Pluto's atmospheric structure from the July 2007 stellar occultation

In July 2007, we observed a stellar occultation by Pluto from three sites in New Zealand and Australia. From these occultation observations, we find that Pluto's atmospheric pressure is still at the increased level measured in 2002 and 2006 with a pressure at a radius of 1275 km of 2.09 ± 0.09 μbar. One of the sites, Mt. John Observatory, was ∼70 km from the shadow center and we recorded the first central-flash occultation by Pluto. We carried out a dual-wavelength observation from this site with two different cameras using filtered high-time resolution observations in the visible from the one-meter telescope at Mt. John Observatory. From our central-flash observations, we find the elliptical shape that best matches the data corresponds to a nearly prolate atmosphere with an ellipticity of 0.09. The flux observed in the central-flash data can be fit equally well with either a haze layer or a thermal gradient in the altitudes probed by the occultation. However, the star light contributing to the central-flash occultation for the haze layer model would pass through a radius of 1130 km from Pluto's center. Given our current best estimate of Pluto's surface radius is greater than 1151 km (Tholen, D.J., Buie, M.W. [1997]. Bulk properties of Pluto and Charon. In: Stern, S.A., Tholen, D.J. (Eds.), Pluto and Charon. The University of Arizona Press), we prefer the thermal gradient solution or a combination of haze and thermal gradient to explain the occultation light curves.