Moving Groups as the Origin of the Vertical Phase Space Spiral in the Solar Neighborhood

Abstract

Using Gaia data release 2 (DR2), we analyzed the distribution of stars in the close vicinity of the Sun in the full 3D position–velocity space. We have found no evidence of incomplete phase mixing in the vertical direction of the disk, which could have originated from external events. We show that the vertical phase-space spiral Z – V z is produced by the well-known moving groups (MGs), mainly by Coma Berenices, Pleiades–Hyades, and Sirius, when the statistical characteristics (mean, median, or mode) of the azimuthal velocity V φ are used to analyze the distribution in the vertical position–velocity plane. This result does not invoke external perturbations and is independent of the internal dynamical mechanisms from which the MGs originate. Our conclusions counterbalance current arguments in favor of short-lived (between 300 and 900 Myr) structures in the solar neighborhood. Contrarily, they support the hypothesis of a longer formation timescale (around a few gigayears) for the MGs.