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IAUC 8649: RINGS OF URANUS

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                                                  Circular No. 8649
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
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RINGS OF URANUS
     M. R. Showalter, SETI Institute; and J. J. Lissauer, Ames
Research Center, NASA, report the discovery of two rings of Uranus,
R/2003 U 1 and R/2003 U 2.  Both are faint, dusty rings orbiting
well beyond Uranus' previously known ring system.  R/2003 U 1 peaks
in brightness at 97700 km from the center of Uranus, coinciding
with the orbit of Uranus XXVI (Mab), which is almost certainly the
ring's primary source body.  The ring has a broad triangular
profile, with an inner limit at 86000 km -- near the orbit of
Uranus XV (Puck) -- and an outer limit at 103000 km.  R/2003 U 2
peaks in brightness at 67300 km, where no known moons are present.
This ring also has a triangular profile, terminating near the
orbits of Uranus XII (Portia) on the inside and Uranus XIII
(Rosalind) on the outside.  The rings are visible in data from the
High Resolution Channel of the Hubble Space Telescope (HST)
Advanced Camera for Surveys.  They appear in 250-s clear-filter
(300-700 nm) images obtained on 2003 Aug. 25 (24 images), 2004 Aug.
20 (40 images), 2004 Aug. 26 (40 images) and 2005 Aug. 20 (18
images).  When images from a single date are co-added, S/N  > 10.
Both rings vary in brightness with the opening angle, as is
expected for optically thin rings.  R/2003 U 1 has a peak
reflectivity at opposition (times the sine of the opening angle) of
9 x 10**-7 and an estimated normal optical depth of 9 x 10**-6.
R/2003 U 2 has a peak of 4 x 10**-7 and an estimated normal optical
depth of 6 x 10**-6.
     I. de Pater, University of California at Berkeley; H. B.
Hammel, Space Science Institute; and S. Gibbard, Lawrence Livermore
National Laboratory, write:  "We report the detection of a faint
uranian ring at a distance of about 67700 km from Uranus (outside
the epsilon ring).  We identify this ring as the first ground-based
detection of R/2003 U 2, reported by Showalter and Lissauer above.
We obtained the images at K' band (2.2 microns) on 2005 Aug. 23 UT
with the 10-m Keck II Telescope (+ NIRC2/AO) on Mauna Kea.  The
small ring opening angle (about 8.4 deg and small phase angle (0.4
deg, close to opposition), along with a full hour of integration
time, combined to make this faint ring detectable.  We also made a
probable detection of this same ring in Oct. 2005, but the ring
opening and phase angles were less favorable.  We searched out to
115000 km from Uranus in Oct. 2005 and saw no additional rings.
This will establish upper limits on the 2-micron brightness of any
material out to that distance, including R/2003 U 1 reported by
Showalter and Lissauer from HST data."

                      (C) Copyright 2005 CBAT
2005 December 29               (8649)            Daniel W. E. Green

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