Presentation #102.208 in the session Poster Session.
Ultra-hot Jupiters have dayside temperatures at which most molecules are expected to thermally dissociate. The dissociation of water vapour results in the production of the hydroxyl radical (OH). We report on the detection of OH in the atmosphere of the ultra-hot Jupiter WASP-76b using high-resolution transmission spectroscopy with CARMENES. We find that the signal is blueshifted with -13 km/s, indicating that it mostly originates from the hotter evening side of the planet, and is under the influence of a strong day-to-nightside wind. We also find that the signal is broad and observe a slightly increased Kp over the expected value. Our detection shows that thermal dissociation may indeed explain the relatively weak water features in ultra-hot Jupiters. OH should therefore be considered when studying similar ultra-hot Jupiters and can be used to trace thermal dissociation in the atmospheres of such planets.