Circular No. 3357 Central Bureau for Astronomical Telegrams INTERNATIONAL ASTRONOMICAL UNION Postal Address: Central Bureau for Astronomical Telegrams Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A. TWX 710-320-6842 ASTROGRAM CAM Telephone 617-864-5758 SATURN'S RINGS P. E. Johnson and J. C. Kemp, Department of Physics, University of Oregon, write: "With the rings close to edge-on, a sensitive probe of the structure has been possible using the polarization of reflected sunlight. Forty observations during five months centered on the Mar. 1 opposition date have revealed a precise position-angle correlation (within 2o), at all phases, with the sun-Saturn-earth plane, and no correlation at all with the ring plane. The rings thus cannot be a monolayer, since that would produce a sizable polarization (> ~ 1 percent) normal to the ring plane, due to double interbody scattering. Instead, the structure is a low-density disk of rock bodies, with mean separation D many times the diameter d of a ring body; this is required in order to make the interbody scattering isotropic. This conclusion rests on the assumption that the large-angle (90o) scattering for the ring-body surfaces exceeds 5 percent; that will be checked by polarimetry from Pioneer 11 during the flyby of Saturn in September. The measured polarization (~ -0.5 percent in the blue) comes then mainly from local double scattering on the rough surfaces of individual ring bodies, i.e., the Wolff mechanism. If D/d > 5, then since the ring transmittance is perhaps 20 percent (for normal incidence), the ring thickness must be at least 25 times D." OY CARINAE N. Vogt, European Southern Observatory, reports the discovery of a new eclipsing ultrashort-periodic dwarf nova. Three eclipses of OY Car were observed with the 100-cm telescope during the rising branch of an eruption on Apr. 30. The three eclipses, which occurred on Apr. 30.05336, 30.11640 and 30.17952 (heliocentric) UT, showed a totality phase of ~ 70 s and depths of 1.4, 1.0 and 0.8 magnitude, respectively; during this time OY Car itself brightened from V = 14.6 to V = 13.2. On the next night, when OY Car was near maximum (V = 12.4), two partial eclipses with depths of 1.3 magnitude were observed, on May 1.00002 and 1.06326. The eclipses follow the preliminary elements: 1979 Apr. 30.05331 + 0.063118E (heliocentric) UT. Further eclipse observations are required in order to establish accurate elements. 1979 May 11 (3357) Brian G. Marsden
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