2,036 research outputs found
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
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
A model of transport nonuniversality in thick-film resistors
We propose a model of transport in thick-film resistors which naturally
explains the observed nonuniversal values of the conductance exponent t
extracted in the vicinity of the percolation transition. Essential ingredients
of the model are the segregated microstructure typical of thick-film resistors
and tunneling between the conducting grains. Nonuniversality sets in as
consequence of wide distribution of interparticle tunneling distances.Comment: 3 pages, 1 figur
SPINSTARS at low metallicities
The main effect of axial rotation on the evolution of massive PopIII stars is
to trigger internal mixing processes which allow stars to produce significant
amounts of primary nitrogen 14 and carbon 13. Very metal poor massive stars
produce much more primary nitrogen than PopIII stars for a given initial mass
and rotation velocity. The very metal poor stars undergo strong mass loss
induced by rotation. One can distinguish two types of rotationnaly enhanced
stellar winds: 1) Rotationally mechanical winds occurs when the surface
velocity reaches the critical velocity at the equator, {\it i.e.} the velocity
at which the centrifugal acceleration is equal to the gravity; 2) Rotationally
radiatively line driven winds are a consequence of strong internal mixing which
brings large amounts of CNO elements at the surface. This enhances the opacity
and may trigger strong line driven winds. These effects are important for an
initial value of of 0.54 for a 60 M at
, {\it i.e.} for initial values of
higher than the one (0.4) corresponding to observations at solar .
These two effects, strong internal mixing leading to the synthesis of large
amounts of primary nitrogen and important mass losses induced by rotation,
occur for between about 10 and 0.001. For metallicities above 0.001
and for reasonable choice of the rotation velocities, internal mixing is no
longer efficient enough to trigger these effects.Comment: 5 pages, 4 figures, to be published in the conference proceedings of
First Stars III, Santa Fe, 200
Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars
(Abridged) We present a detailed analysis of the main physical processes
responsible for the transport of angular momentum and chemical species in the
radiative regions of rotating stars. We focus on cases where meridional
circulation and shear-induced turbulence only are included in the simulations.
Our analysis is based on a 2-D representation of the secular hydrodynamics,
which is treated using expansions in spherical harmonics. We present a full
reconstruction of the meridional circulation and of the associated fluctuations
of temperature and mean molecular weight along with diagnosis for the transport
of angular momentum, heat and chemicals. In the present paper these tools are
used to validate the analysis of two main sequence stellar models of 1.5 and 20
Msun for which the hydrodynamics has been previously extensively studied in the
literature. We obtain a clear visualization and a precise estimation of the
different terms entering the angular momentum and heat transport equations in
radiative zones. This enables us to corroborate the main results obtained over
the past decade by Zahn, Maeder, and collaborators concerning the secular
hydrodynamics of such objects. We focus on the meridional circulation driven by
angular momentum losses and structural readjustements. We confirm
quantitatively for the first time through detailed computations and separation
of the various components that the advection of entropy by this circulation is
very well balanced by the barotropic effects and the thermal relaxation during
most of the main sequence evolution. This enables us to derive simplifications
for the thermal relaxation on this phase. The meridional currents in turn
advect heat and generate temperature fluctuations that induce differential
rotation through thermal wind thus closing the transport loop.Comment: 16 pages, 18 figures. Accepted for publication in A&
Single star progenitors of long gamma-ray bursts I: Model grids and redshift dependent GRB rate
We present grids of massive star evolution models at four different
metallicities (Z=0.004, 0.002, 0.001, 0.00001). The effects of rotation on the
stellar structure and the transport of angular momentum and chemical elements
through the Spruit-Tayler dynamo and rotationally induced instabilities are
considered. After discussing uncertainties involved with the adopted physics,
we elaborate the final fate of massive stars as a function of initial mass and
spin rate, at each considered metallicity. In particular, we investigate for
which initial conditions long gamma-ray bursts (GRBs) are expected to be
produced in the frame of the collapsar model. Then, using an empirical spin
distribution of young massive metal-poor stars and a specified
metallicity-dependent history of star-formation, we compute the expected GRB
rate as function of metallicity and redshift based on our stellar evolution
models. The GRB production in our models is limited to metallicities of Z \lsim
0.004, with the consequence that about 50 % of all GRBs are predicted to be
found at redshifts above z = 4, with most supernovae occurring at redshifts
below z\simeq 2.2. The average GRB/SN ratio predicted by our model is about
1/200 globally, and 1/1250 at low redshift. Future strategies for testing the
considered GRB progenitor scenario are briefly discussed.Comment: 17 pages, 10 figures, 6 tables, accpeted by A&A, corrected, reference
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