2007ApJ...668.1012W

We discuss the results of continued optical and UV spectroscopic monitoring of the variable intermediate-velocity (IV) absorption at v_☉=-38 km/s 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 ≳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) (∼6x10¹⁷/cm2) 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/cm3 to about 45/cm3) and in the electron density n_e(by a factor ≳3) over that 6 yr period. (3) The column densities of C I and C II*–and the corresponding inferred n_H 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 I and Ca II. The relatively low total N(H) and the modest n_H suggest that the -38 km/s 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.