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Abstract: We are reporting on a new accurate photographic light curve of Pluto for1933-1934 when the heliocentric distance was 40 AU. We used 43 B-band andV-band images of Pluto on 32 plates taken on 15 nights from 19 March 1933 to 10March 1934. Most of these plates were taken with the Mount Wilson 60- and 100-telescopes, but 7 of the plates now at the Harvard College Observatory weretaken with the 12- and 16- Metcalf doublets at Oak Ridge. The plates weremeasured with an iris diaphragm photometer, which has an average one-sigmaphotometric error on these plates of 0.08 mag as measured by the repeatabilityof constant comparison stars. The modern B and V magnitudes for the comparisonstars were measured with the Lowell Observatory Hall 1.1-m telescope. Themagnitudes in the plate-s photographic system were converted to the Johnson B-and V-system after correction with color terms, even though they are small insize. We find that the average B-band mean opposition magnitude of Pluto in1933-1934 was 15.73 +- 0.01, and we see a roughly sinusoidal modulation on therotational period 6.38 days with a peak-to-peak amplitude of 0.11 +- 0.03mag. With this, we show that Pluto darkened by 5% from 1933-1934 to 1953-1955.This darkening from 1933-1934 to 1953-1955 cannot be due to changing viewinggeometry as both epochs had identical sub-Earth latitudes, so ourobservations must record a real albedo change over the southern hemisphere. Thelater darkening trend from 1954 to the 1980s has been explained by changingviewing geometry as more of the darker northern hemisphere comes into view.Thus, we now have strong evidence for albedo changes on the surface of Pluto,and these are most easily explained by the systematic sublimation of frostsfrom the sunward pole that led to a drop in the mean surface albedo.



Autor: Bradley E. Schaefer, Marc W. Buie, Luke Timothy Smith

Fuente: https://arxiv.org/







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