Azimuthal Variability at Edges in Saturn’s Rings from Binary Star Occultations

Several of the bright star occultations by Saturn’s rings observed by the Cassini Ultraviolet Imaging Spectrograph (UVIS) were of binary stars where both stars in the system contributed measurable signals to the total signal. Fifteen occultations of the multi-star system Beta Centauri were observed, and the two brightest components are separated by tens to hundreds of meters projected into the ring plane as observed by Cassini. These occultations provide an opportunity to measure azimuthal variability in ring structure on the scale of the separation of the stars, much less than the scale of an imaging pixel. We model the occultation data of Beta Centauri at the sharp edges of ringlets and gaps with a simple two component transparency model for the rings, with one component for each azimuth sampled by one of the stars in the binary system. We choose sharp edges for this initial modeling because the radial separation of the two stars can be directly measured from the radial separation of the two edge features, one for each star passing behind the edge. This provides an estimate for the azimuthal separation based on the total projected ring plane separation of the stars. We also use astrometric determinations of the stars’ coordinates to get the ring plane separation in the radial and azimuthal directions. We analyze the edges of the Huygens ringlet, the Encke gap and Keeler gap edges, and the A and B ring edges and report on variability of the ring structure on the scale of the separation of the stars. Future work will extend this forward modeling approach across the ring system and to include observations of other binary stars with different projected ring plane separations.