[mesa-users] Low mixing length alpha values -> convergence problems

[Lewis I. <lg...@ex...>]

Hello,

I am using the work directory (tutorial folder) to create 0.5 solar mass models, all at different mixing length parameters (to see the change in R when it nears ZAMS). I have these set:

! begin with a pre-main sequence model
    create_pre_main_sequence_model = .true.

  ! starting specifications
    initial_mass = 0.5d0 ! in Msun units

mixing_length_alpha = 0.55d0

I have the following set to end the run:  

  ! stop when the star nears ZAMS (Lnuc/L > 0.99)
    Lnuc_div_L_zams_limit = 0.99d0
    stop_near_zams = .true.

  ! stop when the center mass fraction of h1 drops below this limit
    xa_central_lower_limit_species(1) = 'h1'
    xa_central_lower_limit(1) = 1d-3

 I need to go to real low values of mixing length parameter, e.g. 0.1, 0.01 etc., but when I go to or below alpha = 0.5, I get:

hydro_newton_step failed to converge
       retry cnt, step, log10(dt/yr), retry_factor     1   543      -14.045150        0.500000
                                                     dt    5.6884209599392411D-07
                                     min_timestep_limit    9.9999999999999995D-07

 stopping because of convergence problems dt < min_timestep_limit


terminated evolution: convergence problems

termination code: min_timestep_limit

How can I go about using lower values of alpha without this problem? Is there another test_suite you recommend me to use that would benefit my models or get past this convergence issue?

Cheers,

Lewis