2001MNRAS.321..553P

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~110000) observations of the interstellar D lines of Nai 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 ~15000au (the projected separation of the two stars). Components with the largest Nai 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~900000) Nai D₁ observations and high-resolution (R~110000) Caii 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 Nai absorption component. These new observations, while revealing the further reduction in intensity of the time-variable Nai absorption, indicate constant Caii 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.