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Titanium oxide and chemical inhomogeneity in the atmosphere of the exoplanet WASP-189 b

Subjects

Abstract

The temperature of an atmosphere decreases with increasing altitude, unless a shortwave absorber that causes a temperature inversion exists1. Ozone plays this role in the Earth’s atmosphere. In the atmospheres of highly irradiated exoplanets, the shortwave absorbers are predicted to be titanium oxide (TiO) and vanadium oxide (VO)2. Detections of TiO and VO have been claimed using both low-3,4,5,6 and high-7 spectral-resolution observations, but subsequent observations have failed to confirm these claims8,9,10 or overturned them11,12,13. Here we report the unambiguous detection of TiO in the ultra-hot Jupiter WASP-189 b14 using high-resolution transmission spectroscopy. This detection is based on applying the cross-correlation technique15 to many spectral lines of TiO from 460 to 690 nm. Moreover, we report detections of metals, including neutral and singly ionized iron and titanium, as well as chromium, magnesium, vanadium and manganese (Fe, Fe+, Ti, Ti+, Cr, Mg, V, Mn). The line positions of the detected species differ, which we interpret as a consequence of spatial gradients in their chemical abundances, such that they exist in different regions or dynamical regimes. This is direct observational evidence for the three-dimensional thermochemical stratification of an exoplanet atmosphere derived from high-resolution ground-based spectroscopy.

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Fig. 1: Schematic showing the contributions of the terminators throughout the course of the transit in the example of a toy-planet atmosphere with a hot dayside component, and a cooler nightside component with a clearly reduced scale height (day–night gradient).
Fig. 2: Expected cross-correlation functions and velocity–velocity diagrams for three toy models of the planet atmosphere.
Fig. 3: Overview of the detections of TiO, Ti, Ti+, Fe, Fe+, Cr, Mg, V and Mn.

Data availability

Raw data as well as pipeline-reduced data from which the findings that are presented in this paper are derived are publicly available from the data archives of the European Southern Observatory (ESO) and the Telescopio Nazionale Galileo (TNG). Cross-correlation templates and models are available upon reasonable request. Precomputed opacity functions are publicly available via http://dace.unige.ch/opacity.

Code availability

The computer code for performing cross-correlations is publicly available at https://github.com/hoeijmakers/tayph/. Documentation, instructions and a data demonstration can be found at https://tayph.readthedocs.io.

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Acknowledgements

We acknowledge partial financial support from the PlanetS National Centre of Competence in Research (NCCR) supported by the Swiss National Science Foundation (SNSF), the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (projects Four Aces, EXOKLEIN, Spice Dune and Exo-Atmos with grant agreement numbers 724427 (D.E., J.V.S., H.J.H.), 771620 (K.H., C.F.), 947634 (V.B.) and 679633 (L.P.), respectively), UKRI Future Leaders Fellow Grant (MR/S035214/1) (H.M.C.), Spanish State Research Agency (AEI) Projects PID2019-107061GB-C61 (D.B.) and MDM-2017-0737 Unidad de Excelencia ‘María de Maeztu’—Centro de Astrobiología (CSIC/INTA) (D.B.), the Märta and Eric Holmberg Endowment (B.P.) and FRQNT (R.A). R.A. is a Trottier Postdoctoral Fellow and acknowledges support from the Trottier Family Foundation. The analysis presented in this work has made use of the VALD database, operated at Uppsala University, the Institute of Astronomy RAS in Moscow and the University of Vienna; Ian Crossfields’ Astro-Python Code library and Astropy56,57. C.F. acknowledges a University of Bern International 2021 PhD Fellowship. K.H. acknowledges a Honorary Professorship from the University of Warwick, as well as an imminent chair professorship from the Ludwig Maximilian University in Munich. B.P., H.J.H., D.K., E.S., N.W.B., B.T., C.F., M.H., B.M.M., L.P., S.G. and K.H. are part of the Mantis network.

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Authors and Affiliations

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Contributions

B.P. performed the data analysis (including applying computer code originally written by H.J.H.), produced all of the figures except Extended Data Figs. 2 and 8, co-led the scientific vision and co-led the writing of the manuscript. H.J.H. provided the computer code that was the basis and starting point for the data analysis, mentored B.P. on data analysis techniques, co-led the scientific vision and co-led the writing of the manuscript. D.K. performed radiative transfer calculations used to construct the cross-correlation templates and model spectra. E.S. performed FastChem calculations and produced Extended Data Fig. 8. J.V.S. investigated the fidelity of specific spectral lines, performed supporting EulerCam observations and provided the code, expertise and results to produce Supplementary Figs. 6 and 7. M.L. analysed the EulerCam data and produced Extended Data Fig. 2. N.W.B. cowrote the manuscript. B.T. constructed a model of the stellar spectrum and provided technical support throughout the analysis procedure. H.J.H., D.R.A. and D.B. performed HARPS observations. K.K. performed supporting EulerCam observations. A.G.-M. proofread the manuscript. S.G. provided guidance on opacities. H.M.C., M.H., B.M.M. and L.P. provided substantial feedback on the manuscript. H.J.H., D.K., J.V.S., R.A., V.B., H.M.C., D.E., C.F., C.L., S.G., M.O. and K.H. were all coinvestigators on the ESO proposal for open time in observing period 103 that led to the procurement of the data. K.H. co-led the scientific vision, cowrote the manuscript, guided its narrative and formulation and assisted with formatting.

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Correspondence to Bibiana Prinoth.

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Extended data

Extended Data Fig. 1 Illustration of Doppler shadow subtraction and detrending of the cross- correlation function for the time series observed on April 14, 2019, with the Fe template at 3,000 K (see Methods).

Top panel: Raw two-dimensional cross-correlation function. During the transit, the Doppler shadow emerges as the positive near-vertical structure. Time of first, second, third and fourth contact as predicted using the ephemeris of Lendl et al. (2020)16 are indicated as dashed lines. Middle panel: Best-fit model of the Doppler shadow. Bottom panel: Residuals after subtracting the best-fit model from the raw cross-correlation function (top panel) and application of a detrending algorithm in the vertical direction. The absorption signature of the planet atmosphere is visible as the slanted feature, Doppler-shifted to the instantaneous radial velocity of the planet. The residual of the Doppler shadow at the end of the transit is masked during further analysis.

Extended Data Fig. 2 Phase-folded light-curve as observed with EulerCam on the nights of 2019-04-14 and 2019-04-24.

No astrophysical sources of variability are detected.

Extended Data Fig. 3 Three Fe lines in the spectrum of WASP-189.

The black lines correspond to the observed spectrum, the red dashed lines correspond to the best fit using a metallicity of [Fe/H] = 0.24. The blue shaded regions indicate the fit with ± 0.15 metallicity uncertainty.

Extended Data Fig. 4 Summary of stellar and planetary parameters of the WASP-189 system adopted in this study.

a Lendl et al. 202016, b: Anderson et al. 201814 (HARPS-MCMC), c: fixed parameter.

Extended Data Fig. 5 Signals of Cr+, Sc+, Na, Ni and Ca classified as tentative.

All of these species have previously been observed in other ultra-hot Jupiters19,25. The shaded region indicates the expected 1σ uncertainty. Dashed lines show expected signal strengths, obtained by injecting and recovering the signatures of model spectra, assuming isothermal atmospheres at 2,000 K (red) and 3,000 K (orange) respectively.

Extended Data Fig. 6 Transmission spectrum of WASP-189b at the wavelength of the Na D-doublet.

The lines are fit assuming a Gaussian line-shape, resulting in an average line depth of 15.3 ± 3.1 × 104, equivalent to 4.9σ.

Extended Data Fig. 7 Two injected models of the transmission spectrum of WASP-189 b at 2,500 K (purple) and 3,000 K (blue).

We assumed chemical equilibrium and solar metallicity. The models are sampled at their native resolution as set by intrinsic line broadening, and not additionally broadened to match e.g. the planetary rotation or the instrumental resolving power, although such broadening terms are taken into account when injecting these templates into the data. The inset plot shows the wavelength region between 495.4 and 495.8 nm, where a molecular band head of TiO is visible.

Extended Data Fig. 8 Model of the abundances of key selected species as a function of pressure (inverse altitude) at a temperature of 2,500 K, assuming thermo-chemical equilibrium and solar metallicity, computed with FastChem66.

Solid lines correspond to atomic species, dashed lines to ionised species and the dotted line to TiO.

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Supplementary Tables 1 and 2 and Figs. 1–10.

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Prinoth, B., Hoeijmakers, H.J., Kitzmann, D. et al. Titanium oxide and chemical inhomogeneity in the atmosphere of the exoplanet WASP-189 b. Nat Astron 6, 449–457 (2022). https://doi.org/10.1038/s41550-021-01581-z

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