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

Within the last 10 yr, strong, narrow Na I absorption has appeared at v ☉ ~ -38 km s -1 toward the halo star HD 219188; that absorption has continued to strengthen, by a factor of 2-3, over the past 3 yr. The line of sight appears to be moving into/through a relatively cold, quiescent intermediate-velocity (IV) cloud, because of the 13 mas yr -1 proper motion of HD 219188; the variations in Na I probe length scales of 2-38 AU yr -1 . UV spectra obtained with the Hubble Space Telescope GHRS in 1994-1995 suggest N (H tot ) ~ 4.8 × 10 17 cm -2 , "halo cloud" 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 high fractional ionization, n e / n H ≳ 0.034, implies that hydrogen must be partially ionized. The N (Na I)/ N (H tot ) ratio is very high; in this case, the variations in Na I do not imply large local pressures or densities.