[Mesa-users] Inquiry About MESA Convergence Issues During Convective Region Expansion

[Guo Li]

Dear All,

I am using MESA-24.08.1 to evolve a star with M=1 Msun and [Fe/H]=-3. During the second thermal pulse, a proton ingestion event and H-flash occur. After the lg_LH exceeds 8.0, the convective zone in the He intershell expands outward, and the code fails to converge at the upper boundary of the convective region. I have attached my inlist file, mod file, terminal output, and the mass fraction profile of nuclides at the termination point for reference.

I attempted to improve spatial resolution, particularly at the convective boundaries, but the convergence issue persisted. The problem was only resolved by relaxing the convergence criteria as follows:

tol_correction_norm = 8d-4  
tol_max_correction = 8d-2  

use_gold2_tolerances = .false.  

convergence_ignore_equL_residuals = .true.  

use_gold_tolerances = .true. ! the following only apply when this flag is true  
gold_tol_residual_norm1 = 1d-6  
gold_tol_max_residual1 = 1d-4  
gold_iter_for_resid_tol2 = 5  
gold_tol_residual_norm2 = 1d-4  
gold_tol_max_residual2 = 1d-2  
gold_iter_for_resid_tol3 = 10  
gold_tol_residual_norm3 = 1d-1  
gold_tol_max_residual3 = 1d1  

However, I believe this approach compromises accuracy, as it appears to cause unnaturally rapid expansion of the He intershell convective zone.

Question:
For this convergence issue during the expansion phase of the He intershell convective region, what methods should I try to resolve it without sacrificing accuracy? Any advice would be greatly appreciated.

Best regards,
Guo Li
National Astronomical Observatories, CAS
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