  The formation of optical fluorescent lines in moving media has not yet been
studied in detail, so this work represents a first step in investigating the
fluorescence process in different types of macroscopic velocity fields: (a)
accelerated outflows, (b) accelerated infalls, and (c) non-monotonic velocity
fields (such as an accelerating outflow followed by a deceleration region or an
accretion shock front). We solve the radiative transfer equations for the lines
involved in the fluorescent process, assuming spherical symmetry and a
simplified atomic model. We use the framework of the generalized Sobolev theory
for computing the interacting, non-local source functions. The emergent line
fluxes are then integrated exactly. Because of Doppler shifts in the moving
gaseous envelope, photons of the three lines involved in TTS FeI fluorescence
CaII H, FeI 3969, and H_epsilon interact with each other in a complex way, so
that fluorescent amplification of the line flux occurs not only for FeI 3969,
but also for the other two lines, in all velocity fields that we investigated.
With the assumption of LTE populations, the line source functions of moderately
optically thick lines are not strongly affected by line interactions, while
they are depressed in the inner envelope for optically thick lines because of
stellar photon absorption in the interaction regions. Fluorescent amplification
takes place mainly in the observer's reference frame during the flux
integration. Further comparison with observations will require solving the rate
equations for the atomic populations involved, along with the radiation field
computed with the method presented here. The main product of this research is
the open-source computer code SLIM2 (Spectral Line Interactions in Moving
Media), written in Python/Numpy, which numerically solves the fluorescence
problem for arbitrary 2D velocities.
