2002 PLUTO OCCULTATIONS

1
2002 PLUTO OCCULTATIONS
J. Elliot et al.

• Observations were made of an occultation of the
  star by Pluto.
• Previous observations in 1988 showed an
  isothermal atmosphere above a radius of 1215 km.
  Below this layer there was either a thermal
  gradient or an extinction layer.
• The results from the IRTF and the UH 2.2-m
  telescope show
• Evidence for extinction in the atmosphere of
  Pluto.
• The surface pressure at the surface of Pluto
  increased by a factor of 2.
• The atmosphere has expanded by about 45 km.

The expansion of the atmosphere could be due to a
decrease in the albedo coupled with a surface
temperature rise of 1 K, or to sublimation of the
south polar cap due to seasonal variations.
Reference Elliot, J.L. et al. 2003, Nature, 424,
165.
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Occultation path
The star P131.1 was occulted by Pluto (Rmag
15.7). The Lick obs. and MKO chords were used
for calculating the astrometric solution.
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SPECTRA OF PLUTO-CHARON AND P131.1(SpeX at IRTF
0.5 seeing)
"
This work was made possible by close
collaboration between Elliot and Rayner to
provide occultation capability with SpeX.
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Minimum Stellar flux vs. wavelength
Minimum Stellar Flux
Wavelength (mm)
The trend of less extinction with longer
wavelengths is evidence for extinction in the
atmosphere of Pluto. These observations are best
explained by aggregates of haze particles that
are produced photochemically in the atmosphere.
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IRTF contributions
Planning for stellar occultations with SpeX began
at an early stage when SpeX was being designed by
John Rayner. The specifications for the
electronics and time stamping were set to
accommodate occultations. SpeX provided
simultaneous wavelength coverage from 0.8-2.5
microns and this was the best data set showing
the wavelength dependence of the extinction.
To quote the PI We had been trying for 14
years to get multi-wavelength occultation
observations without success. The high-throughput
, high-speed mode on SpeX was optimally designed
for this observation and gave us great results,
even for such a faint star!
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Occultation light curves
The UH 2.2-m light curve was used in the analysis
because it had the highest signal-to-noise and
time resolution. The KAO data refers to the 1988
occultation. The kink observed in 1988 does
was not observed in 2002.
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Pressure profiles for 1988 and 2002
Inversion of the UH 2.2-m light curve shows a
factor of 2 increase in pressure compared to the
1988 KAO observations. Thermal models predict
the pressure to decrease starting in 2020 or
earlier. Thus future observations are necessary.