Hyperfine structure in H13CO+ and 13CO: Measurement, analysis, and consequences for the study of dark clouds

Abstract

The magnetic moment of the 13C nucleus is shown to provide a potentially useful tool for analysing quiescent cold molecular clouds. We report discovery of hyperfine structure in the lowest rotational transition of H 13CO+. The doublet splitting in H13CO +, observed to be of width 38.5 ± 5.2 kHz or 0.133 km s -1, is confirmed by quantum chemical calculations which give a separation of 39.8 kHz and line strength ratio 3:1 when H and 13C nuclear spin-rotation and spin-spin coupling between both nuclei are taken into account, We improve the spectroscopic constants of H13CO+ and determine the hitherto uncertain frequencies of its low-J spectrum to better precision by analysing the dark cloud L 1512. Attention is drawn to potentially high optical depths (3 to 5 in L 1512) in quiescent clouds, and examples are given for the need to consider the (1-0) line's doublet nature when comparing to other molecular species, redirecting or reversing conclusions arrived at previously by single-component interpretations. We further confirm the hyperfine splitting in the (1-0) rotational transition of 13CO that had already been theoretically predicted, and measured in the laboratory, to be of width about 46 kHz or, again, 0.13 km s-1. By applying hyperfine analysis to the extensive data set of the first IRAM key-project we show that 13CO optical depths can as for H13CO+ 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. Thus, for the core of L 1512 we find an inverse proportionality between linewidth and column density, resp. linewidth and square root of optical depth, and a systematic inside-out increase of excitation temperature and of the 13CO:C18O abundance ratio. Overall motion toward the innermost region is suggested.