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

[David Arnett]

An nice example of things swept under the rug that we uncover with MESA.

Dave

On Wed, Nov 26, 2014 at 7:47 AM, Francis Timmes <fxt44 at mac.com> wrote:

> hi warrick,
>
> thanks for sharing this journey. i learned something new!
>
> fxt
>
>
>
> > On Nov 26, 2014, at 1:53 AM, Warrick Ball <
> wball at astro.physik.uni-goettingen.de> wrote:
> >
> > 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
>
>


-- 
David Arnett
Regents Professor
Steward Observatory
University of Arizona

Facts are stubborn, but statistics are more pliable.  Mark Twain
Facts do not cease to exist because they are ignored.  Aldous Huxley
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