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arXiv · 2105.10513

Where is the Water? Jupiter-like C/H ratio but strong H$_2$O depletion found on $τ$ Boötis b using SPIRou

Stefan Pelletier·Björn Benneke·Antoine Darveau-Bernier·Anne Boucher·Neil J. Cook·Caroline Piaulet·Louis-Philippe Coulombe·Étienne Artigau·David Lafrenière·Simon Delisle·Romain Allart·René Doyon·Jean-François Donati·Pascal Fouqué·Claire Moutou·Charles Cadieux·Xavier Delfosse·Guillaume Hébrard·Jorge H. C. Martins·Eder Martioli·Thomas Vandal
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Abstract

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 due to the limited simultaneous wavelength coverage of current space-based instruments. Here, we present thermal emission observations of the non-transiting hot Jupiter $τ$ Boo b using the new wide wavelength coverage (0.95$-$2.50$\,μ$m) and high spectral resolution ($R=70\,000$) SPIRou spectrograph. By combining a total of 20 hours of SPIRou data obtained over five nights in a full atmospheric retrieval framework designed for high-resolution data, we constrain the abundances of all the major oxygen- and carbon-bearing molecules and recover a non-inverted temperature structure using a new free-shape, nonparametric TP profile retrieval approach. We find a volume mixing ratio of log(CO)$\,\,=-2.46_{-0.29}^{+0.25}$ and a highly depleted water abundance of less than $0.0072$ times the value expected for a solar composition envelope. Combined with upper limits on the abundances of CH$_4$, CO$_2$, HCN, TiO, and C$_2$H$_2$, this results in a gas-phase C/H ratio of 5.85$_{-2.82}^{+4.44}\times\,$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 super-solar 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 due to pebble drift.

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Stefan Pelletier, Björn Benneke, Antoine Darveau-Bernier, Anne Boucher, Neil J. Cook, Caroline Piaulet, Louis-Philippe Coulombe, Étienne Artigau, David Lafrenière, Simon Delisle, Romain Allart, René Doyon, Jean-François Donati, Pascal Fouqué, Claire Moutou, Charles Cadieux, Xavier Delfosse, Guillaume Hébrard, Jorge H. C. Martins, Eder Martioli, Thomas Vandal. 2021-07-28. Where is the Water? Jupiter-like C/H ratio but strong H$_2$O depletion found on $τ$ Boötis b using SPIRou. https://doi.org/10.3847/1538-3881%2Fac0428

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