[Mesa-users] How do I change the initial_z value of a star module

[Bill Wolf]

Hi James,

As the person who ran those simulations in the first place, feel free to reach out to me off-list for additional help.

From a more practical standpoint, I’m not sure what you’re trying to do. Changing the metallicity of the whole model is difficult as indicated by Evan and Pablo, and in my opinion, probably won’t do anything interesting except in the tiny surface accreted layers. And that change will be quickly washed away by the freshly accreted material with the older composition. Finally, I don’t think setting initial_z does anything if you are starting from a saved model. I think that only does anything interesting for creating new models (same goes for initial_mass, but others should correct me if I’m wrong).

The interesting question that you’re probably trying to answer is how the outcomes change when accreting more metal-rich material. Changing the metallicity of the model by whatever means won’t do this, and should instead be done by adjusting the accretion fractions via the following controls (which I think are already in those inlists:

      accrete_same_as_surface = .false.

      accretion_h1 = 0
      accretion_h2 = 0
      accretion_he3 = 0
      accretion_he4 = 0
         !### accretion_zfracs =

         ! One of the following identifiers for different Z fractions from `chem_def`.

         ! + `AG89_zfracs = 1`, Anders & Grevesse 1989
         ! + `GN93_zfracs = 2`, Grevesse & Noels 1993
         ! + `GS98_zfracs = 3`, Grevesse & Sauval 1998
         ! + `L03_zfracs = 4`, Lodders 2003
         ! + `AGS05_zfracs = 5`, Asplund, Grevesse & Sauval 2005

         ! or set `accretion_zfracs = 0` to use the following list of z fractions

      accretion_zfracs = -1


Pick your h1, h2, he3, and he4 fractions such that 1.0 - (h1 + h2 + he3 + he4) = [YOUR DESIRED METALLICITY]. Maybe start with a steadily-burning model and change the accretion composition while its easy to cycle through the material and wait until all the “old” material has been burned through, and then start your exploration by changing mass_change to the values you care about.

If it truly is your desire to start from a more metal-poor white dwarf, I think Pablo is right that you should start from pre-main sequence with a different metallicity. Use something like the make_co_wd test case with a different initial_z and follow the procedures I gave in the paper/repository for getting accretion started.

Regards,

Bill

On April 8, 2018 at 4:48:27 PM, Evan Bauer (ebauer at physics.ucsb.edu) wrote:


On Apr 8, 2018, at 5:56 AM, James via Mesa-users <mesa-users at lists.mesastar.org> wrote:

Also, could I ask why the initial_Z is limited to 0 to 0.04? 

Hi James,

To build on what Pablo said, upgrading to the latest MESA version (10398) is almost certainly the best way to improve your situation. The basic reason that metallicity higher than 0.04 could be problematic on the main sequence for older versions of MESA is that OPAL EOS tables only go up to Z=0.04. Higher metallicity requires extrapolating off the OPAL tables or blending over into another EOS. New EOS options were introduced with release 10398 that greatly improve the EOS situation for regimes not covered by the OPAL tables.

A little more detail can be found in the release notes: https://lists.mesastar.org/pipermail/mesa-users/2018-March/008812.html

Cheers,
Evan
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