[mesa-users] Unexpected metallicity profiles, possibly caused by diffusion

[Warrick Ball]

Okay, I think I resolved this one.  The bumps happen in increasingly heavy 
metals, most noticeable in Ne20 and Mg24.  So I tried setting

       diffusion_calculates_ionization = .false. ! instead of .true.

and the profiles are perfectly smooth.

I should point out that the profiles are also *different*, as is the 
evolution.  For example, with ionization, TAMS happens at 6.21 Gyr.
Without ionization, TAMS is at 6.51 Gyr: 5% later.  With ionization, Teff 
at TAMS is 5807 K; without ionization it's 5903 K.  At least part of this 
is because the diffusion speeds come out faster in the model with 
ionization.

Cheers,
Warrick

On Wed, 1 Oct 2014, Warrick Ball wrote:

> Hi all,
>
> I've been looking more closely at the output of some models I've been 
> fitting.  There was one case I looked at closely (for a different, but also 
> interesting reason), and I found that the model has an unexpectedly bumpy the 
> metallicity profile.  I've attached an appropriate inlist, and the results 
> are taken from the final model of that run, at TAMS.  This is a somewhat 
> metal rich star with M=1.17 Msun, Z=0.042, Y=0.3 and alpha=1.91.
> I copied the diffusion parameters out of the solar calibration test case, but 
> I think they're basically the same as the defaults anyway.
>
> Note that this is a revision 6022.  Obviously, a first thing to try 
> (downloading now!) is to check out the most recent release...
>
> The plot shows metallicity Z as a function of logT for a series of models 
> along the main sequence.  As you can see, around logT=6.5, there's a certain 
> bumpiness to the profile.  I immediately suspected the diffusion was to 
> blame, so I added in the diffusion velocities, plotted in the second file. 
> There, I've plotted the magnitude of the diffusion velocity for O16 as a 
> function of logT.  The diffusion is also bumpy, but I a closer look seems to 
> show that this is in the convection zone where it'd be totally overwhelmed. 
> The only structural feature that I think is of interest is the fact that this 
> region is consistently just below the base of the outer convection zone, 
> though I don't use overshooting or semiconvection.
>
> Anyway, any help on what's going on and what I'm doing wrong here would be 
> greatly appreciated.  Or, advice on why this effect is real.
>
> Cheers,
> Warrick
>
>
> ------------
> Warrick Ball
> Postdoc, Institut für Astrophysik Göttingen
> wball at astro.physik.uni-goettingen.de
> +49 (0) 551 39 5069


------------
Warrick Ball
Postdoc, Institut für Astrophysik Göttingen
wball at astro.physik.uni-goettingen.de
+49 (0) 551 39 5069