THE ANALYSIS OF APPARENT OPTICAL DEPTH PROFILES FOR INTERSTELLAR ABSORPTION-LINES

The apparent optical depth method, a procedure for analyzing interstellar absorption lines, is discussed. The method involves converting observed absorption-line profiles into profiles of apparent optical depth, tau-a(upsilon), and apparent column density per unit velocity, N(a)(upsilon). By comparing N(a)(upsilon) profiles for a given interstellar species having two or more absorption lines which differ in the product f-lambda, it is possible to directly infer the presence or absence of unresolved saturated structure in the profiles. The method is illustrated using interstellar absorption-line data from the IUE satellite for the highly ionized gas toward HD 64760. Additional illustrations and a study of the accuracy of the method are provided through a series of numerical simulations of multicomponent interstellar absorption situations. The method is compared to the standard curve of growth method for deriving interstellar column densities. The principal value of the apparent optical depth method is that the absorption-line data are directly converted into a form (i.e., plots of column density per unit velocity) that provides for direct scientific interpretations of the physical conditions in the interstellar absorbing medium as a function of velocity.