[Mesa-users] MESA's Molecular Hydrogen EOS in low-Temperature & low-Pressure Regime

[Charlie Willard]

Hello all,

I have a question regarding the representation of molecular/atomic/ionized
(etc.) hydrogen in the Mesa EOS tables.

I have extracted the Mesa EOS information for mean molecular weight per gas
particle (mu) and density (rho) for a pure hydrogen atmosphere.
I am roughly working in the regime of 2<log(T)<4.5 Kelvin and 6<log(P)<13
dyne/cm^2 (for SubNeptune Exoplanet Atmospheres), which is primarily
governed by the SVCH/OPAL EOS in Mesa.

The mu output by MESA appears to be different from the mu calculated
directly from SCVH EOS. I've attached a plot where the blue dots show mu
for SCVH while the red-orange surface shows mu output by MESA for pure
hydrogen. Notably in the regime (lowT/lowP) where I had expected mu = 2 amu
(Molecular Hydrogen), MESA output mu = 1 amu (atomic hydrogen).

In the ideal gas regime, the density rho of the gas output by MESA appears
consistent with that of an ideal gas with mu=1amu, instead of mu=2 amu as
expected for molecular hydrogen. In the P-T range spanned by the SCVH EOS
in MESA, the density rho output by MESA appears to agree with the density I
interpolated directly from SCVH (even if the mu's disagree).

Am I misinterpreting something, or is this the expected behavior of
hydrogen in MESA?

I have attached the project folder containing the inlist/run_stars_extras
files used to generate the data in the plot. Also in the folder are the
data-lists extracted as well as the python notebook used to generate the
plot.
My specific use case was to compute the compressibility factor, Z = PV_m/RT
(where V_m = mu/rho), for the gas in a planet's atmosphere.

Thank you for your help,
Charlie Willard
University of Chicago Undergraduate
Mesa version: 22.05.1

[image: Screen Shot 2022-11-06 at 12.40.44 PM.png]

 Z_Test_Heos 2.zip
<https://drive.google.com/file/d/13IxTQUjETxfq-MXb19CoQXxa6mTv3o1H/view?usp=drive_web>
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