We have proposed a framework for interpreting those moving groups in the solar neighbourhood whose members show relatively large deviations from the circular velocity around the galactic center. Our interpretation is based on the possible existence of a large scale shock associated with the density wave responsible for the spiral structure of our Galaxy, which determines the initial velocity of the stars formed by this process. New conditions for moving group membership have been derived, which happen to be equivalent to those based on the similarity of spatial velocities when they are applied to samples of very nearby stars. These conditions imply the existence of velocity gradients, independent of the isoperiodicity condition sometimes assumed for moving groups. An approximation to these gradients is derived, and the distortion due to the gravitational field associated to the density wave (not considered in our treatment) is estimated from numerical integrations. The predictions of our proposed definition of moving group membership may be tested as high precission spatial velocities for more distant stars are available, such as those provided by the Hipparcos satellite.
© European Southern Observatory (ESO) 1997
Online publication: May 5, 1998