paxton at kitp.ucsb.edu
Thu Dec 4 15:34:04 EST 2008
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
> 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
The problem at the moment is the breakdown of the convergence of the
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
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