Spectrum and Relative Intensities of Interstellar OH Lines

Abstract

THE initial detection of the OH lines in the interstellar medium involved the two strongest lines at 1,665.402 Mc/s and 1,667.357 Mc/s of the four hyperfine lines making up the ²Π_3/2, J = 3/2, Λ-doublet¹. Subsequent observations resulted in the detection of the two weaker lines at 1,612.201 Mc/s and 1,720.559 Mc/s². The strong OH absorption in the direction of the galactic centres, noted by the Australian workers³, and the unexpected disagreement between the theoretical and observed intensity ratios of the lines², make it important to consider the detection possibilities of other radio lines of OH in the interstellar medium. A discussion of molecular energy-levels and the physical mechanism responsible for the Λ-doublet levels of OH has been prepared for publication elsewhere⁴; but here we wish to direct attention to 14 microwave lines of various energy-levels and isotopic species of OH, some of which appear to be capable of detection at the present time. The line frequencies, theoretical relative intensities, and transition probabilities are presented in Table 1. The frequencies have been computed by using the theory of Dousmanis, Sanders, and Townes⁵, and the hyperfine coupling constants given by Radford⁶. The dipole moment for all isotopic species has been taken to be (1.60±0.12) × 10⁻¹⁸ E.S.U., although this was determined only for ¹⁶O¹H (ref. 7).