Retrieval analysis of 38 WFC3 transmission spectra and resolution of the normalization degeneracy

Abstract

A comprehensive analysis of 38 previously published Wide Field Camera 3 (WFC3) transmission spectra is performed using a hierarchy of nested-sampling retrievals: with versus without clouds, grey versus non-grey clouds, isothermal versus non-isothermal transit chords and with water, hydrogen cyanide and/or ammonia. We revisit the "normalisation degeneracy": the relative abundances of molecules are degenerate at the order-of-magnitude level with the absolute normalisation of the transmission spectrum. Using a suite of mock retrievals, we demonstrate that the normalisation degeneracy may be partially broken using WFC3 data alone, even in the absence of optical/visible data and without appealing to the presence of patchy clouds, although lower limits to the mixing ratios may be prior-dominated depending on the measurement uncertainties. With James Webb Space Telescope-like spectral resolutions, the normalisation degeneracy may be completely broken from infrared spectra alone. We find no trend in the retrieved water abundances across nearly two orders of magnitude in exoplanet mass and a factor of 5 in retrieved temperature (about 500-2500 K). We further show that there is a general lack of strong Bayesian evidence to support interpretations of non-grey over grey clouds (only for WASP-69b and WASP-76b) and non-isothermal over isothermal atmospheres (no objects). 35 out of 38 WFC3 transmission spectra are well-fitted by an isothermal transit chord with grey clouds and water only, while 8 are adequately explained by flat lines. Generally, the cloud composition is unconstrained. Tsiaras et al. (2018). In order to isolate the spectral feature due to water 1 , they quantified the strength of absorption between 1.3-1.65 µm, relative to the continuum, in terms of the number of pressure scale heights, which they represented by AH . Based on the finding that both AH and the equilibrium temperature (Teq) follow log-normal distributions, Fu et al. (2017) concluded that their sample of AH is affected by observational bias. Tsiaras et al. (2018) defined an Atmospheric Detectability Index (ADI) to quantify the strength of detection of the water feature, but do not explicitly link the ADI to any trends in cloud properties. They concluded that all of their WFC3 transmission spectra, except for WASP-69b, are consistent with the presence of a grey cloud deck.Our intention is to build upon the Fu et al. (2017) and Tsiaras et al. (2018) studies by subjecting their WFC3 sample to a detailed atmospheric retrieval study and elucidating the presence of assumptions, limitations and trends. It follows the principle that the same datasets should be analysed by different groups (using different codes and techniques) 1 Technically, it is due to a collection of unresolved water lines.

HST/STIS transmission spectrum of the ultra-hot Jupiter WASP-76 b confirms the presence of sodium in its atmosphere

“…Indeed, the WFC3 transmission spectrum of WASP-76 b, covering wavelengths between 1.1 and 1.6 microns, was analyzed by Tsiaras et al (2018), who found a significant water feature plus signs of TiO/VO. Contrary to this, Fisher & Heng (2018) used a non-grey cloud model to fit the observed slope, finding a lower water abundance and no detection of TiO/VO. While carrying out our first determination of the wavelength-dependent transit depths, we were not aware of WASP-76's very bright companion.…”

Wind of change: retrieving exoplanet atmospheric winds from high-resolution spectroscopy

“…In more recent work, Fisher & Heng (2018) showed, that this three way degeneracy is partially due to model assumptions in semi-analytical approaches, and Welbanks & Madhusudhan (2019) that the degeneracy with the element abundances can be broken in retrieval analysis. This would require a more rigorous treatment of parameters influencing the continuum of the transmission spectra than presented here, most importantly the inclu-sion of H 2 − H 2 collision induced absorption (CIA), but also cloud decks, hazes and additional absorber species.…”

The Precision of Mass Measurements Required for Robust Atmospheric Characterization of Transiting Exoplanets

“…This simple methodology has been widely used to reproduce the cloud behavior of hot Jupiters (e.g. Sing et al 2016;Fisher & Heng 2018), and within the CHIMERA framework (Schlawin et al 2018).…”