# [mesa-users] Convergence Problems

Bill Paxton paxton at kitp.ucsb.edu
Fri Aug 23 12:52:04 EDT 2013

```Hi Mathieu,

On Aug 23, 2013, at 9:25 AM, Mathieu wrote:

> I am trying to make a simulation of a 15 solar masses star at solar metallicity from pre-main sequence to the onset of core collapse (infall velocity at the iron core boundary 103 km/s). I want to use Ledoux criterion for convection with the parameters you can find in the attached inlist_project file, but I am experiencing convergence problems (in the final phases, when the iron core is already partially present).
> I have tried changing the values of the following parameters (with various combinations):

In the current release you can find an approach to this problem that works pretty well --- at least in terms of convergence.
The idea behind it is that at extreme temperatures (T > 4e9) it is okay to "soften" some of the reactions to make them easier to solve -- the softening takes the form of reducing each of a pair of forward-reverse reactions by the same factor, with the factor dropping to 1 to a low T threshhold (e.g. 2e9) and rising sharply as T approaches NSE (e.g., at 5e9).

Of course, as with all things in computational astrophysics, you'll need to decide if this numerical model provides an adequate picture of the actual physical processes for your particular project -- it seems to me that's the hard part, the computing is easy in comparison!

Here's a 16 Msun example I ran recently -- along with a movie of the abundances from start of si burn to collapse.

Cheers,
Bill

16 Msun, Z = 0.02, no rotation, using approx26 which has the following isos:
!         neut h1 he3 he4 prot
!         c12 n14 o16
!         ne20
!         mg24
!         si28
!          s32
!         ar36
!         ca40
!         ti44           ti50
!         cr48           cr54 cr56
!         fe52 fe54 fe56 fe58
!         ni56 ni58 ni60

5345

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