294 research outputs found
Convectiveâreactive nucleosynthesis of K, Sc, Cl and p-process isotopes in OâC shell mergers
© 2017 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. We address the deficiency of odd-Z elements P, Cl, K and Sc in Galactic chemical evolution models through an investigation of the nucleosynthesis of interacting convective O and C shells in massive stars. 3D hydrodynamic simulations of O-shell convection with moderate C-ingestion rates show no dramatic deviation from spherical symmetry. We derive a spherically averaged diffusion coefficient for 1D nucleosynthesis simulations, which show that such convective-reactive ingestion events can be a production site for P, Cl, K and Sc. An entrainment rate of 10-3Mâs-1features overproduction factors OPsâ 7. Full O-C shell mergers in our 1D stellar evolution massive star models have overproduction factors OPm> 1 dex but for such cases 3D hydrodynamic simulations suggest deviations from spherical symmetry. Îł - process species can be produced with overproduction factors of OPm> 1 dex, for example, for130, 132Ba. Using the uncertain prediction of the 15Mâ, Z = 0.02 massive star model (OPmâ 15) as representative for merger or entrainment convective-reactive events involving O- and C-burning shells, and assume that such events occur in more than 50 per cent of all stars, our chemical evolution models reproduce the observed Galactic trends of the odd-Z elements
The Role of Organizational Capacity in Student-Athlete Development
In-depth interviews were conducted with the life skills coordinators of 9 of 21 institutions identified as being âdedicatedâ to service (Andrassy & Bruening, 2011). As a result of service being one portion of CHAMPS/Life Skills programming, we expanded our investigation to include all aspects of this student development program. In particular, we focused our inquiry on organizational capacity and its role in student involvement. Findings indicate these âdedicatedâ athletic departments were characterized by strong organizational capacity for engaging student-athletes in meaningful service efforts. The critical role of coaches and mutual values among internal stakeholders emerged as the primary strengths of departmentâs human resources capacity. Despite the limited financial capacity, departments were able to creatively secure some funding for development programs. The ability to leverage external relationships, an organizational culture promoting participative decision-making and student-athlete development, and on-going efforts to improve service and life skills opportunities for student-athletes indicated strong structural capacity
Turbulent dynamo action and its effects on the mixing at the convective boundary of an idealized oxygen-burning shell
Convection is one of the most important mixing processes in stellar
interiors. Hydrodynamic mass entrainment can bring fresh fuel from neighboring
stable layers into a convection zone, modifying the structure and evolution of
the star. Under some conditions, strong magnetic fields can be sustained by the
action of a turbulent dynamo, adding another layer of complexity and possibly
altering the dynamics in the convection zone and at its boundaries. In this
study, we used our fully compressible Seven-League Hydro code to run detailed
and highly resolved three-dimensional magnetohydrodynamic simulations of
turbulent convection, dynamo amplification, and convective boundary mixing in a
simplified setup whose stratification is similar to that of an oxygen-burning
shell in a star with an initial mass of . We find that the random
stretching of magnetic field lines by fluid motions in the inertial range of
the turbulent spectrum (i.e., a small-scale dynamo) naturally amplifies the
seed field by several orders of magnitude in a few convective turnover
timescales. During the subsequent saturated regime, the magnetic-to-kinetic
energy ratio inside the convective shell reaches values as high as , and
the average magnetic field strength is . Such strong
fields efficiently suppress shear instabilities, which feed the turbulent
cascade of kinetic energy, on a wide range of spatial scales. The resulting
convective flows are characterized by thread-like structures that extend over a
large fraction of the convective shell. The reduced flow speeds and the
presence of magnetic fields with strengths up to of the equipartition
value at the upper convective boundary diminish the rate of mass entrainment
from the stable layer by as compared to the purely
hydrodynamic case
Towards a self-consistent model of the convective core boundary in upper-main-sequence stars
There is strong observational evidence that convective cores of
intermediate-mass and massive main-sequence stars are substantially larger than
standard stellar-evolution models predict. However, it is unclear what physical
processes cause this phenomenon or how to predict the extent and stratification
of stellar convective boundary layers. Convective penetration is a
thermal-time-scale process that is likely to be particularly relevant during
the slow evolution on the main sequence. We use our low-Mach-number
Seven-League Hydro (SLH) code to study this process in 2.5D and 3D geometries.
Starting with a chemically homogeneous model of a M zero-age
main-sequence star, we construct a series of simulations with the luminosity
increased and opacity decreased by the same factor ranging from to
. After reaching thermal equilibrium, all of our models show a clear
penetration layer. Its thickness becomes statistically constant in time and it
is shown to converge upon grid refinement. As the luminosity is decreased, the
penetration layer becomes nearly adiabatic with a steep transition to a
radiative stratification. This structure corresponds to the adiabatic ,,step
overshoot'' model often employed in stellar-evolution calculations. The
thickness of the penetration layer slowly decreases with decreasing luminosity.
Depending on how we extrapolate our 3D data to the actual luminosity of the
initial stellar model, we obtain penetration distances ranging from to
pressure scale heights, which are broadly compatible with observations.Comment: 10 pages, 12 figures, submitted to A&
Fully compressible simulations of waves and core convection in main-sequence stars
Context. Recent, nonlinear simulations of wave generation and propagation in
full-star models have been carried out in the anelastic approximation using
spectral methods. Although it makes long time steps possible, this approach
excludes the physics of sound waves completely and rather high artificial
viscosity and thermal diffusivity are needed for numerical stability. Direct
comparison with observations is thus limited. Aims. We explore the capabilities
of our compressible multidimensional hydrodynamics code SLH to simulate stellar
oscillations. Methods. We compare some fundamental properties of internal
gravity and pressure waves in 2D SLH simulations to linear wave theory using
two test cases: (1) an interval gravity wave packet in the Boussinesq limit and
(2) a realistic stellar model with a convective core and a
radiative envelope. Oscillation properties of the stellar model are also
discussed in the context of observations. Results. Our tests show that
specialized low-Mach techniques are necessary when simulating oscillations in
stellar interiors. Basic properties of internal gravity and pressure waves in
our simulations are in good agreement with linear wave theory. As compared to
anelastic simulations of the same stellar model, we can follow internal gravity
waves of much lower frequencies. The temporal frequency spectra of velocity and
temperature are flat and compatible with observed spectra of massive stars.
Conclusion. The low-Mach compressible approach to hydrodynamical simulations of
stellar oscillations is promising. Our simulations are less dissipative and
require less luminosity boosting than comparable spectral simulations. The
fully-compressible approach allows the coupling of gravity and pressure waves
to be studied too.Comment: Accepted for publication in A&
A finite-volume scheme for modeling compressible magnetohydrodynamic flows at low Mach numbers in stellar interiors
Fully compressible magnetohydrodynamic (MHD) simulations are a fundamental
tool for investigating the role of dynamo amplification in the generation of
magnetic fields in deep convective layers of stars. The flows that arise in
such environments are characterized by low (sonic) Mach numbers (M_son < 0.01
). In these regimes, conventional MHD codes typically show excessive
dissipation and tend to be inefficient as the Courant-Friedrichs-Lewy (CFL)
constraint on the time step becomes too strict. In this work we present a new
method for efficiently simulating MHD flows at low Mach numbers in a
space-dependent gravitational potential while still retaining all effects of
compressibility. The proposed scheme is implemented in the finite-volume
Seven-League Hydro (SLH) code, and it makes use of a low-Mach version of the
five-wave Harten-Lax-van Leer discontinuities (HLLD) solver to reduce numerical
dissipation, an implicit-explicit time discretization technique based on Strang
splitting to overcome the overly strict CFL constraint, and a well-balancing
method that dramatically reduces the magnitude of spatial discretization errors
in strongly stratified setups. The solenoidal constraint on the magnetic field
is enforced by using a constrained transport method on a staggered grid. We
carry out five verification tests, including the simulation of a small-scale
dynamo in a star-like environment at M_son ~ 0.001 . We demonstrate that the
proposed scheme can be used to accurately simulate compressible MHD flows in
regimes of low Mach numbers and strongly stratified setups even with moderately
coarse grids
Well-balanced treatment of gravity in astrophysical fluid dynamics simulations at low Mach numbers
Accurate simulations of flows in stellar interiors are crucial to improving
our understanding of stellar structure and evolution. Because the typically
slow flows are merely tiny perturbations on top of a close balance between
gravity and the pressure gradient, such simulations place heavy demands on
numerical hydrodynamics schemes. We demonstrate how discretization errors on
grids of reasonable size can lead to spurious flows orders of magnitude faster
than the physical flow. Well-balanced numerical schemes can deal with this
problem. Three such schemes were applied in the implicit, finite-volume
Seven-League Hydro (SLH) code in combination with a low-Mach-number numerical
flux function. We compare how the schemes perform in four numerical experiments
addressing some of the challenges imposed by typical problems in stellar
hydrodynamics. We find that the - and deviation well-balancing
methods can accurately maintain hydrostatic solutions provided that
gravitational potential energy is included in the total energy balance. They
accurately conserve minuscule entropy fluctuations advected in an isentropic
stratification, which enables the methods to reproduce the expected scaling of
convective flow speed with the heating rate. The deviation method also
substantially increases accuracy of maintaining stationary orbital motions in a
Keplerian disk on long timescales. The Cargo-LeRoux method fares substantially
worse in our tests, although its simplicity may still offer some merits in
certain situations. Overall, we find the well-balanced treatment of gravity in
combination with low Mach number flux functions essential to reproducing
correct physical solutions to challenging stellar slow-flow problems on
affordable collocated grids.Comment: Accepted for publication in A&
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