Observations of a successive stellar occultation by Charon and graze by Pluto in 2011: Multiwavelength SpeX and MORIS data from the IRTF

Pluto's lower atmosphere has been observed to evolve since the first definitive occultation detection in 1988. Possibilities for explaining the lower atmospheric structure include a steep thermal gradient and/or extinction, the latter of which can be characterized as a dependence between occultation flux and wavelength. On 2011 June 23, a 14.43 UCAC magnitude star (R = 13.64) was occulted by Pluto as observed from multiple sites. Observations made at NASA's 3-m Infrared Telescope Facility (IRTF) on Mauna Kea, Hawai'i, showed a full occultation of the star by Charon followed by an atmospheric graze by Pluto. Data were taken simultaneously in visible-wavelength images and low-resolution, near-infrared spectra. This dataset is unique in that (i) the double occultation allows astrometric measurements for Pluto and Charon as well as accurate calibration of the Pluto light curve, and (ii) the wavelength-resolved data serve as a test for atmospheric extinction. The graze reached a minimum normalized flux level of roughly 0.35, serving primarily as a probe of Pluto's upper atmosphere (which is typically defined to be above half-light level in occultation light curves). However, the light curve is well fit by atmospheric models with a power-law thermal gradient, a clear upper atmosphere, and haze in the lower atmosphere. We find a negative dependence between flux and wavelength in the deepest part of Pluto's atmosphere probed by the graze and in a spike during emersion. A simple extinction model for spherical, μm-sized tholins matches the observed spectral trends. While the atmospheric fits cannot rule out a clear atmosphere having a steep thermal gradient at the bottom, the flux-wavelength dependence and the feasibility of our particle-scattering fits suggest that Pluto's lower atmosphere contained haze in 2011. These results provide an important link in monitoring Pluto's dynamic atmosphere.