[Mesa-users] Relaxing angular momentum

Thu Oct 24 01:35:02 EDT 2019

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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