[mesa-users] Adding Isotopes to MESA Nuclear Networks

[Josiah Schwab]

Hi All,

Hector Martinez Rodriguez and I had an email exchange about adding
isotopes to MESA networks.  Questions like these come up periodically,
so I wanted to send some of this info to the list for archival purposes.

Thanks to Frank for double checking my explanation.


There are two kinds of nuclear networks in MESA:

"soft nets" are nets where you specify the isotopes you want to track
and let MESA fill in all the reactions between them.  mesa_45.net is a
nice example of a soft net.  In net like this, the isotopes ne20, na23,
and mg23 are included and the c12 + c12 reactions that get used are

  r_h1_na23_to_c12_c12
  r_c12_c12_to_h1_na23

  r_he4_ne20_to_c12_c12
  r_c12_c12_to_he4_ne20

  r_neut_mg23_to_c12_c12
  r_c12_c12_to_neut_mg23


"hard-wired nets" include special reactions which assume equilibrium
abundances for intermediate elements that are not explicitly included in
the net. co_burn_plus.net is a hard net.

If you look, na23 is not included in co_burn_plus and instead there are
auxiliary reactions such as rna23pg_aux.  As you noted, r1212 already
takes into account "c12 + c12 -> na23 + h1", so which makes it the c12 +
c12 reaction suitable for use in this context.

It is a tricky business to modify hard-wired nets.  You have to think
carefully and make sure that you are not violating any of the
assumptions that went into them.  As an example, here are some old
instructions from Bill of what one would need to do to create a version
of co_burn that included f19 explicitly

Three steps (can be done in any order)
  1) add_iso(f19)
  2) add_reactions(r_o16_ap_f19, r_f19_pg_ne20, r_ne20_gp_f19, r_f19_pa_o16)
  3) remove_reactions(ro16ap_to_ne20, ro16ap_aux, rf19pg_aux, rne20gp_to_o16, rne20gp_aux, rf19pa_aux)

That gives you the idea of the sort of surgery you'd need to do if you
wanted to track na23 explicitly in one of these hard nets.


Josiah