Pluto's Atmospheric Structure: Results From The 2006 June 12 Stellar Occultation

Abstract

Observations of the 2006 June 12 occultation by Pluto of P384.2 (McDonald & Elliot, AJ 120, 1599; UCAC2 26039859) were attempted by the MIT-Williams College collaboration from five sites in Australia and New Zealand. Four sites were successful: Black Springs, South Australia (0.8m); Mt. Canopus, Tasmania (1m); Mt. Stromlo, Australian Capital Territory (1.8m); and Siding Spring, New South Wales (2.3m). Data were recorded using Portable Occultation, Eclipse, and Transit Systems (POETS; Souza et al., in preparation). Using these data, we characterize Pluto's atmosphere and compare our results to previous occultation observations. Above half-light level, the light curves exhibit the signature of an isothermal atmosphere. The scale height is consistent at 60 km, equivalent to a temperature of 110 K for an N2 atmosphere. Below half-light level, the light curves resemble those obtained in 2002 (Pasachoff et al., AJ 129, 1718) more than 1988 (Elliot et al., Icarus 77, 148). The data drop significantly below the isothermal curve at this level, due either to a thermal gradient or extinction (or some combination); however, the drop is not as abrupt as in 1988. Data from 2002 demonstrated that at least some extinction is at work, due to the wavelength dependence of the residual flux at the bottom of the light curves (Elliot et al., Nature 424, 165). Unfortunately, we do not have multi-wavelength observations for P384.2. Our highest signal-to-noise ratio data, from the 2.3m, exhibit spikes caused by density variations in Pluto's atmosphere and interesting structure in the bottom of the light curve, when the star is probing around the limb more than vertically. Data from this event are also discussed in Elliot et al., a derivation of Pluto's atmospheric size, and Pasachoff et al., a search for satellites, rings and debris.