Add dynamical friction #346
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…o inherit from general Force class and introduce two subclasses: Potential for all forces that derive from a potential and DissipativeForce for those that do not (like dynamical friction), generalize evaluatePotentials and evaluateXForces to include dissipative forces; generalize FullOrbit integration to pass velocities to forces (for dynamical friction)
…its with a symplectic integrator when the force is dissipative
…setup of ChandrasekharDynamicalFrictionForce)
… DissipativeForces
… error when no velocity given for dissipative force
…en a dissipative force, still raise error in potential, density, and second derivative
…s; start with computing sigma_r(r) using the spherical Jeans equation
…s, as a possibly better approx. for halo objects in a disk+halo potential
…df/ and galpy/df.py --> galpy/df/__init.py; various other necessary adjustments
…le_src/ --> galpy/actionAngle/ and galpy/actionAngle.py --> galpy/actionAngle/__init.py; various other necessary adjustments
…galpy/orbit/ and galpy/orbit.py --> galpy/orbit/__init.py; various other necessary adjustments
…rc/ --> galpy/potential/ and galpy/potential.py --> galpy/potential/__init.py; various other necessary adjustments
…e in Python 2.7, but not in python 3 (?)
…exclude dissipative for now, only ChandrasekharDynamicalFriction) and that inputs to RZToplanarPotential are axisymmetric
…mmetric and raise error if not
…kharDynamicalFrictionForce
…ng tools, and general package updates [ci skip]
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This PR adds dynamical friction using the Chandrasekhar dynamical-friction formula (with a few small recent tweaks to better agree with N-body simulations). As a dissipative force, dynamical friction is not specified by a potential and therefore did not fit into the regular
galpy.potential.Potentialframework. Therefore a new class hierarchy was constructed, withgalpy.potential.Forcebeing the top-level class from whichgalpy.potential.Potential(for forces deriving from potentials) and a newgalpy.potential.DissipativeForce(for dissipative forces) inherit. The newChandrasekharDynamicalFrictionForceclass inherits from thisDissipativeForce.PR includes tests and extensive documentation. A single new line is untested, in
galpy.orbit, because it is the fallback onto the non-symplectic, C-based integrator for dissipative forces. This fallback cannot currently be tested, because there is no support for dissipative forces in C.This PR also made the following changes:
galpy.df.jeanswith tools for dealing with the Jeans equations (used inChandrasekharDynamicalFrictionForcefor computing sigmar)galpy/potential_src/andgalpy/potential.pyforgalpy.potentialfunctions in favor ofgalpy/potential/andgalpy/potential/__init.py.