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

[Jeremy Sakstein]

Hi Øyvind,

I did something similar to what you are trying to do using RSP a while back
(https://ui.adsabs.harvard.edu/abs/2021PhRvD.103b4028D/abstract) but only
for a constant scaling of [image: G] i.e. [image: G\rightarrow G(1+\alpha)].

The way that I did this was to set the value of standard_cgrav in
extras_controls at the start of each run. You might be able to include
time-variations in G by changing standard_cgrav at the start of each
timestep in e.g. extras_start_step but I don't know if this will work or
not so you will have to test it.

For spatial variations, you'll need to see if other_cgrav or other_momentum
is implemented into RSP.

My Zenodo package for the paper is here in case it's helpful:
https://zenodo.org/record/4309065#.Yhe4cPXMK84

Cheers,

Jeremy

On Wed, Feb 23, 2022 at 11:26 PM Rob Farmer via Mesa-users <
mesa-users at lists.mesastar.org> wrote:
>
> Hi
>
> 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.
>
> Rob
>
>
> 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
>>
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>>
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--
Jeremy Sakstein (He/Him/His)
Assistant Professor of Theoretical Physics
Department of Physics & Astronomy
University of Hawaiʻi at Mānoa
Watanabe Hall
2505 Correa Road
Honolulu, HI, 96822, USA
http://www.jeremysakstein.com
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