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We present a new ground-based visible transmission spectrum of thehigh-gravity, hot Jupiter HAT-P-23b, obtained as part of the ACCESS project. Wederive the spectrum from five transits observed between 2016 and 2018, withcombined wavelength coverage between 5200 {\AA} - 9269 {\AA} in 200 {\AA} bins,and with a median precision of 247 ppm per bin. HAT-P-23b's relatively highsurface gravity (g ~ 30 m/s^2), combined with updated stellar and planetaryparameters from Gaia DR2, gives a 5-scale-height signal of 384 ppm for ahydrogen-dominated atmosphere. Bayesian models favor a clear atmosphere for theplanet with the tentative presence of TiO, after simultaneously modelingstellar contamination, using spots parameter constraints from photometry. Ifconfirmed, HAT-P-23b would be the first example of a high-gravity gas giantwith a clear atmosphere observed in transmission at optical/NIR wavelengths;therefore, we recommend expanding observations to the UV and IR to confirm ourresults and further characterize this planet. This result demonstrates howcombining transmission spectroscopy of exoplanet atmospheres with long-termphotometric monitoring of the host stars can help disentangle the exoplanet andstellar activity signals.

ACCESS: An optical transmission spectrum of the high-gravity, hot Jupiter HAT-P-23b