Spatial and Temporal Variations in Interstellar Absorption toward HD 72127AB

TL;DR

This study investigates spatial and temporal variations in interstellar absorption toward HD 72127AB using optical and UV spectra, revealing differences in physical conditions and element abundances on small scales, possibly related to the Vela SNR.

Contribution

The paper provides new optical spectra and analysis of interstellar absorption variations along two sight lines, highlighting differences in physical conditions and element abundances at small spatial scales.

Findings

Similar total hydrogen column densities in both sight lines.

Main components are relatively cool with warm, diffuse cloud-like depletions.

Enhanced neutral species toward HD 72127B suggest dielectronic recombination effects.

Abstract

New optical spectra of Ca II and Na I toward HD 72127AB provide additional evidence for both spatial and temporal variations in the complex interstellar absorption along the two sight lines; archival UV spectra yield information on the abundances, depletions, and physical conditions in the gas toward HD 72127A. Similarities in the strengths of various tracers of interstellar material in the two lines of sight suggest that the total hydrogen column densities [N(H) ~ 2.5 x 10^{20} cm^{-2}] and the depletions and ionization in the main components at low LSR velocities also are similar. Toward HD 72127A, the main components are relatively cool (T < 900 K), but with depletions resembling those found in warm, diffuse disc clouds; the generally weaker components at higher velocities have much milder depletions, more like those found in halo clouds. Several trace neutral species -- Ca I, Cr I, and Fe I -- are much stronger toward HD 72127B, however. The column density of Cr I, for example, is about 30 times the value determined toward zeta Oph (the only previous detection of that species in the ISM). Dielectronic recombination in warmer gas (T > 5000 K) may be largely responsible for the enhanced abundances of those trace neutral species toward HD 72127B. If the main components toward HD 72127AB are associated with material in the Vela SNR, the differences in abundances and physical conditions occur on scales of about 1100 AU.