1,019 research outputs found
Gyroscopic pumping of large-scale flows in stellar interiors, and application to Lithium Dip stars
The maintenance of large-scale differential rotation in stellar convective
regions by rotationally influenced convective stresses also drives large-scale
meridional flows by angular--momentum conservation. This process is an example
of ``gyroscopic pumping'', and has recently been studied in detail in the solar
context. An important question concerns the extent to which these
gyroscopically pumped meridional flows penetrate into nearby stably stratified
(radiative) regions, since they could potentially be an important source of
non-local mixing. Here we present an extensive study of the gyroscopic pumping
mechanism, using a combination of analytical calculations and numerical
simulations both in Cartesian geometry and in spherical geometry. The various
methods, when compared with one another, provide physical insight into the
process itself, as well as increasingly sophisticated means of estimating the
gyroscopic pumping rate. As an example of application, we investigate the
effects of this large-scale mixing process on the surface abundances of the
light elements Li and Be for stars in the mass range 1.3-1.5 solar masses
(so-called ``Li-dip stars''). We find that gyroscopic pumping is a very
efficient mechanism for circulating material between the surface and the deep
interior, so much in fact that it over-estimates Li and Be depletion by orders
of magnitude for stars on the hot side of the dip.However, when the diffusion
of chemical species back into the surface convection zone is taken into
account, a good fit with observed surface abundances of Li and Be as a function
of stellar mass in the Hyades cluster can be found for reasonable choices of
model parameters.Comment: Submitted to Ap
On the Penetration of Meridional Circulation below the Solar Convection Zone II: Models with Convection Zone, the Taylor-Proudman constraint and Applications to Other Stars
The solar convection zone exhibits a strong level of differential rotation,
whereby the rotation period of the polar regions is about 25-30% longer than
the equatorial regions. The Coriolis force associated with these zonal flows
perpetually "pumps" the convection zone fluid, and maintains a quasi-steady
circulation, poleward near the surface. What is the influence of this
meridional circulation on the underlying radiative zone, and in particular,
does it provide a significant source of mixing between the two regions? In
Paper I, we began to study this question by assuming a fixed meridional flow
pattern in the convection zone and calculating its penetration depth into the
radiative zone. We found that the amount of mixing caused depends very
sensitively on the assumed flow structure near the radiative--convective
interface. We continue this study here by including a simple model for the
convection zone "pump", and calculating in a self-consistent manner the
meridional flows generated in the whole Sun. We find that the global
circulation timescale depends in a crucial way on two factors: the overall
stratification of the radiative zone as measured by the Rossby number times the
square root of the Prandtl number, and, for weakly stratified systems, the
presence or absence of stresses within the radiative zone capable of breaking
the Taylor-Proudman constraint. We conclude by discussing the consequences of
our findings for the solar interior and argue that a potentially important
mechanism for mixing in Main Sequence stars has so far been neglected.Comment: 42 pages, 13 figures. Submitted to Ap
Double-Diffusive Convection
Much progress has recently been made in understanding and quantifying
vertical mixing induced by double-diffusive instabilities such as fingering
convection (usually called thermohaline convection) and oscillatory
double-diffusive convection (a process closely related to semiconvection). This
was prompted in parts by advances in supercomputing, which allow us to run
Direct Numerical Simulations of these processes at parameter values approaching
those relevant in stellar interiors, and in parts by recent theoretical
developments in oceanography where such instabilities also occur. In this paper
I summarize these recent findings, and propose new mixing parametrizations for
both processes that can easily be implemented in stellar evolution codes.Comment: To be published in the proceedings of the conference "New Advances in
Stellar Physics: from microscopic to macroscopic processes", Roscoff, 27-31st
May 201
On rotationally driven meridional flows in stars
A quasi-steady state model of the consequences of rotation on the
hydrodynamical structure of a stellar radiative zone is derived, by studying in
particular the role of centrifugal and baroclinic driving of meridional motions
in angular-momentum transport. This nonlinear problem is solved numerically
assuming axisymmetry of the system, and within some limits, it is shown that
there exist simple analytical solutions. The limit of slow rotation recovers
Eddington-Sweet theory, whereas it is shown that in the limit of rapid
rotation, the system settles into a geostrophic equilibrium. The behaviour of
the system is found to be controlled by one parameter only, linked to the
Prantl number, the stratification and the rotation rate of the star.Comment: 5 pages, submitted to MNRAS Letter
2D or not 2D: the effect of dimensionality on the dynamics of fingering convection at low Prandtl number
Fingering convection (otherwise known as thermohaline convection) is an
instability that occurs in stellar radiative interiors in the presence of
unstable compositional gradients. Numerical simulations have been used in order
to estimate the efficiency of mixing induced by this instability. However,
fully three-dimensional (3D) computations in the parameter regime appropriate
for stellar astrophysics (i.e. low Prandtl number) are prohibitively expensive.
This raises the question of whether two-dimensional (2D) simulations could be
used instead to achieve the same goals. In this work, we address this issue by
comparing the outcome of 2D and 3D simulations of fingering convection at low
Prandtl number. We find that 2D simulations are never appropriate. However, we
also find that the required 3D computational domain does not have to be very
wide: the third dimension need only contain a minimum of two wavelengths of the
fastest-growing linearly unstable mode to capture the essentially 3D dynamics
of small-scale fingering. Narrow domains, however, should still be used with
caution since they could limit the subsequent development of any large-scale
dynamics typically associated with fingering convection.Comment: Submitted to Ap
Turbulent transport by diffusive stratified shear flows: from local to global models. III. A closure model
Being able to account for the missing mixing in stellar radiative zones is a
key step toward a better understanding of stellar evolution. Zahn (1974) argued
that thermally diffusive shear-induced turbulence might be responsible for some
of this mixing. In Part I and Part II of this series of papers we showed that
Zahn's (1974, 1992) mixing model applies when the properties of the turbulence
are local. But we also discovered limitations of the model when this locality
condition fails, in particular near the edge of a turbulent region. In this
paper, we propose a second-order closure model for the transport of momentum
and chemical species by shear-induced turbulence in strongly stratified,
thermally diffusive environments (the so-called low P\'eclet number limit),
which builds upon the work of Garaud \& Ogilvie (2005). Comparison against
direct numerical simulations (DNSs) shows that the model is able to predict the
vertical profiles of the mean flow and of the stress tensor (including the
momentum transport) in diffusive shear flows, often with a reasonably good
precision, and at least within a factor of order unity in the worst case
scenario. The model is sufficiently simple to be implemented in stellar
evolution codes, and all the model constants have been calibrated against DNSs.
While significant limitations to its use remain (e.g. it can only be used in
the low P\'eclet number, slowly rotating limit), we argue that it is more
reliable than most of the astrophysical prescriptions that are used in stellar
evolution models today
Turbulent transport in a strongly stratified forced shear layer with thermal diffusion
This work presents numerical results on the transport of heat and chemical
species by shear-induced turbulence in strongly stratified but thermally
diffusive environments. The shear instabilities driven in this regime are
sometimes called "secular" shear instabilities, and can take place even when
the gradient Richardson number of the flow (the square of the ratio of the
buoyancy frequency to the shearing rate) is large, provided the P\'eclet number
(the ratio of the thermal diffusion timescale to the turnover timescale of the
turbulent eddies) is small. We have identified a set of simple criteria to
determine whether these instabilities can take place or not. Generally
speaking, we find that they may be relevant whenever the thermal diffusivity of
the fluid is very large (typically larger than cm/s), which is the
case in the outer layers of high-mass stars () for instance.
Using a simple model setup in which the shear is forced by a spatially
sinusoidal, constant-amplitude body-force, we have identified several regimes
ranging from effectively unstratified to very strongly stratified, each with
its own set of dynamical properties. Unless the system is in one of the two
extreme regimes (effectively unstratified or completely stable), however, we
find that (1) only about 10% of the input power is used towards heat transport,
while the remaining 90% is viscously dissipated; (2) that the effective
compositional mixing coefficient is well-approximated by the model of Zahn
(1992), with where is the thermal
diffusivity and is the gradient Richardson number. These results need to be
confirmed, however, with simulations in different model setups and at higher
effective Reynolds number.Comment: Submitted to Ap
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