Dependence of Gas-Phase Abundances in the Interstellar Medium on Column Density

Sight lines 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 versus N (H I) relation is the same at high and low column densities 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 interstellar medium are (1) because of clumping, over sufficiently long path lengths N (H I) is a reasonable measure of the local density of most of the H atoms along the sight line; (2) 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; (3) 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; and (4) in gas that has a low average density the attachment should occur within denser concentrations.