[mesa-users] Scratchy surface metallicity profile with diffusion in 1.3Msun star

[Warrick Ball]

Hi all,

Here's the update on what seems to be happening in my abundance evolution 
from yesterday.  For those who missed it, I ran a 1.3Msun model with 
diffusion, and found that the surface metallicity gradually decreased (as 
expected) but then suddenly increased around 3.4 Gyr.  The scratchiness is 
resolved by using the control

       T_mix_limit = 1d4

which marks layers cooler than 10^4K as convective if any layer in that
region is convective.  But what about the upward jump in metallicity?

The attached plot shows the profile of the oxygen abundance as a function 
of 1-q (i.e. mass fraction, reversed).  The profiles go roughly from ZAMS 
to TAMS, in the cycle blue, green, red, cyan, magenta, yellowish, black 
(Python default).  As you can see, the abundance is depleted just below 
the very narrow convection zone.  There are also the peaks that we 
identified as where the diffusion velocity decreases slightly as the 
ionization state of the gas changes.

As the star approaches the end of hydrogen burning, the convection zone 
stars penetrating inwards, which starts to increase the surface oxygen 
abundance, because the layers beneath are less depleted.  However, at the 
yellow plot (around 3.4 Gyr), you can see that the convective zone has 
penetrated into one of these peaks, which gives the sudden upward jump in 
the abundance, of similar order to the height of the peak.

Note that the layer is only very small in mass space.  By radius the 
convective zone is about 5-7% and it extends down to about 10^6K in 
temperature.

Hope other people found this journey into the models interesting!

Cheers,
Warrick





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