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_solar~-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)~4.8×10¹⁷ 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. Based in part on observations with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.