Central Bureau for Astronomical Telegrams
IAUC 3357: SATURN'S RINGS; OY Car
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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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