7,891 research outputs found

### Stellar evolution with rotation and magnetic fields:I. The relative importance of rotational and magnetic effects

We compare the current effects of rotation in stellar evolution to those of
the magnetic field created by the Tayler instability. In stellar regions, where
magnetic field can be generated by the dynamo due to differential rotation
(Spruit 2002), we find that the growth rate of the magnetic instability is much
faster than for the thermal instability. Thus, meridional circulation is
negligible with respect to the magnetic fields, both for the transport of
angular momentum and of chemical elements. Also, the horizontal coupling by the
magnetic field, which reaches values of a few $10^5$ G, is much more important
than the effects of the horizontal turbulence. The field, however, is not
sufficient to distort the shape of the equipotentials. We impose the condition
that the energy of the magnetic field created by the Tayler--Spruit dynamo
cannot be larger than the energy excess present in the differential rotation.
This leads to a criterion for the existence of the magnetic field in stellar
interiors. Numerical tests are made in a rotating star model of 15 M$_{\odot}$
rotating with an initial velocity of 300 km$\cdot$s$^{-1}$.Comment: Accepted for Astronomy and Astrophysics, 11 pages, 8 figure

### 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.Comment: 4 pages, 3 figures, accepted by Astronomy and Astrophysic

### 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.Comment: 11 pages, 7 figures, to appear in the proceedings of IAUS 272 "Active
OB stars: structure, evolution, mass loss and critical limits

### Stellar evolution with rotation and magnetic fields II: General equations for the transport by Tayler--Spruit dynamo

We further develop the Tayler--Spruit dynamo theory, based on the most
efficient instability for generating magnetic fields in radiative layers of
differentially rotating stars. We avoid the simplifying assumptions that either
the $\mu$-- or the $T$--gradient dominates, but we treat the general case and
we also account for the nonadiabatic effects, which favour the growth of the
magnetic field. Stars with a magnetic field rotate almost as a solid body.
Several of their properties (size of the core, MS lifetimes, tracks,
abundances) are closer to those of models without rotation than with rotation
only. In particular, the observed N/C or N/H excesses in OB stars are better
explained by our previous models with rotation only than by the present models
with magnetic fields that predict no nitrogen excesses. We show that there is a
complex feedback loop between the magnetic instability and the thermal
instability driving meridional circulation. This opens the possibility for
further magnetic models, but at this stage we do not know the relative
importance of the magnetic fields due to the Tayler instability in stellar
interiors.Comment: 14 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic

### Stellar evolution with rotation XI: Wolf-Rayet star populations at different metallicities

Grids of models of massive stars ($M \ge$ 20 $M_\odot$) with rotation are
computed for metallicities $Z$ ranging from that of the Small Magellanic Cloud
(SMC) to that of the Galactic Centre. The hydrostatic effects of rotation, the
rotational mixing and the enhancements of the mass loss rates by rotation are
included. The evolution of the surface rotational velocities of the most
massive O--stars mainly depends on the mass loss rates and thus on the initial
$Z$ value. The minimum initial mass for a star for entering the Wolf--Rayet
(WR) phase is lowered by rotation. For all metallicities, rotating stars enter
the WR phase at an earlier stage of evolution and the WR lifetimes are
increased, mainly as a result of the increased duration of the eWNL phase.
Models of WR stars predict in general rather low rotation velocities ($< 50$
km s$^{-1}$) with a few possible exceptions, particularly at metallicities
lower than solar where WR star models have in general faster rotation and more
chance to reach the break--up limit.The properties of the WR populations as
predicted by the rotating models are in general in much better agreement with
the observations in nearby galaxies. The observed variation with metallicity of
the fractions of type Ib/Ic supernovae with respect to type II supernovae as
found by Prantzos & Boissier (\cite{Pr03}) is very well reproduced by the
rotating models, while non--rotating models predict much too low ratios.Comment: 20 pages, 16 figure, Astronomy and Astrophysics, in pres

### 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&

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