The bar/bulge traced by RRLs 6D

The analysis of the orbital parameter space was used to distinguish bulge stars that in the bar reference frame have prograde orbits from those on retrograde orbits. We classified the prograde stars into orbital families and estimated the chaoticity (in the form of the frequency drift, log{Delta}{Omega}) of their orbits. RR Lyrae stars with banana-like orbits have a bimodal distance distribution, similar to the distance distribution seen in metal-rich red clump stars.

The fraction of stars with banana-like orbits decreases linearly with metallicity, as does the fraction of stars on prograde orbits (in the bar reference frame). The retrograde-moving stars (in the bar reference frame) form a centrally concentrated nearly spherical distribution. From analyzing an N-body+SPH simulation, we found that some stellar particles in the central parts oscillate between retrograde and prograde orbits and that only a minority stays prograde over a long period of time.

Based on the simulation, the ratio of prograde and retrograde stellar particles seems to stabilize within some gigayears after the bar formation. The nonchaoticity of retrograde orbits and their high numbers can explain some of the spatial and kinematical features of the MW bulge that have been often associated with a classical bulge.