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Large shift of the center of mass of the star
During the relaxation process for long times ($\sim 10^{4}$ [unit]), the center of mass of the neutron star is shifted by a distance comparable to the radius of the star. To reproduce this issue one need to execute:
$PHANTOM_DIR/scripts/writemake.sh nsmerger > Makefile
make
make setup
./phantomsetup star
During the 'setup' action one should choose the following options:
cat star.setup
# Phantom v2022.0.1 (c) 2007-2020 The Authors
# input file for Phantom star setup
iprofile = 2 ! 1=Unif,2=Poly,3=Dens,4=KEPL,5=MESA,6=Piec, 7=Evra
# units
mass_unit = solarm ! mass unit (e.g. solarm)
dist_unit = km ! distance unit (e.g. au)
# resolution
np = 50000 ! approx number of particles (in box of size 2R)
use_exactN = T ! find closest particle number to np
# equation of state
use_var_comp = F ! Use variable composition (X, Z, mu)
ieos = 2 ! 1=isothermal,2=adiabatic,10=MESA,12=idealplusrad
gamma = 2.000 ! Adiabatic index
mu = 1.000 ! mean molecular weight
# star properties
Rstar = 10. ! radius of star
Mstar = 1.000 ! mass of star
# relaxation options
relax_star = F ! relax star automatically during setup
write_rho_to_file = F ! write density profile to file
These parameters are selected to reproduce a typical neutron star.
Here we choose not automated relaxation of the star.
So one needs to run ./phantomsetup star
again.
This will create star.in
In the last file, we change the tmax value from 100, which was set by default, to 3e+04.
After that run command
./phantom star.in
Results are pictured on the following figure:
The rms velocity of sph-particals is smaller than $5 \cdot 10^{-3}$
The coordinates of center mass of the star ${x_{com},y_{com},z_{com}}$ is greater than 30 km at the end of the calculation.
The star seems to shift as a whole, although the velocities of all particles are small (see following figure).
Where can there be a mistake?
Thank you for your work.
Hello
Sorry for the delay in my response.
I have been able to reproduce your issue -- thanks for clearly providing your setup files & information.
I first ran the default case, and I found similar drift, however, that drift was on order 1e-3 compared to the spatial scale on order 1; the drift for the default case is unimportant.
The smaller radius in your case is amplifying the issue. Moreover, I have seen similar drifts in my own star formation simulations. I have tested several patches using your setup without any success. I suspect that any slight asymmetries in the sphere -- either caused by the initial lattice, particle groupings (for gravity or openmp), or computational roundoff, ultimately leads to the shift. At the moment, I don't have any viable patch, however, it seems reasonable that if you 'simply' remove the shift during post-processing, then you should have reasonable data to work with.
I know that this isn't that this isn't the answer that you are looking for, but you can be assured that it is not a mistake on your part.
Cheers
I'm not understanding why the automatic relaxation is not being used here.
The best way to fix this is to use a smaller value for the tree_tolerance parameter. The tree code does not exactly conserve momentum and this is the main source of the drift