[Users] mass estimation of neutron star

[Roland Haas]

Dear Chia-Hui,

> To my understanding, "sum" output multiply dxdydz is the total mass
> of the neutron star(s) in the simulation. Is this correct ? 
That is correct.

> In my case
> which is gallery example,  initial sum = 0.00012605 and final sum is
> 0.00012701 so that the mass increases from the initial binary neutron
> star to the final merged neutron star is (0.00012701-
> 0.00012605)*18*18*18*4=0.0224 M. This is somehow different to what I
> thought. I originally think that during the inspiral and merging .
> some mass would outflow due to tidal force , so the total mass of
> neutron star would decrease. 
The sum is over the full domain so unless some of the tidal outflows reach the edge of the domain or are otherwise removed (eg by numerical effects inside of a black hole where we apply tricks to make the code not crash) then the total rest mass will be conserved.

The slight increase that you see may well be due to either accretion of atmosphere onto the star or due to numerical issues at mesh refinement boundaries since the default mesh refinement technique used in the Einstein Toolkit is not conservative (it uses vertex centering and no refluxing, both of which can be changed by options to the parameter file and by using the Refluxing thorn).

> Contrast to this, the mass gained. IS
> this means accretion mass is larger than the summation of outflow
> mass and energy radiated away by the gravitational wave ?

Energy radiated would not show up in the balance since the rest mass is
literally just the sum of baryons present which does not change as
energy is radiated away.

If you are interested in the amount of matter ejected I would suggest
looking at the Outflow thorn:
https://www.einsteintoolkit.org/thornguide/EinsteinAnalysis/Outflow/documentation.html which is part of the Einstein Toolkit. It measures the amount of matter that passes through spheres of constant radius. It has an example parfile (with very low resolution) in its test directory.

Yours,
Roland

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