Mathematics – Probability
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2204b&link_type=abstract
American Astronomical Society, DPS meeting #42, #22.04; Bulletin of the American Astronomical Society, Vol. 42, p.988
Mathematics
Probability
Scientific paper
Using UVIS stellar occultation data, we analyzed isolated and unexpected high photon counts in different places. These "ghosts" are like holes in a ringlet or plateau through which we directly observe the star: it is therefore characterized by an isolated peak in photon counts with a height equal to photon counts in places without ring material.
In order to detect ghosts, we focus on the plateaus in the Cassini Division and in the C ring and on the B ring. (The A ring contains so many clumps from self-gravity wakes that it consists almost entirely
of clumps and ghosts.) We have identified 45 ghosts in the C ring and 57 in the Cassini Division. We have also identified lower probability potential ghosts (79 in the C ring and 24 in the Cassini Division).
We rule out cosmic rays as a cause of these high count rates by analyzing the frequency of high counting rates above the stellar signal. Applying this process to all our identified potential cosmic rays allowed us to clearly identify 89 cosmic ray events out of 65443330 measured points. Therefore we derived a density of 1 cosmic ray hit per 735318 points. Our detected potential ghosts are very unlikely cosmic rays and rather due to some actual structure in the rings.
We suggest that ghosts are the signatures of ephemeral structures in the rings that could be due to particularly large clumps of material or small moonlets. The usual S-shape around a "propeller" moonlet coincides with the presence of a hole around the clump: these moonlets are probably not massive enough to open full gaps, but could produce azimuthally limited holes in the rings like those seen in the UVIS occultation data.
Baillié Kévin
Colwell Joshua E.
Esposito W. L. W. L.
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