Precise Measurements of CH Maser Emission and Its Abundance in Translucent Clouds

TL;DR

This study provides precise measurements of CH maser emission in translucent clouds, revealing excitation temperatures, column densities, and their correlation with OH and H₂, refining understanding of molecular tracers in diffuse gas.

Contribution

It offers direct excitation temperature measurements and quantifies the impact of LTE assumptions on CH column density estimates, improving molecular cloud diagnostics.

Findings

CH emission detected toward 6 of 18 sources.

CH excitation temperature ranges from -54.5 to -0.4 K.

CH is confirmed as a reliable tracer of H₂ in diffuse clouds.

Abstract

We present high-sensitivity CH 9 cm ON/OFF observations toward 18 extra-galactic continuum sources that have been detected with OH 18 cm absorption in the Millennium survey with the Arecibo telescope. CH emission was detected toward six of eighteen sources. The excitation temperature of CH has been derived directly through analyzing all detected ON and OFF velocity components. The excitation temperature of CH 3335 MHz transition ranges from $-54.5$ to $-0.4$ K and roughly follows a log-normal distribution peaking within [$-$5, 0] K, which implies overestimation by 20% to more than ten times during calculating CH column density by assuming the conventional value of $-60$ or $-10$ K. Furthermore, the column density of CH would be underestimated by a factor of $1.32\pm 0.03$ when adopting local thermal equilibrium (LTE) assumption instead of using the CH three hyperfine transitions. We found a correlation between the column density of CH and OH following log$N$(CH) = (1.80$\pm$ 0.49) log$N$(OH) $-11.59 \pm 6.87$. The linear correlation between the column density of CH and H$_2$ is consistent with that derived from visible wavelengths studies, confirming that CH is one of the best tracers of H$_2$ component in diffuse molecular gas.