[Mesa-users] The formula of energy generation rate of C+C reaction

[Ebraheem Farag]

Hello Wenyu,

MESA's default C12 + C12 rates come from CF88, however  the C12 + C12 - >
Mg24* rate is not one single rate but three separate rate chains, see Pignatari
et al. 2012 <https://iopscience.iop.org/article/10.1088/0004-637X/762/1/31> .
(* means excited state)

The expression presented in CF88 is a single rate approximation for these
three rate chains.  In previous versions of mesa circa r15140 and before,
there was an option to choose between CF88 single rate approximation, and a
separate multi rate chain that combined the rates for n, p, and alpha
channels (c12(c12,n)mg23, c12(c12,p)na23, and c12(c12,a)ne20) with neutron
branching from dayras, switkowski, and woosley, 1976.)

In the current release version r22.11.1, I believe this option was removed
and now MESA defaults to using the single rate approximation for small
nuclear networks. For larger adopted nuclear networks which include mg23,
na23, ne20: MESA will separate the rate into its component channels.

For more information on how these equations are implemented in MESA see
$MESA_DIR/rates/private/rate_lib.f90, where this is implemented. look for
"r1212a" and "r1212p".

How to modify the default CF88 equation for the combined c12+c12 will
depend on how you desire to change the rate.
MESA does not store the analytical formula for these rates during the
evolution, but rather interpolates all reaction rates over a grid of 10,000
points from 0 20 GK. If you'd like to replace the default rates with those
of your choosing, then you can recalculate your chosen rate over a
relevant temperature range, say 0-10Gk, and produce a tabulated rate file
for mesa to read. more details on how to do this are provided here in the
&starjob
<https://docs.mesastar.org/en/release-r22.11.1/reference/star_job.html#adjust-abundances-for-new-isos>
MESA
defaults.

"To provide tabulated rates: create a file of (T8, rate) pairs as in
data/rates_data/rate_tables You can give as many pairs as you want with any
spacing in T8. The first uncommented line of the file should be a number
giving the total number of (T8, rate) pairs in the subsequent lines. The
following lines are your specified values of T8 and rate separated by a
single space, one pair per line. Add the filename to rate_list.txt along
with the name of the rate you want it to govern, either in
data/rates_data/rate_tables or in a local directory specified with the
rate_tables_dir control. Be aware that if you choose to put the modified
rate_list.txt in data/rates_data/rate_tables rather than a local directory,
your custom tabulated rate will override the rate for that reaction for all
future MESA runs.

If the reaction you wish to control does not already have a name that MESA
will recognize, you will also need to add it to the file specified by
net_reaction_filename (defaults to reactions.list). The default version of
this file is located in data/rates_data. If you place a modified copy of
this file in your work directory, it will take precedence."
Hope this helps,
EbF














On Sun, Feb 19, 2023 at 8:32 AM 辛文宇 <xinwenyu16 at mails.ucas.ac.cn> wrote:

> Hi everyone,
>
>
> I notice the energy generation rate of C+C reaction is calculated with the
>
> attached formula in the book of kippenhahn. However, I want to know is this
>
> also the formula used in MESA? If not, what is the formula used in MESA or
>
> where can I find this formula in MESA code.
>
>
> Another question is that in the attached formula, the CF88 rate is used.
>
> If I use other rate of C+C reaction, how this formula should be changed?
>
>
> Thanks very much!
>
> Wenyu
> _______________________________________________
> mesa-users at lists.mesastar.org
>
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