Molecular Clouds at High Galactic Latitudes.

The basic properties of a new class of interstellar object, the high-latitude molecular cloud, are described in this thesis. At least sixty clouds grouped into thirty -eight complexes are found at |b | >=q 25 ^circ. Slightly more than half of the objects are mapped in the CO(J = 1 - 0) transition. Seventy percent of the clouds are associated with optical emission and all of the clouds coincide with the cores of the IRAS 100 mu m "high-latitude cirrus" and with atomic hydrogen clouds. The high-latitude molecular clouds are shown to be distributed asymmetrically with respect to b = 0 ^circ; the distribution is consistent with a displacement of the Sun of 30 pc above the midplane of the galaxy. A gap in the cloud distribution in the northern galactic hemisphere from 180^circ < l < 340^circ mimics that found in HI. Star counts and a statistical analysis of the velocity dispersion of the ensemble of clouds locate the clouds within a few hundred parsecs of the Sun. The clouds are the nearest molecular clouds to the Sun and may contain the nearest regions of star formation. At a mean distance of 100 pc, typical diameters and masses for the clouds are 2 pc and 40 M_ ⊙. Using the CO(J = 1-0) transition as a mass tracer yields a density of ~ 200 cm^{-3} averaged over the entire cloud. Better sampled, high resolution data show a clumpy structure which sometimes extends to the telescope resolution with an average density for these clumps of ~4000 cm^ {-3}. The results of an unbiased survey of CO emission at |b| >=q 25^circ reveal that the number of individual clouds within 100 pc of the Sun is ~120; the surface filling fraction is 4.5 times 10^{-2}. If the distribution of the high-latitude clouds mimics the overall CO emissivity for the inner galaxy, then they would contribute almost 10^8M_⊙, about 10% of the total molecular mass of the galaxy. The high-latitude molecular clouds show characteristics of both dark molecular clouds and diffuse molecular clouds. The molecular abundances of CO, OH, and H_2 CO resemble those in dark molecular clouds. However, the extinction averaged over the face of a high-latitude cloud is similar to the extinction in a diffuse molecular cloud where the molecular abundances are orders of magnitude lower than in dark clouds. The high-latitude molecular clouds show a complicated spatial and velocity structure with examples of resolved and unresolved structure to the limit of the resolution (1^' or 0.03 pc at a distance of 100 pc). The internal velocity structure implies that many of the clouds are not gravitationally bound and are breaking up. The age of these objects based on the kinematic crossing time of the clumps is <=q10 ^6 years implying that these objects are the youngest known molecular clouds in the galaxy.