arXiv subjects

André Maeder

Publications and source records attributed to André Maeder.

At least 37 records · Page 2Linked to original sources

arXiv2011

Grids of stellar models with rotation - I. Models from 0.8 to 120 Msun at solar metallicity (Z = 0.014)

[abridged] Many topical astrophysical research areas, such as the properties of planet host stars, the nature of the progenitors of different types of supernovae and gamma ray bursts, and the evolution of galaxies, require complete and homogeneous sets of stellar models at different metallicities in order to be studied during the whole of cosmic history. We present here a first set of models for solar metallicity, where the effects of rotation are accounted for in a homogeneous way. We computed a grid of 48 different stellar evolutionary tracks, both rotating and non-rotating, at Z=0.014, spanning a wide mass range from 0.8 to 120 Msun. For each of the stellar masses considered, electronic tables provide data for 400 stages along the evolutionary track and at each stage, a set of 43 physical data are given. These grids thus provide an extensive and detailed data basis for comparisons with the observations. The rotating models start on the ZAMS with a rotation rate Vini/Vcrit=0.4. The evolution is computed until the end of the central carbon-burning phase, the early AGB phase, or the core helium-flash for, respectively, the massive, intermediate, and both low and very low mass stars. The initial abundances are those deduced by Asplund and collaborators, which best fit the observed abundances of massive stars in the solar neighbourhood. We update both the opacities and nuclear reaction rates, and introduce new prescriptions for the mass-loss rates as stars approach the Eddington and/or the critical velocity. We account for both atomic diffusion and magnetic braking in our low-mass star models. [...]

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arXiv2010

Effects of anisotropic winds on massive stars evolution

Whenever stars are rotating very fast (Omega/Omega_crit > 0.7, with Omega_crit the Keplerian angular velocity of the star accounting for its deformation) radiative stellar winds are enhanced in polar regions. This theoretical prediction is now confirmed by interferometric observations of fast rotating stars.} Polar winds remove less angular momentum than spherical winds and thus allow the star to keep more angular momentum. We quantitatively assess the importance of this effect. First we use a semi-analytical approach to estimate the variation of the angular momentum loss when the rotation parameter increases. Then we compute complete 9 MSun stellar models at very high angular velocities (starting on the ZAMS with Omega/Omega_crit = 0.8 and reaching the critical velocity during the Main Sequence) with and without radiative wind anisotropies. When wind anisotropies are accounted for, the angular momentum loss rate is reduced by less than 4% for Omega/Omega_crit < 0.9 with respect to the case of spherical winds. The reduction amounts to at most 30% when the star is rotating near the critical velocity. These values result from two counteracting effects: on the one hand polar winds reduce the loss of angular momentum, on the other hand, surface deformations imply that the mass which is lost at high co-latitude is lost at a larger distance from the rotational axis and thus removes more angular momentum. In contrast with previous studies, which neglected surface deformations, we show that the radiative wind anisotropies have a relatively modest effect on the evolution of the angular momentum content of fast rotating stars.

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arXiv2010

Massive stellar models: rotational evolution, metallicity effects

The Be star phenomenon is related to fast rotation, although the cause of this fast rotation is not yet clearly established. The basic effects of fast rotation on the stellar structure are reviewed: oblateness, mixing, anisotropic winds. The processes governing the evolution of the equatorial velocity of a single star (transport mechanisms and mass loss) are presented, as well as their metallicity dependence. The theoretical results are compared to observations of B and Be stars in the Galaxy and the Magellanic Clouds.

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arXiv2010

The GSF Instability and Turbulence do not Account for the Relatively Low Rotation Rate of Pulsars

The aim of this paper is to examine the effects of the horizontal turbulence in differentially rotating stars on the GSF instability and apply our results to pre-supernova models. For this purpose we derive the expression for the GSF instability with account of the thermal transport and smoothing of the mu-gradient by the horizontal turbulence. We apply the new expressions in numerical models of a 20 solar mass star. We show that if N^2_{Omega} < 0 the Rayleigh-Taylor instability cannot be killed by the stabilizing thermal and mu-gradients, so that the GSF instability is always there and we derive the corresponding diffusion coefficient. The GSF instability grows towards the very latest stages of stellar evolution. Close to the deep convective zones in pre-supernova stages, the transport coefficient of elements and angular momentum by the GSF instability can very locally be larger than the shear instability and even as large as the thermal diffusivity. However the zones over which the GSF instability is acting are extremely narrow and there is not enough time left before the supernova explosion for a significant mixing to occur. Thus, even when the inhibiting effects of the mu-gradient are reduced by the horizontal turbulence, the GSF instability remains insignificant for the evolution. We conclude that the GSF instability in pre-supernova stages cannot be held responsible for the relatively low rotation rate of pulsars compared to the predictions of rotating star models.

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arXiv2009

The different progenitors of type Ib, Ic SNe, and of GRB

We discuss the properties of the progenitors of core collapse SNe, as they can be deduced from rotating stellar models of single stars. The type of the SN progenitor was determined from the surface abundances at the pre-SN stage. The type of the SN event was obtained from the masses of hydrogen and helium ejected at the time of the core-collapse SN event. We find that the minimum amount of helium ejected by a core-collapse SN (of whatever type) is around 0.3 MSun. There is no difference between the WC and WO stars in the ejected masses of helium, CNO elements, and heavy elements. Also no difference is expected between the chemical composition of a WC star resulting from a normal or a homogeneous evolution. The progenitors of type Ib SNe are WNL, WNE, or less massive WC stars. Those of type Ic are WC and WO stars. WO stars are produced in a limited mass range (around 60 MSun) and only at low metallicity (for <= 0.010) as already found. The WO stars are the progenitors of only a small fraction of type Ic. Present stellar models indicate that, at solar metallicity, there is about 1 type Ib SN for 1 type Ic, and this ratio rises to 3 type Ic for 1 type Ib SN at twice solar metallicity. At this metallicity, type Ic's are more frequent than type Ib's because most massive stars that go through a WNE stage evolve further into a WC/WO phase. Current models can account for the observed number ratios SN Ib/SN II and SN Ic/SN II and for their observed variation with the metallicity. In case no supernova occurs when a black hole is formed, single-star models can still account for more than half of the observed (SN Ib+SN Ic)/SN II ratio for Z >= ZSun. For the GRB rate, our models produce too large a number for such an event, even if we restrict the progenitor to the WO stars.

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arXiv2008

The basic role of magnetic field in stellar evolution

Magnetic field is playing an important role at all stages of star evolution from star formation to the endpoints. The main effects are briefly reviewed. We also show that O-type stars have large convective envelopes, where convective dynamo could work. There, fields in magnetostatic balance have intensities of the order of 100 G. A few OB stars with strong polar fields (Henrichs et al. 2003a) show large N-enhancements indicating a strong internal mixing. We suggest that the meridional circulation enhanced by an internal rotation law close to uniform in these magnetic stars is responsible for the observed mixing. Thus, it is not the magnetic field itself which makes the mixing, but the strong thermal instability associated to solid body rotation. A critical question for evolution is whether a dynamo is at work in radiative zones of rotating stars. The Tayler-Spruit (TS) dynamo is the best candidate. We derive some basic relations for dynamos in radiative layers. Evolutionary models with TS dynamo show important effects: internal rotation coupling and enhanced mixing, all model outputs being affected.

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arXiv2008

Wind anisotropy and stellar evolution

Mass loss is a determinant factor which strongly affects the evolution and the fate of massive stars. At low metallicity, stars are supposed to rotate faster than at the solar one. This favors the existence of stars near the critical velocity. In this rotation regime, the deformation of the stellar surface becomes important, and wind anisotropy develops. Polar winds are expected to be dominant for fast rotating hot stars. These polar winds allow the star to lose large quantities of mass and still retain a high angular momentum, and they modifie the evolution of the surface velocity and the final angular momentum kept in the star's core. We show here how these winds affect the final stages of massive stars, according to our knowledge about Gamma Ray Bursts. Computation of theoretical Gamma Ray Bursts rate indicates that our models have too fast rotating cores, and that we need to include an additional effect to spin them down. Magnetic fields in stars act in this direction, and we show how they modify the evolution of massive star up to the final stages.

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arXiv2008

Can very massive stars avoid Pair-Instability Supernovae?

Very massive primordial stars (140 Msol < M < 260 Msol) are supposed to end their lives as PISN. Such an event can be traced by a typical chemical signature in low metallicity stars, but at the present time, this signature is lacking in the extremely metal-poor stars we are able to observe. Does it mean that those very massive objects were not formed, contrarily to the primordial star formation scenarios ? Could it be possible that they avoided this tragical fate ? We explore the effects of rotation, anisotropical mass loss and magnetic field on the core size of very massive Population III models. We find that magnetic fields provide the strong coupling that is lacking in standard evolution metal-free models and our 150 Msol Population III model avoids indeed the pair-instability explosion.

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arXiv2008

Convective envelopes in rotating OB stars

We study the effects of rotation on the outer convective zones of massive stars. We examine the effects of rotation on the thermal gradient and on the Solberg--Hoiland term by analytical developments and by numerical models. Writing the criterion for convection in rotating envelopes, we show that the effects of rotation on the thermal gradient are much larger and of opposite sign to the effect of the Solberg-Hoiland criterion. On the whole, rotation favors convection in stellar envelopes at the equator and to a smaller extent at the poles. In a rotating 20 Msun star at 94% of the critical angular velocity, there are two convective envelopes, with the bigger one having a thickness of 13.2% of the equatorial radius. In the non-rotating model, the corresponding convective zone has a thickness of only 4.6% of the radius. The occurrence of outer convection in massive stars has many consequences.

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arXiv2006

CNO production in first generation stars

Big Bang nucleosynthesis produces only light elements and the very first generation stars are thus formed from metal-free clouds. They start the production of heavy elements during their life, and enrich the interstellar medium through their explosive death. Stellar evolution models show that the treatment of rotation has important effects on the evolution of those metal-free stars: for example, rotating models produce up to five orders of magnitude more primary nitrogen than non rotating models, due to internal mixing. This will have an impact in the composition of the second generation stars, some of which may now be observed in the Galactic halo. In the case Population III stars were very massive and would end up as direct black holes, rotation again have an interesting effect of enhancing mass loss through centrifugal force and surface enrichment. CNO composition patterns observed in ultra metal-poor halo stars may be explained by a 'wind only' contribution.

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arXiv2006

On the Origin of the High Helium Sequence in $ω$ Centauri

The blue Main Sequence (bMS) of $ω$ Cen implies a ratio of helium to metal enrichment $ΔY/ΔZ \approx 70$, which is a major enigma. We show that rotating models of low metallicity stars, which account for the anomalous abundance ratios of extremely metal poor stars, are also useful for understanding the very high $ΔY/ΔZ$ ratio in $ω$ Cen. Models of massive stars with moderate initial rotation velocities produce stellar winds with large He-- and N--excesses, but without the large C-- (and O--) excesses made by very fast rotation, in agreement with the observed chemical abundance ratios in $ω$ Cen. It is still uncertain whether the abundance peculiarities of $ω$ Cen result from the fact that the high velocity contributions of supernovae escaped the globular cluster, usually considered as a tidally stripped core of a dwarf galaxy. Another possibility is a general dominance of wind ejecta at very low $Z$, due to the formation of black holes. Some abundance and isotopic ratios like $Mg/Al$, $Na/Mg$, $Ne/N$, $^{12}C/^{13}C$, $^{16}O/^{18}O$ and $^{17}O/^{18}O$ may allow us to further discriminate between these scenarios and between the AGB and massive star contributions.

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arXiv2006

Initial conditions for reaching the critical velocity

The aim of this paper is to determine the initial rotational velocities required on the ZAMS for single stars to reach the critical velocity, sometimes called the break-up velocity, during the Main-Sequence (MS) phase. Some useful relations between $Ω/Ω_{\rm crit}$, $\upsilon/\upsilon_{\rm crit}$ ($\upsilon$ is the velocity at the equator), the moments of inertia, the angular momenta, the kinetic energy in the rotation and various other basic physical quantities are obtained.

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arXiv2005

Effects of Rotation on Mass Loss for Population III stars

The effects of rotation on low-metallicity stellar models are twofold: first, the models reach break-up during main sequence and may lose mass by mechanical process; second, strong internal mixing brings freshly synthesized elements towards the surface and raises the effective metallicity to higher values, so that the initially very low radiative winds are enhanced. Those two effects are also found in Z=0 models, but to a lesser degree because of structural differences. This weak mass loss becomes interesting though in the case of the black hole-doomed stars (25-140 M_\sun and > 260 M_\sun) because it allows these stars to contribute to the enrichment of the medium by stellar winds.

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arXiv2005

Stellar evolution with rotation and magnetic fields:III: The interplay of circulation and dynamo

We examine the effects of the magnetic field created by the Tayler--Spruit dynamo in differentially rotating stars. Magnetic fields of the order of a few $10^4$ G are present through most of the stellar envelope, with the exception of the outer layers. The diffusion coefficient for the transport of angular momentum is very large and it imposes nearly solid body rotation during the MS phase. In turn, solid body rotation drives meridional circulation currents which are much faster than usual and leads to much larger diffusion coefficients than the magnetic diffusivity for the chemical species. The consequence is that the interplay of the thermal and magnetic instabilities favours the chemical transport of elements, while there would be no transport in models with magnetic field only. We also discuss the effects on the stellar interior, lifetimes and HR diagram.

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arXiv2004

Chemical Abundances and Yields from Massive Stars

Stellar rotation produces an internal mixing of the elements due to shear instability and meridional circulation. This leads to observable $N/C$ enhancements in massive stars above about 7--9 $M_{\odot}$. Rotation also favours mass loss by stellar winds. Mass loss effects dominate for masses above 30 $M_{\odot}$, while mixing dominates below that limit. The effects of mixing are also much larger at lower metallicity $Z$, because the internal $Ω$--gradients are steeper. This appears to be in agreement with observations in the SMC. At very low $Z$ and Z=0, mixing between the He--burning core and the H--burning shell leads to the production of primary N in intermediate mass stars. Such enrichments increase the metallicity of the rotating star, also massive Z=0 stars with moderate initial velocities currently reach break--up velocity during a fraction of the MS phase. Both effects favour mass loss in Z=0 stars, which have ejecta with abundance anomalies very similar to those of C--rich very metal poor stars.

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arXiv2004

Effects of Rotation on Presupernovae Models

We show that rotation strongly affects the nature of the supernova progenitor (blue/red supergiant or Wolf--Rayet star), and thus the supernova types. In particular our models well reproduce the variations of the number ratio SNIb/Ic to SNII with metallicity. Rotation also produces envelope enrichments of the N/C ratio, and increases the size of the CO cores. We show the evolution of the specific angular momentum up to the preSN stage and make comparison with neutron stars. We suggest that the rare WO stars, preferentially formed at low metallicity, are the progenitors of GRB.

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arXiv2004

Evolution of the Most Massive Stars

We discuss the physics of the $ΩΓ$-- Limit, i.e. when the star is unbound as a result of {\emph{both}} rotation and radiation pressure. We suggest that the $ΩΓ$-- Limit is what makes the Humphreys--Davidson Limit. Stellar filiations are discussed, with an emphasis on the final stages, in particular on the final masses, their angular momentum and the chemical yields with account of rotation. A possible relation between WO stars and GRB is emphasized.

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arXiv2004

Evolution at very low and zero Z

Rotation deeply affects the evolution of very metal poor massive stars. Indeed, even moderately rotating stars reach the break--up limit during the Main--Sequence (MS) phase, they evolve rapidly to the red after the core H--burning phase and important surface enrichment in CNO elements occurs at the supergiant stage. High rotation and the self enrichment in heavy elements $Z$ tend to enhance the quantity of mass lost by stellar winds. As a numerical example, we obtain that a 60 M$_\odot$ at $Z=0.00001$, with an initial rotation corresponding to 0.7 of the critical velocity, loses half of its initial mass during its lifetime. The ejected material, enriched in helium and CNO elements, has interesting consequences for the early chemical evolution of galaxies.

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