HST GHRS OBSERVATIONS OF THE BETA-PICTORIS SYSTEM .2. EXPLORING THE POTENTIAL FOR TRACKING COMET-LIKE OBJECTS ORBITING THE STAR/

We present a systematic study of the transient absorption features obs erved in the spectra of beta Pictoris. We calculate detailed synthetic spectra of a star, which is partially obscured by a permanent circums tellar disk and by an absorbing body of a given size, position, radial velocity with respect to the star, and total column density. We show that because of the nonuniform surface brightness of the star, the str ength of an absorption feature is an intricate function of both positi on and size. Consequently, one cannot determine the projected area of the body or its position from a single measurement only. However, by s tudying several closely spaced lines simultaneously, like those in the region of the Fe II UV1 multiplet at lambda approximate to 2600 Angst rom, one can determine both the position and size of the body. We pres ent a preliminary analysis of Goddard High Resolution Spectrograph obs ervations of beta Pic in this region and show that while existing spec tra do not have sufficient time coverage to allow us to track a single body during its transit across the stellar disk, our approach provide s a robust means of tracking cometlike objects transiting the star and should be used to interpret future monitoring of beta Pic and other s tars showing similar variable absorption features. Conversely, the eff ect of body position is so great that it must be taken into account; a bsorption-line analyses based on techniques developed for interstellar lines should not be used.