Gas shells and magnetic fields in the Orion-Eridanus superbubble

TL;DR

This study investigates the complex structure, gas composition, magnetic fields, and dynamics of the Orion-Eridanus superbubble using multi-wavelength observations and gas decomposition techniques, revealing multiple shells, magnetic field compression, and hot plasma properties.

Contribution

It introduces a new gas decomposition method to identify multiple shells and magnetic field structures within the superbubble, enhancing understanding of its formation and evolution.

Findings

Multiple gas shells identified at 150-250 pc distances.

A nearly complete shell filled with hot plasma was discovered.

Magnetic fields are compressed and shaped by the superbubble's expansion.

Abstract

The Orion-Eridanus superbubble has been blown by supernovae and supersonic winds of the massive stars in the Orion OB associations. The formation history and current structure of the superbubble are still poorly understood. It possibly consists of a combination of nested shells along the line of sight. We have investigated the composite structure of the Eridanus side of the superbubble in the light of a new decomposition of the atomic and molecular gas. We used HI and CO emission lines to separate coherent gas shells in space and velocity, and we studied their relation to the warm ionised gas probed in H$\alpha$ emission, to the hot plasma emitting X-rays, and to the magnetic fields traced by dust polarised emission. We also constrained the relative distances to the clouds using dust reddening maps and X-ray absorption. We used the dust polarisation data to estimate the plane-of-sky components of the magnetic field in several clouds and along the outer rim of the superbubble. Our gas decomposition has revealed several shells inside the superbubble that span distances from about 150 pc to 250 pc. One of these shells forms a nearly complete ring filled with hot plasma. Other shells likely correspond to the layers of swept-up gas that is compressed behind the expanding outer shock wave. We used the gas and magnetic-field data downstream of the shock to derive a shock expansion velocity close to 20 km/s. Taking the X-ray absorption by the gas into account, we find that the pressure of the hot plasma inside the superbubble exceeds that in the Local Bubble. It comprises a mix of hotter (3-9 MK) and cooler (0.3-1.2 MK) gas along the lines of sight. The magnetic field along the western and southern rims and in the approaching wall of the superbubble appears to be shaped and compressed by the ongoing expansion. We find plane-of-sky magnetic strengths ranging from 3 to 15 $\mu$G along the rim.