The Effect of Spiral Structure on the Stellar Velocity Distribution in the Solar Neighborhood

Abstract

Clumps in the solar neighborhood's stellar velocity distribution could be caused by spiral density waves. In the solar neighborhood, stellar velocities corresponding to orbits that are nearly closed in the frame rotating with a spiral pattern represent likely regions for stellar concentrations. Via particle integration, we show that orbits can intersect the solar neighborhood when they are excited by Lindblad resonances with a spiral pattern. We find that a two-armed spiral density wave with pattern speed placing the Sun near the 4:1 Inner Lindblad Resonance (ILR) can cause two families of nearly closed orbits in the solar neighborhood. One family corresponds to square shaped orbits aligned so their peaks lie on top of, and support, the two dominant stellar arms. The second family correspond to orbits 45 degrees out of phase with the other family. Such a spiral density pattern could account for two major clumps in the solar neighborhood's velocity distribution. The Pleiades/Hyades moving group corresponds to the first family of orbits and the Coma Berenices moving group corresponds to the second family. This model requires a spiral pattern speed of approximately 0.66 +- 0.03 times the angular rotation rate of the Sun or 18.1 +- 0.8 km/s/kpc.