AAS 201st Meeting, January, 2003
Session 61. Pluto-Occultation Studies
Oral, Tuesday, January 7, 2003, 10:00-11:30am, 616-617

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[61.05] Adaptive Optics Imaging of the Pluto-Charon System

D. J. Tholen (Univ. Hawaii)

Previous observations of Charon's orbit acquired with the Hubble Space Telescope revealed a small but significant nonzero eccentricity (Tholen and Buie, Icarus 125, 245-260, 1997), which tidal effects should have removed in less than 10 million years. Part of the apparent eccentricity is probably due to variegation-induced offsets between the center of light and the center of disk, which is assumed to represent the center of mass, though the best available surface albedo models were unable to account for all of the measured eccentricity. In an attempt to confirm the existence of this orbital eccentricity, new observations were made using the Institute for Astronomy's Hokupa'a adaptive optics camera attached to the Gemini North telescope on Mauna Kea, for which diffraction-limited images in the infrared H band would be 0.05 arcsec, enough to resolve the 0.11 arcsec diameter disk of Pluto and solve for the center of disk directly. Images were acquired on eight separate nights spanning a full year (2001 April 19 and 28 UT; 2001 June 23, 25, and 26 UT; and 2002 April 23, 24, and 26 UT). These eight epochs provide a reasonably uniform distribution in orbital longitude, which is essential for a robust eccentricity solution. Diffraction-limited performance was not achieved, with the best images having a FWHM of 0.09 arcsec; the image quality was typically in the 0.15 to 0.20 arcsec range. Calibration of image scale and position angle orientation were accomplished by watching asteroids of known motion move past field stars of suitable brightness. The approximate image scale is 0.02 arcsec per pixel, so the point spread function is well sampled. Initial orbit solutions are consistent with the previous HST data, though the error analysis is not yet complete, so it is premature to say whether the eccentricity is confirmed. Residuals are typically in the 6 milliarcsec range, which is approaching the performance reached with HST. The orbital period for Charon, 6.387233 days, is improved by about an order of magnitude, thanks to the ten-year baseline between the HST and Gemini observations. The latest results will be presented at the meeting.

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