The Wilson-Bappu effect: A tool to determine stellar distances

We study, using direct orbit integrations, the kinematic response of the outer stellar disk to the presence of a central bar, as in the Milky-Way. We find that the barʹs outer Lindblad resonance (OLR) causes significant perturbations of the velocity moments. With increasing velocity dispersion, the radius of these perturbations is shifted outwards, beyond the nominal position of the OLR, but also the disk becomes less responsive. If we follow Dehnen (2000) in assuming that the OLR occurs just inside the Solar circle and that the Sun lags the bar major axis by ~ 20(degree), we find (1) no significant radial motion of the local standard of rest (LSR), (2) a vertex deviation of ~ 10(degree) and (3) a lower ratio ...of the principal components of the velocitydispersion tensor than for an unperturbed disk. All of these are actually consistent with the observations of the Solarneighbourhood kinematics. Thus it seems that at least the lowest-order deviations of the local-disk kinematics from simple expectations based on axisymmetric equilibrium can be attributed entirely to the influence of the galactic bar.