δ Orionis: further temporal variability and evidence for small-scale structure in the interstellar medium

We report here the detection of both spatial and temporal variations in interstellar absorption in the line of sight to δ Orionis. First, we present new high-resolution (R≈110 000) observations of the interstellar D lines of Na I towards both δ Ori A and C. Comparison of these spectra highlights variations in absorption between the two stars, indicative of small-scale spatial structure in the interstellar medium in this direction over distances of less than ≈15 000 au (the projected separation of the two stars). Components with the largest Na I column densities and lowest velocity dispersions are, in general, found to be subject to the greatest differences; in fact the narrowest component detected is only observed in one of the sightlines. This effect has also been reported by Meyer & Blades. Secondly, we present new ultra-high-resolution (R≈900 000) Na I D1 observations and high-resolution (R≈110 000) Ca II H & K observations of δ Ori A which, through ultra-high-resolution work conducted between 1994 and 2000, has been shown to exhibit a time-variable interstellar Na I absorption component. These new observations, while revealing the further reduction in intensity of the time-variable Na I absorption, indicate constant Ca II absorption over the same period. This results in a dramatic reduction in the Na°/Ca+ abundance ratio, perhaps indicating the line of sight to be gradually probing a less-dense outer region of an absorbing filament