Detection of Paschen β absorption in the atmosphere of KELT-9 b: A new window into the atmospheres of ultra-hot Jupiters

Hydrogen and helium transmission signals trace the upper atmospheres of hot gas-giant exoplanets, where the incoming stellar extreme ultraviolet and X-ray fluxes are deposited. Further, for the hottest stars, the near-ultraviolet excitation of hydrogen in the Balmer continuum may play a dominant role in controlling the atmospheric temperature and driving photoevaporation. KELT-9 b is the archetypal example of such an environment as it is the hottest gas-giant exoplanet known to date (T$_{eq}$ $\sim$ 4500 K) and orbits an A0V-type star. Studies of the upper atmosphere and escaping gas of this ultra-hot Jupiter have targeted the absorption in the Balmer series of hydrogen (n$_1$ = 2 $\rightarrow$ n$_2$ $>$ 2). Unfortunately, the lowermost metastable helium state that causes the triplet absorption at 108.3 nm is not sufficiently populated for detection. Here, we present evidence of hydrogen absorption in the Paschen series in the transmission spectrum of KELT-9 b observed with CARMENES. Specifically, we focus on the strongest line covered by its NIR channel, Paschen-$\beta$ at 1282.16 nm (n$_1$ = 3 $\rightarrow$ n$_2$ = 5). The observed absorption shows a contrast of (0.53 $^{+0.12}_{-0.13}$)%, a blueshift of $-$14.8 $^{+3.5}_{-3.2}$ km/s, and a FWHM of 31.9$^{+11.8}_{-8.3}$ km/s. The observed blueshift in the absorption feature could be explained by day-to-night circulation within the gravitationally bound atmosphere or, alternatively, by Paschen-$\beta$ absorption originating in a tail of escaping gas moving toward the observer as a result of extreme atmospheric evaporation. This detection opens a new window for investigating the atmospheres of ultra-hot Jupiters, providing additional constraints of their temperature structure, mass-loss rates, and dynamics for future modeling of their scorching atmospheres.