[Mesa-users] ZAMS profiles

[Richard Townsend]

Hi Huaiyu —

(please keep mesa-users included in the cc list, so others can see the response)

As a criterion for the ZAMS, I often use the requirement that the central hydrogen mass fraction has decreased by 0.25% of its initial value. So, for instance, if your initial central hydrogen mass fraction is 0.7, the appropriate stopping condition is

   xa_central_lower_limit_species(1) = 'h1'
   xa_central_lower_limit(1) =  0.69825 ! 0.25% H burned

The choice of 0.25% is arbitrary — stars don’t themselves know about the ZAMS, it’s a human concept!

cheers,

Rich

> On Jul 17, 2025, at 9:39 PM, Huaiyu Duan <duan at unm.edu> wrote:
> 
> Hi Rich,
> 
> Thank you for the explanation. It makes sense. I also suspect the discrepancy is because of different ZAMS criteria. 
> 
> Besides the discontinuity in the energy production profiles, their values at the center of the MESA models are much higher than those in Fig. 22.4 of Stellar Structure and Evolution. Is this also because the nuclear burning hasn’t been fully established? What may be a better way to terminate the code at the ZAMS?
> 
> Thank you again for your help!
> 
> — Huaiyu (Mike)
> 
>> On Jul 17, 2025, at 6:22 PM, Richard Townsend <townsend at astro.wisc.edu> wrote:
>> 
>> [EXTERNAL]
>> 
>> Hi Mike —
>> 
>> It’s my experience that the “stop near ZAMS” functionality can end up stopping before the nuclear burning has established itself completely.
>> 
>> I think the spike you’re seeing in eps_nuc is the consequence of CNO “pre-burning”, where the 12C in the inner parts of the star undergoes proton capture (this reaction is faster than the first step of the proton-proton chain, because it doesn’t involve a weak decay). The core is convective in this phase (for both masses — yes, solar-mass stars have a small convective core at the beginning of the main sequence!), and depletes its 12C reserves faster than the surrounding envelope. This leads to a composition discontinuity and hence an eps_nuc discontinuity at the convective boundary (with the envelope side of the boundary having a larger eps_nuc because it is still relatively abundant in 12C).
>> 
>> cheers,
>> 
>> Rich
>> 
>>> On Jul 17, 2025, at 4:44 PM, Huaiyu Duan via Mesa-users <mesa-users at lists.mesastar.org> wrote:
>>> 
>>> Hello,
>>> 
>>> I’m new to MESA. After installing the newest version (24.08.1), I ran a couple of test cases by modifying the model under $MESA_DIR/star/work. The models I ran have initial masses 1 and 10 M_sun, respectively, and initial Y = 0.28 and Z = 0.02.
>>> 
>>> I compared the last profiles generated by MESA near ZAMS (stop_near_zams = .true.) to Fig. 22.4 of Stellar Structure and Evolution by Kippenhahn et al (2nd edition). The agreement is good except for the nuclear energy production. (See panel d of the figure from the book and the output from MESA). I enabled and plotted eps_nuc for the MESA profiles. Did I do something wrong?
>>> 
>>> Thank you for your help!
>>> 
>>> — Huaiyu (Mike)
>>> <Screenshot 2025-07-17 at 3.37.38 PM.png> <a231e1e6-ec52-4b13-801a-667821a735df.png> _______________________________________________
>>> mesa-users at lists.mesastar.org
>>> https://urldefense.com/v3/__https://lists.mesastar.org/mailman/listinfo/mesa-users__;!!Mak6IKo!NpbWOUiEPOjEZWy-mSrP319WCZ15AX5suKudbgzD6i0DldVZxT4_WcihQGwXoK0CMJdxnviPyHlJnwr66pYBgPsRLxhkM_0$
>>> 
>> 
>> --
>> Rich Townsend • Professor of Astronomy
>> Astronomy Department • University of Wisconsin-Madison
>> Phone: 608-262-1752 • E-mail: rhtownsend at wisc.edu
>> 
>> 
>> 
>> 
>> 
> 

--
Rich Townsend • Professor of Astronomy
Astronomy Department • University of Wisconsin-Madison
Phone: 608-262-1752 • E-mail: rhtownsend at wisc.edu





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