Water observed in the atmosphere of {\tau} Bootis Ab with CARMENES/CAHA

TL;DR

This study confirms water vapor in the atmosphere of the non-transiting hot Jupiter { au} Bootis Ab using high-resolution spectroscopy, providing detailed orbital and atmospheric parameters, and highlighting discrepancies with previous observations.

Contribution

First detection of water vapor in { au} Bootis Ab's atmosphere using CARMENES, with detailed orbital and atmospheric characterization and comparison to prior studies.

Findings

Detected water vapor with high confidence.

Measured planetary mass and orbital inclination.

Found no evidence for additional minor species.

Abstract

Characterising the atmospheres of hot Jupiters is important in understanding the formation and migration of these exotic planets. However, there are still many open questions about the chemical and physical properties of these atmospheres. Here, we confirm the detection of water vapour in thermal emission from the non-transiting hot Jupiter {\tau} Bootis Ab with the high resolution NIR CARMENES spectrograph. Combining over 17 h of observations (560 spectra) and using a Bayesian cross-correlation to log-likelihood approach, we measure a systemic velocity of $V_{sys} = -11.51^{+0.59}_{-0.60}$ km/s and a radial velocity semi-amplitude of $K_{P} = 106.21^{+1.76}_{-1.71}$ km/s for the planet, which results in an absolute mass of $M_{P} = 6.24^{+0.17}_{-0.18} M_{J}$ and an orbital inclination of $41.6^{+1.0}_{-0.9}$ degrees. Our retrieved $V_{sys}$ shows a significant shift (+5 km/s) from the literature value, which could be caused by an inaccurate time of periastron. Within the explored model grid, we measure a preference for solar water abundance (VMR = $10^{-3}$) and no evidence for additional minor species in the atmosphere. Given the extensive orbital coverage of the data, we searched for a phase dependency in the water signal but found no strong evidence of variation with orbital phase. This detection is at odds with recent observations from SPIRou/CFHT and their tight upper limit on water abundance. We recommend further observations of the atmosphere {\tau} Bootis Ab to try and resolve these discrepancies.