[Mesa-users] about the decay rates of al26, al26-1 and al26-2

[Josiah Schwab]

Hi Ertao,

> Firstly, I noticed that the decay rates (stored in cache/*.bin) of
> al26, al26-1 and al26-2 are constant and do come from
> $MESA/data/rates_data/JINA_reaclib***. But, from the MESA instruction
> paper apj 220 (2015) 15, page 32: \8. WEAK REACTIONS \The rates module
> provides weak reaction rates for \hundreds of isotopes. By default,
> when atoms are fully ionized, \these rates are based (in order of
> precedence) on the tabulations \of Langanke & Martínez-Pinedo...
>
> What I think is that MESA only creates the decay rates from JINA but
> doesn't use them, MESA uses the
> $MESA/data/rates_data/weakreactions.tables to get the decay rates but
> doesn't save them.  Am I correct?

Yes, that's right.  MESA takes the coefficients from reaclib and
evaluates the rates on a fine grid of temperatures.  Then it caches
those results and interpolates in them to get the rate.

When using the weakreactions.tables instead, MESA already has the rate
tables and so it just loads them in memory and interpolates directly.
(There's no need for a cache.)

> Secondly, the partition functions in $MESA/data/chem_data/isotopes.dat
> should affect the decay rates of al26, al26-1 and al26-2. However,
> after I changed the partition functions, the decay rates do not change
> at all. Does MESA use the partition function?

MESA uses the partition function to get some reverse rates.  Off the top
of my head, I do not think it uses them otherwise.

> Thirdly, why is the partition function of al26-1 and al26-2 always
> equal 1.0?

That does seem a bit strange.  The partition functions are from
https://groups.nscl.msu.edu/jina/nucdatalib/

https://lists.mesastar.org/pipermail/mesa-users/2016-January/006182.html

but I don't see any info about the isomeric states there.  So I don't
know.

> Lastly, how can I include the excitation and de_excitation between
> al26-1 and al26-2?

I'm not sure about this either.  And I'm somewhat confused about how
this would work given the current setup.

There's an al26-1 <-> al26-2 rate from FFN that is included in
weakreactions.tables.  It is in the positron capture slot in those
tables, but I think the values make sense for it to be the thermal
excitation/de-excitation.

But I'm a bit confused about how one would actually ask MESA to load/use
it and even about what exactly would happen if you did.

Sorry I can't be of more help.  Perhaps someone else has done this
before and has some more insight.

Josiah