[Mesa-users] ZAMS profiles

[Richard Townsend]

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