AN ASSOCIATION BETWEEN H I CONCENTRATIONS WITHIN HIGH-VELOCITY CLOUD-A AND CLOUD-C AND NEARBY MOLECULAR CLOUDS

Neutral hydrogen concentrations within high-velocity cloud (HVC) A in Ursa Major are found to be morphologically similar to and spatially separated from nearby high-latitude CO clouds. This places these HVCs at distances comparable to those of the CO clouds, which are believed to be of order 100 pc from the Sun. One interpretation of the data suggests that the HVCs are "ghost" images of the CO clouds and represent H I gas swept out of the molecular clouds about 105 yr ago. This age is consistent with HVC lifetimes derived from cloud diameters and line widths. The HVCs in UMa are therefore transient phenomena. A prediction is made regarding where to search for the CO equivalent of another HVC concentration in complex A. Using more limited H I data for HVCs in Draco, spatial separations between morphologically similar HVCs and infrared cirrus clouds have been found. Again, the HVCs must be at distances not very different from the dust clouds in Draco, which according to various observers may be 200-800 pc. The implied age for these HVCs is of order 2 × 105 yr for the nearer distance. A detailed model for the phenomenon of separation of ghost HVCs from parent CO/dust clouds will require a global view of high-resolution observations of low-, intermediate-, and high-velocity H I gas, which, at this time, are severely limited.