0 Dependence of Gas Phase Abundances in the ISM on Column Density

Sightlines through high-and intermediate-velocity clouds allow measurements of ionic gas phase abundances, A, at very low values of H I column density, N(H I). Present observations cover over 4 orders of magnitude in N(H I). Remarkably, for several ions we find that the A vs N(H I) relation is the same at high and low column density and that the abundances have a relatively low dispersion (factors of 2–3) at any particular N(H I). Halo gas tends to have slightly higher values of A than disk gas at the same N(H I), suggesting that part of the dispersion may be attributed to the environment. We note that the dispersion is largest for Na I; using Na I as a predictor of N(H I) can lead to large errors. Important implications of the low dispersions regarding the physical nature of the ISM are: (a) because of clumping, over sufficiently long pathlengths N(H I) is a reasonable measure of the local density of most of the H atoms along the sight line; (b) the destruction of grains does not mainly take place in catastrophic events such as strong shocks, but is a continuous function of the mean density; (c) the cycling of the ions becoming attached to grains and being detached must be rapid, and the two rates must be roughly equal under a wide variety of conditions; (d) in gas that has a low average density the attachment should occur within denser concentrations.