The 4:1 Outer Lindblad Resonance of a long slow bar as a potential explanation for the Hercules stream

TL;DR

This paper proposes that the Hercules stream in the Solar neighbourhood can be explained by the 4:1 Outer Lindblad Resonance of a long, slow galactic bar, supported by analytical modeling including an m=4 Fourier component.

Contribution

It introduces a novel explanation for the Hercules stream involving the 4:1 OLR of a long bar, incorporating an m=4 Fourier component in the model, which aligns with N-body simulation amplitudes.

Findings

Reproduces Hercules-like feature using the 4:1 OLR model.

Predicts symmetric velocity distribution testable with Gaia.

Supports long, slow bar as a source of local stellar streams.

Abstract

There are multiple groups of comoving stars in the Solar neighbourhood, which can potentially be explained as the signatures of one of the fundamental resonances of non-axisymmetric structure such as the Galactic bar or spiral arms. One such stream, Hercules, has been proposed to result from the outer Lindblad resonance (OLR) of a short fast rotating bar as shown analytically, or the corotation resonance (CR) of a longer slower rotating bar as observed in an N-body model. We show that by including an m = 4 Fourier component in an analytical long bar model, with an amplitude that is typical for bars in N-body simulations, we can reproduce a Hercules like feature in the kinematics of the Solar neighbourhood. We then describe the expected symmetry in the velocity distribution arising from such a model, which we will soon be able to test with Gaia.