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

Abstract

Within the last 10 yr, strong, narrow Na I absorption has appeared at v☉ ~ -38 km s-1 toward the 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(Htot) ~ 4.8 × 1017 cm-2, halo depletions, nH ~ 25 cm-3, and ne ~ 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, ne/nH 0.034, implies that hydrogen must be partially ionized. The N(Na I)/N(Htot) ratio is very high; in this case, the variations in Na I do not imply large local pressures or densities.