[Mesa-users] Fwd: [mesa-users] Understanding gravitational settling
Evan Bauer
ebauer at physics.ucsb.edu
Thu Aug 16 15:58:44 EDT 2018
Just realized this thread was from before the list migration, so resending an attempt to set the record straight:
> Begin forwarded message:
>
> From: Evan Bauer <ebauer at physics.ucsb.edu>
> Subject: Re: [mesa-users] Understanding gravitational settling
> Date: August 16, 2018 at 12:55:23 PM PDT
> To: Warrick Ball <wball at bison.ph.bham.ac.uk>
> Cc: Anne Thoul <anne.thoul at ulg.ac.be>, MESA Users List <mesa-users at lists.sourceforge.net>
>
> This thread came up during MESA Summer School yesterday, so I wanted to make sure to update the record for anybody searching back through old threads.
>
> Since Warrick’s original post here, the thermal diffusion terms have been added into the newer diffusion solver (diffusion_use_cgs_solver = .true.). The attached plot shows Fig 15 from the 4th MESA Instrument paper, verifying that both the old and new diffusion solvers give the same results for main sequence stars unless you actively disable the thermal diffusion terms in the new solver.
>
> Cheers,
> Evan
>
>
>
>> On Mar 8, 2017, at 12:44 AM, Warrick Ball <wball at bison.ph.bham.ac.uk> wrote:
>>
>> Hi Anne and Evan,
>>
>> Thanks very much for the quick and useful replies. I had already tried the diffusion_do_thermal_diffusion option but saw no change, which is obvious if it isn't implemented yet. I'll stick to the implementation from Thoul et al. (1994), as advised.
>>
>> Also, my apologies to Evan: I did search the forum archive but my choice of keywords (or a lack of coffee) must've made me miss your post.
>>
>> Cheers,
>> Warrick
>>
>>
>>
>> ------------
>> Warrick Ball
>> Postdoc, School of Physics and Astronomy
>> University of Birmingham, Edgbaston, Birmingham B15 2TT
>> wball at bison.ph.bham.ac.uk
>> +44 (0)121 414 4552
>>
>> On Tue, 7 Mar 2017, Evan Bauer wrote:
>>
>>> You beat me to it. The “cgs” solver does not include thermal diffusion terms from the Burgers equations (yet). So far, I’ve mostly been using it for the degenerate interior of WDs where those terms are
>>> unimportant/wrong. Of course, they’re quite important on the main sequence, and Warrick’s plot shows about the difference I would expect from neglecting them. The Thoul et al. (1994) solver is certainly the
>>> correct one to use in this circumstance. I’ve attempted to address the current status and motivation for different diffusion solver options in the notes attached to this mailing list
>>> post: https://sourceforge.net/p/mesa/mailman/message/35626929/.
>>> As you point out, there is an option that allows setting diffusion_do_thermal_diffusion = .true., however it doesn’t actually do anything right now. That’s why it’s false by default. I haven’t yet finished
>>> adding the code to include the thermal diffusion terms in the Burgers equations. I should clarify that in the comments in the defaults file…
>>> Cheers,
>>> Evan
>>>
>>> On Mar 7, 2017, at 9:47 AM, anne.thoul at ulg.ac.be wrote:
>>> Question: Does the cgs solver include the heat diffusion terms in Burger’s equation?
>>> If not, it could explain the difference.
>>> As I understand the default values, you have to set diffusion_do_thermal_diffusion = .true. to include the heat terms when using the cgs solver. Default value is .false.
>>> Anne
>>>
>>> Le 7 mars 2017 à 08:53, Warrick Ball <wball at bison.ph.bham.ac.uk> a écrit :
>>>
>>> Hi everyone,
>>>
>>> I've been playing around with calibrating solar models and found myself surprised at how much difference the different formulations of gravitational settling can make. I've attached a plot
>>> showing the surface oxygen abundance as a function of age for four 1.25 Msun models using a standard work folder in r9575 and a choice of either:
>>>
>>> 1. no diffusion (do_element_diffusion = .false.)
>>>
>>> 2. the Thoul et al. (1994) formulation (do_element_diffusion = .true.,
>>> everything else default),
>>>
>>> 3. the cgs solver (diffusion_use_cgs_solver = .true.), or
>>>
>>> 4. the Thoul et al. (1994) formulation with Iben & Macdonald (1985)
>>> coefficients (instead of Paquette et al. 1986).
>>>
>>> An example inlist is also attached.
>>>
>>> What surprised me was that switching from the Thoul et al. (1994) formulation to the direct solution of the Burgers equations (the cgs solver, if I understood correctly) changed the maximum
>>> depletion by more than 60%. Is there a simple reason for this? Are there good reasons to use one formulation rather than the other for Sun-like stars? I'm not very familiar with the
>>> literature on the subject so I'm going to start looking into it (and the code) in more detail now. But I'm hoping the vast collective wisdom of this forum might steer me in the right
>>> direction...
>>>
>>> Cheers,
>>> Warrick
>>>
>>> ------------
>>> Warrick Ball
>>> Postdoc, School of Physics and Astronomy
>>> University of Birmingham, Edgbaston, Birmingham B15 2TT
>>> wball at bison.ph.bham.ac.uk
>>> +44 (0)121 414 4552<t_XO_diffusion.png><inlist_project.txt>------------------------------------------------------------------------------
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