[Mesa-users] Relaxing angular momentum

Thu Oct 24 15:32:05 EDT 2019

Hi Pablo, (all),

Do you know if the relaxation timescale (for composition) needs to be in
seconds or years?  I didn't see it specified in the source code...

Thanks,
Morgan

On Thu, Oct 24, 2019 at 5:57 AM Pablo Marchant <pamarca at gmail.com> wrote:

> Hi Roque,
>
> I don't quite understand what you're trying to do with the description you
> gave. I can interpret it as you wanting to use a kind of sub-stepping
> algorithm to estimate the loss of angular momentum. This does not sound
> like an appropriate application for the relaxation routines, which even
> though they use multiple steps, need to know in advance the final angular
> momentum profile. If what I describe is correct, it's something much more
> difficult to implement, I don't think there's a simple way to do so right
> now. So if small timesteps/higher res help your problem, I think you're
> better off just using that than looking for a more complex solution.
>
> If what I describe does not quite reflect what you're trying to do, it
> will be necessary to have much more detail on what exactly you're trying to
> do (physics and implementation wise) before we can help.
>
> Cheers
>
> On Thu, Oct 24, 2019 at 1:23 AM Roque Caballero Navarro <
> rcaballeron at hotmail.com> wrote:
>
>> Hi Pablo, Hi Morgan,
>>
>> What would you recommend in the case of the solar-like model?
>>
>> Many thanks in advance for your advices.
>>
>> Best regards,
>>
>> Roque
>>
>> Obtener Outlook para iOS <https://aka.ms/o0ukef>
>> ------------------------------
>> *De:* Morgan Taylor <taylormorgan32 at gmail.com>
>> *Enviado:* Thursday, October 24, 2019 8:17:02 AM
>> *Para:* Pablo Marchant <pamarca at gmail.com>
>> *Cc:* Roque Caballero Navarro <rcaballeron at hotmail.com>; mesa-users <
>> mesa-users at lists.mesastar.org>
>> *Asunto:* Re: [Mesa-users] Relaxing angular momentum
>>
>> 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
>>
>>
>
> --
> Pablo Marchant Campos
> M.Sc on Astrophysics, Universidad Católica de Chile
> PhD on Astrophysics, Argelander-Institut für Astronomie, Universität Bonn
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20191024/f9ed499d/attachment.html>