[Users] Eccentricity in BNS evolution with Lorene

[Zach Etienne]

Hi Johnny,

I noticed you set
ADMBase::initial_shift           = "zero"
so you eliminated one possible contributor to coordinate eccentricity
(Lorene's initial shift condition results in significant eccentricity).
Good choice.

I don't use GRHydro/ML_BSSN, so I cannot comment on your choices there.
Generic advice:
* Resolution may be too low -- try higher resolution & see if your
eccentricity reduces.
* Initial data is too close (Lorene assumes a helical Killing vector, which
is violated more and more as the initial separation of the stars decreases)
* Check that you are properly tracking the NSs such that they are centered
within high-resolution AMR boxes, and that the radius of the AMR boxes is
at least 1.5x the radius of each NS. A movie visualizing the density of the
stars in the orbital plane would be very very useful.

Hope this helps!

-Zach

*     *     *
Zachariah Etienne
Assoc. Prof. of Physics, U. of Idaho
Adjunct Assoc. Prof. of Physics & Astronomy, West Virginia U.
https://etienneresearch.com
https://blackholesathome.net


On Wed, Oct 5, 2022 at 11:41 AM Bing-Jyun Tsao <johnny.tsao.880724 at gmail.com>
wrote:

> Dear Einstein Toolkit community,
>
> I am Bing-Jyun (Johnny) Tsao, a graduate student at University of Texas at
> Austin. I am currently working on a project comparing our local version of
> BNS initial data (M. Clark, P. Laguna, 2016 Physical Review D 94 064058)
> with Lorene.
>
> When using Lorene, I found that the trajectory of the stars is showing
> eccentricity (as shown in the plot below), and this occurred when I ran
> Lorene with equal-mass BNS with ADM mass = 1.4 Msun, and with the
> publicly available Lorene data on their website "G2_I14vs14_D4R33_45km",
> both of which have an initial separation of 45 km. I use
> VolumeIntegral_GRMHD to track the stars, and GRHydro + ML_BSSN to evolve.
> Additionally, I also found that the ADM angular momentum from Lorene is
> about 20% smaller than that from our code which uses post-Newtonian
> calculations.
>
> My question is:
> From my understanding, Lorene gives quasi-circular initial data. Thus,
> is it abnormal to see eccentricity, or is it a physical artifact, perhaps
> from tidal deformation, that always shows up when the stars are very close
> to each other?
>
> Here I attached my parfile and the trajectory for a run using the Lorene
> publicly available data "G2_I14vs14_D4R33_45km".
>
> Thanks in advance.
>
> Best,
> Johnny Tsao
>
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