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Where Is the Water? Jupiter-like C/H Ratio but Strong H2O Depletion Found on τ Boötis b Using SPIRou

S. Pelletier, B. Benneke, A. Darveau-Bernier, A. Boucher, N. J. Cook, C. Piaulet, L. Coulombe, E. Artigau, D. Lafrenière, S. Delisle, R. Allart, R. Doyon, J.-F. Donati, P. Fouqué, C. Moutou, C. Cadieux, X. Delfosse, G. Hébrard, J. H. C. Martins, E. Martioli, T. Vandal

The present-day envelope of gaseous planets is a relic of how these giant planets originated and evolved. Measuring their elemental composition therefore presents a powerful opportunity to answer long-standing questions regarding planet formation. Obtaining precise observational constraints on the elemental inventory of giant exoplanets has, however, remained challenging owing to the limited simultaneous wavelength coverage of current space-based instruments. Here, we present thermal emission observations of the nontransiting hot Jupiter τ Boo b using the new wide wavelength coverage (0.95-2.50 μm) and high spectral resolution (R = 70,000) CFHT/SPIRou spectrograph. By combining a total of 20 hr of SPIRou data obtained over five nights in a full atmospheric retrieval framework designed for high-resolution data, we constrain the abundances of all the major oxygen- and carbon-bearing molecules and recover a noninverted temperature structure using a new free-shape, nonparametric temperature-pressure profile retrieval approach. We find a volume mixing ratio of log(CO) = - 2.46+0.25−0.29 and a highly depleted water abundance of less than 0.0072 times the expected value for a solar composition envelope. Combined with upper limits on the abundances of CH4, CO2, HCN, TiO, and C2H2, this results in a gas-phase C/H ratio of 5.85+4.44−2.82 × solar, consistent with the value of Jupiter, and an envelope C/O ratio robustly greater than 0.60, even when taking into account the oxygen that may be sequestered out of the gas phase. Combined, the inferred supersolar C/H, O/H, and C/O ratios on τ Boo b support a formation scenario beyond the water snowline in a disk enriched in CO owing to pebble drift.

Exoplanet atmospheres; Exoplanet atmospheric composition; Exoplanet structure; High resolution spectroscopy; Molecular spectroscopy; Astronomical instrumentation; Astronomy data analysis; 487; 2021; 495; 2096; 2095; 799; 1858; Astrophysics - Earth and Pla

The Astronomical Journal
Volume 162(2, Number 73, Page 19
2021 July

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia