High-resolution Doppler-resolved spectroscopy has opened up a new window into the atmospheres of both transiting and non-transiting exoplanets .
Here , we present VLT/UVES observations of a transit of WASP-121b , an ‘ ultra-hot ’ Jupiter previously found to exhibit a temperature inversion and detections of multiple species at optical wavelengths .
We present initial results using the blue arm of UVES ( \approx 3700 – 5000 Å ) , recovering a clear signal of neutral Fe in the planet ’ s atmosphere at > 8 \sigma , which could contribute to ( or even fully explain ) the temperature inversion in the stratosphere .
However , using standard cross-correlation methods , it is difficult to extract physical parameters such as temperature and abundances .
Recent pioneering efforts have sought to develop likelihood ‘ mappings ’ that can be used to directly fit models to high-resolution datasets .
We introduce a new framework that directly computes the likelihood of the model fit to the data , and can be used to explore the posterior distribution of parameterised model atmospheres via MCMC techniques .
Our method also recovers the physical extent of the atmosphere , as well as account for time- and wavelength-dependent uncertainties .
We measure a temperature of 3710 ^ { +490 } _ { -510 } K , indicating a higher temperature in the upper atmosphere when compared to low-resolution observations .
We also show that the \ion Fei signal is physically separated from the exospheric \ion Feii .
However , the temperature measurements are highly degenerate with aerosol properties ; detection of additional species , using more sophisticated atmospheric models , or combining these methods with low-resolution spectra should help break these degeneracies .