FBILI METHOD FOR THE TWO-LEVEL ATOM LINE FORMATION IN MEDIA WITH LOW VELOCITY FIELDS

SUMMARY: In this paper we generalized the fast convergent Forth-and-Back Implicit Lambda Iteration (FBILI) method to the solution of the two-level atom line transfer problems in media with low velocity fields using the observer’s reference frame. In order to test the accuracy and the convergence properties of the method we solved several astrophysically important benchmark problems of the NLTE line formation: in a plan-parallel differentially expanding medium of finite thickness, and in spherically symmetric stellar atmospheres, both static and expanding. We compared our solutions with those obtained by other authors using different numerical methods.