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HI and CO surveys have been undertaken of 13 peculiar comet-shaped, high Galactic latitude clouds identified in the Infrared Astronomical Satellite (IRAS) far-IR sky survey. The far-IR, HI and CO data have been examined for clues to the origin of the cloud shapes. The cloud sample appears to consist of two populations; one resembling far-IR 'Cirrus', the other a mixture of dense molecular clouds and active star-forming regions.
We have examined a variety of mechanisms that could give rise to such elongated, and even streamlined, clouds. Excluded by the data are interactions with ionization fronts, molecular outflows and 'champagne' flows. Ram pressure stripping of the cloud material due to large relative velocities between the clouds and their ambient ISM also seems to be excluded by the HI and CO data which show the clouds to be primarily low-velocity systems with V(LSR) < 30 km/s. Only the Draco Cloud may be a viable candidate for such an interaction. Although supernova remnant-cloud collisions can under some circumstances lead to elongated clouds, the HI data yield no examples of clouds that are clearly associated with SNRs.
The remaining processes involve cloud-cloud collisions and a variety of 'instabilities' which naturally lead to elongated clouds of the kind seen in this survey. We will discuss these possibilities in more detail.
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