Dust Models with Tight Abundance Constraints

The reference abundance of the interstellar medium (the abundances of heavy elements in both the grains and the gas) is perhaps as low as 65% of solar, rather than solar as has been widely assumed in the past. Furthermore, ≈38% of the solar C/H is now known to be in the gas rather than in grains. The evidence for these statements is reviewed. These reductions in the amounts of heavy elements available for grains have profound significance for the nature of interstellar dust.