Non-LTE line transfer with spatially correlated turbulence: Fluctuation against average emergent radiation field

¹ Observatoire d'Alger, CRAAG, BP 63, Bouzareah-Alger, Algeria
² Institut d'Astrophysique de Paris – CNRS, 98bis Bld. Arago, 75014 Paris, France e-mail: loucif@iap.fr; bjaffel@iap.fr

Received: 1 March 2000
Accepted: 3 April 2002

Abstract

The increasing interest in using turbulence to explain anomalies observed in stellar spectral lines is discussed. This turbulence occurs at a finite spatial scale, a key parameter. In our previous work on non-LTE radiation transfer in the presence of a turbulent velocity field (Loucif & Magnan 1982), we calculated individual profiles representing intensity fluctuations around the averaged line shape for photon diffusion with complete redistribution in frequency (CFR), a function well-adapted to media where collisional excitations dominate. We now generalize the problem by accounting for partial redistribution in frequency (PFR). To show the effect of the spatial correlation in the velocity field on the emergent line profiles as well as the importance of fluctuations in the medium's optical properties against the average, exhaustive calculations of emergent non-LTE line profiles are performed in the frame of a systematic comparison between the two processes of redistribution for the three following cases: static medium, averaged turbulent medium, and deviation from the average. In all cases, we confirm the differences and similarities in CFR and PFR descriptions previously derived for static media. These results seem reasonable since light scattering occurs at the atomic level–a scale well below the macroscopic size of the turbulence scale length. In addition, pronounced differences in the emergent radiation field are obtained between the average case and the so-called deviation from average for both frequency redistribution functions. These systematic differences rule out classical treatment of line formation in turbulent fields that assume averaged media to model observations obtained over an exposure time shorter than the turbulent field time scale (). Beyond the different assumptions made to describe the non-LTE line formation in a turbulent medium, our model provides a simple and coherent description of realistic media applicable to different astrophysical observational backgrounds. For this purpose, a unified picture is proposed for the radiation field formation, invoking mainly the medium opacity, the turbulence correlation length, and the velocity. Observations of variable objects, or along extended lines of sights, allowing a high probability for such deviations to occur (e.g., quasars), are suitable for application of our model.