First Search for Low-Frequency CH with a Square Kilometre Array Precursor Telescope

TL;DR

This study conducted a pilot interferometric search for low-frequency CH hyperfine transitions in RCW 38 using ASKAP, setting upper limits on the column density but not detecting the signal, advancing radio observations of interstellar molecules.

Contribution

First interferometric search for low-frequency CH hyperfine transitions with ASKAP, providing new sensitivity limits and demonstrating the environment's radio quietness for future studies.

Findings

No detection of CH at 724.7883 MHz in RCW 38.

Achieved a 5-hour sensitivity limit of 0.09 Jy.

Set upper limits on CH column density in emission and absorption.

Abstract

The diatomic free radical methylidyne (CH) is an important tracer of the interstellar medium and the study of it was critical to our earliest understanding of star formation. Although it is detectable across the electromagnetic spectrum, observations at radio frequencies allow for a study of the kinematics of the diffuse and dense gas in regions of new star formation. There are only two published (single-dish) detections of the low-frequency hyperfine transitions between 700 and 725 MHz, despite the precise frequencies being known. These low-frequency transitions are of particular interest as they are shown in laboratory experiments to be more sensitive to magnetic fields than their high-frequency counterparts (with more pronounced Zeeman splitting). In this work we take advantage of the radio quiet environment and increased resolution of the Australian Square Kilometre Array Pathfinder (ASKAP) over previous searches to make a pilot interferometric search for CH at 724.7883 MHz (the strongest of the hyperfine transitions) in RCW 38. We found the band is clean of radio frequency interference, but we did not detect the signal from this transition to a five sigma sensitivity limit of 0.09 Jy, which corresponds to a total column density upper limit of 1.9x10^18cm^-2 for emission and 1.3x10^14cm^-2 for absorption with an optical depth limit of 0.95. Achieved within 5 hrs of integration, this column density sensitivity should have been adequate to detect the emission or absorption in RCW 38, if it had similar properties to the only previous reported detections in W51.