[Mesa-users] Limiting the final radius of a simulation

[Ebraheem Farag]

Hello Ali,

If you're trying to stop at a specific radius, there are a few &controls
<https://docs.mesastar.org/en/release-r22.11.1/reference/controls.html#photosphere-r-upper-limit>
that
can help:

stop when photosphere_r is greater than this limit, in Rsun units

photosphere_r_upper_limit = 1d99


stop when photosphere_r is less than this limit, in Rsun units

photosphere_r_lower_limit = -1d99



You can set a tolerance on your stopping condition using the &controls
<https://docs.mesastar.org/en/release-r22.11.1/reference/controls.html#when-to-stop-rtol>
 'when_to_stop_rtol':

Relative error criteria when hitting stop target time. The system will
automatically redo with a smaller timestep to hit a stopping target. It
calculates the following “error” term and retries if it is > 1.

error = abs(value - target_value)/ &        (when_to_stop_atol +
when_to_stop_rtol*max(abs(value),abs(target_value)))

when_to_stop_rtol = 1d99


in your case you could set.

when_to_stop_rtol = 1d-2 # for 0.01 precision, I use 1d-3 typically...


'delta_lgR_limit' adjusts how large of a logarithmic change in the stellar
radius is allowed in each solver timestep, so it might help the time
resolution of your models, but won't exactly improve your stopping
condition.

I hope this helps!

-EbF


On Wed, Feb 15, 2023 at 11:24 AM Ali Pourmand via Mesa-users <
mesa-users at lists.mesastar.org> wrote:

> Hello,
>
>  I want to stop several different MESA runs at the same final
> (photospheric) radius (with a certain precision), but since the physics is
> slightly different in each, I can't stop at the exact same radius. I
> realize that one could decrease the timesteps of evolution to achieve this,
> but was wondering if there is an easier way to achieve this.
>
> I am already using the stop to limit the photospheric radius command, but
> it always stops with a certain precision.
>
> What I want is a way to tell the code to stop when the photosphere reaches
> 7+-0.01 solar radii , is there any command that could achieve that? I tried delta_lgR_limit,
> but I don't think that does what I have in my mind.
>
> best
> Ali Pourmand
> M.Sc. Student
> University of Alberta
> _______________________________________________
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