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Astron. Astrophys. 335, 1025-1028 (1998)

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1. Introduction

It has become increasingly clear that magnetic fields must play an important role in supporting molecular clouds against self-gravity, and thereby in the process of star formation. Although the question of the cloud support via magnetic fields is far from being settled, the invocation of magnetic fields seems to be a promising alternative to the deficiencies of the non-magnetic mechanisms that have been proposed so far (see, for example, McKee et al. 1993 for a good review). An essential parameter in this context is the mass-to-flux ratio, [FORMULA]. If it exceeds a certain critical value, the magnetic field is unable to prevent the cloud from gravitational collapse; on the contrary, if it is less than this value, gravitational collapse is impossible, so long as magnetic flux freezing holds (for a detailed discussion of the issues and for full numerical models, see, for example, Mouschovias 1991; Basu & Mouschovias 1995; and the references cited in these papers). The measurement of magnetic fields in molecular clouds is therefore of paramount importance. This led, more than a decade ago, several astronomers to start carrying out an extensive observation program aimed to detect the Zeeman effect in spectral lines arising in the clouds (Crutcher et al. 1994, 1996; Heiles et al. 1993; Troland et al. 1982, 1996). Until recently, these observations were made exclusively in the lines of H i and OH. But, noting that in general [FORMULA] decreases when the cloud density increases (Troland et al. 1996) Crutcher et al. (1996) made a first attempt to detect magnetic fields in molecular cloud cores through the Zeeman effect in the CN lines, without much success however.

Having in mind, like these observers, that the use of just one species (OH, for example) cannot be sufficient to measure magnetic fields in every part of a given molecular cloud, we had started, several years ago, a systematic investigation of the Zeeman splitting in interstellar molecules (Bel & Leroy 1989; hereafter Paper I). In Paper I, we considered virtually all diatomic molecules observed in the interstellar medium; the net result was that only CN, SO and [FORMULA] exhibit Zeeman effects comparable to that of OH. In view of the diversity and complexity of the polyatomic molecules, and also of their comparatively lower abundance, we decided to restrict our study to those that are found in many places and whose structure allows the Zeeman splitting to be calculated with reasonable realiability. As a result, we were left essentially with linear tri-atomic molecules, of which we only retained CCH, the ethynyl radical, whose ubiquity is well established (Tucker et al. 1974; Baudry et al. 1980; Wooten et al. 1980; van Dishoeck et al. 1995). [We excluded CCS essentially because of the difficulty to treat its ground state properly. Technically, CCS is a linear radical with a [FORMULA] ground state which qualifies for a coupling of the angular momenta intermediate between Hund's cases a and b, and exhibits complex selection rules (Fuente et al. 1990; Suzuki et al. 1992), which lessens somewhat the reliability of corresponding Zeeman splitting calculations.]

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998