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

TL;DR

This study presents a high-precision optical transmission spectrum of the high-gravity hot Jupiter HAT-P-23b, suggesting a clear atmosphere with potential TiO presence, and highlights the importance of combined spectroscopic and photometric analysis.

Contribution

First optical transmission spectrum of HAT-P-23b showing evidence for a clear atmosphere with possible TiO, utilizing Bayesian modeling and stellar activity constraints.

Findings

Indicates a likely clear, hydrogen-dominated atmosphere.

Detects a tentative TiO signature in the spectrum.

Demonstrates the effectiveness of combined spectroscopic and photometric analysis.

Abstract

We present a new ground-based visible transmission spectrum of the high-gravity, hot Jupiter HAT-P-23b, obtained as part of the ACCESS project. We derive the spectrum from five transits observed between 2016 and 2018, with combined 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 high surface gravity (g ~ 30 m/s^2), combined with updated stellar and planetary parameters from Gaia DR2, gives a 5-scale-height signal of 384 ppm for a hydrogen-dominated atmosphere. Bayesian models favor a clear atmosphere for the planet with the tentative presence of TiO, after simultaneously modeling stellar contamination, using spots parameter constraints from photometry. If confirmed, HAT-P-23b would be the first example of a high-gravity gas giant with a clear atmosphere observed in transmission at optical/NIR wavelengths; therefore, we recommend expanding observations to the UV and IR to confirm our results and further characterize this planet. This result demonstrates how combining transmission spectroscopy of exoplanet atmospheres with long-term photometric monitoring of the host stars can help disentangle the exoplanet and stellar activity signals.