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