Relative abundance constraints from high-resolution optical transmission spectroscopy of WASP-121b, and a fast model-filtering technique for accelerating retrievals

Abstract

ABSTRACT High-resolution Doppler-resolved spectroscopy has presented new opportunities for studying the atmospheres of exoplanets. While the ‘classical’ cross-correlation approach has proven to be efficient at finding atmospheric species, it is unable to perform direct atmospheric retrievals. Recent work has shown that retrievals are possible using a direct likelihood evaluation or likelihood ‘mappings’. The unique aspect of high-resolution retrievals is that the data-processing methods required to remove the stellar and telluric lines also distort the underlying exoplanet’s signal and therefore the forward model must be pre-processed to match this filtering. This was the key remaining limitation in our previously published framework. This paper directly addresses this by introducing a simple and fast model-filtering technique that can replicate the processing performed by algorithms such as SysRem and PCA. This enables retrievals to be performed without having to perform expensive injection and pre-processing steps for every model. We show that we can reliably constrain quantitative measures of the atmosphere from transmission spectra including the temperature–pressure profile, relative abundances, planetary velocities, and rotational broadening parameters. Finally, we demonstrate our framework using UVES transmission spectroscopy of WASP-121b. We constrain the temperature–pressure profile and relative abundances of Fe, Cr, and V to be log10(χFe/χCr) = 1.66 ± 0.28, log10(χFe/χV) = 3.78 ± 0.29, and log10(χFe/χMg) = −1.26 ± 0.60. The relative abundances are consistent with solar values, with the exception of Fe/Mg, where the large Mg abundance is probably explained by the escaping atmosphere of WASP-121b that is not accounted for in our atmospheric model.