New OH Zeeman Measurements of Magnetic Field Strengths in Molecular Clouds

Abstract

We present the results of a new survey of 23 molecular clouds for the Zeeman effect in OH undertaken with the ATNF Parkes 64-m radio telescope and the NRAO Green Bank 43-m radio telescope. The Zeeman effect was clearly detected in the cloud associated with the H II region RCW 38, with a field strength of 38 ± 3 µG, and possibly detected in a cloud associated with the H II region RCW 57, with a field strength of −203 ± 24 µG. The remaining 21 measurements give formal upper limits to the magnetic field strength, with typical 1σ sensitivities < 20 µG.For 22 of the molecular clouds we are also able to determine the column density of the gas in which we have made a sensitive search for the Zeeman effect. We combine these results with previous Zeeman studies of 29 molecular clouds, most of which were compiled by Crutcher (1999), for a comparsion of theoretical models with the data.This comparison implies that if the clouds can be modeled as initially spherical with uniform magnetic fields and densities that evolve to their final equilibrium state assuming flux-freezing then the typical cloud is magnetically supercritical, as was found by Crutcher (1999). If the clouds can be modeled as highly flattened sheets threaded by uniform perpendicular fields, then the typical cloud is approximately magnetically critical, in agreement with Shu et al. (1999), but only if the true values of the field for the non-detections are close to the 3σ upper limits. If instead these values are significantly lower (for example, similar to the 1σ limits), then the typical cloud is generally magnetically supercritical.When all observations of the Zeeman effect are considered, the single-dish detection rate of the OH Zeeman effect is relatively low. This result may be due to low mean field strengths, but a more realistic explanation may be significant field structure within the beam. As an example, for clouds associated with H II regions the molecular gas and magnetic field may be swept up into a thin shell, which results in a non-uniform field geometry and measurements of the beam-averaged field strength which are significantly lower than the true values. This effect makes it more difficult to distinguish magnetically subcritical and supercritical clouds.

The Formation of Self‐Gravitating Cores in Turbulent Magnetized Clouds

“…McKee (1999), however, points out that ambipolar diffusion might already have altered the mass-to-flux ratio observed by Crutcher. Observations by Bourke et al (2001) yield similar conclusions to Crutcher (1999), although they caution that the measured magnetic field strength may be significantly lower than the true values as a result of the low volume filling fraction of dense cores. Nakano (1998) argues that observed cloud cores cannot be magnetically supported.…”

“…Here B los is the line-of-sight magnetic field strength, G is the gravitational constant, and c È $ 0:12 for a spherical cloud or 0.16 for an isothermal disk (Bourke et al 2001). In these two plots, we choose F ¼ 3 and c È ¼ 0:16 for the extreme case.…”

Angle-dependent radiative grain alignment

“…This could happen if a ridge centered on HD 97300 is oriented parallel to the average magnetic field. However, the low estimated magnetic field of the Chamaeleon I cloud, B = 3±4 μG (Bourke et al 2001) makes this unlikely. Repeating the experiment in other regions where a localized radiation source dominates the grain illumination will allow such caveats to be addressed.…”

The correlation between magnetic pressure and density in compressible MHD turbulence

“…Observational surveys of the magnetic field strength in molecular clouds seem to indicate that the former is essentially uncorrelated from the density at low ( < ∼ 10 3 cm −3 ) densities, with recorrelation seemingly appearing at higher densities (e.g., Crutcher et al 2002). In particular, in molecular clouds nondetections of the Zeeman effect are generally more frequent than detections, and so in many cases only upper limits to the field strength are available (e.g., Crutcher et al 1993;Bourke et al 2001). Although it has been argued that these results are consistent with the standard theory of molecular cloud support (Shu et al 1987) through consideration of statistical corrections to the random orientations of the field with respect to the line of sight (Crutcher et al 1993;Crutcher et al 2002), it is clear that they are also consistent with the lack of correlation between field strength and density found in the present paper and in other numerical simulations Ostriker et al 2001).…”