Detection of Ionized Calcium in the Atmosphere of the Ultra-hot Jupiter KELT-9b

Abstract

Abstract With a dayside temperature in excess of 4500 K, comparable to a mid-K-type star, KELT-9b is the hottest planet known. Its extreme temperature makes KELT-9b a particularly interesting test bed for investigating the nature and diversity of gas giant planets. We observed the transit of KELT-9b at high spectral resolution ( R ∼ 94,600) with the CARMENES instrument on the Calar Alto 3.5 m telescope. Using these data, we detect for the first time ionized calcium (Ca ii triplet) absorption in the atmosphere of KELT-9b; this is the second time that Ca ii has been observed in a hot Jupiter. Our observations also reveal prominent H α absorption, confirming the presence of an extended hydrogen envelope around KELT-9b. We compare our detections with an atmospheric model and find that all four lines form between atmospheric temperatures of 6100 and 8000 K and that the Ca ii lines form at pressures between 50 and 100 nbar while the H α line forms at a lower pressure (∼10 nbar), higher up in the atmosphere. The altitude that the core of H α line forms is found to be ∼1.4 R p , well within the planetary Roche lobe (∼1.9 R p ). Therefore, rather than probing the escaping upper atmosphere directly, the H α line and the other observed Balmer and metal lines serve as atmospheric thermometers enabling us to probe the planet’s temperature profile, thus the energy budget.