Characterizing exoplanetary atmospheres at high resolution with SPIRou: Detection of water on HD 189733 b

TL;DR

This study demonstrates the first detection of water vapor in an exoplanet atmosphere using the SPIRou high-resolution spectro-polarimeter, confirming its capability for detailed atmospheric characterization of transiting exoplanets.

Contribution

It presents the first exoplanet atmosphere detection with SPIRou, showcasing its effectiveness in high-resolution near-infrared spectroscopy for atmospheric studies.

Findings

Detected water vapor at 5.9 sigma significance in HD 189733 b

Constrained atmospheric parameters including water abundance and cloud pressure

Measured a notable blue shift of the planetary signal, suggesting complex atmospheric dynamics

Abstract

We present the first exoplanet atmosphere detection made as part of the SPIRou Legacy Survey, a Large Observing Program of 300 nights exploiting the capabilities of SPIRou, the new near-infrared high-resolution (R ~ 70 000) spectro-polarimeter installed on the Canada-France-Hawaii Telescope (CFHT; 3.6-m). We observed two transits of HD 189733, an extensively studied hot Jupiter that is known to show prominent water vapor absorption in its transmission spectrum. When combining the two transits, we successfully detect the planet's water vapor absorption at 5.9 sigma using a cross-correlation t-test, or with a Delta BIC >10 using a log-likelihood calculation. Using a Bayesian retrieval framework assuming a parametrized T-P profile atmosphere models, we constrain the planet atmosphere parameters, in the region probed by our transmission spectrum, to the following values: VMR[H2O] = -4.4^{+0.4}_{-0.4}, and P_cloud >~ 0.2 bar (grey clouds), both of which are consistent with previous studies of this planet. Our retrieved water volume mixing ratio is slightly sub-solar although, combining it with the previously retrieved super-solar CO abundances from other studies would imply super-solar C/O ratio. We furthermore measure a net blue shift of the planet signal of -4.62^{+0.46}_{-0.44} km s-1, which is somewhat larger than many previous measurements and unlikely to result solely from winds in the planet's atmosphere, although it could possibly be explained by a transit signal dominated by the trailing limb of the planet. This large blue shift is observed in all the different detection/retrieval methods that were performed and in each of the two transits independently.