[Mesa-users] degeneracy parameter

Josiah Schwab jwschwab at ucsc.edu
Tue Feb 23 17:51:51 UTC 2021


Hi Jason,

My main issue now is that I don't have a check for the number density of
> electrons.  I have been trying to use the parameter free_e, but I don't
> think I understand it.  It says it's the "mean number of electrons per
> nucleon." I think this means that I should at least roughly be able to get
> the number density of electrons as
>
> n_e = rho/amu * free_e
>
> where amu is the unit, but that's not even close to right. In fact, just
> counting hydrogen and helium I get 0.86 free electrons per nucleon at the
> center of the ZAMS sun, but free_e says there are 0.67.  Indeed, in
> complete ionization I would expect the number of free electrons to match
> the number of protons, so free_e = ye, but again I get 0.86 for ye,
> matching my expectation but inconsistent with free_e.
>
> So my questions are:
> 1) What does free_e represent?
> 2) Is there a way to extract n_e from MESA output parameters precisely?
>  Perhaps an unlisted parameter like mu_e or n_e?
>

The expression you wrote for n_e is correct and corresponds to the most
consistent way to extract this value from the EOS.

I can reproduce the bogus value you report. As you say, it should be about
0.86 in the center of the ZAMS sun.  I'll investigate.

At first glance, it seems to be a problem with the reporting when we're on
FreeEOS.  If I turn FreeEOS off and fall back to OPAL, the value at the
center looks more like you'd expect.

&eos
  ! eos options
  ! see eos/defaults/eos.defaults

  use_FreeEOS = .false.

/ ! end of eos namelist

Thanks for the report.

Josiah
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