Optical Polarization Mapping toward the interface between the Local Cavity and Loop I

TL;DR

This study uses optical polarization data from 878 stars to investigate the structure and magnetic field geometry of the interaction zone between the Local Cavity and Loop I, revealing a highly irregular and fragmented ring-like feature.

Contribution

It provides the first detailed optical polarization mapping of the suspected interaction zone, revealing its irregular, twisted, and fragmented nature, challenging the idea of a coherent large-scale ring.

Findings

Distances along the ring vary from 100 pc to 250 pc.

Polarization vectors show different orientations on each side of the ring.

The structure appears highly irregular and fragmented.

Abstract

The Sun is located inside an extremely low density and quite irregular volume of the interstellar medium, known as the Local Cavity (LC). It has been widely believed that some kind of interaction could be occurring between the LC and Loop I, a nearby superbubble seen in the direction of the Galactic Center. As a result of such interaction, a wall of neutral and dense material, surrounded by a ring shaped feature, would be formed at the interaction zone. Evidence of this structure was previously observed by analyzing the soft X-ray emission in the direction of Loop I. Our goal is to investigate the distance of the proposed annular region and map the geometry of the Galactic magnetic field in these directions. On that account, we have conducted an optical polarization survey to 878 stars from the Hipparcos catalogue. Our results suggest that the structure is highly twisted and fragmented, showing very discrepant distances along the annular region: approximately 100 pc to the left side and 250 pc to the right side, independently confirming the indication from a previous photometric analysis. In addition, the polarization vectors' orientation pattern along the ring also shows a widely different behavior toward both sides of the studied feature, running parallel to the ring contour in the left side and showing no relation to its direction in the right side. Altogether, these evidence suggest a highly irregular nature, casting some doubt on the existence of a unique large-scale ring-like structure.