Infrared [Fe II] Emission Lines from Radiative Atomic Shocks

TL;DR

This study models infrared [Fe II] emission lines from radiative shocks, compares predictions with observations from various astrophysical objects, and discusses uncertainties affecting line ratio interpretations.

Contribution

It updates shock models for IR [Fe II] lines with new atomic data and compares these with a diverse set of observational data.

Findings

Observed line ratios mostly match model predictions

Some mid-infrared line ratios are significantly offset from models

Atomic rate uncertainties are likely the main source of discrepancies

Abstract

[Fe II] emission lines are prominent in the infrared (IR), and they are important diagnostic tools for radiative atomic shocks. We investigate the emission characteristics of [Fe II] lines using a shock code developed by Raymond (1979) with updated atomic parameters. We first review general characteristics of IR [Fe II] emission lines from shocked gas, and derive [Fe II] line fluxes as a function of shock speed and ambient density. We have compiled the available IR [Fe II] line observations of interstellar shocks and compare them to the ratios predicted from our model. The sample includes both young and old supernova remnants in the Galaxy and the Large Magellanic Cloud and several Herbig-Haro objects. We find that the observed ratios of IR [Fe II] lines generally fall on our grid of shock models, but the ratios of some mid-infrared lines, e.g., [Fe II] 35.35 um/[Fe II] 25.99 um, [Fe II] 5.340 um/[Fe II] 25.99 um, and [Fe II] 5.340 um/[Fe II] 17.94 um, are significantly offset from our model grid. We discuss possible explanations and conclude that the uncertainty in atomic rates might be the major source of uncertainty, while uncertainties in the shock modeling and the observations certainly exist.