[mesa-users] Hook in HR when hitting max_age

[Radek Smolec]

Hi Bill,

Thanks for your explanantion. I will stop at some larger max_age and
interpolate. There is however one think I don't understand. By default,
MESA instantly adjust the time-step. Usually the changes are much smaller
than in the case of interpolation to hit the max_age. I infer that the
problem remains but is much smaller. I guess that for a longer time, the
effect cancels out rather than accumulates. Am I right?
Cheers,
Radek


2014-01-30 Bill Paxton <paxton at kitp.ucsb.edu>:

>
> On Jan 30, 2014, at 1:09 AM, Radek Smolec wrote:
>
> the last time step is adjusted to match the desired age. But in HR diagram
> it produces an artificial hook - see an example  Figure (magnified at the
> end of calculation) and inlist I used. The hook remains there for all the
> parameters I tried, sometimes it is smaller, sometimes it is very large.
> How can I get rid of it?
>
>
> Hi Radek,
>
> Short answer: to get rid of the hook you need to reduce the size of
> changes in timestep at the end of the run:
> either use much smaller timesteps or (perhaps better) keep the same
> timestep size and let the last step go beyond the target age; then
> interpolate the values for things lke L, R, Teff to the target intra-step
> time.
>
> Longer answer: changes in timestep leads to changes in eps_grav because of
> changes in 1st order estimates for Lagrangian time derivatives for lnT,
> lnRho, or lnPgas.  Changes in eps_grav lead to changes in temperature,
> density, luminosity, and radius since everything depends on everything else.
>
> [Someday it would be interesting to see if the eps_grav calulations could
> be improved by going to a 2nd order backward difference estimate for the
> time derivatives -- but I haven't gotten to that yet.]
>
> Here are plots from your case.   The biggest changes happen at the base of
> the convection zone and actually result in a small movement of the base
> (too small to see in these plots).    (model 1730 is the final model; it
> has a slightly smaller timestep than 1729 -- log(dt/yr) of 6.433119 vs
> 7.000 for previous steps)
>
> First, here's the HR hook for the final model (1730):
>
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> Here are profiles for the final 2 models:
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> Those changes in eps_grav are reflected in the luminosity (more negative
> eps_grav in 1730 gives lower L)
>
>
> There are similar changes in the time derivatives for lnT, lnPgas, and lnR
>
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> If the change in dt is small relative to the previous value, then I'd
> expect smaller effects -- conversely, if you have to make a big reduction
> in the final timestep to hit the specified max age, then I'd expect that to
> give a large "hook".
>
>
> -Bill
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