Vol. 627
In section 10. Planets and planetary systems

A spectral survey of an ultra-hot Jupiter

by H.J. Hoeijmakers, D. Ehrenreich, D. Kitzmann, et al. 2019, A&A, 627, A165 alt

KELT-9b is the prototype of the so-called ultra-hot planets, which orbit hot, early-type stars at short distances. The huge stellar irradiation they are exposed to heats their atmospheres to temperatures of ~4,000 K, so they straddle the transition between gas giants and stars. These temperatures prevent aerosol formation and complex molecular chemistry, and models have predicted that the observable atmosphere of KELT-9b is almost purely in atomic form, close to chemical equilibrium. Hence, relatively heavy atomic species such as oxygen, iron, and magnesium should be present in the atmosphere at detectable levels. Previous transit observations with HARPS-N have resulted in the detection of Fe I, Fe II, and Ti II. Once again using the powerful high-resolution cross-correlation technique that employs model spectra based on a chemical equilibrium model as templates, Hoeijmakers et al. analyze additional HARPS-N observations and now report the first detection of several new atomic and ionized species (Na I, Cr II, Sc II, and Y II) in the upper atmosphere of KELT-9 b. Remarkably, in most cases, the detected absorption lines are much stronger than model expectations, possibly pointing to a strong hydrodynamical outflow that lifts the absorbing species to higher altitudes. In addition, these strong absorption lines lend themselves to probing the dynamics of the atmosphere, and provide a direct measure of the orbital velocity of the planet, which the authors used to constrain the system to high accuracy.