LBT transmission spectroscopy of HAT-P-12b: confirmation of a cloudy atmosphere with no significant alkali features

TL;DR

This study used the Large Binocular Telescope to observe HAT-P-12b's atmosphere, confirming it is cloudy with no significant alkali features, and aligning with previous HST findings except for a tentative potassium detection.

Contribution

First ground-based confirmation of HAT-P-12b's cloudy atmosphere using LBT transmission spectroscopy covering 0.4-0.9 μm.

Findings

Spectrum is flat, no significant sodium or potassium features.

High-altitude clouds confirmed by model fitting.

Results consistent with previous HST data, except for potassium detection.

Abstract

The hot sub-Saturn-mass exoplanet HAT-P-12b is an ideal target for transmission spectroscopy because of its inflated radius. We observed one transit of the planet with the multi-object double spectrograph (MODS) on the Large Binocular Telescope (LBT) with the binocular mode and obtained an atmosphere transmission spectrum with a wavelength coverage of $\sim$ 0.4 -- 0.9 $\mathrm{\mu}$m. The spectrum is relatively flat and does not show any significant sodium or potassium absorption features. Our result is consistent with the revised Hubble Space Telescope (HST) transmission spectrum of a previous work, except that the HST result indicates a tentative detection of potassium. The potassium discrepancy could be the result of statistical fluctuation of the HST dataset. We fit the planetary transmission spectrum with an extensive grid of cloudy models and confirm the presence of high-altitude clouds in the planetary atmosphere. The fit was performed on the combined LBT and HST spectrum, which has an overall wavelength range of 0.4 -- 1.6 $\mathrm{\mu}$m. The LBT/MODS spectrograph has unique advantages in transmission spectroscopy observations because it can cover a wide wavelength range with a single exposure and acquire two sets of independent spectra simultaneously.