5.5.1 Initializing

There are a few programs with which orbits can be initialized:

• mkorbit is the most straightforward program. You can give simply give it all 6 phase space coordinates, and an orbit file consisting of this one point is generated. It is also possible to give the potential in which the particle is to move, and 5 phase space coordinates plus the energy, or even 4 phase space coordinates and the energy plus the total angular momentum or angular momentum along the Z axis (for axisymmetric systems).

  Let's start with an example of creating a simple orbit by itself with no associated potential.

  % mkorbit out=orb1 x=1 y=0 z=0 vx=0 vy=0.2 vz=0
  ### Warning [mkorbit]: Potential potname= not used; set etot=0.0
  pos: 1.000000 0.000000 0.000000
  vel: 0.000000 0.200000 0.000000
  etot: 0.000000
  lz=0.200000                              
  
  % tsf orb1
  char History[59] "mkorbit out=orb1 x=1 y=0 z=0 vx=0 vy=0.2 vz=0 VERSION
    =3.2b"
  set Orbit
    set Parameters
      int Ndim 03
      double Mass 1.00000
      double IOM[3] 0.00000 0.200000 0.00000
      int Nsteps 01
    tes
    set Potential
    tes
    set Path
      double TimePath[1] 0.00000
      double PhasePath[1][2][3] 1.00000 0.00000 0.00000 0.00000 0.200000
        0.00000
    tes
  tes
  

• perorb is a program that for given initial conditions (similar to the ones described in mkorbit above) attempts to calculate periodic orbits in that potential. The output file will be a file with one (or more) orbits. This is a bit of an advanced program, and will not be covered here.

• stoo is a program that can take a particle position from a snapshot, and turn it into an orbit. For example, sampling some initial conditions from the positions of stars in a Plummer sphere, we could use the following small C-shell code to find some statistical properties of this selected set of orbits^5.13

      mkplummer out=p100 nbody=p100
      foreach i (`nemoinp 0:100:10`)    
          stoo in=p100 out=orb$i ibody=$i
          orbint orb$i orb$i.out 10000 0.01 10000 potname=plummer
          orbstat orb$i.out
      end
  

  The reverse program, otos turns an orbit into a snapshot, and may come in handy since the snapshot package has far more advanced analysis programs.