Spatial and temporal variations in interstellar absorption towards HD 72127AB

Abstract

New optical spectra of CaII and NaI towards HD 72127AB provide additional evidence for both spatial and temporal variations in the complex interstellar absorption along the two sightlines; archival ultraviolet spectra yield information on the abundances, depletions and physical conditions in the gas towards 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 ~ 2.5 × 10²⁰cm^-2) and the depletions and ionization in the main components at low local standard of rest (LSR) velocities also are similar. Towards 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 - CaI, CrI and FeI - are much stronger towards HD 72127B, however. The column density of CrI, for example, is about 30 times the value determined towards ζ 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 towards HD 72127B. If the main components towards HD 72127AB are associated with material in the Vela supernova remnant, the differences in abundances and physical conditions occur on scales of about 1100 au.

Based on observations collected at the European Southern Observatory, Chile, under programme 72.C-0682, and 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.

Visiting observer, European Southern Observatory. ‡

E-mail: welty@oddjob.uchicago.edu (DEW); thuso@unm.edu (TS); hobbs@yerkes.uchicago.edu (LMH)