[mesa-users] Curious generation of Hydrogen - Interplay of convection and nuclear burning

[Jan-Torge S. <jts...@em...>]

Hi,

motivated by Joseph's email I wanted to report a curious behaviour of 
MESA that I encountered in my simulations. I have to say that I am using 
version 4442 at the moment which is outdated. But the behaviour I 
encountered also involves nuclear burning and convection and might be 
interesting for current discussions.

I am working on the simulation of subdwarfs B stars. The first inlist 
starts the evolution of the progenitor star and saves a model at model 
number 16000. With the second inlist I load this model and then take off 
most of the envelope using the relax mass option. The two inlists are 
attached.

Running the second inlist the star contracts further until it encounters 
a Helium flash and then starts Helium core burning. At least this is, 
what it in my opinion is supposed to do (and in most models does).
But there are some models in which Hydrogen is erratically generated. 
There is no process in my current understanding that would generate 
Hydrogen in a Helium flash.

I suggest that this behaviour is a consequence of how MESA handles the 
interplay of convection and nuclear burning.

The attachments are the two inlists, one plot showing the h1_total_mass 
vs. model number and three plots showing convective diagrams with some 
general properties of the stars. The three convective plots correspond 
to the models portrayed in the H1 mass plot.

I am well aware that this problem might not exist with newer versions of 
MESA but if it does it would be interesting to find out what exactly 
happens here. I would like to find out if it's a problem of the code, a 
problem of not well enough understood physics or my fault by mostly 
using just the MESA star default parameters.

As soon as I update my MESA version I will report back if this problem 
still exists. Meanwhile I am open for suggestions and comments to solve 
this mystery.

Thank you for your attention,
best regards,

Jan-Torge Schindler


On 02/13/2013 10:00 AM, Joseph Wang wrote:
> Hi there,
>
> I'm curious about what you've seen, and I was wondering if you could
> send me your infiles.  Something
> that I've found to be useful in looking at convective related issues
> is to plot out the convective zones.
>
> One thing that I've noticed in massive stars is how quickly the
> convective structure of the star can
> change once you hit a nuclear threshold.  Say you have a massive star
> with a large convective core
> that's burning hydrogen.  The convective core insures that there is a
> fresh supply of hydrogen.  Then
> the interior of the star hits stars burning helium.  What ends up
> happening is that you end up with
> carbon ash in the center of the star, and since this ash isn't
> radiating this causes the center to suddenly
> becomes convectively stable.  Once the center of the star stops
> mixing, the nuclear burning shell
> quickly burns up the the remaining unburned material, creating more
> ash, which causes the radiative
> shell to move outward, and you can very quickly have the convective
> core disappear.  Now if you
> put in overshooting or Ledoux, then the interaction between the
> burning and the mixing is different.
>
> Once you hit more nuclear thresholds, things get very complicated, as
> you have an interaction between
> nuclear burning and convection, and I've seen runs where there are
> bursts of convection which move
> material around and change the nuclear burning which changes the
> convective regions, and there
> are situations in which I suspect are chaotic.
>
> I haven't seen how and if this affects the H-R diagram, so I'm quite
> interested in running your models
> and seeing what happens.  In addition, my quick impression is that the
> code is "misbehaving" but
> it's misbehaving for reasons that telling us something deep and
> significant about how stars work.
>
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