Evidence for stellar contamination in the transmission spectra of HAT-P-12b

¹ CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, PR China
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
e-mail: czjiang@pmo.ac.cn, guochen@pmo.ac.cn
³ CAS Center for Excellence in Comparative Planetology, Hefei 230026, PR China
⁴ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
⁶ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

Received: 19 July 2021
Accepted: 22 September 2021

Abstract

Context. Transmission spectroscopy characterizes the wavelength dependence of transit depth, revealing atmospheric absorption features in planetary terminator regions. In this context, different optical transmission spectra of HAT-P-12b reported in previous studies exhibited discrepant atmospheric features (e.g., Rayleigh scattering and alkali absorption).

Aims. We aim to understand the atmosphere of HAT-P-12b using two transit spectroscopic observations by the Gran Telescopio Canarias (GTC) and to search for evidence of stellar activity contaminating the transmission spectra, which might be the reason behind the discrepancies.

Methods. We used Gaussian processes to account for systematic noise in the transit light curves and used nested sampling for Bayesian inferences. We performed joint atmospheric retrievals using the two transmission spectra obtained by GTC OSIRIS, as well as previously published results, coupled with stellar contamination corrections for different observations.

Results. The retrieved atmospheric model exhibits no alkali absorption signatures, but shows tentative molecular absorption features including H₂O, CH₄, and NH₃. The joint retrieval of the combined additional public data analysis retrieves similar results, but with a higher metallicity.

Conclusions. Based on Bayesian model comparison, the discrepancies of the transmission spectra of HAT-P-12b can be explained by the effect of different levels of unocculted stellar spots and faculae. In addition, we did not find strong evidence for a cloudy or hazy atmosphere from the joint analysis, which is inconsistent with prior studies based on the observations of the Hubble Space Telescope.