Atmosphere escape inferred from modelling the H$\alpha$ transmission spectrum of WASP-121b.

The escaping atmospheres of hydrogen driven by stellar X-ray and extreme Ultraviolet (XUV) have been detected around some exoplanets by the excess absorption of Ly$\alpha$ in far ultraviolet band. In the optical band the excess absorption of H$\alpha$ is also found by the ground-based instruments. However, it is not certain so far if the escape of the atmosphere driven by XUV can result in such absorption. Here we present the XUV driven hydrodynamic simulation coupled with the calculation of detailed level population and the process of radiative transfer for WASP-121b. Our fiducial model predicts a mass loss rate of $\sim$1.28$\times$10$^{12}$g/s for WASP-121b. Due to the high temperature and Ly$\alpha$ intensity predicted by the fiducial model, many hydrogen atoms are populated into the first excited state. As a consequence, the transmission spectrum of H$\alpha$ simulated by our model is broadly consistent with the observation. Comparing with the absorption of H$\alpha$ at different observation times, the stellar XUV emission varies in the range of 0.5-1.5 times fiducial value, which may reflect the variation of the stellar activity. Finally, we find that the supersonic regions of the planetary wind contribute a prominent portion to the absorption of H$\alpha$ by comparing the equivalent width of H$\alpha$, which hints that a transonic outflow of the upper atmosphere driven by XUV irradiation of the host star can be detected by the ground-based telescope and the H$\alpha$ can be a good indicator of escaping atmosphere.