Effect of orbital trapping by bar resonances in the local U–V velocity field

Abstract

ABSTRACT The effects in the local U–V velocity field due to orbital trapping by bar resonances have been studied computing 15 resonant families in a non-axisymmetric Galactic potential, considering the bar’s angular velocity between 35 and 57.5 ${\rm \, km\, s^{-1}{kpc}^{-1}}$. Only cases in the low, 37.5, 40 ${\rm \, km\, s^{-1}{kpc}^{-1}}$, and high, 55, 57.5 ${\rm \, km\, s^{-1}{kpc}^{-1}}$, velocity ranges give trapping structures that have some similarity with observed features in the velocity distribution. The resulting structures in the local U–V plane form resonant bands appearing at various levels in velocity V. Cases with angular velocity 40 and 55 ${\rm \, km\, s^{-1}{kpc}^{-1}}$ show the greatest similarity with observed branches. Our best approximation to the local velocity field by orbital trapping is obtained with a bar angular velocity of 40 ${\rm \, km\, s^{-1}{kpc}^{-1}}$ and a bar angle of 40°. With this solution, three main observed features can be approximated: (i) the Hercules branch at V = $-50\, {\rm \, km\, s^{-1}}$ produced by the resonance 8/1 outside corotation, and the close features produced by resonances 5/1 and 6/1, (ii) the newly detected low-density arch at V ≃ 40 ${\rm \, km\, s^{-1}}$ produced approximately by the resonance 4/3, (iii) the inclined structure below the Hercules branch, also observed in the Gaia DR2 data, produced by tube orbits around Lagrange point L5 at corotation. Some predicted contributions due to orbital trapping in regions of the U–V plane corresponding to the Galactic halo are given, which could help to further restrict the value of the angular velocity of the Galactic bar. No support by orbital trapping is found for the Arcturus stream at V$\approx \, -100\, {\rm \, km\, s^{-1}}$.