The GAPS programme at TNG

Abstract

Aims. We aim to extract the transmission spectrum of the HI Balmer lines of the ultra-hot Jupiter (UHJ) KELT-20b/MASCARA-2b from observations and to further compare the results with what was obtained through forward modelling, accounting for non-local thermodynamic equilibrium (NLTE) effects. Methods. We extracted the line profiles from six transits obtained with the HARPS-N high-resolution spectrograph attached to the Telescopio Nazionale Galileo telescope. We computed the temperature-pressure (TP) profile employing the HELIOS code in the lower atmosphere and the CLOUDY NLTE code in the middle and upper atmosphere. We further used CLOUDY to compute the theoretical planetary transmission spectrum in LTE and NLTE for comparison with observations. Results. We detected the Hα (0.79±0.03%; 1.25 Rp), Ηβ (0.52±0.03%; 1.17 Rp), and Ηγ (0.39±0.06%; 1.13 Rp) lines, and we detected the Ηδ line at almost 4σ (0.27±0.07%; 1.09 Rp). The models predict an isothermal temperature of ≈2200 K at pressures >10−2 bar and of ≈7700 K at pressures <10−8 bar, with a roughly linear temperature rise in between. In the middle and upper atmosphere, the NLTE TP profile is up to ~3000 K hotter than in LTE. The synthetic transmission spectrum derived from the NLTE TP profile is in good agreement with the observed HI Balmer line profiles, validating our obtained atmospheric structure. Instead, the synthetic transmission spectrum derived from the LTE TP profile leads to significantly weaker absorption compared to the observations. Conclusions. Metals appear to be the primary agents leading to the temperature inversion in UHJs, and the impact of NLTE effects on them increases the magnitude of the inversion. We find that the impact of NLTE effects on the TP profile of KELT-20b/MASCARA-2b is larger than for the hotter UHJ KELT-9b, and thus NLTE effects might also be relevant for planets cooler than KELT-20b/MASCARA-2b.

High-resolution Transmission Spectroscopy of Ultrahot Jupiter WASP–33b with NEID

“…Yan & Henning (2018) detected 1.15% ± 0.05% of the Hα absorption in the atmosphere of which of H and Ca II in the UHJ KELT-20b, orbiting an A2-type star, implies an extended atmosphere produced by irradiation far from the Roche lobe of this planet. Fossati et al (2023) argued that KELT-9b and KELT-20b host hydrostatic atmospheres and the Balmer series of H and Ca II lines do not probe the exosphere. This is because their host stars are early A type and do not have strong X-ray and EUV emission to turn a hydrostatic atmosphere into hydrodynamics.…”

“…Given an A5 type for WASP-33, the upper atmosphere of WASP-33b could be strongly heated to the hydrodynamic regime (Fossati et al 2018). In addition, Fossati et al (2023) shows that it is necessary to consider NLTE effects in the atmospheric modeling of the Balmer series of H and Ca II. Overall, the absorption of Ca II and/or Balmer series of H can be linked to the state of the planet's upper atmosphere but is highly model dependent and requires consideration of hydrodynamic and/or NLTE effects.…”

“…We note that the maximum temperature allowed by our adopted H line opacity is 6000 K, which might lead to the underestimation. On the other hand, the discrepancy could be due to the lack of consideration of nonlocal thermodynamic equilibrium (NLTE) effects and/or hydrodynamic escape (e.g., García Muñoz & Schneider 2019; Fossati et al 2021Fossati et al , 2023Huang et al 2023).…”