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

[Ali Pourmand]

Thank you Ebraheem for replying!

Would you have a suggestion for someone who has to deal with that
non-blended region I mentioned anyway?

best
Ali

On Wed, May 13, 2026 at 12:07 PM Farag, Ebraheem <ebraheem.farag at yale.edu>
wrote:

> 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
>
>
> [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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