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

• Published in: The Astrophysical journal 2007-10, Vol.668 (2), p.1012-1027
• Main Author: Welty, Daniel E
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/521269

We discuss the results of continued optical and UV spectroscopic monitoring of the variable intermediate-velocity (IV) absorption at [unk] = -38 km s super(-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 equal 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 sub(17) cm super(-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 sub(H) (from about 20 cm super(-3) to about 45 cm super(-3)) and in the electron density n sub(e) (by a factor greater than or equal to 3) over that 6 yr period. (3) The column densities of C I and C II*-and the corresponding inferred n sub(h) and n sub(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 sub(H) suggest that the -38 km s super(-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.