Monitoring the Variable Interstellar Absorption toward HD 219188 with HST/STIS

TL;DR

This study monitors variable interstellar absorption toward HD 219188 over a decade, revealing changes in trace species linked to density and ionization variations, while the overall hydrogen column remains stable.

Contribution

It provides detailed temporal analysis of interstellar absorption lines, showing how trace species vary independently of total hydrogen content, highlighting ionization effects.

Findings

Na I and Ca II absorption declined by factors >= 2 from 2000 to 2006.

C I and C II* increased by factors of 2-5 between 1995 and 2001.

Total hydrogen column density remained roughly constant over ten years.

Abstract

We discuss the results of continued 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 HST/STIS echelle spectra obtained in 2001, 2003, and 2004 and HST/GHRS echelle spectra obtained in 1994--1995 indicate the following: (1) The absorption from the dominant species S II, O I, Si II, and Fe II is roughly constant in all four sets of spectra -- suggesting that the total N(H) and the (mild) depletions have not changed significantly over a period of nearly ten 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-year 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 appear to reflect differences in density and ionization [and not N(H)] over scales of tens of AU.