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

Tuguldur Sukhbold sukhbold at ucolick.org
Fri Apr 19 02:27:10 EDT 2013

Dear Bill,

many thanks for these! 

> 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.   

So in the new version it is going 'crazy' after my stopping condition, while in the older version it is going 'crazy' before?

At that central T condition for this model - 1.2GK - I don't think we should expect Ne flashes to take place. The balanced power condition gives Ne burning temperature at around 1.5GK.

I don't know if you have seen my older email with convection plots (attached again), where what you described was seen. In the alpha=1 case you see at around model#3600, O is ignited off-center.

I agree that the difference is the off-center burning, essentially. But there might be other subtle things (i am not 100$ sure). With KEPLER I have definitely seen similar stuff with smaller progenitors (as they have more degenerate cores), see the attached section of a plot for 12Msun alpha=1 model. There O also burns off-center, but the burning front migrates inward before the convection ceases. If you have inefficient semi convection, then the He-core will be bifurcated resulting in smaller CO core and thus it will also promote off-center O burning. 

Anyways, I think you set me up on the right track. I am running alpha=2 model a bit further now.


(below) 12Msun alpha_mlt=1 model:

> 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.
> <hecoreflash.tiff>
> 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
> <15M_alpha2.mov>
> alpha = 1
> <15M_alpha1.mov>
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