Hyperfine Structure in H^{13}CO^{+} and ^{13}CO: measurement, analysis, and consequences for the study of dark clouds

Abstract

The magnetic moment of the ^{13}C nucleus is shown to provide a potentially useful tool for analysing quiescent cold molecular clouds. We report discovery of hyperfine structure in the H^{13}CO^{+}(1-0) transition. The doublet splitting is observed to be 38.5 +/- 5.2 kHz, and is confirmed by calculations which give 39.8 kHz and a line strength ratio of 3:1. We improve the spectroscopic constants of H^{13}CO^{+} and determine the frequencies of its low-J spectrum to better precision by analysing the dark cloud L1512. Potentially high optical depths (3 to 5 in L1512) in quiescent clouds require to consider the line's doublet nature when comparing to other molecular species, possibly reversing previous conclusions from single-component interpretations. We further confirm the hyperfine splitting in the ^{13}CO(1-0) transition that had been predicted, and measured in the laboratory, to be about 46 kHz. By applying hyperfine analysis to the data set of the first IRAM key-project we show that ^{13}CO optical depths can be estimated in narrow linewidth regions without recourse to other transitions nor to assumptions on beam filling factors, and linewidth and velocity determinations can be improved. For the core of L1512 we find an inverse proportionality between linewidth and column density resp. square root of optical depth, and a systematic inside-out increase of excitation temperature and of the ^{13}CO:C^{18}O abundance ratio. Overall motion toward the innermost region is suggested.