DPS Pasadena Meeting 2000, 23-27 October 2000
Session 45. Pluto and Triton
Oral, Chairs: L. Young, D. Tholen, Thursday, 2000/10/26, 11:10am-12:10pm, C106

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[45.05] Continued Evolution in the Lightcurve of Pluto

M.W. Buie, W.M. Grundy (Lowell Observatory)

We have been working for many years on monitoring observations of the Pluto-Charon system. The present geometry of Pluto is changing to a more northerly vantage looking down on regions that have been poorly or un-illuminated for the past century. Combining the geometric change with the recent perihelion passage would indicate that the surface and atmosphere of Pluto could be undergoing the most rapid epoch of change during its year. Our efforts to monitor Pluto for changes have been split between two basic observational approaches: photometry and spectroscopy. The companion presentation from Grundy and Buie at this meeting will cover the spectroscopic approach. This presentation will show results and interpretations gleaned from over 170 nights of photometric observations spanning the past 9 years, all collected on the 0.9- and 1.1-m telescopes at Lowell Observatory.

Our observations indicate that the lightcurve of Pluto has indeed changed from the system lightcurve in 1986. Our current static models predict that the mean brightness should now be increasing by roughly 1% per year. At the same time, the lightcurve amplitude should be decreasing as our viewpoint moves closer to the north rotation pole. We do see a decrease in the lightcurve amplitude. The current amplitude is now 0.26 mag compared to 0.30 mag in 1986. However, the mean brightness has not increased as expected. Instead, the mean has decreased. All longitudes are lower but the difference is very small (2%) at minimum light. The greatest change is at maximum light where the peak seems to be roughly 6.5% fainter now. In addition to the above data taken in Johnson V, we also have data from these nights in B and in a methane band filter centered on the 8900Å\ band. The (B-V) lightcurve may show weak variations (<1%) and the CH4 lightcurve (referenced to V) shows an 8-10% variation with longitude.

This work was supported by NASA Grant NAG5-4210, STScI grant HF-01091.01-97A, and the NSF REU Program grant to NAU.

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