A possibly solar metallicity atmosphere escaping from HAT-P-32b revealed by H$\alpha$ and He absorption

Abstract

This paper presents a hydrodynamic simulation that couples detailed non-local thermodynamic equilibrium (NLTE) calculations of the hydrogen and helium level populations to model the H$\alpha$ and He 10830 transmission spectra of the hot Jupiter HAT-P-32b. A Monte Carlo simulation is applied to calculate the number of Ly$\alpha$ resonance scatterings, which is the main process for populating H(2). In the examined parameter space, only the models with H/He $\geq$ 99.5/0.5, $(0.5 \sim 3.0)$ times the fiducial value of $F_{\rm XUV}$, $\beta_m = 0.16\sim 0.3$, can explain the H$\alpha$ and He 10830 lines simultaneously. We find a mass-loss rate of $\sim (1.0\sim 3.1) \times 10^{13}$ g s$^{-1}$, consistent with previous studies. Moreover, we find that the stellar Ly$\alpha$ flux should be as high as $4 \times 10^{5}$ erg cm$^{-2}$ s$^{-1}$, indicating high stellar activity during the observation epoch of the two absorption lines. Despite the fact that the metallicity in the lower atmosphere of HAT-P-32b may be super-solar, our simulations tentatively suggest it is close to solar in the upper atmosphere. The difference in metallicity between the lower and upper atmospheres is essential for future atmospheric characterisations.