[mesa-users] EoS & partial ionization

Ken Shen kenshen at astro.berkeley.edu
Wed Aug 3 20:20:40 EDT 2011

Hi all.  I've got a question about how the MESA EoS handles partial
ionization.  In particular, are the changes in free electron fraction
taken into account when the EoS returns, e.g., chiT and chiRho?  As a
test, here are the results for two calls to the EoS for X_12C = X_16O
= 0.5 and rho = 2e-8 g/cm^3:

T = 6.5e4 K
P = 9.022e4 dyne/cm^2
free_e = 0.3463
chiT = 2.499
mu = 1.745

T = 6.4e4 K
P = 8.577e4 dyne/cm^2
free_e = 0.3364
chiT = 2.482
mu = 1.745

A crude calculation from these two calls gives dlnP/dlnT = 3.26, which
is significantly different from the returned value of chiT = 2.5.
Also, the mean molecular weight that is returned seems to assume full
ionization; it should be 2.39 for the first call and 2.44 for the


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