Monitoring the variable interstellar absorption toward HD 219188 with Hubble Space Telescope STIS

We discuss the results of continued optical and UV spectroscopic monitoring of the variable intermediate-velocity ( IV) absorption at v(circle dot) = -38 km s(-1) toward HD 219188. After reaching maxima in mid-2000, the column densities of both Na I and Ca II in that IV component declined by factors greater than or similar to 2 by the end of 2006. Comparisons between echelle spectra from HST STIS ( 2001, 2003, 2004) and HST GHRS ( 1994-1995) indicate the following: ( 1) The absorption from the dominant species S II, O I, Si ii, and Fe ii is roughly constant for all four epochs suggesting that the total N( H) (similar to 6 x 10(17) cm(-2)) and the ( mild) depletions did not change significantly over a period of nearly 10 years. ( 2) The column densities of the trace species C I ( both ground and excited fine-structure states) and of the excited state C II* all increased by factors of 2-5 between 1995 and 2001-implying increases in the hydrogen density n(H) ( from about 20 cm(-3) to about 45 cm(-3)) and in the electron density ne ( by a factor greater than or similar to 3) over that 6 yr period. ( 3) The column densities of C I and C II*-and the corresponding inferred nH and n(e)-then decreased slightly between 2001 and 2004. ( 4) The changes in C I and C II* are very similar to those seen for Na II and Ca II. The relatively low total N(H) and the modest n(H) suggest that the -38 km s(-1) cloud toward HD 219188 is not a very dense knot or filament. Partial ionization of hydrogen appears to be responsible for the enhanced abundances of Na I, C I, Ca II, and C II*. In this case, the variations in those species reflect differences in density and ionization [ and not N( H)] over scales of tens of AU.