Hydrodynamic solutions for radiation-driven winds in transition regions

The theoretical framework for modeling radiation-driven winds in hot stars is the “m-CAK theory”, which describes the radiation force using three parameters: α, δ, and k. In particular, δ introduces changes in the ionization of the material and can lead to two different types of solutions for the hydrodynamic equations of slowly or non-rotating winds. These solutions, known as the “fast” and “δ-slow” solutions, are principally distinguished by their markedly different terminal velocities. Both solutions are separated by a gap in the parameter space of δ, where no stationary solutions have been found so far. In this study, we employ the time-dependent hydrodynamic code ZEUS-3D, to solve the equation of motion, tracking the temporal evolution from a specified initial solution to obtain solutions in the gap. These novel solutions exhibit a stationary kink in the velocity profile at a fixed distance from the star, the position of which depends on the value of δ. Here we investigated if this discontinuity can lead to Discrete Absorption Components (DACs), solving the transfer equation in the comoving frame along with the non-LTE rate equations to derive UV line profiles for Si iv.