delta 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 int erstellar absorption in the line of sight to delta Orionis. First, we prese nt new high-resolution (R approximate to 110 000) observations of the inter stellar D lines of Na I towards both delta Ori A and C. Comparison of these spectra highlights variations in absorption between the two stars, indicat ive of small-scale spatial structure in the interstellar medium in this dir ection over distances of less than approximate to 15 000 au (the projected separation of the two stars). Components with the largest Na i column densi ties and lowest velocity dispersions are, in general, found to be subject t o the greatest differences; in fact the narrowest component detected is onl y observed in one of the sightlines. This effect has also been reported by Meyer & Blades. Secondly, we present new ultra-high-resolution (R approxima te to 900 000) Na I D-1 observations and high-resolution (R approximate to 110 000) Ca II H & K observations of delta 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 observati ons, while revealing the further reduction in intensity of the time-variabl e Na I absorption, indicate constant Ca II absorption over the same period. This results in a dramatic reduction in the Na degrees /Ca+ abundance rati o, perhaps indicating the line of sight to be gradually probing a less-dens e outer region of an absorbing filament.