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

Evan Bauer evan.bauer.astro at gmail.com
Sat Dec 10 01:43:45 UTC 2022


Hi Diogo,

You're right that there are a few different ways one might imagine defining a degeneracy parameter. In the case of MESA, you can find the precise definition in $MESA_DIR/eos/public/eos_def.f90

      integer, parameter :: i_eta = i_lnfree_e+1
	    ! electron degeneracy parameter (eta > 1 for significant degeneracy)
	    ! eta = ratio of electron chemical potential to kT

Cheers,
Evan

> On Dec 8, 2022, at 10:32 AM, Diogo Capelo via Mesa-users <mesa-users at lists.mesastar.org> wrote:
> 
> 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
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20221209/fee63725/attachment.htm>


More information about the Mesa-users mailing list