[mesa-users] crazy C shell burning -- not crazy, just off-center neon burning

[Bill Paxton]

Hi Tug,

Looks like it isn't a mesa version issue after all.  Or least that's not the important thing.
With my current version, it hits your T center stopping criterion before going into Ne burning
whereas the older version does not.   When I let the current version keep going,
it shows the same "crazy" behavior as the older version.   

However --- it is "crazy" only in that the alpha=1 case ignited Ne off center
while for alpha=2 it ignites at the center.   THAT MAKES ALL THE DIFFERENCE.

What happens it is very similar to the off-center ignition for 1Msun at tip of RGB. 
The center cools and expands adiabatically, and that's the "crazy" T-Rho line you see.  
You get the same adiabatic cooling and expanding in the center T-Rho plot for 
the helium core flash -- here's figure 14 from the 1st mesa paper.

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For the He flash case, the burning moves to the center in not-too-many timesteps,
and then there is an almost constant pressure rise in center T making the 
distinctive "V" in the plot.  Your 15 M with alpha=1 is suffering from the 
same problem that afflicts the 12M alpha=2 case -- it is very computationally 
painful to get the neon burning to crawl to the center.  So you understandably
gave up before getting to center burning and the constant P rise in T.

Were you watching the pgplots during the run?  Did you notice the off center ignition?
I've attached movies showing it.   For the 15M alpha=2 case, you'll see in model 2715
that the neon burning starts at the center.   For the alpha=1 case, model 2515
shows the neon ignition starting at around 0.2M.

Why off-center for alpha=1? --- look at the sizes of the C/O cores for alpha=1 vs alpha=2.
The alpha=1 C/O is quite a bit smaller, more like you'd get for a lower mass with alpha=2.
So it behaves more like a 12M with alpha=2, and that's VERY different than a 15M with alpha=2.

So when you changed alpha from 2 to 1, you moved into a very different
burning scenario --- from nice center neon ignition to the nasty-nasty off-center case.

At least that's what I think is going on.  What do you think?

-Bill




alpha = 2
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alpha = 1
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