High-latitude molecular clouds

We report the detection of a large number of molecular clouds at high galactic latitude, a few of which were previously cataloged as dark nebulae. We have mapped a large fraction of the clouds detected at Vertical BarbVertical Bar>30/sup 0/ in order to determine the properties of this class of objects. The clouds are nearby (d-bar100 pc) with typical diameters and masses of about 2 pc and roughly 65 M. Allowing for incompleteness in sky coverage, there are approximately 64/epsilon such clouds withon 100 pc, where epsilon is the fractional completeness of our survey. A large fraction, perhaps a majority, of the clouds are not gravitationally bound. The internal velocity structure implies that many of the clouds are very young with ages < or =10/sup 6/ yr. The clouds are invariably associated with atomic hydrogen clouds, but the molecular column densities appear to dominate in many cases. The mean molecular density of the clouds is roughly 170 cm/sup -3/. The one-dimensional cloud-to-cloud velocity dispersion is 5.7 +- 1.2 km s/sup -1/. The surface density of these clouds projected on the galactic plane is approximately 0.1/epsilon M pc/sup -2/, a small fraction of the H I surface density, but an appreciablemore » fraction of the smoothed surface density of giant molecular clouds. Individual clouds subtend areas from a fraction of a square degree to several square degrees, implying that they may be responsible for at least some of the diffuse high-latitude emission detected by IRAS.« less