Arms coinciding with resonances and star formation in the Galactic disk
D. A. Barros, T. C. Junqueira, D. Falceta-Gonçalves, C. A. de Souza Braga.
The understanding of resonances in the Galactic disk has improved as evidences have accumulated in favor of a well defined pattern speed. These include the discovery of a ring-shaped gap in the HI distribution at corotation, and the identification of the 4:1 resonance, which has a corner of the square-shaped arm close to the Sun. We shall also report the identification of the 5:1 resonance. What determines the aspect of several arms is the form of the underlying stellar orbits, not other effects like shock waves. These spiral arms are understood as grooves in the potential of the disk, caused by an accumulation of stars at resonant orbits. We discuss mechanisms that collect stars into resonant orbits. Hydrodynamic simulations of gas of the disk crossing a gravitational potential groove are presented, showing that the gas gets trapped into the groove and flows along it, so that newly born stars start their life in the resonant orbit. Another mechanism is a phase-locking effect on stellar orbits passing close to a groove.