Multi-station Observations of the 6 September 1996 Occultation of N72 by Neptune in the Vicinity of the Ring Arcs

R. G. French, R. J. Danos (Wellesley), I. S. Glass (SAAO), S. Hodgkin (WHT), C. A. McGhee (Cornell), B. Sicardy (Obs. de Paris), B. Stecklum (Max Planck Inst.)

We observed the 6 September 1996 Neptune occultation of N72 ( tex2html_wrap_inline13 ) as part of an ongoing campaign to detect ring material in the vicinity of the known ring arcs. The geometry of this occultation was particularly favorable for studying both the rings and atmosphere of Neptune. To provide as much coverage as possible of the ring region, we obtained IR observations from four stations. From the South African Southern Observatory (SAAO), we used a high-speed IR aperture photometer in the tex2html_wrap_inline15 tex2html_wrap_inline17 (K band), and from three other sites, we observed with IR arrays: Pic du Midi (OPMT), the William Herschel Telescope (WHT) and Calar Alto. Observing conditions were good at all four sites, although Neptune was low in the sky as seen from the northern hemisphere. We obtained planetary occultation data from several of the stations, which yielded stratospheric temperature profiles of Neptune. From astrometric fits to the half-light times of the atmospheric light curves, we found that the SAAO occultation track crossed the Adams Ring at an orbital longitude very near to the predicted location of the trailing arc. However, no measurable ring occultation was detected in the SAAO data, or in any of the IR imaging observations. N72 was substantially brighter than the star occulted during the June 1985 event in which the arcs were detected at SAAO with this same instrument (Covault et al. 1986, Icarus 67, 126-133). We conclude that the trailing arc region probed by the 1996 occultation had a substantially smaller radially integrated optical depth than the arc detected ten years earlier. By combining the results from all four observatories, we can set upper limits on the opacity of the rings in the vicinity of the arcs, which should help to establish whether they are stable ring features or are ephemeral and change on the timescale of years. This work was supported in part by NASA Grant NAGW-1368.