Carbon monoxide emission lines reveal an inverted atmosphere in the ultra hot Jupiter WASP-33 b and indicate an eastward hot spot

Abstract

We report the first detection of CO emission lines at high spectral resolution in the day-side infrared thermal spectrum of an exoplanet. These emission lines, found in the atmosphere of the ultra hot Jupiter WASP-33 b, provide unambiguous evidence of its thermal inversion layer. Using spectra from the MMT Exoplanet Atmosphere Survey (MEASURE, 𝑅 ∼ 15, 000), covering pre-and post-eclipse orbital phases (0.33 < 𝜙 < 0.73), we performed a cross-correlation analysis with 1D PHOENIX model atmospheres to detect CO at S/N=7.9 at 𝑣 𝑠𝑦𝑠 = 0.15 +0.64 −0.65 km/s and 𝐾 𝑝 = 229.5 +1.1 −1.0 km/s. However, using the framework of Cross-Correlation-to-log-Likelihood mapping, we further find that the spectral line depths, as probed by the scaling parameter, change with phase: the line contrast is larger after the eclipse than before. We then use the general circulation model SPARC/MITgcm post-processed by the 3D gCMCRT radiative transfer code and interpret this variation as due to an eastward-shifted hot spot. Before the eclipse, when the hot spot is facing Earth, the thermal profiles are shallower, leading to a smaller line depth despite greater overall flux. After the eclipse, the western part of the day-side is facing Earth, where the thermal profiles are much steeper, leading to larger line depth despite less overall flux. We thus demonstrate that even relatively moderate resolution spectra can be used to understand the 3D nature of close-in exoplanets, if assessed within the log-likelihood framework, and that resolution can be traded for photon collecting power when the induced Doppler-shift is sufficiently large. We highlight that CO in ultra hot Jupiters is a good probe of their thermal structure and corresponding dynamics, and does not suffer from stellar activity unlike some atomic species, such as iron, that also appear in the hot host star spectrum.

Atmospheric characterization of the ultra-hot Jupiter WASP-33b

“…To our knowledge, we performed the first retrieval of the thermal profile of WASP-33b's atmosphere using high-resolution spectroscopy. Similar work was recently carried out by van Sluijs et al (2022), who maximized the S/N detection strengths of the CO emission lines to study the atmosphere of the same planet. We retrieve temperatures that are consistent with their results at low atmospheric pressures, but are able to get tighter constraints on the thermal profile at higher pressures.…”

“…For instance, neutral or ionic species of H, Li, O, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Sr, and Y have been identified in the atmospheres of several UHJs, including KELT-9b (Yan & Henning 2018;Hoeijmakers et al 2018Hoeijmakers et al , 2019Wyttenbach et al 2020;Borsa et al 2021a), WASP-33b (Yan et al 2019(Yan et al , 2021Nugroho et al 2020a;Borsa et al 2021b;Cont et al 2021Cont et al , 2022Herman et al 2022), WASP-189b (Yan et al 2020;Stangret et al 2022;Prinoth et al 2022), WASP-76b (Seidel et al 2019;Ehrenreich et al 2020;Tabernero et al 2021;Deibert et al 2021;Casasayas-Barris et al 2021;Kesseli et al 2022), and KELT-20b/MASCARA-2b (Casasayas-Barris et al 2018, 2019Stangret et al 2020;Nugroho et al 2020b;Hoeijmakers et al 2020a;Rainer et al 2021;Yan et al 2022b;Borsa et al 2022). Also, the spectral signatures of an increasing number of molecular species such as CO, HCN, H 2 O, OH, SiO, or TiO have been identified in UHJ atmospheres (e.g., Nugroho et al 2021;Cont et al 2021;Landman et al 2021;Fu et al 2022;Prinoth et al 2022;Sánchez-López et al 2022;Yan et al 2022a;van Sluijs et al 2022;Lothringer et al 2022). Detecting the spectral lines of different species not only...…”

“…Different studies have shown that the planet possesses a temperature inversion in its dayside hemisphere (e.g., Haynes et al 2015;Nugroho et al 2017;Cont et al 2021). To date, the hydrogen Balmer lines, Si i, Ca ii, Fe i, OH, CO, H 2 O, and TiO have been identified via transmission or emission spectroscopy in the planetary atmosphere (Nugroho et al 2017(Nugroho et al , 2020a(Nugroho et al , 2021Yan et al 2019Yan et al , 2021Cont et al 2021Cont et al , 2022Herman et al 2022;van Sluijs et al 2022). The WASP-33 system parameters used in this work are summarized in Table 1.…”

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

“…3D models are also able to capitalize on the 3D effects present in HRS, such as the Doppler shifting of lines, temperature gradients, and chemical inhomogenieties (Flowers et al 2019;MacDonald & Lewis 2022;Prinoth et al 2021). Other recent work has also been investigating other routes for retrieving 3D structure from high-resolution emission spectra (Herman et al 2022;van Sluijs et al 2022) 3D models are especially necessary for ultrahot Jupiters (UHJs), given the large spatial variations across these planets, due to their extreme irradiation regime. These planets, with equilibrium temperatures that exceed ∼2200 K, orbit so close to their host star that they are expected to have tidally synchronized rotation states (Showman & Guillot 2002).…”

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

“…3D models are also able to capitalize on the 3D effects present in HRS such as Doppler shifting of lines, temperature gradients, and chemical inhomogenieties (Flowers et al 2019;MacDonald & Lewis 2021;Prinoth et al 2021). Other recent work has also been investigating other routes to retrieve 3D structure from high-resolution emission spectra (Herman et al 2022;van Sluijs et al 2022) 3D models are especially necessary for ultrahot Jupiters (UHJs), given the large spatial variations across these planets due to their extreme irradiation regime. These planets, with equilibrium temperatures that exceed ∼ 2200 K, orbit so close to their host star that they are expected to have tidally-synchronized rotation states (Showman & Guillot 2002).…”