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

[Natasha Ivanova]

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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