[Mesa-users] Regarding r_b8_wk_be8 and reaction rates units

[Meisel, Zach]

Greetings,



Note that ReacLib does include the 8B decay rate, but since 8Be more or less instantaneously breaks up into two alpha-particles, the rate is included as 8B->4He+4He  (search “b8  he4  he4” in the ReacLib data file).



For the reaction rate units, in the MESA code I believe it’s referred to as the variable “rate_screened”. “rate_screened” is defined in the “Net_info” type within the file “mesar9793/net/public/net_def.f90”. The text of that file (on line 187) says, regarding “rate_screened”:

“The units here depend on the number of reactants. In all cases, the rate_screened times as many molar fractions as there are reactants gives a number with the same units as dy/dt. So for a 2body reaction, there are 2 Y factors, each with units [moles/gram] and the rate_screened units for such a reaction are [grams/(mole sec)], which when multiplied by [moles/gram]^2 gives the same units as dydt.



For a 1body reaction (e.g., a decay), there is only 1 Y factor, so the units are [1/second]. Similarly, a 3body reaction will have rate_screened with units of [gram^2/(mole^2sec)].”



So the units above are those you would specify in a table to include a custom reaction rate in tabular form.



If instead you’re using the ReacLib format to calculate the reaction rate directly from the ReacLib data file that’s located in data/rates_data/, then the units are those specified on the ReacLib page: s^-1 for decays and cm^3 mol^-1 s^-1 for reactions.



-Zach



----
Zach Meisel
Assistant Professor of Physics & Astronomy

Director, Edwards Accelerator Laboratory
Ohio University
Email: meisel at ohio.edu
Web: http://inpp.ohio.edu/~meisel



________________________________
From: Mesa-users <mesa-users-bounces at lists.mesastar.org> on behalf of Diogo Capelo via Mesa-users <mesa-users at lists.mesastar.org>
Sent: Tuesday, November 6, 2018 11:38:06 AM
To: mesa-users at lists.mesastar.org
Subject: [Mesa-users] Regarding r_b8_wk_be8 and reaction rates units

Dear MESA users,

I've been trying to reconstruct the solar neutrino radial flux profile
by source using MESA (-r10398), specifically by exploiting the weak
reaction rates and element abundances. I've noticed that the B8 decay
(r_b8_wk_be8) reaction is not present in pp_extras.net, and the decay
reaction itself is omitted in weak_info.list. In skip_warnings.list, the
reaction is there (twice, actually), so I understand that it's not in
reaclib. However, I also know that MESA calculates the total neutrino
loss, of which the B8 decay neutrino would be a part. My question is, if
the reaction is not present specifically in the net but its energy
accounted for, is it assumed to be in equilibrium with another (like in
the basic network)? It doesn't seem likely, because printing out the
abundance of B8, I could see it varying. For the same reason, when I
tried adding Be7 and B8 to the basic network without specifying their
independent reactions, it didn't work, since they are considered in
equilibrium there. Is there no way to get the B8 decay rate and the B8
abundance in the same model, using the same net?

Additionally, I'd like to ask what units the reaction rates are in: I
assumed they were absolute number of events per unit time and per unit
volume for the reaction in question, but I've had relatively more
success replicating the results from the literature (for instance,
arXiv:1308.3346, Fig. 2 on page 5) by taking into account the densities
of the reactant(s) (with the exception of the pep and hep neutrinos
which I still have to figure out).

Thank you very much for your time.
_______________________________________________
mesa-users at lists.mesastar.org
https://lists.mesastar.org/mailman/listinfo/mesa-users

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