Detection of OH in the ultra-hot Jupiter WASP-76b

Abstract

Context. Ultra-hot Jupiters have dayside temperatures at which most molecules are expected to thermally dissociate. The dissociation of water vapour results in the production of the hydroxyl radical (OH). While OH absorption is easily observed in near-infrared spectra of M dwarfs, which have similar effective temperatures as ultra-hot Jupiters, it is often not considered when studying the atmospheres of ultra-hot Jupiters. Ground-based high-resolution spectroscopy during the primary transit is a powerful tool for detecting molecular absorption in these planets. Aims. We aim to assess the presence and detectability of OH in the atmosphere of the ultra-hot Jupiter WASP-76b. Methods. We use high-resolution spectroscopic observations of a transit of WASP-76b obtained using CARMENES. After validating the OH line list, we generate model transit spectra of WASP-76b with petitRADTRANS. The data are corrected for stellar and telluric contamination and cross-correlated with the model spectra. After combining all cross-correlation functions from the transit, a detection map is constructed. Constraints on the planet properties from the OH absorption are obtained from a Markov chain Monte Carlo analysis. Results. OH is detected in the atmosphere of WASP-76b with a peak signal-to-noise ratio of 6.1. From the retrieval we obtain Kp = 232 ± 12 km s−1 and a blueshift of − 13.2 ± 1.6 km s−1, which are offset from the expected velocities. Considering the fast spin rotation of the planet, the blueshift is best explained with the signal predominantly originating from the evening terminator and the presence of a strong dayside-to-nightside wind. The increased Kp over its expected value (196.5 km s−1) is, however, a bit puzzling. The signal is found to be broad, with a full width at half maximum of 16.8−4.0+4.6 km s−1. The retrieval results in a weak constraint on the mean temperature of 2700–3700 K at the pressure range of the OH signal. Conclusions. We show that OH is readily observable in the transit spectra of ultra-hot Jupiters. Studying this molecule can provide insights into the molecular dissociation processes in the atmospheres of such planets.

Searching for the origin of the Ehrenreich effect in ultra-hot Jupiters: Evidence for strong C/O gradients in the atmosphere of WASP-76b?

“…The H 2 O signal is blueshifted by -14.3 ± 2.6 km s −1 , indicative of a strong day to nightside wind at the atmospheric pressures probed by this molecule. This Doppler shift is in agreement with that observed for neutral iron towards the end the transit (Ehrenreich et al 2020) and for OH (Landman et al 2021).…”

“…In particular, the S/N of the observed H 2 O signal is con-sistent with injection recovery tests of models created using the retrieved abundance from Hubble Space Telescope observations (Tsiaras et al 2018;Edwards et al 2020). For H 2 O and NH 3 , the K P uncertainties do not encompass the expected velocity, which is in line with the OH results of Landman et al (2021) and the tentative H I signal in . In the case of HCN, while the peak K P is also located at ∼250 km s −1 , the expected value is inside the large error bars produced by its multiplepeak configuration in the K Psys map.…”

“…If confirmed, the HCN signal would provide evidence of a clear morning terminator in WASP-76 b. -In the three cases (i.e., H 2 O, HCN, and NH 3 ), the maximum significance CCF peaks are consistently displaced towards higher-than-expected K P values, which is in line with results presented in Landman et al (2021) and . The origin of the asymmetries producing the Ehrenreich effect in these signals is still uncertain as the latest GCM calculations for this planet do not predict such scenarios.…”

“…cently used in Landman et al (2021) and Casasayas-Barris et al (2021) to perform atmospheric characterization of this UHJ. A second transit was observed on October 30, 2019, although with significantly poorer conditions (S/N ∼ 37), which did not allow us to recover any planet signal (not shown).…”

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

“…Our models are all based on parameters from the UHJ WASP-76b, an inflated planet orbiting its host star every 1.81 days. This planet has been the target of many high-resolution transit analyses, and a multitude of different species have been detected in its atmosphere, including Fe I, Na, Li I, Ca II (Seidel et al 2019;Deibert et al 2021;Casasayas-Barris et al 2021;Kawauchi et al 2021;Tabernero et al 2021), and recently molecules like OH (Landman et al 2021).Analyses of high resolution emission spectra have yet to be published. The work by Kesseli et al (2021) explores an entire atomic spectral survey of the atoms and ions that have been detected on this UHJ and finds evidence for temperature and chemical gradients on the planet.…”