2001ApJ...551L.175W

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)∼4.8x10¹⁷ 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.