ACCESS: Tentative detection of H$_2$O in the ground-based optical transmission spectrum of the low-density hot Saturn HATS-5b

TL;DR

This study presents a ground-based optical transmission spectrum of HATS-5b, suggesting the presence of water vapor and a clear, solar-metallicity atmosphere with sub-solar C/O ratio, highlighting its potential for future JWST observations.

Contribution

First ground-based optical spectrum of HATS-5b with improved precision using second-order light analysis, indicating water vapor detection and atmospheric composition insights.

Findings

Detection of water vapor in HATS-5b's atmosphere

Evidence for a clear, solar-metallicity atmosphere with sub-solar C/O ratio

HATS-5b as a prime target for JWST follow-up observations

Abstract

We present a precise ground-based optical transmission spectrum of the hot-Saturn HATS-5b ($T_{eq} =1025$ K), obtained as part of the ACCESS survey with the IMACS multi-object spectrograph mounted on the Magellan/Baade Telescope. Our spectra cover the 0.5 to 0.9 micron region, and are the product of 5 individual transits observed between 2014 and 2018. We introduce the usage of additional second-order light in our analyses which allows us to extract an "extra" transit light curve, improving the overall precision of our combined transit spectrum. We find that the favored atmospheric model for this transmission spectrum is a solar-metallicity atmosphere with sub-solar C/O, whose features are dominated by H$_2$O and with a depleted abundance of Na and K. If confirmed, this would point to a "clear" atmosphere at the pressure levels probed by transmission spectroscopy for HATS-5b. Our best-fit atmospheric model predicts a rich near-IR spectrum, which makes this exoplanet an excellent target for future follow-up observations with the James Webb Space Telescope, both to confirm this H$_2$O detection and to superbly constrain the atmosphere's parameters.