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Paper: Midplane Stellar Orbits and the Galactic Mass Distribution in the Presence of 3D Spiral Arms
Volume: 275, Disks of Galaxies: Kinematics, Dynamics and Perturbations
Page: 359
Authors: Pichardo, B.; Martos, M.; Moreno, E.; Espresate, J.
Abstract: We have modeled galactic stellar spiral arms as a superposition of inhomogeneous oblate spheroids, and added its contribution to an axisymmetric model of the Galactic mass distribution. The physical characteristics and the spatial extent of the spiral arms, were selected by observations giving us a plausible model for the Galactic potential outside the ILR. With the selected pitch angle, self-consistent spiral arms models terminate at the 4/1 resonance (at a galactocentric radius of 9.5 kpc) as predicted by Contopoulos & Grosbøl for strong spirals. Then our Galaxy is seemingly much closer to a strong spiral than to a weak spiral and nonlinear theory applies. In this model, we have explored the stellar orbital structure in the midplane of symmetry. We found that chaos occurs only in prograde orbits defined in an inertial frame. In this frame, the sign of the azimuthal velocity of each orbit is preserved with time. We constructed then the usual Poincaré diagrams in the rotating frame but labeled in the diagram prograde and retrograde orbits according to the sense determined following the same orbits in an inertial frame with origin at the Galactic center. We found then a sharp separation between prograde and retrograde motion in Poincaré diagrams. The separatrix is a region corresponding to orbits around zero angular momentum, as defined in the inertial frame. Additionally, we have performed 1D, MHD simulations. The usual local models of the spiral potential are found in substantial error as the contribution of the entire spiral pattern distorts, even for the weak perturbation regime, the shape of the potential, which becomes a rapidly varying function of the position in the midplane. These local distorsions have a strong impact on the gas flow. Probing the response for flows encountering the arms at various locations, we show that shocks can only occur inside certain galactocentric radii determined by the azimuthal angle, for an arm does not act at each point as an effective potential well.
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