Setting up stars and tidal disruption events

Setting up and relaxing a star

First, follow the usual procedure for initiating a new simulation with phantom. We’ll use the “star” setup, but you can also use the “polytrope” or “neutronstar” configurations (the first two use self-gravity for the star, the last one uses an external potential). For tidal disruption events in general relativity use “grtde”. That is:

make a new directory and write a local Makefile

$ mkdir star
$ cd star
$ ~/phantom/scripts/writemake.sh star > Makefile

compile phantom and phantomsetup

$ make
$ make setup
$ ls
Makefile    phantom*    phantomsetup*

run phantomsetup

 $./phantomsetup star

 star.setup not found: using interactive setup

 1) Uniform density profile
 2) Polytrope
 3) Density vs r from ascii file
 4) KEPLER star from file
 5) MESA star from file
 6) Piecewise polytrope
 7) Evrard collapse
Enter which density profile to use ([1:7], default=1): 2
Setting up Polytrope
Enter mass unit (e.g. solarm,jupiterm,earthm) (blank="blank",default="solarm"):
Enter distance unit (e.g. au,pc,kpc,0.1pc) (blank="blank",default="solarr"):
Enter the approximate number of particles in the sphere ([0:], default=100000):
Enter the desired EoS for setup (default=2):
Enter gamma (adiabatic index) ([1.000:7.000], default=1.667):
Enter the mass of the star (code units) ([0.000:], default=1.000):
Enter the radius of the star (code units) ([0.000:], default=1.000):
Relax star automatically during setup? (default=no): y
 Writing star.setup
STOP please check and edit .setup file and rerun phantomsetup

once you have answered the questions once, they are written to a file called star.setup, which you can use for subsequent runs of phantomsetup without having to answer prompts (see below)

relax the star

Open the star.setup file and make sure the parameter “relax_star” to True:

relax_star =       T    ! artificial damping of velocities (if on, v=0 initially)

Then run phantomsetup:

./phantomsetup star.setup

Which will generate a sequence of snapshots of the relaxation process:

RELAX-A-STAR-O-MATIC: Etherm:  0.499     Epot: -0.854     R*:   1.00
    WILL stop WHEN: dens error <   1.00% AND Ekin/Epot <   1.000E-07 OR Iter=0

-------->   TIME =    0.000    : full dump written to file relax_00000   <--------

 Relaxing star: Iter   1/1000, dens error: 20.39%, R*:  0.978     Ekin/Epot:  3.157E-03
 Relaxing star: Iter   2/1000, dens error: 15.91%, R*:  0.978     Ekin/Epot:  4.858E-03
 Relaxing star: Iter   3/1000, dens error: 13.05%, R*:  0.978     Ekin/Epot:  3.303E-03
 Relaxing star: Iter   4/1000, dens error: 11.11%, R*:  0.977     Ekin/Epot:  2.276E-03
 Relaxing star: Iter   5/1000, dens error:  9.69%, R*:  0.975     Ekin/Epot:  1.655E-03

-------->   TIME =   0.3756E-01: full dump written to file relax_00001   <--------
 Relaxing star: Iter   6/ 500, dens error: 22.17%, R*:   1.03     Ekin/Epot:  1.280E-03

This process will stop when either the density error is less than 1% and the ratio of kinetic to potential energy is less than the specified tolerance, OR the number of iterations reaches the specified maximum.

Once complete, you should obtain a relaxed initial conditions snapshot with the correct density and pressure profiles:

-------->   TIME =    0.000    : full dump written to file star_00000.tmp   <--------

 input file poly.in written successfully.
 To start the calculation, use:

 ./phantom star.in

evolve the star

If you really want to, you can evolve the star in isolation with the regular code to double-check that the relaxation process worked:

./phantom star.in

check the output

splash star_0*

(if you have version 2 of splash, the relevant command is “ssplash”)

Putting the star on an orbit for a tidal disruption event

If you used the “tde” or “grtde” setup then simply compile moddump:

$ make moddump

otherwise you need to specify the tidal moddump file:

$ make moddump MODFILE=moddump_tidal.f90

Then run moddump on your relaxed star:

$ ./phantommoddump star_00000 tde 0.0
...
...
...
 writing moddump params file tde.tdeparams
  Edit tde.tdeparams and rerun phantommoddump

When you first run this, a “tde.tdeparams” file will be created. Edit this to set the star on your desired orbit, and then rerun phantommoddump:

# parameters file for a TDE phantommodump
                beta =       1.000    ! penetration factor
                  mh =   1.000E+06    ! mass of black hole (code units)
                  ms =       1.000    ! mass of star       (code units)
                  rs =       1.000    ! radius of star     (code units)
               theta =       0.000    ! stellar rotation with respect to x-axis (in degrees)
                 phi =       0.000    ! stellar rotation with respect to y-axis (in degrees)
                  r0 =        490.    ! starting distance

After this you can simply run phantom:

$ ./phantom tde.in

Adding a magnetic field to the star

compile phantommoddump

The module used to compile this utility is specified using MODFILE= in build/Makefile_setups. The default for the “polytrope” setup is currently moddump_spheres.f90:

MODFILE=moddump_spheres.f90

Change this to moddump_default.f90. You can do this temporarily on the command line by compiling phantommoddump as follows:

make moddump MODFILE=moddump_default.f90 MHD=yes

run phantommoddump

$ ./phantommoddump
PhantomSPH: (c) 2007-2023 The Authors

 Usage: moddump dumpfilein dumpfileout [time] [outformat]

in our case we want:

./phantommoddump star_00010 magstar_00000

which will give some errors:

ERROR! MHD arrays not found in Phantom dump file: got            0

but then prompt you to add magnetic fields:

add/reset magnetic fields? (default=no): yes

you can follow the prompts to add uniform magnetic fields using this routine.

now implement something decent in src/setup/set_Bfield.f90

you can either use the pre-cooked magnetic field setups in this routine, or you can just make a new moddump module that sets up the magnetic field in a custom way.