Retrieval Analysis of 38 WFC3 Transmission Spectra
I am a first year PhD student in the group of Kevin Heng at the University of Bern. We have been working on a retrieval analysis of WFC3 transmission spectra, the paper for which will be submitted by the time of Exoplanets II. Using a validated analytical model developed by Heng & Kitzmann (2017) we have performed a comprehensive retrieval analysis of the transmission spectra of 38 objects, ranging from Super Earths to Hot Jupiters. We treated WASP-12b as a test case and verified our results against Kreidberg et al. (2015). However, there is an underlying ‘normalization degeneracy’ in the method that is regularly ignored in previous studies. For a few planets that happen to be cloud-free in the optical bandpass we were able to break this degeneracy and obtain a relationship between the reference radius and pressure. This allowed us to calculate the pressure probed by the WFC3 bandpass as roughly 10mbar. After our retrieval analysis of the 38 objects we investigated any potential trends in the retrieved quantities, but found no correlations. Specifically, we found no relationship between the retrieved water abundance and the planet mass. Using Bayesian Evidences we were also able to demonstrate that more complicated models, for example non-isothermal atmospheres or non-grey clouds, are rarely favoured over the more simplistic versions for the WFC3 data.