[Mesa-users] Doubts from a master's student about MESA

[Francis Timmes]

hi fran,

i didn't see an attachment, so just a few comments.

stars of ~1 msun don't hot enough to alter iron from nuclear reactions. the iron-input is equal to the iron-output. if you are doing an s-process on iron seed nuclei, that is something else.

to set a precise initial composition in mesa, one goes something like

! set specific abundances - these are set after the pre-ms relaxation
       set_uniform_initial_xa_from_file = .true.
       file_for_uniform_xa = 'gch_0p0_mesa_79.dat'

where gch_0p0_mesa_79.dat is attached. the mass fractions listed should sum to unity and usually depend on the chosen reaction network. note that the resulting initially flat iron profile can change if element diffusion is active, for example. without a precise initial composition, mesa re-normalizes the initial mass fractions such that the sum is unity - this may be what you are reporting.

fxt




> On May 23, 2024, at 12:57 PM, francandon via Mesa-users <mesa-users at lists.mesastar.org> wrote:
> 
> Dear MESA members,
> 
> I’m a master student at the university of Zaragoza, currently finishing my master thesis. I have just started using MESA to run stellar simulations, and I read the documentation and tutorials.
> 
> For my project, I need to calculate the abundance of some isotopes in the Sun. For example, I want to know the abundance of Fe57. What I did was the following: I used te documentation example (https://docs.mesastar.org/en/latest/using_mesa/running.html) and changed some parameters. In addition, I used the nuclear network “http://aprox21.net” and I added Fe57 to consider all the possible reactions that can produce it.
> However, my problem arises at the first step of the simulation. The abundance rate of Fe57 is 10 times larger than I expected, because the presence of the Fe57 isotope in nature is just 2% of that of Fe. I even tried changing the set_uniform_initial_composition with the initial_zfracs = 3, corresponding to GS98. I obtain the following for the first step:
> 
> total_mass_fe56                                       total_mass_fe57
> 1.4142415754937801E-003                 3.0840908076103230E-004
> which means that Fe57 is the 21.43% if we assume Fe56 to be the 98%.
> 
> I would appreciate it if someone could clarify this for me. You can find attached to this email the inlist and the nuclear network files that I’m using.
> Thank you for your attention.
> 
> Best regards,
> Fran
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
> 





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