Seminars & Colloquia Calendar
Odd Harmonics in Exoplanet Photometry: Weather or Artifact?
Victoria Chayes - Rutgers University
Location: Hill 423
Date & time: Wednesday, 25 April 2018 at 11:00AM - 12:15PM
Abstract: This talk reports the result of an undergraduate research project done with Professor Hal Haggard of Bard College and Professor Nicholas Cowan of McGill University. Direct observations of short-period planetary systems in missions like the Kepler mission allow astronomers to monitor exoplanets, their host stars, and their mutual interactions. Certain patterns are expected in the light curves: flux variations at the orbital frequency and Doppler beaming can create one peak per orbit, while ellipsoidal variations of a tidally distorted star produce two maxima per orbit. Researchers have also reported significant photometric variability at three times the orbital frequency, sometimes much greater than the predictions of tidal theory. The reflected phase variations of a homogeneous planet contains power at even orbital harmonics—important for studies of ellipsoidal variations—but cannot contain odd orbital harmonics. We show that odd harmonics can, however, be produced by an edge-on planet with a time-variable map, or an inclined planet with a North- South (N-S) asymmetric map. Either of these scenarios entail weather: short-period planets are expected to have zero obliquity and hence N-S symmetric stellar forcing. North-South asymmetry in a giant planet would therefore suggest stochastic localized features, such as weather. However, we find that previous claims of large-amplitude odd modes in Kepler photometry are artifacts of removing planetary transits rather than modeling them. The only reliable claims of odd harmonics remain HAT-P-7b and Kepler-13Ab, for which the third mode amplitude is 6–8% of the planetary flux. Although time-variable albedo maps could in principle explain these odd harmonics, upper-limits on the infrared variability of hot Jupiters make this hypothesis unlikely. We therefore believe the effects of close-in planets on stellar atmospheres in these two cases remains the only plausible hypothesis.