The link between magnetic fields and filamentary clouds: bimodal cloud orientations in the Gould Belt

The orientations of filamentary molecular clouds in the Gould Belt and their local intercloud media (ICM) magnetic fields are studied using near-infrared dust extinction maps and optical stellar polarimetry data. These filamentary clouds are a few-to-10 pc in length, and we find that their orientations tend to be either parallel or perpendicular to the mean field directions of the local ICM. This bimodal distribution is not found in cloud simulations with super-Alfvénic turbulence, in which the cloud orientations should be random. ICM magnetic fields that are dynamically important compared to inertial range turbulence and self-gravity can readily explain both field–filament configurations. Previous studies commonly recognize that strong magnetic fields can guide gravitational contraction and result in filaments perpendicular to them, but few discuss the fact that magnetic fields can also channel sub-Alfvénic turbulence to form filaments aligned with them. This strong-field scenario of cloud formation is also consistent with the constant field strength observed from ICM to clouds and is possible to explain the ‘hub-filament’ cloud structure and the density threshold of cloud gravitational contraction.