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M.W. Buie (Lowell Obs.), J.L. Elliot (MIT), M.R. Kidger (Ast. Inst. Canary Islands), A.S. Bosh (BU), O. Saá (CTIO), R. Van Malderen, K. Uytterhoeven, G. Davignon (Katholieke U. Leuven), E.W. Dunham, C.B. Olkin, B.W. Taylor, L.H. Wasserman (Lowell Obs.), K. Clancy, M.J. Person (MIT), S.E. Levine, R.C. Stone (USNOFS), P.G. Peréz González (U. Complutense de Madrid), J.M. Pasachoff, S.P. Souza, D.R. Ticehurst (Williams College), A. Fitzsimmons (Queens U.)
We will report the results from an occultation of P126A by Pluto on the night of 2002 July 20 UT. The event was successfully observed with a 0.4-m telescope at Mamiña, Chile under photometric conditions. Additional data were collected at CTIO on a 0.6-m telescope with heavy interference from clouds. The CTIO observations preclude an occultation at that location thus forcing the Mamiña chord to lie south of the centerline. Less-constraining negative results were obtained from the Canary Islands.
The Mamiña lightcurve shows an occultation profile that clearly indicates the continued presence of a substantial atmosphere around Pluto. The profile does not show any trace of the ``kink'' seen in the 1988 occultation data at a similar distance from the centerline of the shadow. Depending on the specific model assumptions, the minimum distance of Mamiña from the center of Pluto's shadow lies in the range 1025-1130 km, which corresponds to a range of 1180-1260 km for the deepest radius probed in Pluto's atmosphere. These new occultation data cannot be well fitted with models derived from the 1988 data. Hence one or more changes have occurred in Pluto's atmosphere in the past 14 years. Either the haze/thermal gradient altitude has decreased (or disappeared altogether), or the temperature above this level has increased (accompanied by an increase in pressure), or some combination of the these two. These results challenge the current level of understanding of the nature of Pluto's atmosphere and its surface-atmosphere interaction.
This work was supported, in part, by NASA Grants NAG5-10444, NAG5-9008, by NOAO and CTIO operated for NSF by AURA, NASA through the New Horizons project, and Research Corporation.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.