Spiral instabilities provoked by accretion and star formation

A description of spiral structure is presented that appears to account for both the imperfect grand-design nature of the patterns and the importance of gas. It is argued that most visible spirals are short-lived features organized by gravitational forces. The recurrent transient patterns are self-regulating since they cause the velocity dispersion of the stars to rise secularly until, in the absence of gas, the disk becomes stable. However, any method of cooling will ensure that the disk will never completely stabilize and will, consequently, exhibit recurrent spiral structure. Both dissipation in the gas and accretion will cool the disk and provoke instabilities. Transient spiral patterns in computer simulations are observed which recur repeatedly only when processes designed to mimic accretion and star formation are included. The models evolve at an equilibrium Q, implying that the rate of heating by the spirals balances the steady rate of cooling. It is also found that the growth of spiral patterns is in reasonable quantitative agreement with the predictions of 'swing amplification' theory. In a companion paper it is shown that this continuous activity leads naturally to the type of age-velocity dispersion relation observed for the solar neighborhood stars.