2004A&A...419..949S

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 H¹³CO⁺. The doublet splitting in H¹³CO⁺, observed to be of width 38.5±5.2kHz or 0.133km/s, is confirmed by quantum chemical calculations which give a separation of 39.8kHz 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 L1512. Attention is drawn to potentially high optical depths (3 to 5 in L1512) 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 46kHz or, again, 0.13km/s. 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 L1512 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.