Order and chaos in the local disc stellar kinematics induced by the Galactic bar

Abstract

(Abridged) The Galactic bar causes a characteristic splitting of the disc phase space into regular and chaotic orbit regions which is shown to play an important role in shaping the stellar velocity distribution in the Solar neighbourhood. A detailed orbital analysis within an analytical 2D rotating barred potential reveals that this splitting is mainly dictated by the value of the Hamiltonian H and the bar induced resonances. Test particle and N-body simulations reveal how the decoupled evolution of the disc distribution function in the two kind of regions and the process of chaotic mixing lead to overdensities in the H>H_12 chaotic part of the u-v velocity distributions outside corotation, where H_12 is the effective potential at the Lagrangian points L_1/2. In particular, for realistic space positions of the Sun near or slightly beyond the outer Lindblad resonance and if u is defined positive towards the anti-centre, the eccentric quasi-periodic orbits trapped around the stable x1(1) orbits - i.e. the bar-aligned closed orbits which asymptotically become circular at larger distances - produce a broad u<0 regular arc in velocity space extending within the H>H_12 zone, whereas the corresponding u>0 region appears as an overdensity of chaotic orbits forced to avoid that arc. This chaotic overdensity provides an original interpretation, distinct from the anti-bar elongated quasi-periodic orbit interpretation proposed by Dehnen (2000), for the prominent stream of high asymmetric drift and predominantly outward moving stars clearly emerging from the Hipparcos data. The effects of spiral arms and of molecular clouds are also briefly discussed within this context.