73 research outputs found
Hydrodynamic Processes in Massive Stars
The hydrodynamic processes operating within stellar interiors are far richer
than represented by the best stellar evolution model available. Although it is
now widely understood, through astrophysical simulation and relevant
terrestrial experiment, that many of the basic assumptions which underlie our
treatments of stellar evolution are flawed, we lack a suitable, comprehensive
replacement. This is due to a deficiency in our fundamental understanding of
the transport and mixing properties of a turbulent, reactive, magnetized
plasma; a deficiency in knowledge which stems from the richness and variety of
solutions which characterize the inherently non-linear set of governing
equations. The exponential increase in availability of computing resources,
however, is ushering in a new era of understanding complex hydrodynamic flows;
and although this field is still in its formative stages, the sophistication
already achieved is leading to a dramatic paradigm shift in how we model
astrophysical fluid dynamics. We highlight here some recent results from a
series of multi-dimensional stellar interior calculations which are part of a
program designed to improve our one-dimensional treatment of massive star
evolution and stellar evolution in general.Comment: 10 pages, 4 figures, IAUS 252 Conference Proceeding (Sanya) - "The
Art of Modeling Stars in the 21st Century
Chaos and Turbulent Nucleosynthesis Prior to a Supernova Explosion
Three-dimensional (3D), time dependent numerical simulations, of flow of
matter in stars, now have sufficient resolution to be fully turbulent. The late
stages of the evolution of massive stars, leading up to core collapse to a
neutron star (or black hole), and often to supernova explosion and
nucleosynthesis, are strongly convective because of vigorous neutrino cooling
and nuclear heating. Unlike models based on current stellar evolutionary
practice, these simulations show a chaotic dynamics characteristic of highly
turbulent flow. Theoretical analysis of this flow, both in the
Reynolds-averaged Navier-Stokes (RANS) framework and by simple dynamic models,
show an encouraging consistency with the numerical results. It may now be
possible to develop physically realistic and robust procedures for convection
and mixing which (unlike 3D numerical simulation) may be applied throughout the
long life times of stars. In addition, a new picture of the presupernova stages
is emerging which is more dynamic and interesting (i.e., predictive of new and
newly observed phenomena) than our previous one.Comment: 11 pages, 2 figures, Submitted to AIP Advances: Stardust, added
figures and modest rewritin
Toward a consistent use of overshooting parametrizations in 1D stellar evolution codes
Several parametrizations for overshooting in 1D stellar evolution
calculations coexist in the literature. These parametrizations are used
somewhat arbitrarily in stellar evolution codes, based on what works best for a
given problem, or even for historical reasons related to the development of
each code. We bring attention to the fact that these different parametrizations
correspond to different physical regimes of overshooting, depending whether the
effects of radiation are dominant, marginal, or negligible. Our analysis is
based on previously published theoretical results, as well as multidimensional
hydrodynamical simulations of stellar convection where the interaction between
the convective region and a stably-stratified region is observed. Although the
underlying hydrodynamical processes are the same, the outcome of the
overshooting process is profoundly affected by radiative effects. Using a
simple picture of the scales involved in the overshooting process, we show how
three regimes are obtained, depending on the importance of radiative effects.
These three regimes correspond to the different behaviors observed in
hydrodynamical simulations so far, and to the three types of parametrizations
used in 1D codes. We suggest that the existing parametrizations for
overshooting should coexist in 1D stellar evolution codes, and should be
applied consistently at convective boundaries depending on the local physical
conditions.Comment: 5 pages, 2 figures, to appear in A&A as a regular paper. Last
version: language editing + typos in Eq. (6) & (9) correcte
Turbulent convection in stellar interiors. III. Mean-field analysis and stratification effects
We present 3D implicit large eddy simulations (ILES) of the turbulent
convection in the envelope of a 5 Msun red giant star and in the oxygen-burning
shell of a 23 Msun supernova progenitor. The numerical models are analyzed in
the framework of 1D Reynolds-Averaged Navier-Stokes (RANS) equations. The
effects of pressure fluctuations are more important in the red giant model,
owing to larger stratification of the convective zone. We show how this impacts
different terms in the mean-field equations. We clarify the driving sources of
kinetic energy, and show that the rate of turbulent dissipation is comparable
to the convective luminosity. Although our flows have low Mach number and are
nearly adiabatic, our analysis is general and can be applied to photospheric
convection as well. The robustness of our analysis of turbulent convection is
supported by the insensitivity of the mean-field balances to linear mesh
resolution. We find robust results for the turbulent convection zone and the
stable layers in the oxygen-burning shell model, and robust results everywhere
in the red giant model, but the mean fields are not well converged in the
narrow boundary regions (which contain steep gradients) in the oxygen-burning
shell model. This last result illustrates the importance of unresolved physics
at the convective boundary, which governs the mixing there.Comment: 26 pages, 20 figures, Accepted for publication in Ap
A new stellar mixing process operating below shell convection zones following off-center ignition
During most stages of stellar evolution the nuclear burning of lighter to
heavier elements results in a radial composition profile which is stabilizing
against buoyant acceleration, with light material residing above heavier
material. However, under some circumstances, such as off-center ignition, the
composition profile resulting from nuclear burning can be destabilizing, and
characterized by an outwardly increasing mean molecular weight. The potential
for instabilities under these circumstances, and the consequences that they may
have on stellar structural evolution, remain largely unexplored. In this paper
we study the development and evolution of instabilities associated with
unstable composition gradients in regions which are initially stable according
to linear Schwarzschild and Ledoux criteria. In particular, we explore the
mixing taking place under various conditions with multi-dimensional
hydrodynamic convection models based on stellar evolutionary calculations of
the core helium flash in a 1.25 \Msun star, the core carbon flash in a
9.3\,\Msun star, and of oxygen shell burning in a star with a mass of
23\,\Msun. The results of our simulations reveal a mixing process associated
with regions having outwardly increasing mean molecular weight that reside
below convection zones. The mixing is not due to overshooting from the
convection zone, nor is it due directly to thermohaline mixing which operates
on a timescale several orders of magnitude larger than the simulated flows.
Instead, the mixing appears to be due to the presence of a wave field induced
in the stable layers residing beneath the convection zone which enhances the
mixing rate by many orders of magnitude and allows a thermohaline type mixing
process to operate on a dynamical, rather than thermal, timescale. We discuss
our results in terms of related laboratory phenomena and associated theoretical
developments.Comment: accepted for publication in Astrophysical Journal, 9 pages, 8 figure
The First 3D Simulations of Carbon Burning in a Massive Star
We present the first detailed three-dimensional hydrodynamic implicit large
eddy simulations of turbulent convection for carbon burning. The simulations
start with an initial radial profile mapped from a carbon burning shell within
a 15 solar mass stellar evolution model. We considered 4 resolutions from 128^3
to 1024^3 zones. These simulations confirm that convective boundary mixing
(CBM) occurs via turbulent entrainment as in the case of oxygen burning. The
expansion of the boundary into the surrounding stable region and the
entrainment rate are smaller at the bottom boundary because it is stiffer than
the upper boundary. The results of this and similar studies call for improved
CBM prescriptions in 1D stellar evolution models.Comment: 5 pages, 3 figures. Published in IAUS 329 on 28/07/1
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