Magnetic fields in the Eos Cloud: dynamically important fields in the interface between atomic and molecular gas

Abstract

The recently-discovered Eos molecular cloud, is a CO-dark, low-density cloud located at a distance of approximately 94 pc from the Sun which does not appear to have formed stars at any point in its history. In this paper we investigate the magnetic fields in the Eos cloud, near the interface between the atomic Cold Neutral Medium (CNM) and molecular gas, using dust emission and extinction polarimetry. A Histogram of Relative Orientation analysis shows that the magnetic field is preferentially parallel to the density structure of the cloud, while a Davis-Chandrasekhar-Fermi analysis finds magnetic field strengths of 6$\pm$3 $\mu$G across the Eos cloud and 12$\pm$4 $\mu$G in the somewhat denser MBM 40 sub-region. These results are consistent with a previous estimate of magnetic field strength in the Local Bubble and suggest that the fields in the Eos cloud are dynamically important compared to both gravity and turbulence. Our findings are fully consistent with the expected behavior of magnetized, non-self-gravitating gas near the CNM/molecular cloud boundary.