Neptune's rings, 1983–1989: Ground-based stellar occultation observations: I. Ring-like arc detections

Abstract

A systematic campaign of stellar occultation observations by Neptune was conducted by our group between 1983 and 1989, and led to the initial discovery of Neptune's rings. These observations provide 24 independent scans across Neptune's equatorial plane. Two of them give evidence for ring-like arcs around the planet. The two detections, made respectively on July 22, 1984 and August 20, 1985, are the only ones ever observed simultaneously by two or more telescopes. The corresponding lightcurves are analyzed using Neptune's pole position recently determined after the Voyager 2 spacecraft observations. Assuming that the arcs lie in Neptune's equatorial plane, the 1984 detection corresponds to material orbiting 65,300 ± 3000 km from the center of the planet, and the 1985 detection corresponds to material which lies at 63,160 ± 200 km, so that both events are compatible with the Voyager 2 observations of the three arc structures observed near the 63,000-km radius (we propose the names Liberty, Equality, and Fraternity for these arcs). The arc detected in 1984 has a radial width of W r = 15.1 ± 0.1 km, an azimuthal extension greater than 100 km, and a normal optical depth of 0.074 ± 0.003 (these quantities are projected in Neptune's equatorial plane). The arc detected in 1985 has a radial width of W r = 15.3 ± 0.2 km and a normal optical depth of 0.058 ± 0.001. Our observations do not show evidence for any additional ring-like arc, nor for any perceptible diffuse, narrow, or broad rings with normal optical depth larger than about 4 × 10 −3 near the 63,000- and 53,000-km radii, where the Voyager 2 spacecraft observed continuous Neptunian rings. Finally, a preliminary search on our best lightcurves near these two radii does not show any short, correlated event with equivalent width larger than ∼75 m.