ACCESS: Tentative detection of H2O in the ground-based optical transmission spectrum of the low density hot-Saturn HATS-5b

One of the most important questions to answer in the field of exoplanet atmospheres is to know which parameters shape their cloudiness levels. These not only define the general feedback mechanisms, but also their observational prospects: cloudy worlds typically have muted atmospheric features in transmission, making them observationally challenging to study. Recent works have predicted a window of “clearer” atmospheres for exoplanets at an equilibrium temperature around 1000 K, which is in between the regimes where silicate clouds and hydrocarbon hazes are believed to dominate. With an estimated T~1025 K, HATS-5b is in this “clear” regime, which also sits at the interesting dividing temperature between “warm” and “hot” giant planets. In this contribution, we present a new, precise ground-based optical transmission spectrum of the hot-Saturn HATS-5b, 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, for the first time in our collaboration, the usage of additional second order light in our analyses. This allows us to extract an “extra” transit lightcurve from our data, improving the overall precision of our combined transit spectrum. In addition, we also highlight how optimal spectral extraction can be much more effective than simple extraction, even in the case of high signal-to-noise ratios, as they can naturally compensate for bad pixels and cosmic rays. We find that the favored atmospheric model for this transmission spectrum is a solar-metallicity atmosphere with C/O = 0.39 +- 0.18, whose features are dominated by H2O, and so we report a tentative detection of H2O in the atmosphere of HATS-5b. If confirmed, this does indeed point to a “clearer” atmosphere at the pressure levels probed by transmission spectroscopy. This is an excellent target for James Webb Space Telescope follow-up, which would allow for a much more precise constraint on the atmospheric C/O, metallicity, and absolute water abundance.