Dependence of gas-phase abundances in the interstellar medium on column density

Sight lines through high- and intermediate-velocity clouds allow measuremen ts 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). R emarkably, for several ions we find that the A versus N(H I) relation is th e same at high and low column densities and that the abundances have a rela tively low dispersion (factors of 2-3) at any particular N(H I). Halo gas t ends 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 predicto r of N(H I) can lead to large errors. Important implications of the low dis persions regarding the physical nature of the interstellar medium are (I) b ecause of clumping, over sufficiently long path lengths N(H I) is a reasona ble measure of the local density of most of the H atoms along the sight lin e; (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 dens ity; (3) the cycling of the ions becoming attached to grains and being deta ched must be rapid, and the two rates must be roughly equal under a wide va riety of conditions; and (4) in gas that has a low average density the atta chment should occur within denser concentrations.