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First search for low-frequency CH with a Square Kilometre Array precursor telescope

Published online by Cambridge University Press:  15 December 2020

Chenoa D. Tremblay
Affiliation:
CSIRO Astronomy and Space Science, PO Box 1130, Bentley WA 6102, Australia
J. A. Green
Affiliation:
CSIRO Astronomy and Space Science, PO Box 1130, Bentley WA 6102, Australia
Stacy L. Mader
Affiliation:
CSIRO Astronomy and Space Science, Parkes Radio Telescope, Parkes NSW, Australia
Chris J. Phillips
Affiliation:
CSIRO Astronomy and Space Science, PO Box 76, Epping NSW 1710, Australia
Matthew Whiting
Affiliation:
CSIRO Astronomy and Space Science, PO Box 76, Epping NSW 1710, Australia
Corresponding
E-mail address:

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 is 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 1.9 $\times 10^{18}$ cm–2 for emission and 1.3 $\times 10^{14}$ cm–2 for absorption with an optical depth limit of 0.95. Achieved within 5 h 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.

Type
Research Article
Copyright
© The Author(s), 2020. Published by Cambridge University Press on behalf of the Astronomical Society of Australia

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First search for low-frequency CH with a Square Kilometre Array precursor telescope
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