Adding Up the Missing Atoms: A Dust Mass Budget from Elemental Depletions

Atoms missing from interstellar gas have gone somewhere, and summing them tests whether depletion alone accounts for the canonical dust mass. This paper sums the depleted fractions of nine elements, using solar reference abundances and representative diffuse-cloud gas fractions, into a dust mass per hydrogen nucleus. The sum gives a dust-to-gas mass ratio of 0.00576, which is 0.576 of the canonical value of 0.01, with O the largest single mass contributor at 2743.8 atomic mass units per million hydrogen atoms, iron contributing 1711.8 and oxygen 2743.8 on the same scale. Depletion accounting therefore supports the canonical dust abundance on moderately depleted sightlines without invoking hidden reservoirs, while also showing how sensitive the total is to oxygen and carbon, whose depletions are the hardest to measure. All values are computed from stated inputs, not measured.