[Mesa-users] inaccuracies from large mesh_delta_coeff and convergence helpers

[Rob Farmer]

Hi,
I'm surprised that setting okay_to_reduce_gradT_excess=.true. is doing
anything for your models of 1-2msun stars. That option is needed to turn on
mlt++ but that isn't necessary until about 20msun. Pextra_factor = 2 is
also not a resolution control, that sets an extra pressure term at the
surface to keep the envelope more tightly bound, that might help if you
have pulsations which are causing large changes in the radius.

Why decreasing the mesh_delta_coeff helps, depends on exactly why/when your
model is failing. Can you make some plots? HR diagram and a Kippenhan so we
can see when the model is failing (and what the outer envelope is doing).
If you're getting down to using mesh_delta_coeff = 0.1 then I would suggest
looking instead at mesa's other mesh controls to put the resolution just
where your model needs it (instead of using mesh_delta_coeff which is
hammer and increases the resolution everywhere).

Rob

On Fri, 3 Sept 2021 at 09:26, Thomas Steindl <thomas.steindl at uibk.ac.at>
wrote:

> Hello Simon,
>
> There are multiple reasons why such models may experience convergence
> issues. I assume you are using T-Tau boundary conditions since you are
> interested in pulsations. I have seen that setting the error tolerance of
> the atmospheric integration can help (atm_T_tau_errtol = 1d-9, for
> example). If the convergence issues happen very early in the evolution,
> setting scale_max_correction = 0.1 can help the solver to not overshoot the
> „correct“ solution in the convergence process.
>
> That said, it is usually very hard to help without having the inlists that
> you are using. I am happy to take a look at them if your issues persist.
>
> If you change mesh_delta_coeff later in the run (you can do that in
> run_star_extras for example), you will obtain a better resolution in the
> model going forward. However, the error that has been made in the beginning
> of the run will propagate through to some degree.
>
> Best,
> Thomas
>
> __________________________________________________________________
>
> Thomas Steindl, MSc
> University of Innsbruck | Institute for Astro- and Particle Physics
> Tel: +43 512507 52031
> Mail: thomas.steindl at uibk.ac.at
> Homepage: www.thomassteindl.com
>
> Am 03.09.2021 um 08:53 schrieb Simon Murphy via Mesa-users <
> mesa-users at lists.mesastar.org>:
>
> Hi folks,
>
> I'm supervising a student project to investigate systematic modelling
> uncertainties on pulsation frequencies of 1.5--2.0 Msun stars.
>
> This involves evolving some models from very young ages
> (create_pre_main_sequence_model = .true.) to the late main sequence. A
> major difficulty is having models converge, because the solver eventually
> gives up with "hydro_failed" a lot of the time.
>
> There are of course things that help. Having "Pextra_factor > 1", having
> "okay_to_reduce_gradT_excess = .true.", and, it seems, having larger
> "mesh_delta_coeff". I'm not sure why the latter helps, but it seems that it
> does. Would love to know why.
>
> My question is this: if at some point in the evolution the run uses, say, Pextra_factor
> = 2 and mesh_delta_coeff = 1.0, is there any point in later using Pextra_factor
> = 1 and mesh_delta_coeff = 0.1? Or is that accuracy permanently lost?
>
> The student or I would check ourselves, but we can't get anything to
> converge with Pextra_factor = 1 and mesh_delta_coeff = 0.1 all the way
> through.
>
> Thanks,
> Simon
>
> -----------------------------------------
> *Dr Simon J. Murphy*
> Incoming ARC Future Fellow
> School of Physics
> University of Sydney
> NSW 2006, Australia
> tel: (+61) 0290365104
> -----------------------------------------
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