Modeling the Neutral Hydrogen Interstellar Medium: A Better Kinematic Distance Tool

An advanced approach to the kinematic distance method is developed. The method is applicable to second- and third-quadrant Galactic objects with known velocities. It is based on fitting a model of the density and velocity features in an isothermal H I disk to observed H I data. The velocity field of the gas is modeled with a power law for basic circular rotation, underlying noncircular motions from a two-armed density wave pattern. With a reasonable number of adjustable parameters and constraints the model reproduces observations toward many Galactic objects, and accurate distances are found from the modeled velocity field. High-resolution H I spectral line data from the Canadian Galactic Plane Survey (CGPS; Taylor et al.) are used to discriminate clouds from the intercloud medium (the stratum) for which the model is intended. The ability of the model to reproduce these data is demonstrated in one- [Tb(v)] and two- [Tb(l, v)] dimensional fits. Distances to 22 H II regions and SNRs calculated by the fitted velocity field compare extremely well with other kinematically independent distances.