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

[Natasha Ivanova]

Hi Ebraheem and all,

Thank you. I used that code and added the component call to my wrapper.

It appears that FreeEOS returns the same physically implausible result at
the test point:

logRho = -10.2
logT = 3
logU = 24.687258
mu = 0.615707
logP = 0.793140
logS = 12.867720

The EOS fractions confirm that this is indeed a direct FreeEOS result:

frac_HELM = 0
frac_OPAL_SCVH = 0
frac_FreeEOS = 1
frac_PC = 0
frac_Skye = 0
frac_CMS = 0

freeeos_ok = 1
ierr = 0

At this density and temperature, a fully ionized solar-composition gas is
not physically plausible in LTE. Thermal ionization of hydrogen is
negligible at logT = 3, helium is even less ionized, and pressure
ionization is not relevant at logRho = -10.2. Therefore, the returned value
mu = 0.6157, which is essentially the fully ionized solar-composition
value, appears unphysical for this EOS regime.

So my question remains: can the MESA EOS be considered reliable in this
low-temperature, low-density regime? In my scientific application, this
regime is relevant to AGB-star pulsations and luminous red novae.
I also attach a zoom-in of the questionable region, showing only the points
where FreeEOS returns a value successfully, i.e. points with frac_FreeEOS =
1 and ierr = 0.

Thanks
Natasha



On Tue, May 12, 2026 at 6:02 PM Farag, Ebraheem <ebraheem.farag at yale.edu>
wrote:

> Hello Natasha,
>
> Perhaps experimenting with the eos plotter in $MESA_DIR/eos/plotter. It
> wraps the eos and returns values for plotting or printing. It allows you to
> choose any eos option or blended.
>
> -EbF
> ------------------------------
> *From:* Mesa-users <mesa-users-bounces at lists.mesastar.org> on behalf of
> Natasha Ivanova via Mesa-users <mesa-users at lists.mesastar.org>
> *Sent:* Tuesday, May 12, 2026 7:37 PM
> *To:* mesa-users <mesa-users at lists.mesastar.org>
> *Subject:* [Mesa-users] EOS: unphysical low-temperature μ from OPAL/SCVH
> and HELM?
>
> 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
>
>
>
>
>
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