ON THE DYNAMICS OF THE BARRED SPIRAL NEBULAE

Abstract

A general survey of the dynamical problem of spiral nebulae,1 in which the development of spiral structure has been connected with certain types of wave motions in a rotating stellar system, has led to some interesting consequences relating to the barred spirals. It seems that the conditions leading to the formation of spiral structure in these objects may be stated in a very clearcut and simple way. In what follows, a few points of general interest will be briefly summarized. In the ordinary spirals the wave motion which is the ultimate cause of the spiral structure appears to be the Riemann wave, which consists in an elongation of the system along a certain equatorial diameter and a contraction along a diameter at right angles to this one. This wave will be unstable in a greatly flattened system ; at the point of instability it moves in a fixed co-ordinate system with half the angular speed of the system itself. By resonance between this wave and the radial oscillations of particles in the outer regions of the system a great number of particles will be expelled. It seems likely that a great part of these particles will belong to the subsystem which corresponds to Baade's population type II. The tidal forces from the ejected matter will then assist in breaking up the ''subsystem of great flattening and of maximum speed of rotation, i.e., the subsystem corresponding to Baade's population type I, which contains also a great amount of dark matter. This dark matter plays an important part in forming an effective of the system. In the outer parts of the resulting spiral structure the inindividual particles will followr orbits which are approximately asymptotic to original circular motions close to the edge of the system. This is particularly clear for the spiral nebula M 81,