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 ICM (inter-cloud media) magnetic fields are studied using near-infrared
dust extinction maps and optical stellar polarimetry data. These filamentary
clouds are a few-to-ten parsecs 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-Alfvenic 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-Alfvenic 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 (Crutcher et al. 2010) and is possible to explain
the "hub-filament" cloud structure (Myers 2009) and the density threshold of
cloud gravitational contraction (Kainulainen et al. 2009).
