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

Abstract

The orientations of filamentary molecular clouds in the Gould Belt and their\nlocal ICM (inter-cloud media) magnetic fields are studied using near-infrared\ndust extinction maps and optical stellar polarimetry data. These filamentary\nclouds are a few-to-ten parsecs in length, and we find that their orientations\ntend to be either parallel or perpendicular to the mean field directions of the\nlocal ICM. This bimodal distribution is not found in cloud simulations with\nsuper-Alfvenic turbulence, in which the cloud orientations should be random.\nICM magnetic fields that are dynamically important compared to inertial-range\nturbulence and self-gravity can readily explain both field-filament\nconfigurations. Previous studies commonly recognize that strong magnetic fields\ncan guide gravitational contraction and result in filaments perpendicular to\nthem, but few discuss the fact that magnetic fields can also channel\nsub-Alfvenic turbulence to form filaments aligned with them. This strong-field\nscenario of cloud formation is also consistent with the constant field strength\nobserved from ICM to clouds (Crutcher et al. 2010) and is possible to explain\nthe "hub-filament" cloud structure (Myers 2009) and the density threshold of\ncloud gravitational contraction (Kainulainen et al. 2009).\n