Retrieval of planetary and stellar properties in transmission spectroscopy with Aura

Abstract

Transmission spectroscopy provides a powerful probe of the atmospheric properties of transiting exoplanets. To date, studies of exoplanets in transit have focused on inferring their atmospheric properties such as chemical compositions, cloud/haze properties, and temperature structures. However, surface inhomogeneities in the host stars of exoplanets in the form of cool spots and hot faculae can in principle imprint signatures on the observed planetary transit spectrum. Here we present Aura, a new retrieval paradigm for inferring both planetary and stellar properties from a transmission spectrum. We apply our retrieval framework to a sample of hot giant exoplanets to determine the significance of stellar heterogeneity and clouds/hazes in their spectra. The retrieval analyses distinguish four groups of planets. First, the spectra of WASP-6b and WASP-39b are best characterised by imprints of stellar heterogeneity and hazes and/or clouds. HD 209458b and HAT-P-12b comprise the second group for which there is weak evidence for stellar heterogeneity and a high significance of hazes and/or clouds. The third group constitutes HAT-P-1b and WASP-31b and shows weak evidence against stellar heterogeneity but weak to substantial indications of clouds/hazes. The fourth group -WASP-19b, WASP-17b, and WASP-12b -is fit best by molecular and alkali absorbers with H 2 scattering without evidence for stellar heterogeneity and weak to no evidence for clouds/hazes. Our retrieval methodology paves the way to simultaneous information on the star and planet from higher resolution spectra using future facilities such as the James Webb Space Telescope and large ground-based facilities.

The Influence of Stellar Contamination on the Interpretation of Near-infrared Transmission Spectra of Sub-Neptune Worlds around M-dwarfs

“…Figure 2 illustrates the influence of unocculted spot-contamination on sub-Neptune transmission spectra. Consistent with recent studies Pinhas et al 2018), there can be a significant shape change in the transmission spectrum, where the wavelength-dependent stellar contamination parameters could mimic atmospheric effects on the planet's spectrum (e.g., molecular features and haze-like power-law slopes).…”

“…Note, the awkward crossing of the 30ppm point below 1% is due to the inaccuarcies in computing Bayes factors in low evidence regimes (e.g., Lupu et al 2016). Pinhas et al (2018)). This result however, may only be true if the stellar models are accurate representations of the true photosphere, which we explore in the next section.…”

Detection of Na, K, and H2O in the hazy atmosphere of WASP-6b

“…Firstly by Barstow et al (2017) who utilize the NEMESIS retrieval code to demonstrate that the atmosphere of WASP-6b is best described by Rayleigh scattering clouds at high altitudes. In addition, Pinhas et al (2018) perform a retrieval using the AURA code, demonstrating that the atmosphere of WASP-6b is best described as a combination of the effects of stellar heterogeneity and atmospheric hazes. However, in an effort to fit the widely disparate STIS and Spitzer points this retrieval predicts a very low H 2 O abundance, a claim that has not been possible to verify or refute until the recent acquisition of HST WFC3 data from this study.…”

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

“…We performed a retrieval of the atmospheric properties of WASP-76 b from the optical transmission spectrum derived in this work, using an adaptation of the AURA retrieval code (Pinhas et al 2018;Welbanks & Madhusudhan 2019). The code consists of two main components: a forward model, which com- putes theoretical transmission spectra assuming a plane-parallel atmosphere in hydrostatic equilibrium, and a statistical sampling algorithm, which explores the full parameter space of possible models to carry out parameter estimation and model comparison.…”