# Re: [Users] problems of reproducing the simulation of binary neutron star with different mass

```Dear Roland,

Thank you for the detailed information.```
```
I have tried to simulate unequal mass binary neutron star with new setting of
initial position of the two stars.It seems fine at least in first several
orbits .I will try the thorn you mentioned to check the difference.

Best regards,

Chia-Hui

________________________________

with different mass

Hello Chia-Hui,

> I do not quite understand what causes ,in your example,
>
> CarpetRegrid2::position_x_1 = 42.
> CarpetRegrid2::position_x_2 = -17
>
> to become fail.
This has to do with the way that the thorn GRHydro_Analysis that looks
for the location of the star works. It first determines the location on
the grid with the largest density, then it computes a center of mass
integral in a region around that point. This computed center of mass if
used as the location of the star and to move boxes. There is code in
thorn NSTracker that uses this single found location and moves two
refined regions to (x,y,z) and (-x,-y,z) where (x,y,z) is the found
location of the star.

There are two reasons why this does not work well with unequal masses:

1. it only looks for a single star and bases that star's location on
the highest density
2. NSTracker makes the hard-coded assumption that the two regions that
cover the star are mirror immages of each other

In your case since the star at -17 (always assuming you actually have
set up a grid that has no symmetries) is the heavier one (since it is
closer to the center of rotation) and thus has the higher central
density, therefore GRHydro_Analysis will use it as the star to track
and thus will find the star location to be (-17,0,0) when it looks for
the star.

>  In my case, if the initial location of the binary star is (-34.59
> km , 45.41 km) , I should set
>
> CarpetRegrid2::position_x_1 = -34.59/1.48=-23.37
> CarpetRegrid2::position_x_2 = 45.41/1.48=30.68
>
> right?
Yes that is correct.

> Would this soon be set to (-23.37,23.37)? Because the mass difference
> is not so much, I am wondering whether there exists some maximum value
> of  mass ratio which can tolerate small mass difference.
>
> By the way ,the link https://trac.einsteintoolkit.org/ticket/1974
> seems not work. I would like to check it up.
Thank you for pointing this out. It seems that there is an endless
redirection loop in the trac website already for
https://trac.einsteintoolkit.org

Yours,
Roland

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