The Effect of Scintillation on Ground-Based Exoplanet Transit Photometry

Abstract

In this thesis, the effect of scintillation arising from atmospheric optical turbulence on exoplanet transit and secondary eclipse photometry is examined.
Atmospheric scintillation arises from the propagation of phase aberrations resulting from wavefront perturbations due to optical turbulence high in the atmosphere. Scintillation causes intensity variations of astronomical targets, which is a problem in exoplanet transit photometry, where the measurement of a decrease in brightness of 1% or less is required. For this reason, ground-based telescopes have inferior photometric precision compared to their space-based counterparts, despite having the advantage of a reduced cost.

In contrast with previous work on the detection limits of fast photometry, which is obtained for an atmosphere averaged over time, the actual scintillation noise can vary considerably from night to night depending on the magnitude of the high-altitude turbulence. From simulation of turbulent layers, the regimes where scintillation is the dominant source of noise on photometry are presented. These are shown to be in good agreement with the analytical, layer based, equations for scintillation.
Through Bayesian analysis, the relationship between the errors on the light and the uncertainties on the astrophysical parameters are examined. The errors on the light curve arising from scintillation linearly increase the scatter on the astrophysical parameters with a gradient in the range of 0.68 -0.80.

The noise due to the photometry aperture is investigated. It is found that for short exposure in times in good seeing, speckle noise contributes to noise in photometry for aperture sizes of up to approximately 2.3xFWHM.

The results from simultaneous turbulence profiling and time-series photometry are presented. It is found that turbulence profiling can be used to accurately predict the amount of scintillation noise present in photometric observations.

An investigation of the secondary eclipse of WASP-12b on the William Herschel Telescope (WHT) is performed, resulting in a high quality z’-band light curve for WASP-12b consistent with a carbon-rich model and with no evidence for strong thermal inversion.