Low Frequency Carbon Radio Recombination Lines II: The Diffuse Interstellar Medium

TL;DR

This paper models low frequency carbon radio recombination lines (CRRL) to probe the physical conditions of the diffuse interstellar medium, aiding future LOFAR surveys with insights into electron density, temperature, and cloud properties.

Contribution

It introduces improved modeling of CRRL line-to-continuum ratios and demonstrates their effectiveness in diagnosing physical conditions of the diffuse interstellar medium.

Findings

CRRL line width and optical depth are sensitive to electron density and temperature.

CRRL to [CII] line ratio strongly depends on cloud temperature and density.

Models align with existing CRRL observations, validating the approach.

Abstract

In the second paper of the series, we have modeled low frequency carbon radio recombination lines (CRRL) from the interstellar medium. Anticipating the LOw Frequency ARray (LOFAR) survey of Galactic CRRLs, we focus our study on the physical conditions of the diffuse cold neutral medium (CNM). We have used the improved departure coefficients computed in the first paper of the series to calculate line-to-continuum ratios. The results show that the line width and integrated optical depths of CRRL are sensitive probes of the electron density, gas temperature, and the emission measure of the cloud. Furthermore, the ratio of CRRL to the [CII] at 158 $\mu$m line is a strong function of the temperature and density of diffuse clouds. Guided by our calculations, we analyze CRRL observations and illustrate their use with data from the literature.