[Mesa-users] Time step problems

[Ebraheem Farag]

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
<https://iopscience.iop.org/article/10.1088/0067-0049/208/1/4#apjs477373s7> or
section 7.2 of MESA VI
<https://iopscience.iop.org/article/10.3847/1538-4365/acae8d/meta> (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
<https://iopscience.iop.org/article/10.3847/1538-4365/acae8d/meta>

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
>> _______________________________________________
>> mesa-users at lists.mesastar.org
>>
>> https://urldefense.com/v3/__https://lists.mesastar.org/mailman/listinfo/mesa-users__;!!IKRxdwAv5BmarQ!Zt8ZEgIll2Di3zr-hxXTDwqvzQTxfs5du_41m1tlVhJF92MtJEA1vdQtovNlgdkMCqv37QHvKXE9Jnf1dWE1$
>>
>>
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