Occultation of U138 by Uranus' Rings

A.A. Sickafoose, A.S. Bosh (Lowell Obs.)

The occultation by Uranus of U138 (SAO 163583; Klemola and Mink, 1991, Astron. J. 102, 389) was observed on 10 April 1996 using the Faint Object Spectrograph on the Hubble Space Telescope. The tex2html_wrap_inline13 ring was observed on ingress, while 6, 5, 4, tex2html_wrap_inline15 , tex2html_wrap_inline17 , tex2html_wrap_inline19 , tex2html_wrap_inline21 , tex2html_wrap_inline23 , and tex2html_wrap_inline25 were clearly detected during egress. One of the strengths of this occultation observation is the fact that it is spectrally resolved, including data over a wavelength range of 3546 - 7080 tex2html_wrap_inline27 with resolution  25 tex2html_wrap_inline27 . The unusually high signal-to- noise ratio of these data allows detection of optically thin features normally hidden in the noise as well as calculation of equivalent depths for at least 140 different wavelength bins over this range. We use these exceptional data to explore several characteristics of the rings: the azimuthal asymmetry of the tex2html_wrap_inline25 ring, the particle size distributions in all rings (via dependence of equivalent depth on wavelength), and the structure of the tex2html_wrap_inline23 - and tex2html_wrap_inline19 -ring shelves.