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We analyse the transmission spectra of KELT-20b/MASCARA-2b to search forpossible thermal inversion agents. The data consist of three transits obtainedusing HARPSN and one using CARMENES. We removed stellar and telluric linesbefore cross-correlating the residuals with spectroscopic templates producedusing a 1D plane-parallel model assuming an isothermal atmosphere and chemicalequilibrium at solar metallicity. Using a likelihood-mapping method, we detectFe\,{\sc i} at $>$ 13-$\sigma$, Ca\,{\sc ii} H$\&$K at $>$ 6-$\sigma$ andconfirm the previous detections of Fe\,{\sc ii}, Ca\,{\sc ii} IRT and Na\,{\sci} D. The detected signal of Fe\,{\sc i} is shifted by -3.4$\pm$0.4 km s$^{-1}$from the planetary rest frame, which indicates a strong day-night wind. Ourlikelihood-mapping technique also reveals that the absorption features of thedetected species extend to different altitudes in the planet's atmosphere.Assuming that the line lists are accurate, we do not detect other potentialthermal inversion agents (NaH, MgH, AlO, SH, CaO, VO, FeH and TiO) suggestingthat non-chemical equilibrium mechanisms (e.g. a cold-trap) might have removedTi- and V-bearing species from the upper atmosphere. Our results, therefore,shows that KELT-20b/MASCARA-2b cannot possess an inversion layer caused by aTiO/VO-related mechanism. The presence of an inversion layer would thereforelikely be caused by metal atoms such as Fe\,{\sc i} and Fe\,{\sc ii}. Finally,we report a double-peak structure in the Fe\,{\sc i} signal in all of ourdata-sets that could be a signature of atmospheric dynamics. However, furtherinvestigation is needed to robustly determine the origin of the signal.

monthly notices of the royal astronomical society

Searching for Thermal Inversion Agents in the Transmission Spectrum of  MASCARA-2b/KELT-20b: Detection of Neutral Iron and Ionised Calcium H$\&$K  Lines