Stellar Atmospheres Near an AGN: The Importance of Radiation Pressure from Trapped Lyman-α Photons

We derive an analytic expression for the intensity of resonance-line radiation “trapped” in a semi-infinite medium. Given a source function and destruction probability per scattering, the radiation pressure due to trapped photons can be calculated by numerically integrating over analytic functions. We apply this formalism to a plane-parallel model stellar atmosphere to calculate the radiation pressure due to Lyman-α photons produced following absorption of UV and X-rays from an AGN. For low surface gravity stars near the AGN (g ∼ 10 cm s−2, r ∼ 0.25 pc), we find that the pressure due to Lyman-α photons becomes an appreciable fraction of that required for hydrostatic support. If the broad emission line emitting gas in AGNs and QSOs consists of stellar outflows, it may be driven, in part, by Lyman-α pressure. Subject headings: galaxies: active — line: formation — radiative transfer — stars: atmospheres