|
Cococubed.com
|
| Helium Shell Flash Convection |
Home
Commercial:
Artwork
Software
Teaching materials
Bicycle sag support
Free:
Family Album
Pretty astronomy pictures
Some astronomy codes
Some astronomy talks
Some research
... Fallback
... Ti44 + Ni56
... He shell flash
... SNIa ignition
... Flames with 22Ne
... P-rich NSE
... Verification
Bicycle adventures
Contact us:
J.D. Maldonado
F.X.Timmes, my vitae
We present the first hydrodynamic, multi-dimensional simulations of He-shell flash convection. Specifically, we investigate the properties of shell convection at a time immediately before the He-luminosity peak during the 15th thermal pulse of a stellar evolution track with initially two solar masses and metallicity Z=0.01. This choice is a representative example of a low-mass asymptotic giant branch thermal pulse.
We construct the initial vertical stratification with a set of polytropes to resemble the stellar evolution structure. Convection is driven by a constant volume heating in a thin layer at the bottom of the unstable layer. We calculate a grid of 2D simulations with different resolutions and heating rates. Our set of simulations includes one low-resolution 3D run. The computational domain includes 11.4 pressure scale heights. He-shell flash convection is dominated by large convective cells that are centered in the lower half of the convection zone. Convective rolls have an almost circular appearance because focusing mechanisms exist in the form of the density stratification for downdrafts and the heating of localized eddies that generate upflows. Nevertheless, downdrafts appear to be somewhat more focused.
The He-shell flash convection generates a rich spectrum of gravity waves in both stable layers above and beneath the convective shell. The magnitude of the convective velocities from our 1D mixing-length theory model and the rms-averaged vertical velocities from the hydrodynamic model are consistent within a factor of a few. However, the velocity profile in the hydrodynamic simulation is more asymmetric, and decays exponentially inside the convection zone. An analysis of the oscillation modes shows that both g-modes and convective motions cross the formal convective boundaries, which leads to mixing across the boundaries.
Our resolution study shows consistent flow structures among the higher resolution runs, and we see indications for convergence of the vertical velocity profile inside the convection zone for the highest resolution simulations. Many of the convective properties, in particular the exponential decay of the velocities, depend only weakly on the heating rate. However, the amplitudes of the gravity waves increase with both the heating rate and the resolution.
Time evolution of the radial location of the He-shell flash convection zone based on the 1D stellar evolution model. Time is set to zero at the peak of the He-burning luminosity. The ordinate is zero at the stellar center. The grey shaded area represents the He-shell flash convection zone. Dots along the boundary indicate individual time steps in the 1D evolution model sequence. The vertical solid line at t=-0.07yr indicates the position and extent of the hydrodynamic simulation box. Dashed and dotted lines correpsond to the Lagrangian coordinates given in the legend, and visualize the expansion of the He-shell as a result of the He-shell flash. |
Time evolution of the He-burning luminosity and the pressure at the boundary of the convection zone for the He-shell flash. The time axis, grey shades, and dots have the same meaning as in the figure to the left. |
Fully developed convection in a high-resolution 2D run with standard heating rate. The flow field is represented by 25x75 pseudo-streamlines, integrated over 100Mm for the constant velocity field of this single snapshot of evolved convection during the lc0gh run. The color indicates the corresponding pressure inhomogeneities. The pressure itself does not deviate much from the initial values. Therefore, to make the fluctuations visible, the horizontal average of the pressure has been subtracted from the pressure value of every grid point. Bright means over-pressure (prominently where a ``mushroom'' approaches the upper boundary of the unstable region). Dark color indicates low-pressure (in the ``eyes'' of the vortices). The boundaries of the unstable layer at y=1.7Mm, and y=7.7Mm are clearly marked in the flow field and the pressure inhomogeneities. |
Entropy inhomogeneities for the same model of the lc0gh sequence as in the figure to the left. Again, the horizontal average of the entropy has been subtracted to render the small fluctuations visible. A bright color indicates material with higher entropy (and in fact higher temperature -- due to the near pressure-equilibrium). Dark means low entropy (or temperature). The boundaries of the entropy plateau at y=1.7Mm and y=7.7Mm are again clearly visible in the change of the patterns. The subtraction of the horizontal mean causes bright features to be accompanied by dark horizontal stripes. These are pure artifacts of the visualization procedure and do not exist in the simulation data itself. |
k-omega diagrams for horizontal planes of the 3D RAGE run. Panels y = 1.62, 1.79, 4.70 and 7.45Mm are in the unstable layer. The signature of convection is a prominent blob at low wave number and frequency marked with white ellipses. g-modes can be identified best in the panels representing stable layers, and are marked with a green ellsipse in thepanel for y = 8.00Mm. p-modes are visible in the far left columns of the panels. |
Pressure fluctuations with 25kK temperature fluctuation lines, 14MB mpg |
Entropy fluctuations with bubbles that trace the fluid flows, 11MB mpg |
5MB mpg |
3 MB mpg |
9MB divx (mp4) |
|
Volume rendering of the vertical velocity in a horizontal slab with an approximate thickness of ~ 1200km (corresponding to 20% of the convectively unstable layer) including the top of the convection zone for the RAGE 300x300x200 (left) and the scPPM 512x512x256 (right) simulation at t = 1990s. The view is from top. Blue are positive (upward) and yellow are negative (downward) velocities. |