Monitoring the Variable Interstellar Absorption toward HD 219188 with Hubble Space Telescope STIS

Abstract

We discuss the results of continued optical and UV spectroscopic monitoring of the variable intermediate-velocity (IV) absorption at v_solar=-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 >~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) (~6×10¹⁷ 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 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^-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.

Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Based in part on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium.