HAT-P-32 b: what can be deduced from transit observations in H{\alpha} and He I lines?

Abstract

HAT-P-32 b is the first exoplanet for which absorption in hydrogen and helium lines has been measured simultaneously. In addition to the relatively large maximum depth of ~5% in both lines, observations have revealed very long pre-transit signatures. In this paper, we apply a 3D aeronomy model to simulate the detailed spectrally resolved absorption profiles at mid-transit, formed in the upper atmosphere, as well as the integral transit curves formed by atmospheric material accumulating around the star. By fitting the observations, we derive key atmospheric characteristics, including the atmospheric composition and metallicity, and constrain the stellar XUV, hard X-ray, and Ly{\alpha} fluxes, as well as the intensity of the stellar plasma wind. We show that the presence of metals crucially affects the absorption in hydrogen and helium lines by accelerating the dissociation of H2. We also demonstrate that several processes not previously included in the modeling of HAT-P-32 b are important for the interpretation of the observations, such as the 3D asymmetric structure of the atmosphere, heating and cooling by excited states of hydrogen and metals, and the stellar VUV flux.