The chemical composition of the interstellar medium

Our knowledge of the abundances of heavy elements with nuclear charge Z > 2 in the interstellar medium is surprisingly incomplete. Several factors contribute to this state of affairs. A substantial but unknown fraction of heavy elements is locked up in interstellar dust, but the total mass of interstellar grains, as well as their size distribution and exact composition, are still uncertain. The use of the chemical compositions of stellar atmospheres as a reference for the interstellar medium has become questionable, as the range in stellar compositions is becoming more fully known. The study of the stellar nucleosynthetic sources of heavy elements also provides only uncertain constraints, given that many different types of processes have contributed to the enrichment of the interstellar medium. The solution to the present dilemma may reside in the in situ detection and chemical characterization of interstellar grains themselves, which could be accomplished in the near future.