mesa-users] Break-evens

[Bill Paxton]

Hi Marcin,

On Dec 4, 2008, at 11:57 AM, Marcin @ astrophysics wrote:

> I'm trying to reach Helium break-even point for stars with masses  
> around 1 Msun. None of my runs reached it. The lg_dt_yr value is  
> getting smaller and smaller and I wonder how much time would be  
> needed to reach Helium flash.

One of the wonderful aspects of EZ was how fast it could do evolution  
up the RGB.  EZ was based on Eggleton's code, and that was designed  
specifically for thin shell burning up the RGB.  But I found that I  
couldn't generalize it to let me do other stuff reliably.  So I've  
abandoned the EZ methods.

So mesa/star is slow up the RGB as it follows the shell burning in  
detail.  However, I have been able to reach the helium core flash and  
even continue through it to reach steady helium burning.   But it  
currently requires some changes in parameter settings.  Since its  
been some time since I've tried this, let me run some tests before I  
suggest what you should do.

So, mesa/star is sometimes (much) slower than EZ, but there are many  
interesting things that mesa/star can do that I couldn't get EZ to do  
at all.


> About 10 Msun stars cross Helium and Carbon break-evens.

Great.   I get that too.


> And if we're talking about break-evens. What are the capabilities  
> of mesa in this area?  Maybe it is possible to reach the point near  
> to supernova expolsion?

The eos at high T is based on Frank Timmes' code and should be fine  
for advanced burning.
The other parts of the mesa microphysics should also be capable of  
supporting this.

The problem at the moment is the breakdown of the convergence of the  
structure equations.
I'm currently working on a new approach to that problem, but it's too  
early to tell if it will work.

In the meantime, I think you should be able to get stars of up to  
about 15Msun to go through carbon burning.

And stars over about 2.5Msun should be fine to go through helium  
burning.  And they should actually show helium shell flashes on the AGB.


Here are some nets to try for helium and carbon burning:

During helium burning, the nitrogen (n14) gets converted to o18 and  
then to ne22.  That's worth following by itself for stars that will  
not reach carbon burning.  For those, I use
	new_net_num = 65 ! Basic + o18 + ne22

The advanced burning stages after c12 are o16, ne20, mg24, si28, s32,  
etc.  For carbon burning stars, I use
	new_net_num = 83 ! Basic + o18 + ne22 + alpha_mg24_to_s32


Cheers,
Bill

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