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Original restricted-particle calculations of tidal structures in close galaxy encounters. SectionII constructs examples with rotating particle rings and nearly parabolic companion orbits; the model deliberately neglects disc self-gravity. Original paper: articles.adsabs.harvard.edu/pdf/1972ApJ...178..623T .

The primary calculation treats a collisionless stellar disk and obtains the critical radial velocity dispersion $3.36G\Sigma /\kappa$. A fluid disk has a related but numerically different criterion involving $\pi G\Sigma /\kappa$. The distinction matters when a fluid dispersion relation is used to approximate stellar random motion.