[mesa-users] asteroseismic application of mesa results

Bill Paxton paxton at kitp.ucsb.edu
Sat Jun 29 13:56:52 EDT 2013


On Jun 28, 2013, at 9:10 AM, G J Savonije wrote:

> hi Bill,
> 
> There remains a problem with the asteroseismic application of mesa results.
> In the region outside the retreating convective core with a composition gradient (6 Msun MS star)
> the composition and density profile is not very smooth but rather wiggly (as usual with evolution codes) .
> This gives rather fluctuating  density (and pressure) derivatives in that region which introduces noise
> in the oscillation calculations.
> 
> In the "old" version of say 2 years ago I managed to cure that but in the current version I am stuck.
> Is there a simple way to smooth the composition profile (eg by an artificially enhanced diffusion coefficient) in that region during the evolution calculations?
> 
> regards,
> Gertjan Savonije


Hi,

I've Cc'd mesa-users since this is a topic of wide interest.  And the results are quite striking.

These plots are from the test case you sent -- 6 Msun from zams until Teff reaches about 16,000.

Using the basic defaults results in a "stair-case" in the He4 abundance behind the retreating
center convection zone.  And the derivatives of stair steps are nasty for brunt!
So what can be done?  Here's one possible answer.

Starting from the basic defaults, we'll do a series of additions:
1) turn on element diffusion,
2) switch from density to gas pressure as basic variable, and
3) change from pure Schw. to Ledoux with semiconvection.

In practice, you might do fine just by switching on semiconvection
without doing the other changes.  You'll have to try it and judge for yourself.

Attached are plots of the He4 composition and the brunt N2 composition term
to show the abundance gradients.

Cheers,
Bill


initial -- "vanilla" -- just pure Schwarzschild and no element diffusion.
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with element diffusion enabled

      do_element_diffusion = .true. ! determines whether or not we do diffusion      
      diffusion_calculates_ionization = .true.
      diffusion_num_classes = 4 ! number of classes of species for diffusion calculations
      diffusion_class_representative(1) = 'h1'
      diffusion_class_representative(2) = 'he4'
      diffusion_class_representative(3) = 'o16'
      diffusion_class_representative(4) = 'fe56'
      ! in ascending order.  species goes into 1st class with A_max >= species A
      diffusion_class_A_max(1) = 2
      diffusion_class_A_max(2) = 4
      diffusion_class_A_max(3) = 16
      diffusion_class_A_max(4) = 10000

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next, switch from density to gas pressure as basic variable.

      change_lnPgas_flag = .true.
      new_lnPgas_flag = .true.

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finally, change from pure Schw. to Ledoux with semiconvection

      use_Ledoux_criterion = .true.
      alpha_semiconvection = 1d-3
      semiconvection_option = 'Langer_85'
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