H\alpha\ Absorption in Transiting Exoplanet Atmospheres

摘要: Absorption of stellar H\alpha\ by the upper atmosphere planet HD189733b has recently been detected Jensen et al. Motivated this observation, we have developed a model for atomic hydrogen in n=2 state and compared resulting line profile to observations. The is hydrostatic balance, as well thermal photoionization equilibrium. Collisional radiative transitions are included determination level population. We find that absorption dominated an optical depth \tau\ ~ 1 shell, composed metastable 2s located below ionization layer. number density within shell found vary slowly with radius, while 1s falls rapidly. Thus Ly\alpha absorption, certain wavelength, occurs inside relatively defined impact parameter, contribution roughly uniform over entire can approximately reproduce observed Ly\alpha\ integrated transit depths using rate enhanced expected star order magnitude. For HD 209458b, unable explain asymmetric al., produces symmetric comparable 189733b. In appendix, study effect on net cooling rate.