[Users] mass estimation of neutron star

[Roland Haas]

Hello Chia-Hui,

I am very sorry for the late response.

There are (at least) two notions of mass that are relevant for neutron
stars

1. the baryon mass which is the integral \int \sqrt{\gamma} \rho W d^3x 
2. the ADM mass (or a similar object) which is the mass measured by an
observer far away

the former can be computed as the integral of the "dens" variable of
GRHydro for example by requesting "sum" scalar output for it:

CarpetIOScalar::outScalar_vars = "GRHydro::dens"
CarpetIOScalar::outScalar_every = 1

and then multiplying the numbers in there by the volume of a cell on
the coarsest grid (ie the product of the values of CoordBase::dx,
CoordBase::dy, and CoordBase::dz in the parameter file).

The later can be computed using the QuasiLocalMeasures thorn on a
spherical surface or using the ADMMass thorn
(https://www.einsteintoolkit.org/thornguide/EinsteinAnalysis/ADMMass/documentation.html
see the example parfile here:
https://bitbucket.org/einsteintoolkit/einsteinanalysis/raw/39b8ae09c3b55d842e19ed5e89a716d3534883d0/ADMMass/test/tov_carpet.par).

Yours,
Roland

> Dear whom it may concern,
> 
> I simulated the binary neutron star merger as in gallery code(https://einsteintoolkit.org/gallery/bns/index.html). I tried to find out the mass of the merged neutron star. For test, I calculated the initial mass of each neutron star  from the file rho.xy.h5 which is the rest mass density. What I computed is about 0.9 solar mass , however it supposed to be much larger because the baryonic mass of the initial data of each neutron star is about 1.45 solar mass. I also calculated magnetic energy which corresponds to about 1e-6 solar mass  which is not a possible reason of the discrepancy. I also considered velocity which may increase mass due to the relativity but the maximum of the initial velocity is only about 0.1c which also does not raise the mass too much. What is the possible reason that the total rest mass is so much smaller than the baryonic mass ?
> 
> Thank you.
> 
> 
> Best regards,
> 
> Chia-Hui



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