Variable Interstellar Absorption toward the Halo Star HD 219188: Implications for Small-Scale Interstellar Structure

Within the last 10 years, strong, narrow Na I absorption has appeared atv_sun ~ -38 km/s toward the halo star HD 219188; that absorption has continuedto strengthen, by a factor 2-3, over the past three years. The line of sightappears to be moving into/through a relatively cold, quiescent intermediatevelocity (IV) cloud, due to the 13 mas/yr proper motion of HD 219188; thevariations in Na I probe length scales of 2-38 AU/yr. UV spectra obtained withthe HST GHRS in 1994-1995 suggest N(H_tot) ~ 4.8 X 10^{17} cm^{-2}, ``halocloud'' depletions, n_H ~ 25 cm^{-3}, and n_e ~ 0.85-6.2 cm^{-3} (if T ~ 100 K)for the portion of the IV cloud sampled at that time. The relatively highfractional ionization, n_e/n_H >~ 0.034, implies that hydrogen must bepartially ionized. The N(Na I)/N(H_tot) ratio is very high; in this case, thevariations in Na I do not imply large local pressures or densities.Comment: 12 pages; aastex; to appear in ApJ