Modeling Radio Recombination in MWC 349A. I. Saturation Properties

Strong nonlocal thermodynamic equilibrium recombination line emission from the envelope of MWC 349A has been studied observationally and theoretically and has been used to derive information about the physical and kinematic properties of the circumstellar environment around this object. In this paper, we construct a detailed model of radiative transfer of the commonly observed maser lines H26α and H30α in MWC 349A, specifically taking into account the effect of maser saturation. The envelope is modeled as a combination of an ionized Keplerian rotating disk and a disk wind expanding at a velocity of 60 km s-1. Our modeling results show that millimeter-line H26α is significantly affected by the saturation effect. The line intensity is predicted to be greatly reduced in comparison with previous models which ignore the saturation effect. The H30α maser is predicted to be unsaturated. Our model shows that the predicted intensity and line shape are in agreement with observations. The predicted position-velocity diagrams are also consistent with observations and the interpretation that the H26α maser appears at smaller radii in comparison to the H30α maser. We also emphasize that the wind component is important in shaping the line profiles of the recombination masers. © 2021. The American Astronomical Society. All rights reserved.