[Mesa-users] Producing a medium to high mass pure Helium star

[Charlie Sharpe]

Hi Aaron and Josiah,

I took your suggestion Josiah with regards to using
okay_to_reduce_gradT_excess. I have attached my new inlist if you were
curious to have a look. I also decided to increase varcontrol_target
from 1d-4 to 1d-3. While this does aid in making the timesteps larger, they
do not increase to an extent where the evolution is able to progress by a
significant amount, still freezing out essentially when Carbon burning
starts. What are your thoughts?

In response to Aarons suggestion, I did take a look at the 'time step
controls' in controls.defaults, but the terminal error I am typically
getting is 'max increase' and this output isn't mentioned anywhere in
controls.defaults so I am not sure what would be the suitable parameter to
change. Do you know what this refers to?

Regards,
Charlie

On Thu, May 20, 2021 at 4:29 AM Josiah Schwab <jwschwab at ucsc.edu> wrote:

> Hi Charlie,
>
>
>> The evolved state of this model is likely to explain the small
>> timesteps.
>>
>
> Building on Aaron's point and your plot, you can see that this model has a
> near- (or even maybe even super-) Eddington luminosity (b/c of the He shell
> burning on a massive degenerate core).  Correspondingly, it has developed
> an inflated envelope with outer layers that are radiation pressure
> dominated and have a density inversion.  The steep entropy gradient at the
> base of this (inefficient) convection zone is usually what is responsible
> for driving the MESA timestep down.
>
> This is similar to the behavior in massive stars that the MLT++ "hack"
> described in Section 7 of Paxton et al. (2013) is designed to numerically
> circumvent (but not physically solve).  You could consider
> using okay_to_reduce_gradT_excess and related controls, depending on what
> your goals for the model are.
>
> You may also find it numerically helpful to include some mass loss in
> these models.  While one may be hard pressed to make a precise physical
> prediction of the mass loss rate, I don't think it is much of a stretch to
> imagine driving a wind in this circumstance.
>
> Josiah
>
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