[Mesa-users] Definition of eta (electron degeneracy parameter)

diogo.capelo at tecnico.ulisboa.pt diogo.capelo at tecnico.ulisboa.pt
Thu Dec 8 18:32:37 UTC 2022

Of course, 40 minutes after I send the email, I find the definition (in the
equation of state files) in MESA.


eta = (efermi - me*clight*clight)/(kerg*T)


Sorry for disturbing you!



Diogo Capelo

Instituto Superior Técnico – Universidade de Lisboa



From: Mesa-users <mesa-users-bounces at lists.mesastar.org> On Behalf Of Diogo
Capelo via Mesa-users
Sent: 8 de dezembro de 2022 17:52
To: mesa-users at lists.mesastar.org
Subject: [Mesa-users] Definition of eta (electron degeneracy parameter)


Dear MESA users,


First of all, my apologies for what I think will be a trivial question. I’ve
been recently studying the cores of RGB stars using the (aging) version
12115, and would like to use a metric for prevalence of degeneracy there. I
know MESA already allows users to select eta as the electron degeneracy
parameter in the profiles_columns.list, and even indicates that eta >> 1
indicates significant degeneracy, but I was unable to find the actual
definition of how it is calculated.


My usual method of using grep in the MESA directories was also not very
useful in this case, since it appears there are several instances of “etas”
in the code. Strangely, searching for ‘eta =’ returned no results. I was
also unable to find a definition in the literature or online that  behaves
like the MESA eta. The behaviour suggests to me something like eta =
ln(T_F/T), with T_F the Fermi temperature and T the local temperature, but
I’m seeing variations of several (15+) orders of magnitude in the parameter
over different regions of the same (1-6 solar masses) star, which seems
perhaps too much.


I was wondering if anyone knew where the definition is, or where in the MESA
code eta should be calculated, to give me some pointer on where to look.


Thank you all very much for your time,


Diogo Capelo

Instituto Superior Técnico – Universidade de Lisboa

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