In order to prospect the possible detection of the Zeeman splittings in the CCH radical, we shall estimate the minimum line-of-sight magnetic field that could be observed with typical facilities such as, e.g., the telescope of the National Radio Astronomy Observatory or the telescope of the Five College Astronomy Department Observatory.
For a Zeeman splitting of , the antenna output obtained by substracting the two opposite circularly polarized signals is given by
where is the line-of-sight component of the magnetic field, the antenna temperature and the FWHM of the transition observed. The minimum temperature that is detectable by observing over a period of time t with a pre-dectection bandwith is
so that the minimum detectable line-of-sight magnetic field is
As reported in the literature (Ziurys et al. 1982, Gottlieb et al. 1983, Ungerechts et al. 1997, Bergin et al. 1997), the observed rotational transitions typically have a FWHM of the order of a few MHz and yield an antenna temperature of a few Kelvins. With the progresses achieved in receiver technology, a typical system temperature at the frequencies under consideration is of the order of a few . This leads to minimum detectable line-of-sight magnetic fields of the order of a few for a typical observing time . In view of these crude estimates, one can conclude that the rotational transitions of CCH exhibit a sensitivity sufficient for detecting magnetic fields of the order of magnitude one could expect from clouds with a molecular hydrogen density of the order of , typical, e.g., of the regions traced by CCH in IRAS 16293-2422 (van Dishoeck et al. 1995).
Putting together these results and those of Paper I, we arrive at the conclusion that the only molecules for which a sufficient sensitivity can be reached to detect interstellar magnetic fields with the strengths expected in molecular clouds are: OH, CN, SO, , CCH and, possibly, CCS (if one relies on rough estimates for the Zeeman splitting). The decrease in sensitivity of OH with density makes it necessary to use other molecules if one wants to probe magnetic fields in regions with typical densities (Crutcher et al. 1996). Observational data on the dense giant molecular cloud cores in Orion, M 17 and Cepheus A (Bergin et al. 1997; Ungerechts et al. 1997), on the dark cloud TMC-1 (Irvine et al. 1991; Pratap et al. 1997) reveal that CN and CCH have very similar distributions and that CCH is more abundant than CN (by a factor of about 3-4). Besides, if one notices that the transition of CCH is at a slightly lower frequency than the same transition in CN, and is therefore intrinsically marginally more sensitive to magnetic fields, one concludes that CCH should be worth as much consideration as CN as a molecule usable to detect magnetic fields in the dense cores of molecular clouds.
© European Southern Observatory (ESO) 1998
Online publication: June 26, 1998