The dynamical origin of the Local arm and the Sun's trapped orbit

Abstract: The Local arm of the Milky Way, a short spiral feature near the Sun whose existence is known for decades, was recently observed in detail with different tracers. Many efforts have been dedicated to elaborate plausible hypotheses concerning the origin of the main spiral arms of the Galaxy; however, up to now, no specific mechanism for the origin of the Local arm was proposed. Here we explain, for the first time, the Local arm as an outcome of the spiral corotation resonance, which traps arm tracers and the Sun inside it. We show that the majority of maser sources belonging to the Local arm, together with the Sun, evolve inside the corotation resonance, never crossing the main spiral arms but instead oscillating in the region between them. This peculiar behavior of the Sun could have numerous consequences to our understanding of the local kinematics of stars, the Galactic Habitable Zone, and the Solar System evolution.

• The paper establishes that the Local arm forms as maser sources are trapped by the spiral corotation resonance.
• The paper reveals the Sun’s orbit is confined within a corotation island, ensuring stability without crossing main spiral arms.
• The paper validates its findings through a two-dimensional Hamiltonian model and alignment with key observational data.

The Dynamical Origin of the Local Arm and the Sun's Trapped Orbit

This paper provides a comprehensive examination of the origin of the Local arm in the Milky Way and proposes a dynamical mechanism involving the corotation resonance which offers a new perspective on the structure and kinematics of our Galaxy. The authors, Lepine et al., build a model utilizing the spiral corotation resonance to explain the peculiar positioning and behavior of the Local arm and the Sun's orbit, challenging previous conceptions and offering insights into the evolution of the Milky Way.

Overview

The study begins by acknowledging the well-documented nature and existence of the Local arm, yet points out the absence of an accepted explanation for its origin. It introduces the hypothesis that the Local arm results from the spiral corotation resonance, which traps both star-forming regions (as evidenced by maser sources) and the Sun itself within it. The model suggests these objects oscillate between the main spiral arms, without crossing them, indicating a significant effect on local stellar kinematics, the potential for a Galactic Habitable Zone, and the evolutionary path of the Solar System.

Methodology

The paper models the Galactic disk using a two-dimensional Hamiltonian framework, incorporating both an axisymmetric potential derived from Galactic rotation curves and perturbations from spiral arms modeled as Gaussian potential wells. This analytical setup allows the exploration of the stability zones created by the corotation resonance which governs the dynamics of stars and star-forming regions within these perturbed potentials.

Key Findings

1. Corotation Resonance as the Local Arm's Origin: The modeling demonstrates that a majority of maser sources linked to the Local arm are trapped within the corotation zone. This indicates the Local arm's formation and persistence are directly tied to the dynamical processes driven by this resonance.
2. Sun's Trapped Orbit: Numerical integrations of solar motion suggest the Sun resides in the corotation island, oscillating without crossing the main spiral arms. This stable trapping suggests long-term stability and may have implications for understanding potential habitability and environmental conditions of the early Solar System.
3. Support from Observational Data: The choice of a four-armed spiral potential and a particular pattern speed ($\Omega_p = 28.5$ km/s/kpc) aligns with observational data regarding solar and galactic dynamics, supporting the resonance hypothesis.

Implications

The implications span both theoretical and practical realms. On a theoretical level, the work provides a dynamical foundation for understanding secondary spiral arm structures such as the Local arm, proposing they may arise naturally from resonances rather than passing stellar objects or local perturbations. Practically, this can influence how galactic surveys interpret spiral arm data and their association with stellar kinematics, aiding in mapping out the potential habitability in different Galactic regions.

Future Directions

The paper points toward several future research directives, including exploring gas dynamics within these resonant zones, and further observational validation through high-precision data such as those expected from upcoming GAIA mission releases. Additional studies on the potential effects of resonant trapping on metallicity gradients across the Galaxy could provide further context on the evolutionary history of the Milky Way and its spiral arm structures.

In summary, the paper by Lepine et al. introduces significant refinements to our understanding of Galactic dynamics by proposing a resonance-driven model for the Local arm, potentially transforming prevailing views on spiral arm formation and stability in our galactic neighborhood.