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

Abstract

Ultra-hot Jupiters (UHJs) are gas giants with very high equilibrium temperatures. In recent years, multiple chemical species, including various atoms and ions, have been discovered in their atmospheres. Most of these observations have been performed with transmission spectroscopy, although UHJs are also ideal targets for emission spectroscopy due to their strong thermal radiation. We present high-resolution thermal emission spectroscopy of the transiting UHJ KELT-20b/MASCARA-2b. The observation was performed with the CARMENES spectrograph at orbital phases before and after the secondary eclipse. We detected atomic Fe using the cross-correlation technique. The detected Fe lines are in emission, which unambiguously indicates a temperature inversion on the dayside hemisphere. We furthermore retrieved the temperature structure with the detected Fe lines. The result shows that the atmosphere has a strong temperature inversion with a temperature of 4900 ± 700 K and a pressure of 10−4.8−1.1+1.0 bar at the upper layer of the inversion. A joint retrieval of the CARMENES data and the TESS secondary eclipse data returns a temperature of 2550−250+150 K and a pressure of 10−1.5−0.6+0.7 bar at the lower layer of the temperature inversion. The detection of such a strong temperature inversion is consistent with theoretical simulations that predict an inversion layer on the dayside of UHJs. The joint retrieval of the CARMENES and TESS data demonstrates the power of combing high-resolution emission spectroscopy with secondary eclipse photometry in characterizing atmospheric temperature structures.

The PEPSI-LBT Exoplanet Transit Survey (PETS). II. A Deep Search for Thermal Inversion Agents in KELT-20 b/MASCARA-2 b with Emission and Transmission Spectroscopy

“…We note that these results are contingent upon the choice of P-T profile, as the strength of the inversion affects the strength of the emission lines, and thus the inferred VMR limit. However, Yan et al (2022b), Fu et al (2022), and Borsa et al (2022 all independently retrieved broadly similar P-T profiles using independent methodology and data, giving us some confidence in the broad accuracy of our P-T profile. Additionally, the presence of additional or stronger continuum opacity sources than we have assumed could affect the limits.…”

“…Several species have already been detected in emission for KELT-20 b at high resolution: Fe i (Yan et al 2022b;Borsa et al 2022), Si i (Cont et al 2021a, Fe ii, and Cr i (Borsa et al 2022). Fu et al ( 2022) also detected H 2 O and CO emission at low resolution.…”

“…We assumed a P-T profile of the form given by Eqn. 29 of Guillot (2010) as implemented in petitRADTRANS, with values of κ IR = 0.01 and γ = 5 chosen to approximate the P-T profiles retrieved for the dayside atmosphere of KELT-20 b by Yan et al (2022b), Fu et al (2022), andBorsa et al (2022). We show this profile in Fig.…”

“…Here, we aim to detect or constrain which species may be responsible for this inversion. Yan et al (2022b) already obtained non-detections for TiO, VO, and FeH in their CARMENES data, while Borsa et al (2022) did the same for AlO, TiO, and VO with HARPS-N; however, both of those spectrographs are fed by 4-mclass telescopes, motivating an independent search with 8-m-class telescopes and including additional potential inversion agents.…”

Patchy nightside clouds on ultra-hot Jupiters: General Circulation Model simulations with radiatively active cloud tracers

“…This includes absorption of incident stellar radiation by atomic metals in the planetary atmosphere that can lead to "inverted" temperature-pressure profiles which increase in temperature with decreasing pressure (Fortney et al 2008, Lothringer et al 2018, Kitzmann et al 2018, Gandhi & Madhusudhan 2019, Malik et al 2019 and can be especially strong for ultra-hot Jupiters that orbit early-type stars (Lothringer & Barman 2019, Fu et al 2022. Recent high spectral resolution observations of ultra-hot Jupiters have found a wealth of metallic species along with evidence for thermal inversions (Nugroho et al 2017, Hoeijmakers et al 2018, Jensen et al 2018, Seidel et al 2019, Cabot et al 2020, Hoeijmakers et al 2020, Nugroho et al 2020, Yan et al 2020, Kasper et al 2021, Kesseli & Snellen 2021, Tabernero et al 2021, Yan et al 2022. The hot daysides of ultra-hot Jupiters should be sufficiently ionized that magnetohydrodynamic mechanisms can affect their atmospheric circulation (Perna et al 2010, Menou 2012, Batygin et al 2013, Rauscher & Menou 2013, Rogers & Showman 2014, Rogers & Komacek 2014, Hindle et al 2019, Beltz et al 2022, potentially causing large-amplitude time-variability due to induced atmospheric magnetic fields (Rogers 2017, Rogers & Mcelwaine 2017, Hindle et al 2021a.…”