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Issue A&A Volume 419, Number 3, June I 2004 Page(s) 949 - 964 Section Interstellar and circumstellar matter DOI 10.1051/0004-6361:20035589

A&A 419, 949-964 (2004)
DOI: 10.1051/0004-6361:20035589

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

J. Schmid-Burgk¹, D. Muders¹, H. S. P. Müller² and B. Brupbacher-Gatehouse<sup>3

1</sup>  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²  I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany
³  Lab. für Phys. Chemie, ETH-Hönggerberg, 8093 Zürich, Switzerland

(Received 28 October 2003 / Accepted 26 January 2004)

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 H ¹³CO ⁺, observed to be of width  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 H ¹³CO ⁺ 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 H ¹³CO ⁺ 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:C ¹⁸O abundance ratio. Overall motion toward the innermost region is suggested.

Key words: molecular data -- line: profiles -- radio lines: ISM -- ISM: molecules -- ISM: individual objects: L 1512, L 1544

Offprint request: J. Schmid-Burgk, schmid-burgk@mpifr-bonn.mpg.de

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