Please see this thread on the mailing-list initially raised by Natasha:
https://lists.mesastar.org/pipermail/mesa-users/2026-May/015873.html
She highlights two regions of concern
In the low density temperature regime, the mean molecular weight and likely electron fraction returned by the eos is that of an ionized gas, which is unphysical. This is likely because the tabulated eos table files (the data files themselves) for Free_EOS and OPAL_SCVH in MESA fall back to HELM in their tabulation, and return :
mu = 0.66667
log_free_e = -99.00000
The Free EOS table floor is logT = 3, at and below which HELM values are tabulated
The same is true for the SCVH routines below the logP = -0.6 boundary (logRho ~ 10 -11 ish) below which HELM values are tabulated in the table.
These data tables get generated through MESA, so it looks like MESA is falling back to HELM before the table edge routines can activate for SCVH. Since the blend is something like:
if (logRho >= logRho5) then
pure_opal_scvh
else if (logRho > logRho6) then
blend
else
pure_helm
end if
with
logRho5 = -9.0d0
logRho6 = -9.99d0
Please see this thread on the mailing-list initially raised by Natasha:
https://lists.mesastar.org/pipermail/mesa-users/2026-May/015873.html
She highlights two regions of concern
In the low density temperature regime, the mean molecular weight and likely electron fraction returned by the eos is that of an ionized gas, which is unphysical. This is likely because the tabulated eos table files (the data files themselves) for Free_EOS and OPAL_SCVH in MESA fall back to HELM in their tabulation, and return :
mu = 0.66667
log_free_e = -99.00000
The Free EOS table floor is logT = 3, at and below which HELM values are tabulated
The same is true for the SCVH routines below the logP = -0.6 boundary (logRho ~ 10 -11 ish) below which HELM values are tabulated in the table.
These data tables get generated through MESA, so it looks like MESA is falling back to HELM before the table edge routines can activate for SCVH. Since the blend is something like:
with