Stellar rotation: Evidence for a large horizontal turbulence and its effects on evolution

We derive a new expression for the coefficient $D_{\mathrm{h}}$ of diffusion by horizontal turbulence in rotating stars. This new estimate can be up to two orders of magnitude larger than given by a previous expression. As a consequence the differential rotation on an equipotential is found to be very small, which reinforces Zahn's hypothesis of shellular rotation. The role of the so--called $\mu$--currents, as well as the driving of circulation, are reduced by the large horizontal turbulence. Stellar evolutionary models for a 20 M${\odot}$ star are calculated with the new coefficient. The new and large $D_{\mathrm{h}}$ tends to limit the size of the convective core and at the same time it largely favours the diffusion of helium and nitrogen to the surface of rotating OB stars, a feature supported by recent observations.Comment: 8 pages, 5 figures, accepted for publication in A&

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.Comment: 11 pages, 10 figures, accepted by Astronomy & Astrophysic

On the Origin of the High Helium Sequence in ω\omega Centauri

The blue Main Sequence (bMS) of $\omega$ Cen implies a ratio of helium to metal enrichment $\Delta Y/\Delta Z \approx 70$, 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 $\Delta Y/\Delta Z$ ratio in $\omega$ 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 $\omega$ Cen. It is still uncertain whether the abundance peculiarities of $\omega$ 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.Comment: 5 pages, 3 figures, accepted for publication in A&

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.Comment: 6 pages, 4 figures, accepted for publication in A&

Concluding Remarks

Highlights of this outstanding meeting are emphasized, as well as important open questions for future researc

DICUSSION C. The stellar yields in He-3, He-4 and Li-7: main sources, observational constraints, and problems

I am pleased to recall that the first determination of the Li-abundance in the Sun was made at the Geneva Observatory in 1975 by Edith Müller, Eric Peytremann and Ramiro de la Reza on the basis of spectra taken at Kitt Peak. The first Be determination was also made in Geneva the same year by Y. Chmielewski, J. Brault (Kitt Peak National Observatory) and E. Müller. These two outstanding works opened the door for all further investigations on these light element