The GAPS Programme at TNG XXXIII. HARPS-N detects multiple atomic species in emission from the dayside of KELT-20b

The detection of lines in emission in planetary atmospheres provides direct evidence of temperature inversion. We confirm the trend of ultra-hot Jupiters orbiting A-type stars that show temperature inversions on their daysides by detecting metals emission lines in the dayside of KELT-20b. We first detected the planetary emission by using the G2 stellar mask of the HARPS-N pipeline, which is mainly composed of neutral iron lines, as a template. Using neutral iron templates, we performed a retrieval of the atmospheric temperature-pressure profile of the planet, confirming a thermal inversion. Then we created models of planetary emission of different species using the retrieved inverted temperature-pressure profile. By using the cross-correlation technique, we detected Fe I, Fe II, and Cr I at signal-to-noise ratio levels of 7.1, 3.9, and 3.6, respectively. The latter was detected in emission in the atmosphere of an exoplanet for the first time. Contrary to Fe I, Fe II and Cr I were detected only after the occultation and not before, hinting at different atmospheric properties in view during the pre- and post-occultation orbital phases. A further retrieval of the temperature-pressure profile performed independently during the pre- and post-occultation phases, while not highly significant, points to a steeper thermal inversion in the post-occultation.