Non-local thermodynamic equilibrium effects determine the upper atmospheric temperature structure of the ultra-hot Jupiter KELT-9b

Abstract

Context. Several observational and theoretical results indicate that the atmospheric temperature of the ultra-hot Jupiter KELT-9b in the main line formation region is a few thousand degrees higher than predicted by self-consistent models. Aims. Our aim was to test whether non-local thermodynamic equilibrium (NLTE) effects are responsible for the presumably higher temperature. Methods. We employed the Cloudy NLTE radiative transfer code to self-consistently compute the upper atmospheric temperature-pressure (TP) profile of KELT-9b, assuming solar metallicity and accounting for Roche potential. In the lower atmosphere, we used an updated version of the HELIOS radiative-convective equilibrium code to constrain the Cloudy model. Results. The Cloudy NLTE TP profile is ≈2000 K hotter than that obtained with previous models assuming LTE. In particular, in the 1–10−7 bar range the temperature increases from ≈4000 to ≈8500 K, remaining roughly constant at lower pressures. We find that the high temperature in the upper atmosphere of KELT-9b is driven principally by NLTE effects modifying the Fe and Mg level populations, which strongly influence the atmospheric thermal balance. We employed Cloudy to compute LTE and NLTE synthetic transmission spectra on the basis of the TP profiles computed in LTE and NLTE, respectively, finding that the NLTE model generally produces stronger absorption lines, particularly in the ultraviolet, than the LTE model (up to 30%). We compared the NLTE synthetic transmission spectrum with the observed Hα and Hβ line profiles obtaining an excellent match, thus supporting our results. Conclusions. The NLTE synthetic transmission spectrum can be used to guide future observations aiming at detecting features in the KELT-9b transmission spectrum. Metals, such as Mg and Fe, and NLTE effects shape the upper atmospheric temperature structure of KELT-9b, and thus affect the mass-loss rates derived from it. Finally, our results call for checking whether this is the case also for cooler planets.

Detection of iron emission lines and a temperature inversion on the dayside of the ultra-hot Jupiter KELT-20b

“…The large error of the retrieved value indicates that there is a certain degree of degeneracy between [Fe/H] and the T -P profile. In addition, Fossati et al (2021) found that nonlocal thermodynamic equilibrium (NLTE) can affect the upper atmosphere of UHJs. NLTE effects at low pressures can alter the Fe level population and thereby affect the retrieval of the Fe abundance and T -P profile.…”

“…Simulations also indicate that the dayside hemispheres of these planets have temperature inversion layers because the absorption of the stellar radiation by species such as metals and metal oxides is strong (e.g., Lothringer & Barman 2019;Baxter et al 2020). It has recently been shown that for the hottest exoplanet KELT-9b, the deviation from local thermodynamic equilibrium in the level population of Fe II is the main driver of strong temperature inversion in the high-altitude atmosphere (Fossati et al 2021).…”

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

“…It is thus probably not a casualty that all the detections of temperature inversion with high-resolution spectroscopy up to now are for UHJs orbiting A-type stars. These stars have strong UV emission, which is the wavelength range where the metals causing temperature inversions present the largest number of lines (Fossati et al 2021).…”

“…In particular, one key characteristic is the atmospheric thermal inversion, which appears to happen when the equilibrium temperature reaches ∼1700 K, with observational evidence for a transition between the two regimes (Baxter et al 2020). Fossati et al (2021) showed that non-LTE effects play a significant role in determining the shape of the temperature inversion. While previous studies predicted the temperature inversion to be caused mainly by TiO and VO (e.g.…”

Magnetic Drag and 3-D Effects in Theoretical High-Resolution Emission Spectra of Ultrahot Jupiters: the Case of WASP-76b

“…For pressures greater than ∼ 10 −6 bar (which encapsulates all of the pressures probed in our GCM) LTE and NLTE models were nearly identical in predicting hydrogen level populations. In Fossati et al (2021), they use NLTE as a framework for pressures less than 10 −4 bars when modeling temperaturepressure profiles. For pressures greater than 10 −4 bars, they note that the retrieved temperature profiles are very similar.…”