Variable Interstellar Absorption toward the Halo Star HD 219188 - Implications for Small-Scale Interstellar Structure
Abstract
Within the last 10 years, strong, narrow Na I absorption has appeared at vsun ~ -38 km/s toward the halo star HD 219188; that absorption has continued to strengthen, by a factor 2-3, over the past three years. The line of sight appears to be moving into/through a relatively cold, quiescent intermediate velocity (IV) cloud, due to the 13 mas/yr proper motion of HD 219188; the variations in Na I probe length scales of 2-38 AU/yr. UV spectra obtained with the HST GHRS in 1994-1995 suggest N(Htot) ~ 4.8 X 1017 cm-2, ``halo cloud'' 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.
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