Interpretation of Infrared Vibration-Rotation Spectra of Interstellar and Circumstellar Molecules

TL;DR

This paper develops a more realistic radiative transfer model for interpreting infrared vibration-rotation spectra of interstellar molecules, improving accuracy over traditional isothermal slab models by accounting for non-LTE and non-isothermal effects.

Contribution

A new non-LTE, non-isothermal radiative transfer model that better matches observations of interstellar and circumstellar molecules, especially symmetric ones.

Findings

Model aligns better with observed spectra, including absorption and emission features.

Isothermal slab models can be misleading, but LTE assumptions are less problematic.

Radiative transfer effects are crucial for accurate interpretation of spectra.

Abstract

Infrared vibration-rotation lines can be valuable probes of interstellar and circumstellar molecules, especially symmetric molecules, which have no pure rotational transitions. But most such observations have been interpreted with an isothermal absorbing slab model, which leaves out important radiative transfer and molecular excitation effects. A more realistic non-LTE and non-isothermal radiative transfer model has been constructed. The results of this model are in much better agreement with the observations, including cases where lines in one branch of a vibration-rotation band are in absorption and another in emission. In general, conclusions based on the isothermal absorbing slab model can be very misleading, but the assumption of LTE may not lead to such large errors, particularly if the radiation field temperature is close to the gas temperature.