[mesa-users] Understanding gravitational settling

[Warrick Ball]

Hi everyone,

I've been playing around with calibrating solar models and found myself 
surprised at how much difference the different formulations of 
gravitational settling can make.  I've attached a plot showing the surface 
oxygen abundance as a function of age for four 1.25 Msun models using a 
standard work folder in r9575 and a choice of either:

1. no diffusion (do_element_diffusion = .false.)

2. the Thoul et al. (1994) formulation (do_element_diffusion = .true.,
    everything else default),

3. the cgs solver (diffusion_use_cgs_solver = .true.), or

4. the Thoul et al. (1994) formulation with Iben & Macdonald (1985)
    coefficients (instead of Paquette et al. 1986).

An example inlist is also attached.

What surprised me was that switching from the Thoul et al. (1994) 
formulation to the direct solution of the Burgers equations (the cgs 
solver, if I understood correctly) changed the maximum depletion by more 
than 60%.  Is there a simple reason for this?  Are there good reasons to 
use one formulation rather than the other for Sun-like stars?  I'm not 
very familiar with the literature on the subject so I'm going to start 
looking into it (and the code) in more detail now.  But I'm hoping the 
vast collective wisdom of this forum might steer me in the right 
direction...

Cheers,
Warrick


------------
Warrick Ball
Postdoc, School of Physics and Astronomy
University of Birmingham, Edgbaston, Birmingham B15 2TT
wball at bison.ph.bham.ac.uk
+44 (0)121 414 4552
-------------- next part --------------
A non-text attachment was scrubbed...
Name: t_XO_diffusion.png
Type: image/png
Size: 268612 bytes
Desc: 
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20170307/37f551b3/attachment.png>
-------------- next part --------------
&star_job

  ! begin with a pre-main sequence model
    create_pre_main_sequence_model = .true.
    pre_ms_T_c = 9d5

/ !end of star_job namelist


&controls

  ! starting specifications
    initial_mass = 1.25 ! in Msun units

  ! 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

    do_element_diffusion = .true.
    diffusion_use_iben_macdonald = .true.
    diffusion_use_cgs_solver = .false.

/ ! end of controls namelist