[Mesa-users] Suppressing CNO reactions

[Bill Paxton]

> On Mar 11, 2019, at 10:40 AM, RICHARD H D TOWNSEND <townsend at astro.wisc.edu> wrote:
> 
> 
>> On Mar 11, 2019, at 12:36 PM, Bill Paxton <paxton at kitp.ucsb.edu> wrote:
>> 
>> i’m not sure that max_abar_for_burning will do what Rich wants. 
>> 
>> here are the lines from mesa/star/private/net.f90 where it is used:
>> 
>>         if ((s% eps_nuc_factor == 0d0 .and. s% dxdt_nuc_factor == 0d0) .or. &
>>              s% abar(k) > s% max_abar_for_burning) then
>>            return
>>         end if
>> 
>> it is checking the abar for the cell to decide whether or not to do nuclear burning for that particular cell.
>> 
>> so it won’t turn off one kind of burning while leaving another kind.
>> 
>> the special rate factors should do the job however.
>> 
>> assuming you are using basic.net, open that file (mesa/data/net_data/nets/basic.net)
>> 
>> in the “add_reactions” section, you’ll find these lines:
>> 
>>         ! cno cycles
>> 
>>         rc12_to_n14         ! c12(p g)n13(e+nu)c13(p g)n14
>>         rn14_to_c12         ! n14(p g)o15(e+nu)n15(p a)c12
>>         rn14_to_o16         ! n14(p g)o15(e+nu)n15(p g)o16
>>         ro16_to_n14         ! o16(p g)f17(e+nu)o17(p a)n14
>> 
>> those are the ones you want to disable, so set their special_rate_factors to 0 in your inlist
>> 
>> num_special_rate_factors = 4
>> reaction_for_special_factor(1) = ‘rc12_to_n14'
>> special_rate_factor(1) = 0d0
>> reaction_for_special_factor(2) = ‘rn14_to_c12'
>> special_rate_factor(2) = 0d0
>> reaction_for_special_factor(3) = 'rn14_to_o16'
>> special_rate_factor(3) = 0d0
>> reaction_for_special_factor(4) = ‘ro16_to_n14'
>> special_rate_factor(4) = 0d0
>> 
>> i haven’t tested this myself, so please let us know if it actually works.
>> 
> 
> Thanks for the info -- but I confess I'm a little confused, as I had been changing the rates thus:
> 
>   num_special_rate_factors = 7
>   reaction_for_special_factor(1) = 'r_c12_pg_n13'
>   reaction_for_special_factor(2) = 'r_c13_pg_n14'
>   reaction_for_special_factor(3) = 'r_n14_pg_o15'
>   reaction_for_special_factor(4) = 'r_n15_pa_c12' 
>   reaction_for_special_factor(5) = 'r_n15_pg_o16' 
>   reaction_for_special_factor(6) = 'r_o16_pg_f17'
>   reaction_for_special_factor(7) = 'r_o17_pg_f18'
>   special_rate_factor(1) = 0.
>   special_rate_factor(2) = 0.
>   special_rate_factor(3) = 0.
>   special_rate_factor(4) = 0.
>   special_rate_factor(5) = 0.
>   special_rate_factor(6) = 0.
>   special_rate_factor(7) = 0.
> 
> I found the reaction names in data/rates/reactions.list. But it seems there's a different set of names I should be looking at. Can you explain?
> 

different nets use different rates.  in particular, the basic.net <http://basic.net/> uses rates that combine several basic reactions in order to reduce the number of species used.  that makes a difference in performance of course, and for many applications the results based on assuming equilibrium amounts of intermediate species are good enough.  so basic net doesn’t include n13, c13, o15, n15, f17, o17, or f18.  turning off reactions that are not used in the net will of course have no effect.

so for basic.net <http://basic.net/>, you need to look at the definition to see what reactions it uses so you can turn off the ones for CNO.

-b

> cheers,
> 
> Rich
> 
>> cheers,
>> bill
>> 
>> 
>> 
>> 
>>> On Mar 11, 2019, at 10:20 AM, Aaron Dotter via Mesa-users <mesa-users at lists.mesastar.org> wrote:
>>> 
>>> 
>>> Hi Rich,
>>> 
>>> In &star_job you can set these:
>>> 
>>>         !### num_special_rate_factors
>>>         !### reaction_for_special_factor
>>>         !### special_rate_factor
>>>         ! For using other special rate factors.
>>>         ! `num_special_rate_factors` must be <= `max_num_special_rate_factors`.
>>> 
>>>      num_special_rate_factors = 0
>>>      reaction_for_special_factor(:) = ''
>>>      special_rate_factor(:) = 1
>>> 
>>> or in &controls you can set this:
>>> 
>>>         !### max_abar_for_burning
>>> 
>>>         ! if abar > this, suppress all burning
>>>         ! e.g., if want an "inert" core heavy elements, set this to 55
>>>         ! or, if want to turn off the net, set this to -1
>>> 
>>>      max_abar_for_burning = 199
>>> 
>>> Either one should give you the desired effect.
>>> 
>>> Aaron
>>> 
>>> 
>>> 
>>> On Mon, Mar 11, 2019 at 1:17 PM RICHARD H D TOWNSEND via Mesa-users <mesa-users at lists.mesastar.org> wrote:
>>> Hi folks --
>>> 
>>> A question that I'm sure I've asked before, but I can't seem to find it in the archives: what's a straightforward approach to suppressing CNO reactions in a MESA run? I'd like to do this to demonstrate to my class what would happen to the luminosity of massive ZAMS stars if they were 'forced' to use the PP chain.
>>> 
>>> cheers,
>>> 
>>> Rich
>>> 
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>> 
> 

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