[Mesa-users] Time step problems

[Shunyi Lan]

Dear Ebraheem:

Thank you for your detailed and helpful response to my questions. I appreciate your suggestions and will test them out.

Best regards,

Shunyi Lan


-----原始邮件-----
发件人:"Ebraheem Farag" <ekfarag at asu.edu>
发送时间:2023-04-10 14:41:00 (星期一)
收件人: "Shunyi Lan" <lanshunyi at ynao.ac.cn>
抄送: mesa-users at lists.mesastar.org
主题: Re: Re: [Mesa-users] Time step problems


No worries,


It looks like you have developed a large radiative envelope with a weak (flat) temperature gradient at logT< 4.0,  by depositing extra heat into the surface regions. (Just a guess) Inefficient heat transport through this region could be causing your timesteps to become prohibitively small.
I'm not sure, but this could be potentially relieved through adopting some MLT++ treatment in your model (artificially reducing grad_T closer to grad_a), & control: 'okay_to_reduce_gradT_excess = .true.').
see section 7.1/7.2 in MESA II or section 7.2 of MESA VI (for an implicit method employed in r22.11.1 of MESA, useful for binaries with Mass loss).
--------
Another approach that might or might not help, albeit more unphysical, would be to apply some extra pressure to the surface. If you are using a 'fixed' or 'iterated' T-tau relationship for your atmosphere, you can apply some artificial extra pressure to the surface, which could help decrease gradT near the surface, &control : "Pextra_factor = 2" ! You can set this value higher to flatten log T vs log Rho gradient near the surface, but it can produce incorrect radii, see section 6.1 of MESA VI


If one fix does not work on its own, you could try adopting both 'okay_to_reduce_gradT_excess' (MLT++) and 'Pextra_factor' together... or, you could use the "superad_reduction" instead of mlt++, if you can run your models in r22.11.1. This scheme is especially useful if there are large step to step variations in mass loss.


Hope this helps,
EbF


On Sun, Apr 9, 2023 at 9:31 PM Shunyi Lan <lanshunyi at ynao.ac.cn> wrote:


Dear Ebraheem:

Thanks for your reply.

The attachments are some selected profiles during the evolution. It should be noticed that this star is currently in a binary system and losing mass while the outer region is heated. The single star energy injection also tested, the timestep is also very small.

BTW, the " heat = " is not a typo, just indicate the heat is a free parameter.

Best regards,

Shunyi Lan

Sorry for not sending to the main thread. Please forgive me for my first MESA community inquiry.





-----原始邮件-----
发件人:"Ebraheem Farag" <ekfarag at asu.edu>
发送时间:2023-04-10 11:43:55 (星期一)
收件人: "兰顺义" <lanshunyi at ynao.ac.cn>
抄送:mesa-users at lists.mesastar.org
主题: Re: [Mesa-users] Time step problems


Hello 兰顺义,


Could you try making a plot of rho(k) vs T(k)?
also, " heat = " - Is this a typo?


-EbF 


On Sun, Apr 9, 2023 at 8:14 PM 兰顺义 <lanshunyi at ynao.ac.cn> wrote:


Dear MESA users and developers:




I'm using extra_energy hook to inject energy to stars. My MESA version is 10398. What I am encountering is, when the star is low mass star (0.5 Msun MS star, for example) and heating is relatively strong,(for example 10 times of luminosity of star) the time step will be very small. I tried to use control force_timestep_min_years to increase the timestep, e.g. 1d3 years, but it still can lower than this limit.

So my question is: how can I increase the timestep while securing the convergence at the same time? And how the exactly the force_timestep_min_years works?

Follow is part of my extra hook. All the variable have stated in right type and the hook works well (at least for other cases, e.g. inermediate mass MS stars, RG, etc.)

*****************************************

 heat = 

start = 0
s% extra_heat(:) = 0
tau_0 = 0.99
do k = 1, s% nz
if(s% tau(k)>tau_0) then
layer_max = k
exit
end if
end do

do k = 1, layer_max
      heat_current = heat*(s% tau(k+1) - s% tau(k) + &
      exp(-1.0*s% tau(k+1))-exp(-1.0*s% tau(k)))/ &
      (s% tau(layer_max)-1+exp(-1.0*s% tau(layer_max)))
      s% extra_heat(k) = heat_current/s% dm(k)
end do
*****************************************

Any suggestion would be very helpful!







Best regards,

Shunyi Lan

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