[Mesa-users] EOS: unphysical low-temperature μ from OPAL/SCVH and HELM?

[Ali Pourmand]

Hello all and Natasha,

I am writing just to confirm that we are facing similar problems, in a
different regime and for different compositions however. Our mean molecular
weight suddenly jumps from partially ionized to fully non-ionized in the
wedge-like region you can see (where mu is roughly mu=14 for a
Carbon-Oxygen mixture), which also corresponds to jumps in everything else.
The reason is obvious if you look at the fractions of contribution from
different underlying EOS tables. There do seem to be some regions that
aren't covered smoothly but on the other hand fall out of conventional
"stellar" regimes for (rho,u).

best regards
Ali


[image: EOS_rhoE_z0.9988_x0.0000.png]


On Wed, May 13, 2026 at 9:37 AM Natasha Ivanova via Mesa-users <
mesa-users at lists.mesastar.org> wrote:

> Hello all,
>
> I wrote earlier about this, but have not received a response yet. After
> digging further, the issue looks more serious.
>
> I am testing MESA EOS through an external wrapper. I scan the (ρ,T) plane
> and record the values returned by eosDT_get. The attached plots show the
> returned μ and logP.
>
> At low temperatures, some returned values look clearly unphysical. For
> example, at
>
> log rho = -13, log T = 3
>
> MESA returns
>
> mu = 0.6157
>
> which corresponds to fully ionized solar-composition gas. At this density
> and temperature, neither thermal ionization nor pressure ionization should
> apply. The EOS fractions show that this point comes fully from OPAL/SCVH:
>
> frac_HELM      = 0
> frac_OPAL_SCVH = 1
> frac_FreeEOS   = 0
> frac_PC        = 0
> frac_Skye      = 0
> frac_CMS       = 0
>
> There is also another low-temperature region, also circled in the attached
> plot, where the fractions show that HELM is responsible, and the returned
> μ is again physically implausible.
>
> My questions are:
>
>    1. If OPAL/SCVH or HELM return clearly unphysical μ in these regions,
>    should pressure and internal energy returned there be considered reliable?
>    2. Is res(i_mu) intended to represent the physical mean molecular
>    weight corresponding to the ionization state, or can it be only an
>    effective/composition-based value?
>    3. Is there a supported way, from an external wrapper, to call FreeEOS
>    only, without fallback to OPAL/SCVH or HELM, so that I can test whether
>    FreeEOS behaves correctly in these regions?
>
>
> thank you,
> Natasha
>
>
>
>
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
>
>
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