Azimuthal Transparency Profiles of Ring Edges from UVIS Occultations of Binary Stars

The Cassini Ultraviolet Imaging Spectrograph (UVIS) Highspeed Photometer (HSP) measured occultations of UV bright stars by Saturns rings during the mission, recording the number of UV photons in the wavelength range 110nm<<190nm captured within the 6.0 x 6.0 mrad field of view (FOV) of the instrument every 12 ms. In some of these occultations, a UV-bright binary companion star was also in the FOV. The positions of the stars are generally offset in ring plane radius and ring longitude. At sharp ring or ringlet edges, the radial offset of the stars can be directly measured and exhibits itself in the radial occultation profile as a stairstep at the ring or ringlet edge. Knowledge of the right-ascension, declination, and UV magnitude of the stars at the time of the occultation allows for the modeling of the transparency profile of the ring or ringlet edge at one ring longitude and another downstream by only a few tens to hundreds of meters depending on the angular separation of the two stars. Beta Centauri was one of the multi-star systems for which 15 UVIS HSP occultation profiles were measured. The two brightest components are separated by tens to hundreds of meters projected into the ring plane radial direction as observed by Cassini. We model the occultation data of Beta Centauri at the sharp edges of the Huygens ringlet, Encke Gap and the A and B ring outer edges with a simple two component transparency model for the rings, one component for each azimuth sampled by one of the stars in the binary system. In some of the occultations, we find significant variations in ring transparency between the two stars while others are fit well by a single ring transparency profile. While the data cannot provide a unique solution to the radial structure at the location of the two stars, we can show whether or not the data are consistent with an unchanging radial profile. We find cases near these edges that require significant changes in ring transparency over the short azimuthal distances separating the two stars.