148 research outputs found
Stellar rotation: Evidence for a large horizontal turbulence and its effects on evolution
We derive a new expression for the coefficient 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 --currents, as well as the driving of circulation, are
reduced by the large horizontal turbulence. Stellar evolutionary models for a
20 M star are calculated with the new coefficient. The new and large
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
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 Centauri
The blue Main Sequence (bMS) of Cen implies a ratio of helium to
metal enrichment , 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 ratio in 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 Cen. It is still uncertain whether the abundance
peculiarities of 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 , due to the formation of black holes.
Some abundance and isotopic ratios like , , ,
, and 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
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