completely disruptive PCSN.

Fundamental properties of core-collapse supernova and GRB progenitors: predicting the look of massive stars before death

We investigate the fundamental properties of core-collapse Supernova (SN) progenitors from single stars at solar meta llicity. For this purpose, we combine Geneva stellar evolutionary models with initial masses of Mini = 20− 120 M⊙ with atmospheric/wind models using the radiative transfer code CMFGEN. We provide synthetic photometry and high-resolution spectra of hot stars at t he pre-SN stage. For models with Mini = 9− 20 M⊙, we supplement our analysis using publicly available MARCS model atmospheres of RSGs to estimate their synthetic photometry. We employ well-established observational criteria of spectroscopic classifi cation and find that massive stars, depending on their initial mass and rotation , end their lives as red supergiants (RSG), yellow hypergian ts (YHG), luminous blue variables (LBV), and Wolf-Rayet (WR) stars of the WN and WO spectral types. For rotating models, we obtained the following types of SN progenitors: WO1‐3 (Mini ≥ 32 M⊙), WN10‐11 (25 40 M⊙), WN7‐8 (25< Mini ≤ 40 M⊙), WN11h/LBV (20< Mini ≤ 25 M⊙), and RSGs (9≤ Mini ≤ 20 M⊙). Our rotating models indicate that SN IIP progenitors are all RSG, SN IIL/b progenitors are 56% LBVs and 44% YHGs, SN Ib progenitors are 96% WN10-11 and 4% WOs, and SN Ic progenitors are all WO stars. We find that n ot necessarily the most massive and luminous SN progenitors are the brighter ones in a given filter, since this depends on t heir luminosity, temperature, wind density, and how the spectral energy distribution compares to a filter bandpass. We find that SN IIP progenitors (RSGs) are bright in the RI JHKS filters and faint in the U B filters. SN IIL /b progenitors (LBVs and YHGs), and SN Ib progenitors (WNs) are relatively bright in optical/infrared filters, while SN Ic progenitors (WOs) are faint in all optical filters. We argu e that SN Ib and Ic progenitors from single stars should be undetectable in the available pre-explosion images with the current magnitude limits, in agreement with observational results.

Massive star models with magnetic braking

Magnetic fields at the surface of a few early-type stars have been directly detected. These fields have magnitudes between a few hundred G up to a few kG. In one case, evidence of magnetic braking has been found. We investigate the effects of magnetic braking on the evolution of rotating (

Massive star evolution: luminous blue variables as unexpected supernova progenitors

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Imprints of fast-rotating massive stars in the Galactic Bulge

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The s-process in the Galactic halo: the fifth signature of spinstars in the early Universe?

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YELLOW AND RED SUPERGIANTS IN THE LARGE MAGELLANIC CLOUD

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RED SUPERGIANTS IN THE ANDROMEDA GALAXY (M31)

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THE YELLOW AND RED SUPERGIANTS OF M33

stellar models at solar metallicity during the main-sequence (MS) phase. The magnetic braking process is included in our stellar models according to the formalism deduced from 2D MHD simulations of magnetic wind confinement by ud-Doula and co-workers. Various assumptions are made regarding both the magnitude of the magnetic field and of the efficiency of the angular momentum transport mechanisms in the stellar interior. When magnetic braking occurs in models with differential rotation, a strong and rapid mixing is obtained at the surface accompanied by a rapid decrease in the surface velocity. Such a process might account for some MS stars showing strong mixing and low surface velocities. When solid-body rotation is imposed in the interior, the star is slowed down so rapidly that surface enrichments are smaller than in similar models with no magnetic braking. In both kinds of models (differentially or uniformly rotating), magnetic braking due to a field of a few 100 G significantly reduces the angular momentum of the core during the MS phase. This reduction is much greater in solid-body rotating models.