[Mesa-users] Magnetic field with MESA

[Matteo Cantiello]

Alberto,
The implementation of the Spruit-Tayler dynamo in MESA is described (including relevant references) in Sec. 6 of Paxton et al. 2013 ( http://adsabs.harvard.edu/abs/2013ApJS..208....4P <http://adsabs.harvard.edu/abs/2013ApJS..208....4P> ).
This dynamo operates in radiative regions and taps into differential rotation to produce toroidal and poloidal magnetic fields. 
You can read the amplitude of these components in the output files (if you look into star/defaults/profile_columns.list you’ll find 
!dynamo_log_B_r ! (Gauss)
!dynamo_log_B_phi ! (Gauss)
…)

Note however that the Spruit-Tayler might not be the relevant dynamo process at work in your M stars, which are mostly(fully) convective. 
For sure in your models you won’t find interesting fields, since as I mentioned, the Spruit-Tayler dynamo is only discussed in radiative regions, and MESA doesn’t calculate any ST B-fields in convective regions.

So MESA won’t do it for you, however you can probably estimate magnetic fields in convective regions e.g. assuming equipartition with kinetic energy.
This is usually the case for small Rossby numbers (~ the ratio between stellar rotation period / convective turnover time ). 
If you are not too familiar with these concepts, I advice you first have a look at some introduction on dynamo action theories in convective stellar regions.

Cheers,

-M   

> On Oct 23, 2017, at 2:27 PM, Alberto Rojo Cid via Mesa-users <mesa-users at lists.mesastar.org> wrote:
> 
> Hello everybody,
> 
> I am doing a study on the rotation in M type stars with MESA, and I get the following doubt that I am not able to resolve: I want to observe the effect of the magnetic field during the evolution of the star and I dont know if MESA calculates the magnetic field a from the speed of rotation and the internal conditions of the star, or a certain value must be included through a condition that I am not able to find. For the case of the magnetic field, I think it is included following the Spruit-Tayler dynamo model (see MESA Paxton article et al. 2013, section 6.2) that corresponds to the option “am_nu_ST” of the inlist_project file, but I hoped that someone could confirm me this. I would also like to know if there is any way to obtain the value of the magnetic field in the output files.
> 
> Im running MESA 9575 with Ubuntu 16.04(64 bit)
> 
> Thank you in advance to everyone for your help.
> Greetings,
> Alberto Rojo.
> 
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