Striations in the Taurus molecular cloud: Kelvin-Helmholtz instability or MHD waves?

Abstract

The origin of striations aligned along the local magnetic field direction in the translucentenvelope of the Taurus molecular cloud is examined with new observations of 12CO and 13CO J = 2-1 emission obtained with the 10-m Submillimeter Telescope of the ArizonaRadio Observatory. These data identify a periodic pattern of excess blue and redshifted emissionthat is responsible for the striations. For both 12CO and 13CO, spatial variations of theJ = 2-1 to J = 1-0 line ratio are small and are not spatially correlated with the striationlocations. A medium comprised of unresolved CO emitting substructures (cells) with a beamarea filling factor less than unity at any velocity is required to explain the average line ratiosand brightness temperatures.We propose that the striations are generated from the modulationof velocities and beam filling factor of the cells as a result of either the Kelvin-Helmholtzinstability or magnetosonic waves propagating through the envelope of the Taurus molecularcloud. Both processes are likely common features in molecular clouds that are sub-Alfvénicand may explain low column density, cirrus-like features similarly aligned with the magneticfield observed throughout the interstellar medium in far-infrared surveys of dust emission.