Where is the Water? Jupiter-like C/H ratio but strong H${}_2$O depletion found on $\ddot{I}„$ Boötis b using SPIRou

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 $\ddot{I}„$ Boo b using the new wide wavelength coverage (0.95$-$2.50$\hat{I}¼$m) and high spectral resolution ($R=70000$) SPIRou spectrograph. By combining a total of 20 hours of SPIRou data obtained over five nights in a full atmospheric retrieval framework designed for high-resolution data, we constrain the abundances of all the major oxygen- and carbon-bearing molecules and recover a non-inverted temperature structure using a new free-shape, nonparametric TP profile retrieval approach. We find a volume mixing ratio of log(CO)$=-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 $\ddot{I}„$ Boo b support a formation scenario beyond the water snowline in a disk enriched in CO due to pebble drift.

27 pages, 14 figures, 3 tables, Accepted for publication in The Astronomical Journal