Gas shells and magnetic fields in the Orion-Eridanus superbubble

Aims: The Orion-Eridanus superbubble has been blown by supernovae and supersonic winds of the massive stars in the Orion OB associations. It is the nearest site at which stellar feedback on the interstellar medium that surrounds young massive clusters can be studied. The formation history and current structure of the superbubble are still poorly understood, however. It has been pointed out that the picture of a single expanding object should be replaced by a combination of nested shells that are superimposed 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.
Methods: We used H I 21 cm and CO (J = 1-0) emission lines to separate coherent gas shells in space and velocity, and we studied their relation to the warm ionised gas probed in H_α emission, the hot plasma emitting X-rays, and 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 applied the Davis-Chandrasekhar-Fermi method to 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.
Results: Our gas decomposition has revealed several shells inside the superbubble that span distances from about 150-250 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 the shock expansion velocity, which is close to ~20 km s^-1. Taking the X-ray absorption by the gas into account, we find that the hot plasma inside the superbubble is over-pressured compared to plasma in the Local Bubble. The plasma comprises a mix of hotter and cooler gas along the lines of sight, with temperatures of (3-9) and (0.3 - 1.2) × 10⁶ K, respectively. 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 field strengths from 3 to 15 μG along the rim.