[mesa-users] Too much mesh points

Ehsan Moravveji e.moravveji at gmail.com
Mon Aug 8 06:02:53 EDT 2016

Dear Dave,

Thanks for your insightful remarks; it is always great to hear your words.
Indeed, nothing fills up the void of "missing physics” in massive star evolution, but the physics itself; to be developed, implemented and tested against observations.
Thus, I never plea to increasing number of mesh points, to compensate for our lack of knowledge of stellar interiors.

The reason for tuning the number of mesh is that I pipe MESA output into GYRE, for later seismic modelling of beta Cephei and SPB stars.
Back in 2008, and in preparation for seismic interpretation of CoRoT data, an interesting exercise was done among several pulsation code developers, and it is published in Moya et al. (2008, Ap&SS) <http://adsabs.harvard.edu/abs/2008Ap&SS.316..231M>. I quote from their abstract:

“… Two equilibrium models with different grids, 2172 and 4042 mesh points, have been used, .... Comparing the results for these two models illustrates the effect of the number of mesh points and their distribution in particularly critical parts of the model, such as the steep composition gradient outside the convective core. ...”

So, to prepare for modelling Kepler data (which exceeds CoRoT in precision), we have to ensure that such numerical issues are better taken care of. That is the only reason that I put several mesh adjustment weights around "regions of interest”, which is actually one of MESA’s niches.

If you’re still reading his email with interest, then, I have attached a figure I made quite some time ago, showing the influence of meshing in MESA on the period spacing of g-modes in SPB models (3 Msun). All used models have identical physics (same inlist, same age). The only difference is in the meshing: in the first 6 top panels, I only change the mesh_delta_coeff (delta) between 1 and 0.02, giving 863 to 47000 mesh points. A visual inspection quickly tells how the dips in the period spacing (Delta P) are modified. In the latter two (bottom) panels, I use MESA’s mesh refinements with a modest mesh_delta_coeff value. Then, the Delta P for the last model with 4100 mesh looks “similar” to some of high-resolution models (in the observed SPB period range of 1 to 3 days). Then, I stop adding more mesh.

The Kepler SPB stars (with tens of observable g-modes easily probing the near-core environment) are only one among many other applications, where going beyond MLT is urgently needed. We’re desperately waiting for it … 
Fortunately, asteroseismology can quantitatively test boundary mixing. 

Thanks again Dave for your motivating email.


> On 07 Aug 2016, at 17:46, David Arnett <wdarnett at gmail.com> wrote:
> Hi Ehsan,
> You are asking for more resolution than the physics in MESA can provide. MESA is great, but it is only as good as the accepted science we give Bill to implement. Mixing-length theory (MLT) fails at convective boundaries (it is singular there, and as 3D simulations have shown a boundary layer develops, for braking). None of this is in MLT, so we have to use various patches to get over this embarrassment. Finer zoning does not capture missing physics. I suspect that the default mesh would contain as much correct information as the 3,000 mesh points you seek.  
> If you wish to do convective boundaries better, you must go beyond MLT.
> -- 
> 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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