Specifying a potential

The code includes a simple class called logpotential, corresponding to a circular velocity that is constant in radius. Other potentials may be implemented in the future or by the user, provided they implement the same methods present in the logpotential class.

The logpotential or similar specifies the spherically-symmetric part of the potential. For particles orbiting near a Milky Way-like disk, the particle is subject to vertical oscillations at a characteristic frequency sqrt(4 pi G rho0), where G is Newton’s constant and rho0 is the midplane density, of order 0.2 solar masses per cubic parsec for the Solar neighborhood. The code additionally allows this midplane density to vary as a powerlaw, so that the frequency of vertical oscillation will depend on the particle’s position.

For particles whose orbits rarely take them near the disk because of high inclinations or large semi-major axes, the orbital plane of the particle will change very little as the result of gravitational interactions with the disk. To adopt this assumption, one can pass tilt=True in the constructor for particleLB. The particle’s position and velocity will still be in the usual disk-centered coordinate system (see below), but internally the particle’s coordinate system will be rotated such that it has no “z” motion, and when the particle’s position is requested at a different time, the particle’s internal coordinates will be rotated back to the global coordinate system.