[Mesa-users] magnetic breaking in hot stars

[Kareem El-Badry]

Thanks, Josiah. Indeed I did almost exactly what you suggested and it seems
to work fine. Following Podsiadlowski+2002
<https://ui.adsabs.harvard.edu/abs/2002ApJ...565.1107P/abstract>, I check
the mass fraction "q_conv" of the convective envelope, and, if it's below
0.02, multiply the magnetic braking torque by a factor exp(1 -
q_conv/0.02).

But yes, It might be good to add a note somewhere to the documentation
that, as far as I can tell, the code doesn't check for a convective
envelope by default.

On Wed, Sep 2, 2020 at 7:00 AM Josiah Schwab <jwschwab at ucsc.edu> wrote:

> Hi Kareem,
>
> In Section 2.2.4 of Paxton et al. 2015, it says:
>>
>> It is normally assumed that once a star becomes fully convective, the
>>> dynamo process that regenerates the field will stop working or at least
>>> behave in a qualitatively different
>>> manner. Similarly, magnetic fields in stars with radiative envelopes are
>>> of a significantly different nature than those seen in stars with
>>> convective envelopes, and there is no simple way
>>> to predict even the presence of magnetism (Donati & Landstreet 2009). By
>>> default, MESAbinary only accounts for magnetic braking as long as the star
>>> being braked *has a convective envelope and a radiative core,* though
>>> the process might still operate outside of these conditions.
>>>
>>
>> (emphasis mine). However, I have found that with do_jdot_mb = .true.
>> and keep_mb_on = .false., magnetic braking seems to operate even when the
>> donor is hot (say, Teff = 10^4 K) and has a radiative envelope.  Poking
>> around binary_jdot.f90, there is a check_radiative_core() function, but I
>> don't see any obvious check whether the envelope is convection.
>>
>> Is this the desired implementation?
>>
>
> Sounds a bit surprising that the code doesn't match the instrument paper
> description.  Pablo is on vacation right now, but when he returns, he may
> be able to give more insight about the intentions.
>
> Is there a good way to turn off mb when the donor has a radiative envelope
>> (i.e. is above the Kraft break)?
>>
>
> You should be able to add your own test in run_binary_extras.  You can
> access the star_info structure for the donor star with b% s_donor and then
> manually toggle magnetic braking.
>
> You might add something like this to extras_binary_finish_step
>
>          ! will be false when envelope is radiative
>          b% do_jdot_mb = b% do_jdot_mb .and. has_convective_envelope(b%
> s_donor)
>
> and then define a function that looks at the stellar structure and decides
> if there's a convective envelope
>
>       ! return true if envelope is convective
>       logical function has_convective_envelope(s)
>          type(star_info), pointer :: s
>          has_convective_envelope = ???
>       end function has_convective_envelope
>
> you could start with just a simple test on Teff (i.e.,
> has_convective_envelope = s% Teff < 6200) to check if this approach does
> what you want and then make it more elaborate if needed.
>
> Hope that helps,
> Josiah
>
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