[mesa-users] Odd problem with semiconvective regions

Kevin Moore klmoore at soe.ucsc.edu
Thu Jan 22 17:10:38 EST 2015


When I’m looking at relatively inefficient semiconvection I usually do something similar, setting remove_small_D_limit to something < 1d-8 and have not run into any problems with semiconvective regions being removed or anything else (yet). I'd agree that setting the default to something smaller would be better since it also took me some time to notice this same effect.


On Jan 22, 2015, at 2:01 PM, Bill Paxton <paxton at kitp.ucsb.edu> wrote:

> 
> On Jan 21, 2015, at 11:51 AM, Pablo Marchant wrote:
> 
>> Hi Bill, could you help me a bit with this odd issue I have?
>> 
>> I'm dealing with some massive binaries that accrete plenty of mass, which causes the superadiabatic core to increase in size. However, as there is a mu gradient due to the receding convective core before accretion, semiconvection is needed for rejuvenation to actually proceed. I'm running some systems to model this and I find that when the semiconvective region is just reduced to a few zones, MESA stops considering those as semiconvective mixing regions.
>> 
>> I attach a simple saved model + inlist to show this (MESA version 7385). The saved profile for model number 3002 (the saved model starts at 3001) shows that between zones ~920-940 there is a region that is ledoux stable and swarzschild unstable, so it should be marked as semiconvective. However most of it is marked as a zone with no mixing, and the mixing coefficient is zero there. Is there any odd control that should produce this result? I couldnt find any... Anyhow, I'm not that familiar on debugging the mlt module to check these things.
>> 
> 
> 
> 
> This is a nice one, so I"m Cc'ing mesa-users.
> 
> The question is about why we don't get semiconvection in a region where we expect to see it.
> 
> Here's the mu gradient:
> <mu.pdf>
> Here's the plot of mixing coefficients showing a no-mixing gap where we'd expect to see semiconvection.
> <mixing_Ds.pdf>
> Note the abrupt cutoff in semiconvection -- turns out there is yet another knob in mesa for setting a min for D.
> 
> 
>         !### remove_small_D_limit
> 
>         ! If MLT diffusion coeff D (cm^2/sec) is less than this limit,
>         ! then set D to zero and change the point to `mixing_type == no_mixing`.
> 
>      remove_small_D_limit = 5
> 
> 
> When I change that and rerun, we get something more like what we expected:
> 
>    remove_small_D_limit = 1d-6
> 
> <mixing_Ds.pdf>
> 
> However note that we've also widened the gap --- or at least it is wider in terms of cells.
> 
> It seems it might be a good idea for me to drop the default value for remove_small_D_limit -- wonder what that will cause to break?
> 
> Cheers,
> Bill
> 
> 
> 
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