[mesa-users] approx13

[Francis Timmes]

i’ll chime in on this thread.

> But it would be good to figure out why the network is wrong without them,
> even though those isotopes don't participate!

hand-crafted hardwired networks like aprox13 and friends make a number
of approximations, hence their name, to provide estimates of the 
energy generation rate and rough nucleosynthesis within the assumptions made.
for example, alpha-centric networks can only have Ye=0.5. if the model 
under consideration has Ye /= 0.5, then the estimates provided by aprox13 degrade, 
sometimes considerably depending on how far from 0.5 the model resides.

more to the case at hand, alpha-centric networks carefully apply equilibrium
conditions to eliminate explicitly evolving the proton and neutron abundances
while at the same time, somewhat paradoxically, attempting to account for 
their major effects such as various (a,p)(p,g), double (n,g), and (a,p)(g,p) sequences,
photodisintegration protons and neutrons, and si to fe in one hop.

mesa can do the aprox* magic, such as the aprox20 or aprox25 that bill mentioned.
mesa will also do general softwired networks, an area that has seen a lot of 
development lately and which i’ll leave to bill to announce. unfortunately but 
with good intentions, mesa currently allows one to try and emulate a 
hardwired network or modify a hardwired network using the softwired constructs 
add_isos and add_reactions. this will not be very successful because the 
assumptions that are carefully hand-crafted into the hardwired networks 
will either be missing or violated or both.  

i’m not surprised at all goofy results occur when mixing these two oil & water 
modes of operation. if you want a hardwired network, use one supported by mesa 
or get an alchemist to craft a speciality item (prepare to pay in gold). if you 
want a general softwired network, and i wholeheartedly second dave arnett’s 
call for using ~100 isotope networks, mesa will do it nicely. 
but don’t mix the two. 

---

to openly share knowledge with the mesa community, the attached pdf walks through 
the mathematics of the above mentioned “equilibrium conditions” for (a,p)(p,g), 
double (n,g), and asymmetric (a,p)(g,p) sequences and how they are applied to 
fine hand-crafted brews like aprox13.

fxt



-------------- next part --------------
A non-text attachment was scrubbed...
Name: effective_reaction_rates.pdf
Type: application/pdf
Size: 113709 bytes
Desc: not available
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20140515/023b8288/attachment.pdf>