[Mesa-users] Caveats to implementing time- and space-varying gravitational constant to MESA RSP Cepheid

Rob Farmer robert.j.farmer37 at gmail.com
Thu Feb 24 09:26:26 UTC 2022


Assuming you are using the other_cgrav hook then changes in G should be
propagated correctly through mesa/star.  GYRE doesn't take into account a
changing G (at least by default), if you look in one of the gyre_* test
cases in star/test_suite and look in the run_star_extras.f90 you'll see
lines like:

call gyre_set_constant('G_GRAVITY', standard_cgrav)

where we are setting G at the start of the run to the standard value.

You'll get energy errors as other_cgrav assumes G is constant so its change
won't be taken into account.
So you would want to look at the other_momentum_implicit hook where you can
provide a change to cgrav as well as the derivatives of your change. Though
I'm not sure how this will interact with RSP.


On Mon, 21 Feb 2022 at 13:43, Øyvind Christiansen <
oyvind.christiansen at astro.uio.no> wrote:

> Dear Mesa users,
> I am working on a project where we intend to study the effect of
> temporally and possibly spatially varying gravitational constant on
> Cepheids using the radially pulsating stellar library, basing it on the
> test_suite rsp_Cepheid working folder. We are updating the gravitational
> constant every time step according to our model, and the results seem
> well-behaved and reasonable. However, I wanted to ask if anyone with more
> experience in working with Mesa are aware of possible caveats to doing so?
> Possible difficulties that I see are
> 1) Energy conservation constraints and termination criterion, if the time
> varying G transfers energy into or out of the system.
> 2) External libraries like gyre, and whether they are using the updated
> value for G at every time step
> 3) Equations such as the hydrostatic equilibrium where the gravitational
> pressure is calculated assuming constant G.
> The variations that we intend to look at for now are around percentage and
> below, so possibly we will be able argue that these effects are negligible.
> Any input is appreciated!
> Best wishes,
> Øyvind Christiansen
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
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