[mesa-users] On the MLT++

[Mathieu]

Hi Bill,

Thanks, as always your answer is rapid and enlightening.
One could interpret the "plus 1" as convection (described using 
classical MLT) plus the unknown mechanism carrying the L_unknown!
It seems quite obvious that a change in the local temperature gradient 
will translate in a change of the stellar structure and in the end of 
its properties (such as total luminosity, and effective temperature).
If anyone has a deeper understanding of how these changes happen, I 
would be very interested in knowing more.

Thank you again!

Mathieu

On 10/28/2014 05:21 PM, Bill Paxton wrote:
> On Oct 28, 2014, at 4:08 PM, Mathieu wrote:
>
>> Hi everyone,
>>
>> I would like some more detailed information regarding the MLT++, that I
>> could not find neither in the second MESA paper, nor in the documentation.
>>
>> First of all: Why call it MLT*++*? As far as I understand, it is just a
>> trick to help convergence and avoid pressure inversions which in
>> principle could be physical (or something like mass loss, or photon
>> bubbles could prevent them).
> My naming was not meant to be profoundly significant.
> But the idea is that MLT++ is MLT plus 1, (following C language notation),
> where the addition is the option to limit the superadiabaticity --
> or, equivalently, to limit the inefficiency.
>
>> Second, and more important: What happens to the energy flux when MLT++
>> sets in?
> Recall that the MLT routine gets L as an argument and returns gradT,
> the expected dlnT/dlnP, as a result. MLT++ can tweak the gradT, but
> again, it doesn't directly set L.  L happens as part of the overall solution.
> It emerges from the newton iterations along with the profiles for T and P.
>
>> MLT++ artificially decreases the superadiabaticity to enforce a
>> near-adiabatic stratification of the structure, so what happens in
>> convective regions where the superadiabaticity is reduced? Will MESA
>> shut down convection there (it seems to me it doesn't)? And what about
>> the convective energy flux, which is proportional to the
>> superadiabaticity to the 3/2-power? Is it changed by the MLT++, and if
>> yes, does the reduced flux imply that a larger amount of energy is
>> trapped inside the star?
> I hope that this question will be resolved by the previous comments.
> L is set; MLT and MLT++ decide how it will be split between L_conv + L_rad.
> The reduction in superadiabaticity means that L_conv gets larger and
> L_rad gets smaller, but the sum remains = L.
>
> Of course, that's on the local level -- for a particular point at a particular location.
> The global impact is a different question.  The increase in convective efficiency
> from MLT++ might well lead to structural changes at a global stellar level
> that might includes changes in total luminosity --- since it can be expected
> to lead to changes in T gradients, it wouldn't be surprise to me if it also changed L.
> But keep in mind that I'm a computer scientist with just a tiny smattering
> of knowledge about stars.  So I better stick just to answering questions about the code!  ;D
> Perhaps someone else will step forward to explain how things might change
> in response to a less steep temperature gradient.
>
>
>>  From this email exchange
>> http://sourceforge.net/p/mesa/mailman/message/32229940/ it seems that
>> the energy is nevertheless carried out by the unknown mechanism
>> represented by the MLT++, but I am still confused.
> The idea is just that in MLT++ we artificially boost L_conv in order to reduce L_rad while keeping the sum L_conv + L_rad = L.
> But one can equally well view this as leaving L_conv small as in standard MLT and introducing L_unknown so that
> L_conv_MLT + L_rad + L_unknown = L.  This allows a smaller L_rad than the L_rad_MLT = L - L_conv_MLT.
> Compared to standard MLT, we've reduced L_rad by the amount L_unknown.  In that view, convection remains inefficient,
> L_conv remains small as set by MLT, but L_rad doesn't have to make up for all of the inefficiency of convection.
> Some unknown mechanism comes into play to carry the difference L - (L_conv_MLT + L_rad).
> This unknown mechanism seems to enter only when the convection region is radiation dominated and there is a high L/M ratio.
> Strong mass loss may also be a part of all of this.  And maybe all of this is a numerical side effect of doing 1D.
> Perhaps it would all go away in 3D, and something would prevent the situations that lead to problems we're solving with MLT++.
> We really need something better to use in our 1D codes.  MLT has been great.  But we may be running into its limits.
>
> But see my warning above -- don't believe anything I say about anything other than what is going on in the code. ;D
>
> Does that help, or is it all just as confusing as before?
>
>> Thanks in advance,
>>
>> Mathieu
>>
> Cheers,
> Bill
>
>
>
>
>
>>
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