[mesa-users] Hook in HR when hitting max_age

Thu Jan 30 17:38:56 EST 2014

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):

Here are profiles for the final 2 models:

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

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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