[mesa-users] pseudo-Lagrangian scheme in MESA

[Dean Townsley]

Hi Jean-Claude,

As Bill has eluded to, terminology on what is done in the outer layers 
in MESA, and other stellar evolution codes, is, well, messy.

This is also not easy to track through the code, since it is basically 
buried in the computation of the eps_grav term in the energy evolution 
equation and has so many tie-ins to how the grid is being handled.

One note of caution, as mentioned in the third instrument paper, while 
things are now in good shape for the thermal evolution equation, 
composition is not yet addressed appropriately. Pragmatically :), this 
means that accreting material of similar composition to the surface is 
fine, but adding material of very different composition is tricky.  It 
can work, but it is tricky to get started.  I currently believe that 
improving the composition treatment would make it easier to get 
accreting models started, but I haven't had a chance to do anything yet.

As Bill said, please ask any questions you have, since the terminology 
is quite a minefield, and Bill and I might give different answers as to 
why any particular thing is done :)

Dean

On 04/18/2016 08:43 AM, Jean-Claude Passy wrote:
> Hi Bill,
>
> thanks a lot for pointing this out.
> I think Section 7 is very clearly written, and together with Townsley et Bildsten (2004), it answered my questions.
>
> Cheers,
>
> JC
>
>> On 18 Apr 2016, at 01:41, Bill Paxton <paxton at kitp.ucsb.edu> wrote:
>>
>> Hi JC,
>>
>> Dean Townsley and I struggled with the terminology in section 7 of the recent mesa instrument paper where is topic is covered in some detail.  Could you please refer to that with a set of more specific questions.  Dean and I will try to answer, although we aren't likely to completely agree about how to say what we both know is going on in the code!
>>
>> Cheers,
>> Bill
>>
>>
>>
>>
>> On Apr 17, 2016, at 4:35 PM, Jean-Claude Passy wrote:
>>
>>> Dear all,
>>>
>>> I have some conceptual questions regarding the numerical scheme used in MESA for mass loss/accretion.
>>> MESA (among other codes) is said to use a pseudo-lagrangian approach in the outer region of the star to solve the stellar equations. My questions are the following:
>>>
>>> 	1) Does it mean that a) the equations are written and solved as a function of q (so there is an extra term in the partial time derivative); or b) the equations are still written in the Lagrangian form and some rescaling is applied (Section 6.6 in Paxton et al. 2011)?
>>>
>>> 	2) Why doing that? It must be for numerical reasons, and I’d love to get the details.
>>>
>>> 	3) Where are the relevant pieces of code doing that?
>>>
>>>
>>> Hope that someone can clarify things for me.
>>>
>>> Cheers,
>>>
>>> JC
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