Strange eigenfunctions (l=9) for subgiant model

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Strange eigenfunctions (l=9) for subgiant model

Postby wiebke » Tue Jun 24, 2014 9:38 am

Hi Rich,

I am having some trouble with the eigenfunction calculations for a subgiant (MESA-) model using GYRE. During the calculation of l=9 modes I noticed some improper values for n_pg. I tried to fix this issue by playing around with the different grid options as proposed on the GYRE website 'Understanding grids', but in this case it did not help. Even worse, when keeping the model grid the same (with RESAMP_DISPERSION enabled) and just using a finer frequency scan grid (n_freq=1500 -> n_freq=3500) I got the same frequencies as a result, but for some of them different eigenfunctions.
Even for some frequencies where the frequency values are exactly the same for both cases of n_freq, and the n_pg values are also the same and seem to be correct, there are differences in the eigenfunctions depending on the parameter n_freq!

I then had a closer look at the differing eigenfunctions, and it turns out that for the same frequency, they show in principle the same oscillatory behavior, but the p-mode part is shifted somehow (see attached figure "xir_mode17.png"), so the oscillations are not centered around zero anymore (which actually would explain the problem with the n_pg values in some of the cases, when the shift gets so large that the whole function is shifted above/below zero). In the file "diff_xir_mode17.png" the difference of the two eigenfunctions is plotted and you can see, that in the inner part, the eigenfunctions are nearly identical, but the difference gets larger in the outer part of the model. When zooming into the g-mode region, close to the core, there is actually also a difference present, but it is very small (~1e-10, see "diff_xir_mode17_core.png").

I find it quite unsettling that this whole problem occurs only for some of the modes but not all of them: for 10 of the 17 modes that were calculated, the eigenfunctions are exactly identical for both settings of n_freq (as I would expect).

Do you know what might cause this behavior of the eigenfunctions? Or how to solve this problem? I would be grateful for any help. Attached you can find the MESA model file and GYRE inlist I used.

Best wishes,
xir_mode17.png (49.79 KiB) Viewed 80 times
diff_xir_mode17.png (32.93 KiB) Viewed 80 times
diff_xir_mode17_core.png (32.2 KiB) Viewed 80 times
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Re: Strange eigenfunctions (l=9) for subgiant model

Postby rhtownsend » Sun Jul 06, 2014 10:47 am

Dear Wiebke --

Apologies for the delay in answering you, I've been trying to come up with a better answer than the one I give here: the problem is with the model, and not with GYRE.

Experience over the past few years has shown that small departures from hydrostatic equilibrium, and other equilibria, in the input stellar model can cause distortions to the mode eigenfunctions, which in turn lead to mode mis-classifications. Figuring out how to suppress these departures is an area of ongoing research; but as a possible workaround, you might want to try increasing the number of grid points MESA is using for the model, to ensure that it's properly resolving the stellar structure.

Best wishes,

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