[mesa-users] stellar-structural noise

[Alfred Gautschy]

I attach a plot showing spatial profiles (measured in log T) of
homology invariant U in the deeper interior of a sequence of solar-mass
models evolving off the main sequence. 
Problem: Whatever I do to the steering of MESA, I cannot get rid of the
"noise" in the U profiles.  

I played with various opacity tables (gn93, gs98, a09, OP): to no
avail. I played with the two EoS choices (mesa, macdonald): to no
avail. I played with different nuclear networks: to no avail.  
Changing num_threads, even going down to 1 - no parallelism at all - did
make no difference. Using P rather than rho as principle dependent
variable makes no difference either. I required tighter tolerances in
the convergence of the Newton-Raphson iterations: to no avail at all. It
is interesting to note that the ripples have a systematics which
repeats, slightly shifted in temperature, from one model to the next
(indicated in the attached plot in two cases with light yellow lines).
The regular pattern of the wiggles is somewhat reminiscent of an
underlying grid (not that of the stellar models though!).

Since the "noise" is more expressed in U and much less in V and since
the choice of opacity has essentially no effect, in particular not on
the regularity pattern, I presume the problem is associated with the
density.

The wiggliness of U (in regular PopI or PopII stars) is mostly
confined to roughly 6.5 and 7.3 in log T; a higher temperature window,
however, appears to be involved in helium stars. So it is not unlikely
that this hints at compositional profiles being involved. The amplitudes
of the wiggles diminish considerably outside of the mentioned domain.

I wonder what I am doing wrong here or what point I miss. Is there
anybody around out there in the MESAverse who encountered the same and
who found a way to avoid it. To me, currently, the scattering of the U
profiles are only a minor annoyance, as I cannot achieve the perfectly
smooth-looking models. However, should I aim at *high-precision*
asteroseismology, this behavior likely affects/disturbs some mode
frequencies.

Or is what I plotted here the price we must pay for using tabular EoS
and having to interpolate in them?

BTW: I am using MESA version 4723; relevant parts of an inlist as
usually going into the computations is attached. Star models and
tracks look at first sight as we are used to them from our textbook
experience.

Alfred Gautschy
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! inlist_project 
! contains the parameters controling a MESA job

&star_job
      mesa_dir = '' ! use value from environment variable

      save_star_job_namelist = .false.

      history_columns_file = 'afg_history_col.list' 
      profile_columns_file = 'afg_profile_col.list' 

      pgstar_flag = .true.

      ! Macro-physics part
      !-------------------
      ! use ln P as primary variable, instead of ln rho
      change_lnPgas_flag = .true.
      new_lnPgas_flag    = .true.

      ! Microphysics part
      !------------------
      !change_net        = .true.     
      !new_net_name      = 'o18_and_ne22.net'

      !eos_file_prefix   = 'macdonald'

      kappa_file_prefix = 'gn93'

/ ! end of star_job namelist

&controls
      write_controls_info_with_profile = .false.

      initial_mass = 1.0
      initial_z    = 0.02d0

      mixing_length_alpha = 1.8d0 
      use_Henyey_MLT      = .true.

      ! Newton-Raphson accuracy
      tol_correction_norm = 1d-5
      tol_max_correction  = 1d-3


      ! output specifications
      !-----------------------
      photostep = 500

      profile_interval = 2
      max_num_profile_models = 5000

      history_interval = 1
      terminal_cnt     = 5
      write_header_frequency = 50


      !stop condition
      !--------------
      max_model_number = 1500 


      ! Mass loss processes
      !--------------------
      RGB_wind_scheme = ''
      AGB_wind_scheme = ''

/ ! end of controls namelist
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