A SUPERSHELL MODEL FOR THE HIGH-VELOCITY AND INTERMEDIATE-VELOCITY NEUTRAL HYDROGEN CLOUDS

The velocities and spatial distribution of high- and intermediate-velocity neutral hydrogen gas in the second quadrant of Galactic longitude in the northern hemisphere are accounted for in terms of a model in which the ''clouds'' appear to be part of a vast supershell with an elliptical cross section normal to the Galactic disk. Its origin appears to be in the Perseus arm toward l = 131-degrees +/- 4-degrees with an apparent radius of 1920 pc and an expansion velocity of 281 +/- 10 km s - 1. The supershell is treated, to first order, as an elongated structure coaxial with the Perseus spiral arm in order to facilitate comparison between available high-velocity cloud (HVC) velocities and the model. Further observations of intermediate-velocity H I are essential for refining the characteristics of the supershell model in three dimensions. If the supershell is uniformly filled with gas, the total mass of H I is 3.2 x 10(6) M. with kinetic energy 2.5 x 10(54) ergs. This is greater than values found in other supershells. If, instead, the gas is filamentary with typical structures less-than-or-equal-to 5 pc wide, the total energy is less-than-or-equal-to 2.0 x 10(53) ergs. Gas in the near side of the shell could be as close as 30 pc from the Sun. However, in the solar neighborhood the supershell has impacted a local structure marked by the radio continuum spur known as Loop III. Evidence is presented that the incoming stream of high-velocity gas from the Perseus arm has been decelerated at the surface of Loop III, which therefore acts to modulate the observed distribution of HVCs. Taking into account the influence of Loop III, the parameters describing the supershell are derived by comparing model predictions with the latitude-velocity distribution of the HVCs in 5-degrees longitude intervals using data from the Hulsbosch & Wakker survey. The model is then used to predict the spatial distribution of the H I brightness in several velocity regimes and compared with the Bell Laboratories H I survey data. The good agreement between the model and the northern hemisphere data suggests that the brightest HVCs result from column density enhancements occurring over a large depth in space, in many cases of order several hundred parsecs. An exception appears to be stream A, which may represent gas still being decelerated at the interface between the supershell and loop III. The distances to the high- and intermediate-velocity H I structures follow from the model. Distances are dependent on location and velocity, ranging from 100 pc or less for gas in the near face of the supershell in directions away from Loop III to as much as 1 kpc for material in the - 100 km s-1 regime in certain directions. Finally, it is suggested that high-latitude, low-velocity H I and molecular ''clouds'' and their associated cirrus structures result from transient density enhancements at the interface between the incoming stream of high-velocity material associated with the supershell and loop III.