Escapees from the bar resonances

S Khoperskov,M Haywood,O N Snaith,P Di Matteo,A Gómez

doi:10.1051/0004-6361/201937188

Abstract

Understanding radial migration is a crucial point for building relevant chemical and dynamical evolution models of the Milky Way disk. In this paper we analyze a high-resolution N-body simulation of a Milky Way-type galaxy to study the role that the slowing down of a stellar bar has in generating migration from the inner to the outer disk. Stellar particles are trapped by the main resonances (corotation and outer Lindblad resonance, OLR) which then propagate outward across the disk due to the bar slowing down. Once the bar strength reaches its maximum amplitude, some of the stars delivered to the outer disk escape the resonances and some of them settle on nearly circular orbits. The number of escaped stars gradually increases, also due to the decrease in the bar strength when the boxy/peanut bulge forms. We show that this mechanism is not limited to stars on nearly circular orbits; stars initially on more eccentric orbits can also be transferred outward (out to the OLR location) and can end up on nearly circular orbits. Therefore, the propagation of the bar resonances outward can induce the circularization of the orbits of some of the migrating stars. The mechanism investigated in this paper can explain the presence of metal-rich stars at the solar vicinity and more generally in the outer Galactic disk. Our dynamical model predicts that up to 3% of stars between corotation and the OLR can be formed in the innermost region of the Milky Way. The epoch of the Milky Way bar formation can be potentially constrained by analyzing the age distribution of the most metal-rich stars at the solar vicinity.

Highlights

It is widely accepted that stars, once formed, do not necessarily stay at their initial birth radius and can migrate across disks (Goldreich & Lynden-Bell 1965; Lynden-Bell & Kalnajs 1972), a phenomenon usually referred to as radial migration
Sellwood & Binney (2002) proposed that the fast appearance and dissolution of spiral arms is an efficient mechanism for radial migration that was later confirmed in some N-body models of transient spiral arms dynamics
We suggest that the presence of these high-[Fe/H] stars at the solar vicinity can be explained in a scenario where the Milky Way experienced the most intense epoch of radial migration at the time of the bar formation

Summary

Introduction

It is widely accepted that stars, once formed, do not necessarily stay at their initial birth radius and can migrate across disks (Goldreich & Lynden-Bell 1965; Lynden-Bell & Kalnajs 1972), a phenomenon usually referred to as radial migration. In recent decades, this phenomenon has had much attention in theoretical and numerical studies of galaxy dynamics. The resonance overlap of spiral arms and bar has been proposed as an efficient driver of radial migration (see Minchev & Famaey 2010; Daniel et al 2019). Gravitational interactions with satellite galaxies can cause a substantial radial migration, or stellar diffusion, especially in the outer disk (Quillen et al 2009)

Objectives

Results

Conclusion