2,751 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 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
rotating with an initial velocity of 300 kms.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
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 -- or the --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 X: Wolf-Rayet star populations at solar metallicity
We examine the properties of Wolf--Rayet (WR) stars predicted by models of
rotating stars taking account of the new mass loss rates for O--type stars and
WR stars (Vink et al. \cite{Vink00}, \cite{Vink01}; Nugis & Lamers
\cite{NuLa00}) and of the wind anisotropies induced by rotation. We find that
the rotation velocities of WR stars are modest, i.e. about 50 km s,
not very dependant on the initial and masses. For the most massive stars,
the evolution of is very strongly influenced by the values of the mass loss
rates; below 12 M the evolution of rotation during the MS phase
and later phases is dominated by the internal coupling. Massive stars with
extreme rotation may skip the LBV phase.
Models having a typical for the O--type stars have WR lifetimes on the
average two times longer than for non--rotating models. The increase of the WR
lifetimes is mainly due to that of the H--rich eWNL phase. Rotation allows a
transition WN/WC phase to be present for initial masses lower than 60
M. The durations of the other WR subphases are less affected by
rotation. The mass threshold for forming WR stars is lowered from 37 to 22
M for typical rotation. The comparisons of the predicted number ratios
WR/O, WN/WC and of the number of transition WN/WC stars show very good
agreement with models with rotation, while this is not the case for models with
the present--day mass loss rates and no rotation. As to the chemical abundances
in WR stars, rotation brings only very small changes for WN stars, since they
have equilibrium CNO values. However, WC stars with rotation have on average
lower C/He and O/He ratios. The luminosity distribution of WC stars is also
influenced by rotation.Comment: 17 pages, 20 figures, accepted for publication in A&
Physics of rotation in stellar models
In these lecture notes, we present the equations presently used in stellar
interior models in order to compute the effects of axial rotation. We discuss
the hypotheses made. We suggest that the effects of rotation might play a key
role at low metallicity.Comment: 32 pages, 7 figures, lectures, CNRS school, will be published by
Springe
Stellar evolution with rotation XI: Wolf-Rayet star populations at different metallicities
Grids of models of massive stars ( 20 ) with rotation are
computed for metallicities 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
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 (
km s) 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 evolution with rotation VII: Low metallicity models and the blue to red supergiant ratio in the SMC
We calculate a grid of models with and without the effects of axial rotation
for massive stars in the range of 9 to 60 M and metallicity =
0.004 appropriate for the SMC. Remarkably, the ratios
of the angular velocity to the break-up angular
velocity grow strongly during the evolution of high mass stars, contrary to the
situation at = 0.020. The reason is that at low , mass loss is smaller
and the removal of angular momentum during evolution much weaker, also there is
an efficient outward transport of angular momentum by meridional circulation.
Thus, a much larger fraction of the stars at lower reach break-up
velocities and rotation may thus be a dominant effect at low . The models
with rotation well account for the long standing problem of the large numbers
of red supergiants observed in low galaxies, while current models with mass
loss were predicting no red supergiants. We discuss in detail the physical
effects of rotation which favour a redwards evolution in the HR diagram. The
models also predict large N enrichments during the evolution of high mass
stars. The predicted relative N-enrichments are larger at lower than solar
and this is in very good agreement with the observations for A-type supergiants
in the SMC.Comment: 18 pages, 16 figures, in press in Astronomy and Astrophysic
Can very massive stars avoid Pair-instability Supernovae?
Very massive primordial stars () are
supposed to end their lives as pair-instability supernovae. 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 did not form,
contrarily to the primordial star formation scenarios? Could they avoid this
tragical fate?
We explore the effects of rotation, anisotropic mass loss and magnetic fields
on the core size of a very massive Population III model, in order to check if
its mass is sufficiently modified to prevent the pair instability.
We obtain that a Population III model of with
computed with the inclusion of wind
anisotropy and Tayler-Spruit dynamo avoids the pair instability explosion.Comment: to be published in the conference proceedings of First Stars III,
Santa Fe, 200
GRB progenitors at low metallicities
We calculated pre-supernova evolution models of single rotating massive
stars. 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 (Z). Using
these models we found the following results concerning long and soft gamma--ray
burst (GRB) progenitors:
- GRBs coming from WO--type (SNIc) WR stars are only produced at low Z (LMC
or lower).
- The upper metallicity limit for GRBs is reduced to Z ~ 0.004 (SMC) when the
effects of magnetic fields are included.
- GRBs are predicted from the second (and probably the first) stellar
generation onwards.Comment: 5 pages, 1 figure, to appear in the proceedings of "Swift and GRBs:
Unveiling the Relativistic Universe", San Servolo, Venice, 5-9 June 200
Research Note: Rotation and the wind momentum-luminosity relation for extragalactic distances
The effects of axial stellar rotation on the wind-momentum relation (WLR) for
determining the extragalactic distances are investigated. Despite the fact that
the mass loss rates grow quite a lot with rotation, remarkably the effects on
the WLR are found to be very small on the average. As an example, for an
average orientation angle between the rotation axis and the line of sight, the
luminosity would be overestimated by 5.9 % for a star rotating at 90% of its
break-up rotational velocity. Different orientation angles between the rotation
axis and the line of sight produce some limited scatter.Comment: 4 pages, 1 figure, in press in A&
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