EARTH-BASED DETECTION OF URANUS LAMBDA-RING

The 11 July 1992 occultation of Uranus by U103 (m(K) = 10.1) was obser ved using the 5-m Hale telescope on Palomar Mountain in California, th e 4-m telescope at Cerro Tololo Inter-American Observatory (CTIO) in C hile, and the nearby 2.2-m telescope at European Southern Observatory (ESO). All observations were taken using InSb aperture photometers and K (lambda similar to 2.2 mu m) filters and provided high-quality prof iles of the uranian rings. The ESO and Palomar ring event times have b een incorporated into a comprehensive orbit model for the elliptical r ings using all available Earth-based and Voyager occultation data, The kinematical model was used to determine the radius scale and geometry of the U103 occultation, as well as to provide absolute timing calibr ation for the CTIO light curve, which was reconstructed from two-dimen sional digital scans of a high-speed strip chart, All nine classical r ings were clearly observed during both ingress and egress. In addition , a sharp feature appeared in the CTIO ingress light curve with an orb ital radius of 50,026.89 +/- 0.39 km, very close to the mean radius of the lambda ring of 50,026.3 +/- 0.6 km found by M. R, Showalter (Scie nce 267, 490-493, 1995) from an analysis of Voyager 2 images. The CTIO feature is well above the noise: its equivalent width of E = 0.32 +/- 0.14 km corresponds to a 5-sigma detection. It is the only such signa ture in the entire CTIO lightcurve, other than the previously known ri ngs. The characteristic ring-like shape of the profile, its high SNR, and its radial location all support the identification of this feature as an Earth-based detection of the lambda ring. At the same time, if the ring were azimuthally homogeneous with E greater than or similar t o 0.32 km, it would have been clearly visible in the CTIO egress and P alomar light curves, but no such features were found, The A ring is th us azimuthally clumpy, resembling to some degree Neptune's are-like Ad ams ring. At the nearby ESO station, a suggestive ingress feature appe ared with a radius of 50,023.01 +/- 0.28 km, but the data are too nois y to permit a positive identification; The orbital characteristics of the A ring and its azimuthal brightness profile cannot be uniquely ext rapolated from the Voyager encounter epoch to the time of the U103 occ ultation because the mean motion and orbital radius of the ring are to o uncertain. However, additional detections in archival Earth-based oc cultation data or from future observations could allow a much better d etermination of the dynamics of this dusty and clumpy ring. (C) 1996 A cademic Press, Inc.