Spectral line fluorescence in moving envelopes of stars

TL;DR

This paper investigates how optical fluorescent lines form in moving stellar envelopes with various velocity fields, using radiative transfer equations and a new open-source code to model complex line interactions and fluorescence amplification.

Contribution

It introduces a novel method for modeling spectral line fluorescence in moving media, including a new open-source Python code for arbitrary 2D velocity fields.

Findings

Fluorescent amplification occurs in all investigated velocity fields.

Line interactions are complex due to Doppler shifts and velocity gradients.

The code SLIM2 enables detailed simulations of spectral line interactions in moving media.

Abstract

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.