  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$3700-5000A), recovering a clear signal of neutral Fe in the
planet's atmosphere at >8$\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 Fe I signal is physically separated from the exospheric Fe II.
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.
