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

The magnetic moment of the C-13 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 (HCO+)-C-13. The doublet splitting in (HCO+)-C-13, 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 C-13 nuclear spin-rotation and spin-spin coupling between both nuclei are taken into account. We improve the spectroscopic constants of (HCO+)-C-13 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 (CO)-C-13 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 (CO)-C-13 optical depths can as for (HCO+)-C-13 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 (CO)-C-13:(CO)-O-18 abundance ratio. Overall motion toward the innermost region is suggested.