Is there Evidence for Depletion of CNO in the Interstellar Medium?

Abstract

In any discussion of interstellar abundances it is crucial to know to what extent the gas phase abundances are representative of the true interstellar abundances. Extensive interstellar absorption data has been obtained in the last few years with the UV telescope on the Copernicus satellite. It has been claimed that this data provides evidence for depletion for many elements, including CNO (see the review by Spitzer and Jenkins 1975 and references therein). My purpose here is to point out that the conclusion that CNO are depleted from the interstellar gas is based on an analysis of the data which is excessively naive. I will conclude that there is no reliable evidence for depletion of these elements in the relatively thin and diffuse clouds (N_H≲10²¹cm⁻², n_H≲10³cm⁻³) studied in the Copernicus observations. It should be borne in mind that I am not claiming that CNO are not depleted, rather, I will simply argue there is no good evidence for their depletion. Although attention is restricted here to CNO, my remarks will apply, in general, to any abundance derived from saturated absorption features.