[Mesa-users] Relaxing angular momentum

[Morgan Taylor]

Thank you for explaining Pablo!  I will talk to Frank about how to proceed.

Thanks,
Morgan

On Wed, Oct 23, 2019 at 10:35 PM Pablo Marchant <pamarca at gmail.com> wrote:

>
>
> On Wed, Oct 23, 2019, 23:06 Morgan Taylor <taylormorgan32 at gmail.com>
> wrote:
>
>> Hi Pablo, Roque, all,
>>
>> I'll jump in with another question if that's okay.  I'm working with
>> Frank and we want to relax the composition starting at a hot white dwarf.
>> I should be able to obtain the thermal timescale, but when I do, is there a
>> rule of thumb on how many steps to relax the composition? Also, I know the
>> relaxation timescale should be much less than the thermal, but I don't know
>> how much less.  I was planning to just guess it as like
>> 1e-4*thermal_timescale, but I'm assuming that's not very good physics...
>>
>
> Hi Morgan,
>
> As I mentioned before, relaxing WDs is trickier. If you're relaxing
> composition, think that during the pseudo-evolution composition changes
> would essentially operate as nuclear burning without any energy generation.
> Even if you go from one hot white dwarf to another one in the relaxation,
> you are not ensured a physically meaningful starting point even if you make
> the change faster than the thermal timescale; it will completely depend on
> what model you started the relaxation with.
>
> If you're doing only minor composition adjustements then it might not
> neccesarily be that much of an issue, and rather than using a specific
> number of timesteps you can use instead the
> option timescale_for_relax_composition in star_job.
>
> But if you really want to capture the complete cooling track of such an
> object id strongly recommend artificially heating your white dwarf until
> lifting degeneracy, and letting it go into the cooling track. You can
> ensure your composition remains frozen in while doing this by setting
>
> dxdt_nuc_factor = 0d0
> mix_factor = 0dp
>
> Cheers
>
>
>> Any help is appreciated!
>>
>> Thanks,
>> Morgan
>>
>>
>> On Sun, Oct 20, 2019 at 9:24 AM Pablo Marchant via Mesa-users <
>> mesa-users at lists.mesastar.org> wrote:
>>
>>> Hi Roque,
>>>
>>> a general thing about all the relaxation procedures is that there is no
>>> guarantee about what is the thermal state of the star after the relaxation.
>>> If you were to do this with a star in the main sequence, allowing for a
>>> relaxation timescale much longer than the thermal timescale of the star,
>>> then you would obtain a main sequence star in thermal equilibrium. This is
>>> because during the relaxation procedure mixing and composition changes from
>>> nuclear burning are turned off, but the energy generation from nuclear
>>> reactions is kept on. If you were to do the same thing with a white dwarf,
>>> it's much more uncertain since if you take a very long timescale the object
>>> would not settle into thermal equilibrium but simply go down its cooling
>>> track, making the final result sensitive to the timescale for the
>>> relaxation. In this case using a very short timescale would resemble an
>>> adiabatic process, happening much faster than energy can be transported
>>> through the interior, and should give a meaningful post-relaxation state.
>>>
>>> So it all depends on your application, this is just stellar engineering,
>>> a method to get a specific outcome fro the code, so it depends entirely on
>>> what outcome yu're looking for. In applications where 3d models are
>>> imported for example, the entropy of the star can be relaxed as well to
>>> provide a very specific thermal state for the star.
>>>
>>> Probably if you specify in more detail what you're trying to achieve,
>>> and what you've tried so far, I could provide additional help.
>>>
>>> Cheers!
>>>
>>> On Sun, Oct 20, 2019 at 3:53 AM Roque Caballero Navarro via Mesa-users <
>>> mesa-users at lists.mesastar.org> wrote:
>>>
>>>> Dear all,
>>>>
>>>> I'm aware of the existence of a number of "relax" options aimed to let
>>>> some modifications be done gradually over several steps. In particular, I'm
>>>> interesting in relaxing the angular momentum loss calculated in my
>>>> other_torque routine.
>>>>
>>>> According to MESA documentation, the following parameters are the
>>>> relevant for my purpose:
>>>>
>>>> relax_initial_angular_momentum = .false.
>>>> max_steps_to_relax_angular_momentum = 1000
>>>> timescale_for_relax_angular_momentum = 1d-10
>>>> max_dt_for_relax_angular_momentum = 1d-9
>>>> num_timescales_for_relax_angular_momentum = 1000
>>>> relax_angular_momentum_filename = ''
>>>>
>>>> Additionally, it's mentioned that for relaxing angular momentum from
>>>> current to specified over a certain amount of relaxation timescales. This
>>>> is done by adding an extra torque term of the form and evolving the star
>>>> without changing the composition for
>>>> num_timescales_for_relax_angular_momentum times
>>>> timescale_for_relax_angular_momentum.
>>>>
>>>> s% extra_jdot(k) =  &
>>>>     (1d0 - exp_cr(-s% dt / (s% job%
>>>> timescale_for_relax_angular_momentum*secyer))) * &
>>>>     (desired_angular_momentum(k) - s% j_rot(k)) / s% dt
>>>>
>>>> I've some questions about how to modified the aforementioned parameters
>>>> in a proper way, meaning that to make longer/shorter the period of time in
>>>> which the angular momentum loss is applied:
>>>>
>>>>    1. If a want to extend the relaxing period, is it enough to assign
>>>>    to max_steps_to_relax_angular_momentum a bigger number, let's say 2000.
>>>>    What about num_timescales_for_relax_angular_momentum, should it be assigned
>>>>    the same value used in max_steps_to_relax_angular_momentum?
>>>>    2. Which is the effect of modifying
>>>>    timescale_for_relax_angular_momentum and max_dt_for_relax_angular_momentum?
>>>>
>>>> Basically I don't understand how to modify those parameters in the
>>>> right way. I don't want just to throw there some values and try to figure
>>>> out what's going on, so any hint about that will be very appreciated.
>>>>
>>>>
>>>> Kind regards,
>>>>
>>>> Roque
>>>>
>>>> _______________________________________________
>>>> mesa-users at lists.mesastar.org
>>>> https://lists.mesastar.org/mailman/listinfo/mesa-users
>>>>
>>>>
>>>
>>> --
>>> Pablo Marchant Campos
>>> M.Sc on Astrophysics, Universidad Católica de Chile
>>> PhD on Astrophysics, Argelander-Institut für Astronomie, Universität Bonn
>>> _______________________________________________
>>> mesa-users at lists.mesastar.org
>>> https://lists.mesastar.org/mailman/listinfo/mesa-users
>>>
>>>
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