Determination of Interstellar Abundance by Correlated X-Ray and Ultraviolet Observations

Ultraviolet observations of interstellar absorption lines obtained with Copernicus have provided important new insights into the structure and composition of the interstellar medium (ISM). These observations have also raised important questions about interstellar matter which are difficult to resolve by ultraviolet observations alone. These questions include: (1) What fraction of the interstellar abundance of the important elements (C, N, O, Mg, Si, S, Ar, Fe) is bound in the interstellar grains?; (2) What is the absolute (i.e. relative to H) abundance of C, N, O, Mg, Si, S, Ar, Fe?; (3) What is the interstellar abundance of Ne?; (4) What is the ionization structure of the interstellar gas?; (5) What is the composition and size of the interstellar grains? X-ray photoabsorption in the interstellar medium introduces observable edge structure in the spectra of galactic x-ray sources at the wavelengths of the Kand L-edges of the abundant elements (these edges occur at wavelengths between 1.5 and 50 A for the light abundant elements and for iron) which contain information on the structure and composition of the interstellar matter which is complementary to that contained in the ultraviolet. Interpretation of moderate resolution x-ray spectra of strong galactic sources has already provided unique information on the total column density of heavy elements (i.e. elements heavier than H and He) in the ISM. The prospect of higher resolution x-ray spectra to be obtained with the transmission grating experiment on HEAO-B and with a rocket experiment planned at Stanford will provide data of sufficiently high spectral resolution to allow a direct measurement of the total column density of several important elements (O, N, C, Ne, Fe) in the ISM. If ultraviolet and x-ray observations of the column density in the interstellar gas are available for the same sources, it should be possible to significantly constrain ISM models with regard to: (1) the fraction of the abundance of number of elements which is bound in grains; (2) the absolute ISM abundance of a number of elements; (3) the relative abundance of elements such as Ne which do not have convenient absorption lines in the ultraviolet; (4) the ionization structure of those elements for which line saturation makes accurate determination of total column density difficult in the ultraviolet; and (5) the typical size and composition of interstellar grains. We propose to analyze IUE observations of several of the strongest x-ray sources which have well established optical counterparts in conjunction with both moderate resolution observations (which are already available from SAS-C, Copernicus and HEAO-A) and high resolution observations which are expected from our rocket program at Stanford and from HEAO-B in order to refine present models of the ISM.