[Users] Eccentricity in BNS evolution with Lorene

[Samuel Tootle]

Hi Johnny,

This is quite interesting given my understanding is that the inspiral for equal mass Lorene ID is usually quite reasonable. Specifically your note regarding the 20% difference between ADM Angular momentum for quasi-equilibrium and that resulting from your PN estimate ID seems quite wrong. Maybe the Lorene ID is at a poor resolution?

I did a test with the FUKA BNS code for a 1.4-1.4 BNS using a polytope at 45km and the difference in the ADM Angular momentum between a coarse solution using quasi-equilibrium and 3.5PN estimates was < 1%.

Cheers,
Samuel Tootle
Goethe Universität Frankfurt

*From: *Zach Etienne <zachetie at gmail.com>
*To: *Bing-Jyun Tsao <johnny.tsao.880724 at gmail.com>
*CC: *users at einsteintoolkit.org
*Date: *Oct 5, 2022 21:02:01
*Subject: *Re: [Users] Eccentricity in BNS evolution with Lorene

> 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[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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