A re-evaluation of profile shapes from resonance line scattering in spherical stellar winds

It is common to treat the scattering of light by resonance lines as isotropic, but in fact it has been known for some time that general resonance line scattering is partially isotropic and partially dipolar, the relative strength of the two components depending on the specific transition.As a result, the profile shapes of lines that scatter with strong dipole distributions could in principle differ markedly from those that scatter isotropically. This paper explores the consequences of general resonance line scattering in spherically symmetric stellar envelopes. As a simplified example, a resonance line profile arising in a constant expansion wind is shown not to be flat-top in shape, as commonly accepted, but can in fact exhibit a symmetric double-horned shape. Although interesting, the case of constant expansion has limited application. Using the Sobolev-P method, sample line profiles are computed for a typical wind velocity distribution that is often assumed in hot star winds. These simulated lines reveal that anisotropic scattering yields profile shapes that are centrally depressed and broadened. Taking account of finite star effects, the emission profile of an optically thin resonant line that is purely Rayleigh scattering (like a free electron) differs from that of the isotropic scattering case at only the 10% level, and the two types of profiles become indistiguishable for increasing line optical depths owing to the effects of multiple scattering. Relative to the case of isotropic scattering, the anisotropic scattering has little effect in altering the emission profile shape, a consequence of (a) stellar occultation and (b) finite star depolarization. Thus, except for the case of a shell of considerable radius, the accuracy of resonance line profile computations in spherically symmetric extended envelopes are hardly compromised by the effects of anisotropic scattering.