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IAUC 8586: 2005dk; URANUS

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                                                  Circular No. 8586
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
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SUPERNOVA 2005dk IN IC 4882
     P. Luckas, Perth, W. Australia; O. Trondal, Oslo, Norway; and
M. Schwartz, Patagonia, AZ, report the discovery of an apparent
supernova on unfiltered CCD frames taken with the 0.35-m Tenagra
telescope at Perth; available approximate magnitudes for 2005dk:
July 10.61 UT, [18.5; Aug. 21.53, 17.8; 22.62, 16.6.  SN 2005dk is
located at R.A. = 19h40m23s.71, Decl. = -55o12'05".1 (equinox
2000.0), which is 3".4 east and 16".2 south of IC 4882.

     L. A. Sromovsky and P. M. Fry, Space Science and Engineering
Center, University of Wisconsin, report the discovery of the
brightest cloud feature ever observed on Uranus at near-infrared
wavelengths, in adaptive-optics (AO) images obtained with the 10-m
Keck II telescope (+ NIRC2) on Aug. 14 and 15 (though the feature
is large enough and bright enough to be observed without AO, even
without excellent seeing).  On Aug. 15.433 UT, when it was near
latitude 30o N (planetocentric) and crossed the central meridian,
the feature was a complex of four parallel components extending
over 8 deg of latitude and 13 deg of longitude.  Other central
meridian crossings can be predicted from its rotation period about
the planet's spin axis (16.83-16.90 hr).  In the K' band (1.95-2.3
microns), the cloud feature reflected nearly 17 percent of all the
light reflected by Uranus.  In K' AO images (resolution near 0".05),
the peak brightness of the cloud feature was nearly fifty times the
brightness of the background atmosphere.  Images with the H filter
(1.485-1.78 microns) show a lower contrast ratio of about 600
percent at AO resolution, which would drop to about 30 percent at
0".5 seeing and to 10 percent at 1".  In a methane-band-filter
image covering 1.61-1.75 microns, about 7 percent of the light
reflected from Uranus came from this single cloud feature.  The
previous record fractional cloud brightness was observed in 1999
for the first discrete cloud feature to appear in groundbased
digital images of Uranus (Sromovsky et al. 2000, Icarus 146, 307);
that feature, recorded at the NASA Infrared Telescope Facility in a
somewhat-higher-contrast filter (1.68-1.78 microns), reflected
almost 5 percent of the total light reflected by Uranus.  The
lifetime of this feature is unpredictable, though smaller bright
features in the northern hemisphere have persisted for 100 days or
more (Karkoschka 1998, Science 280, 570), and more recent
observations have identified features of intermediate brightness
that survived at least 30 days (Sromovsky and Fry 2005, Icarus, in

                      (C) Copyright 2005 CBAT
2005 August 22                 (8586)            Daniel W. E. Green

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