2,025 research outputs found
Internally heated and fully compressible convection: flow morphology and scaling laws
In stars and planets natural processes heat convective flows in the bulk of a
convective region rather than at hard boundaries. By characterizing how
convective dynamics are determined by the strength of an internal heating
source we can gain insight into the processes driving astrophysical convection.
Internally heated convection has been studied extensively in incompressible
fluids, but the effects of stratification and compressibility have not been
examined in detail. In this work, we study fully compressible convection driven
by a spatially uniform heating source in 2D and 3D Cartesian, hydrodynamic
simulations. We use a fixed temperature upper boundary condition which results
in a system that is internally heated in the bulk and cooled at the top. We
find that the flow speed, as measured by the Mach number, and turbulence, as
measured by the Reynolds number, can be independently controlled by separately
varying the characteristic temperature gradient from internal heating and the
diffusivities. 2D simulations at a fixed Mach number (flow speed) demonstrate
consistent power at low wavenumber as diffusivities are decreased. We observe
convection where the velocity distribution is skewed towards cold, fast
downflows, and that the flow speed is related to the length scale and entropy
gradient of the upper boundary where the downflows are driven. We additionally
find a heat transport scaling law which is consistent with prior incompressible
work.Comment: 22 pages, 12 figures, submitted to Phys. Rev. Fluid
Rotation reduces convective mixing in Jupiter and other gas giants
Recent measurements of Jupiter's gravitational moments by the Juno spacecraft
and seismology of Saturn's rings suggest that the primordial composition
gradients in the deep interior of these planets have persisted since their
formation. One possible explanation is the presence of a double-diffusive
staircase below the planet's outer convection zone, which inhibits mixing
across the deeper layers. However, hydrodynamic simulations have shown that
these staircases are not long-lasting and can be disrupted by overshooting
convection. In this paper we suggests that planetary rotation could be another
factor for the longevity of primordial composition gradients. Using rotational
mixing-length theory and 3D hydrodynamic simulations, we demonstrate that
rotation significantly reduces both the convective velocity and the mixing of
primordial composition gradients. In particular, for Jovian conditions at
after formation, rotation reduces the convective
velocity by a factor of 6, and in turn, the kinetic energy flux available for
mixing gets reduced by a factor of . This leads to an entrainment
timescale that is more than two orders of magnitude longer than without
rotation. We encourage future hydrodynamic models of Jupiter and other gas
giants to include rapid rotation, because the decrease in the mixing efficiency
could explain why Jupiter and Saturn are not fully mixed.Comment: Accepted for publication in the Astrophysical Journal Letter
Observed Extra Mixing Trends in Red Giants are Reproduced by the Reduced Density Ratio in Thermohaline Zones
Observations show an almost ubiquitous presence of extra mixing in low-mass
upper giant branch stars. The most commonly invoked explanation for this is the
thermohaline instability. One dimensional stellar evolution models include
prescriptions for thermohaline mixing, but our ability to make direct
comparisons between models and observations has thus far been limited. Here, we
propose a new framework to facilitate direct comparison: Using carbon to
nitrogen measurements from the SDSS-IV APOGEE survey as a probe of mixing and a
fluid parameter known as the reduced density ratio from one dimensional stellar
evolution programs, we compare the observed amount of extra mixing on the upper
giant branch to predicted trends from three-dimensional fluid dynamics
simulations. By applying this method, we are able to place empirical
constraints on the efficiency of mixing across a range of masses and
metallicities. We find that the observed amount of extra mixing is strongly
correlated with the reduced density ratio and that trends between reduced
density ratio and fundamental stellar parameters are robust across choices for
modeling prescription. We show that stars with available mixing data tend to
have relatively low density ratios, which should inform the regimes selected
for future simulation efforts. Finally, we show that there is increased mixing
at low values of the reduced density ratio, which is consistent with current
hydrodynamical models of the thermohaline instability. The introduction of this
framework sets a new standard for theoretical modeling efforts, as validation
for not only the amount of extra mixing, but trends between the degree of extra
mixing and fundamental stellar parameters is now possible.Comment: 19 pages, 7 figures, submitted to Ap
Stellar Convective Penetration: Parameterized Theory and Dynamical Simulations
Most stars host convection zones in which heat is transported directly by fluid motion, but the behavior of convective boundaries is not well-understood. Here, we present 3D numerical simulations that exhibit penetration zones: regions where the entire luminosity could be carried by radiation, but where the temperature gradient is approximately adiabatic and convection is present. To parameterize this effect, we define the "penetration parameter" , which compares how far the radiative gradient deviates from the adiabatic gradient on either side of the Schwarzschild convective boundary. Following Roxburgh and Zahn, we construct an energy-based theoretical model in which controls the extent of penetration. We test this theory using 3D numerical simulations that employ a simplified Boussinesq model of stellar convection. The convection is driven by internal heating, and we use a height-dependent radiative conductivity. This allows us to separately specify and the stiffness of the radiative–convective boundary. We find significant convective penetration in all simulations. Our simple theory describes the simulations well. Penetration zones can take thousands of overturn times to develop, so long simulations or accelerated evolutionary techniques are required. In stars, we expect , and in this regime, our results suggest that convection zones may extend beyond the Schwarzschild boundary by up to ∼20%–30% of a mixing length. We present a MESA stellar model of the Sun that employs our parameterization of convective penetration as a proof of concept. Finally, we discuss prospects for extending these results to more realistic stellar contexts.
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The photometric variability of massive stars due to gravity waves excited by core convection
Massive stars die in catastrophic explosions, which seed the interstellar
medium with heavy elements and produce neutron stars and black holes.
Predictions of the explosion's character and the remnant mass depend on models
of the star's evolutionary history. Models of massive star interiors can be
empirically constrained by asteroseismic observations of gravity wave
oscillations. Recent photometric observations reveal a ubiquitous red noise
signal on massive main sequence stars; a hypothesized source of this noise is
gravity waves driven by core convection. We present the first 3D simulations of
massive star convection extending from the star's center to near its surface,
with realistic stellar luminosities. Using these simulations, we make the first
prediction of photometric variability due to convectively-driven gravity waves
at the surfaces of massive stars, and find that gravity waves produce
photometric variability of a lower amplitude and lower characteristic frequency
than the observed red noise. We infer that the photometric signal of gravity
waves excited by core convection is below the noise limit of current
observations, so the red noise must be generated by an alternative process.Comment: As accepted for publication in Nature Astronomy except for final
editorial revisions. Supplemental materials available online at
https://doi.org/10.5281/zenodo.7764997 . We have also sonified our results to
make them more accessible, see
https://github.com/evanhanders/gmode_variability_paper/blob/main/sound/gmode_sonification.pd
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