Abstract
The presence of spiral structure in rapidly and differentially rotating disc galaxies is currently attributed to the phenomenon of unstable (that is to say, growing) Lin-Shu compression-type waves, or density waves, rotating at a constant angular velocity around the system's centre. It is important that when a density-wave structure is present in a galaxy, the gravitational field of the spiral arms will systematically deflect the motion of gas and young stars away from their mean circular rotation, and point masses in such a model react by streaming motions that are of spiral shape too. We examine the kinematics of Milky Way's 233 Cepheid stars on the assumption that the system is subject to moderately growing spiral density waves by taking into account small-amplitude perturbations of the Galactic gravitational potential. Using Cepheid line-ofsight velocities, we propose new estimates of the parameters of solar motion and Galactic rotation corrected for the effects of density waves, the radial and azimuthal components of systematic stellar motion due to the spiral arms as well as the dynamical parameters of the waves. A basis is given for preferring the dominant one-armed spiral structure in the solar neighbourhood of the Galaxy. © 2013 The Authors.