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K2 observations of Neptune
Patrick Gaulme (New Mexico State University)
We proposed to observe Neptune with K2 for three reasons: 1) studying the evolution of cloud features by monitoring the rotational photometric variability; 2) studying its interior structure through the possible detection of global acoustic oscillations; 3) detecting the solar acoustic oscillations imbedded in the Sun’s reflected light to understand how Kepler would see the Sun. As regards clouds, we compare results extracted from the light curve with contemporaneous disk-resolved imaging of Neptune from the Keck 10-m telescope at 1.65 microns and Hubble Space Telescope visible imaging acquired nine months later. We conclude that a single large discrete storm seen in Keck imaging dominates the K2 and Hubble light curves, and that smaller or fainter clouds likely contribute to short-term brightness variability. As regards seismology of Neptune, the observation principle stipulates that Neptune behaves as a mirror for solar light and that slight distortions of its disk caused by the oscillations would lead to detectable photometric fluctuations. This is very challenging and K2 was the first opportunity ever to test this approach. Even though a low amplitude periodic signal is detected by autocorrelation of the time series in the frequency range where we expect oscillations, it is likely to be a artifact related to instrumental noise. Finally, the detection of the solar oscillations is the major result of these observations as it is the first time that solar oscillations are detected in photometry on any other planet (including the Moon). This is another great performance of Kepler, K2 in particular. We clearly identify eight mode overtones with degrees l=0,1,2, which we compare to SOHO’s VIRGO and GOLF data, as well as BiSON. We also measure the frequency at maximum amplitude and mean large spacing, from which we infer the solar mass and radius as Kepler would have estimated it. Uranus was observed by K2 in early 2016. Depending on the date we get the data, we might be able to show some first results.