Detection of a variable interstellar absorption component towards delta Orionis A

Observations of delta Ori A made with the UHRF in its highest resolution mo de (R approximate to 900 000) have revealed the presence of a cool (T(k)les s than or equal to 350 K) variable absorption component at a heliocentric v elocity of +21.3 km s(-1). The component is detected in Na I D-1, where cle ar hyperfine splitting is seen, and Ca ii K. Comparison of our data with ex isting spectra suggests that the component has consistently increased in st rength from 1966 to 1994, and subsequently reduced in intensity by 1999. Fo llowing a discussion of the possible origins of this component it is conclu ded that an interstellar, rather than circumstellar, origin is most likely. This is one of very few detections of variable interstellar absorption rep orted in the literature, and we suggest an origin within filamentary materi al associated with the expanding H I shell surrounding the Orion-Eridanus s uperbubble.