[mesa-users] Question about abundances in the 1 Msun test problem

Francis Timmes fxt44 at mac.com
Wed Aug 6 18:08:28 EDT 2014


alternative options, which may not be the best options for this
particular case, are a) renormalizing the abundances of whatever 
network is being used to sum to unity and b) putting any nonconservation 
into an existing istope, say h1. both approaches avoid adding a dumping ground.

fxt



On Aug 6, 2014, at 2:49 PM, Bill Paxton <paxton at kitp.ucsb.edu> wrote:

> Hi Manos,
> 
> Turns out that we need to treat create_pre_main_sequence_model differently when it comes to changing the net.
> 
> Normally, change net works with an existing model and adds or removes isotopes from an existing set.  And when it encounters a newly added isotope it gives it an adjusted solar abundance, but it only makes minimal adjustments to isotopes that were already in the net.
> 
> That works fine most of the time, but it leads to the problems you've noted.  I think it will work if we change things so that the create PMS routine knows to use the new net rather than what it does now which is to use "basic.net".   
> 
> That's part of the solution.  The other part is to use a net that contains heavier elements.  All heavy elements not in the net get dumped into the abundance of the heaviest one that is in the net.  So we need to provide at least 1 element heavier than mg24 -- e.g. mg26 or si28 or even zn60, it doesn't really matter what you pick since it will just be an inert placeholder for "all things heavy".  That will give a better value for mg24 but will give a strangely large abundance for whatever you have added since it now is the dumping place for all heavier isotopes.
> 
> For example, here's a possible net you can create:
>          include 'o18_and_ne22.net'
>          add_iso(zn60)
> it just adds zn60 to the o18_and_ne22 net.
> then with a few magic changes to mesa/star so that create_pre_main_sequence_model uses the new net instead of basic.net, I get these starting abundances:
> 
>   h1    6.9999999999999996D-01
>  he3    2.9797635251138618D-05
>  he4    2.7997020236474884D-01
>  c12    3.4416106108120406D-03
>  n14    1.0081585468366041D-03
>  o16    9.3393731281944965D-03
>  o18    2.1073480945478206D-05
> ne20    1.9423584493049016D-03
> ne22    1.5710150039274719D-04
> mg24    7.9962893561223670D-04
> zn60    3.2906953479015129D-03
> 
> 
> So now we're getting initial mg24 < ne20 as you expect.  Do the abundances look okay to you?
> 
> These changes will be the next release.  If you'd like to play with an untested pre-release version let me know.
> 
> Cheers,
> Bill
> 
> 
> 
> 
> 
> 
> 
> 
> On Aug 6, 2014, at 12:41 PM, Manos Chatzopoulos wrote:
> 
>> Dear all,
>> 
>>  I have a question with regards to the initial abundances of the solar 
>> (1M_pre_ms_to_wd) problem in the test_suite. Basically
>> the issue is that the initial surface abundance of Mg24 should always be 
>> smaller than that of Ne20. I do know from previous
>> communication that this is an issue with the size of the network used in 
>> MESA but it actually does not solve the problem.
>>  If one just runs the test problem as it is the Mg24/Ne20 ratio is 
>> wrong. Even if I switch to using approx21.net instead of
>> the default (in the test inlist_1.0) o18_and_ne22.net the ratio is still 
>> wrong. Now when I add the following lines in the inlist_1.0:
>> 
>>       set_uniform_initial_composition = .true.
>>       initial_zfracs = 3
>>       initial_h1 = 6.9979637695268626D-01
>>       initial_h2 = 0
>>       initial_he3 = 2.7973106548401051D-05
>>       initial_he4 = 2.7970309237746210D-01
>> 
>> and I run with the original o18_and_ne22.net the ratio is still wrong. I 
>> only get the correct ratio of Mg/Ne when I run
>> with approx21.net AND the above lines included in the inlist.
>> 
>>  My question is why when using a large network from the beginning of 
>> the run (that includes both isotopes, like approx21.net) does
>> not assign the correct solar abundance ratios for these isotopes in the 
>> model? Why are those lines of code needed for this and it
>> is not done by default since I input inital_z = 0.02d0?
>> 
>> Regards and thank you in advance,
>> 
>> Manos
>> 
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