[Mesa-users] Suppressing CNO reactions

[Evan Bauer]

I can confirm that Bill’s suggestion works. This is a classic exercise in the UCSB graduate stars class, so I’ve done it many times.

The max_abar_for_burning control definitely won’t do what you want in this case. If you set it to allow hydrogen-rich material to burn at all, then those zones will burn with any chain available to them, including CNO. So you need to make a custom net that cuts out those reactions, or set their rate factors to zero. Actually, in the case of CN(O), all you really need to do is turn off enough reactions to break the cycle, which can be fun to play around with.

Cheers,
Evan


> On Mar 11, 2019, at 10:36 AM, 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 <http://basic.net/>, open that file (mesa/data/net_data/nets/basic.net <http://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.
> 
> cheers,
> bill
> 
> 
> 
> 
>> On Mar 11, 2019, at 10:20 AM, Aaron Dotter via Mesa-users <mesa-users at lists.mesastar.org <mailto: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 <mailto: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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