[mesa-users] Unrealistic metallicity ([Fe/H] < -13) in the main-sequence evolution of a low-mass star

[Earl Bellinger]

Hello everyone,

I ran a track using MESA r7623 with
    M = 0.921686
    Y = 0.31379
    Z = 0.0009
    αMLT = 1.93045
with an Eddington atmosphere, atomic diffusion enabled, a tiny mesh
coef (0.2) and a tiny timestep (1d5 Gyr). Inlist attached.

I calculate [Fe/H] as
    log10(10**log_surf_z / surface_h1 / Z_div_X_solar)
where
    Z_div_X_solar = 0.02293.
In this track, [Fe/H] is -1.24 at ZAMS and reaches a value of less
than -13 at its main-sequence terminal age (Xc=0.001, age=2.45 Gyr).
This is many orders of magnitude more metal-poor than any star ever
observed. Is something going wrong in this evolution?

This is not the only track where this happens. This behavior is common
even in tracks with more metal-abundant initial conditions. For
example, a [Fe/H] of -6.4 is achieved on the main sequence with
    M = 1.273248
    Y = 0.314728
    Z = 0.014244
    αMLT = 1.828888.

The diffusion controls I use were taken from the solar-calibration
inlist and are as follows:

      do_element_diffusion = .true.
      diffusion_use_isolve = .true.

      diffusion_class_representative(1) = 'h1'
      diffusion_class_representative(2) = 'he3'
      diffusion_class_representative(3) = 'he4'
      diffusion_class_representative(4) = 'c12'
      diffusion_class_representative(5) = 'n14'
      diffusion_class_representative(6) = 'o16'
      diffusion_class_representative(7) = 'ne20'
      diffusion_class_representative(8) = 'mg24'

      diffusion_class_A_max(1) = 2
      diffusion_class_A_max(2) = 3
      diffusion_class_A_max(3) = 4
      diffusion_class_A_max(4) = 12
      diffusion_class_A_max(5) = 14
      diffusion_class_A_max(6) = 16
      diffusion_class_A_max(7) = 20
      diffusion_class_A_max(8) = 10000

I tried with and without diffusion_use_isolve, but it didn't have a
major impact.

Should I be using different diffusion settings to get more realistic
evolutionary tracks?

Thanks for any assistance,
Earl Bellinger
Stellar Ages & Galactic Evolution Group
Max-Planck-Institut für Sonnensystemforschung
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