[mesa-users] mesa release 4906 -- with radiative levitation

[Bill Paxton]

Hi,

We now have an option to include radiative levitation effects in element diffusion.
The implementation includes routines to calculate opacities for specific abundances
on a cell-by-cell basis during a stellar evolution run.

A big thank you goes to Haili Hu for providing code that guided
the mesa implementation both for radiative levitation and for
the opacities option.   Please reference her paper if you
use these new mesa features:

 Haili Hu, C. A. Tout, E. Glebbeek and M.-A. Dupret,
 "Slowing down atomic diffusion in subdwarf B stars: mass loss or turbulence?",
 Mon. Not. R. Astron. Soc. 418, 195–205 (2011)


The new opacity options are a part of the kap module and can be used 
from mesa/star for evolution independently of radiative levitation.   
Since it is doing a lot of computation on the fly rather than
using prebuilt tables, it is much slower, but there are cases where the standard
faster scheme just won't do the job.  Here's an example -- this is an sdb star where
radiative levitation has pushed fe56 and ni58 to the surface where their contribution
to the opacity results in a small convective zone in the outer 10^-10 Msun of the envelope.
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That plot represents basically all of the testing that I've done!
It seems to be okay, but it is up to you the users to verify that
it is actually working before you publish.  ;-)

There are 2 new test cases in star/test_suite related to this.
The "make_sdb" case starts from just before he core flash
at tip of RGB and evolves with an extra strong wind to remove
the envelope as the star begins core helium burning.
The "sdb" test case continues this with radiative levitation
turned on and evolves long enough to develop the distinctive 
surface convective layer enriched with fe56 and ni58.



The new opacities use data from the OP website.  Because it is
such a large amount of data we've decided not to include it in the
standard mesa download.  You'll need to get it yourself if you
decide to try this:  http://cdsweb.u-strasbg.fr/topbase/TheOP.html
Get the 669 Mb tar file for OPCD_3.3

	http://cdsweb.u-strasbg.fr/topbase/OPCD_3.3.tar

Put it anyplace you want on your disk.  Set the inlist controls
to tell mesa where to find the "mono" directory with the data files.
(It's the directory with files named like "a06.140")
For example, in my case it looks like this, but you can put
the files anywhere you like -- they don't need to be in the
mesa/data directory.

         op_mono_data_path = '/Users/bpaxton/mesa/data/kap_data/op_mono'
         op_mono_data_cache_filename = '/Users/bpaxton/mesa/data/kap_data/op_mono_cache.bin'

There are some controls for how the mono kap option is used:

         ! op mono full on if log10T <= logT_op_mono_full_on
         ! op mono full off if log10T >= logT_op_mono_full_off
         logT_op_mono_full_off = -1d99
         logT_op_mono_full_on = -1d99
         
         op_mono_min_X_to_include = 1d-20 ! skip iso if mass fraction < this
         
         use_op_mono_alt_get_kap = .false.

Check mesa/kap/public/kap_lib for more information.


The addition of radiative levitation was an opportunity to rework element
diffusion that I couldn't resist.   For example, there is no longer a
separate "diffusion" module -- it has been eaten by mesa/star.

If you use diffusion, you'll need to do a careful check to find out 
what I've broken in the process.  One big change is that the code 
is much better at conserving total species abundances (people 
mentioned that the old version sometimes did alchemy by changing 
one element into another).  We also now have the option to use 
atomic diffusion coefficients according to Paquette et al. (1986)
instead of assuming a pure Coulomb treatment.

There are several test cases that use diffusion, and they are all working okay.
For example, here are the final abundances at the end of the wd_diffusion test case.
We're getting much cleaner results now down to very low abundances.
The abrupt break at the bottom of the envelope is because we explicitly
turned off diffusion for Z > 0.5.   That is a help with performance for
a case like this where our focus is on the envelope.
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As usual, with improvements come bugs, so be on the lookout.

Cheers,
Bill