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

[Farag, Ebraheem]

Hello Ali, Natasha (this reply is in reference to Ali's message)

Your figure appears to show the blending boundary region between the ideal gas (left) and helm (right) eos regions. I attach a reference image of the eos at Z=1 to illustrate where we are. In your image, the blending boundary does indeed look particularly unsmooth.

Both your region and Natash's problematic region (logT =3, logRho = -13) straddle the ideal gas boundary.

OPAL/SCVH and Free_EOS handle partial ionization
Helm EOS assumes full ionization
Ideal EOS assume no ionization

In reference to the jump in mu in your figure:
abar = 1/sum_i (X_i / A_i)
zbar = abar * sum_i (X_i Z_i / A_i)

For Helm with Z = 1, equal proportions C and O we get:
abar = 1 / ( 0.5/12 + 0.5/16 = 0.0729167) = 13.714
zbar = abar * (0.5*6/12 + 0.5*8/16) = 6.857

mu_ideal = abar = 13.714
mu_HELM = abar / (1 + zbar) = 13.714 / (1 + 6.857) = 1.745

The EOS does not blended in this regionat all, (hence there is no green in the figure I attached), but those narrow jagged features mightt deserve a closer look as it seems MESA is switching back and forth between HELM and ideal in your image. This could be related to some sort of interpolation in Z, although I'm not sure.

-EbF
________________________________
From: Mesa-users <mesa-users-bounces at lists.mesastar.org> on behalf of Ali Pourmand via Mesa-users <mesa-users at lists.mesastar.org>
Sent: Tuesday, May 12, 2026 8:58 PM
To: Natasha Ivanova <nata.ivanova at ualberta.ca>
Cc: mesa-users <mesa-users at lists.mesastar.org>
Subject: Re: [Mesa-users] EOS: unphysical low-temperature μ from OPAL/SCVH and HELM?

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


[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<mailto: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




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