2006ApJS..162..388T

We have searched for the C₄H Zeeman effect toward the cold dense TMC-1 cyanopolyyne peak (CP) core and obtained an upper limit of 14.5±14 µG for a magnetic field B=|B|cosθ, where θ is the angle between the field and the plane of the sky. C₄H is particularly suited to the detection of small dense cores that result from the evolution of magnetized clumps undergoing ambipolar diffusion (AD) to smaller denser cores that directly form stars. These are the so-called quasi-static models. We discuss three distinct types of model for such systems, those of the Mouschovias, Shu, and Myers groups, respectively. We see no indications of line-broadening or high densities in the core, such as those predicted to follow the ambipolar diffusion and free-fall contractions, but the dearth of observational data for B fields in low-mass protostars makes it difficult to form definite conclusions from our results. We also discuss several models emphasizing recent formulations of large-scale (100 pc) flows of supersonic turbulence that appear to surmount earlier difficulties such as too large a star formation (SF) rate and the need for a magnetically subcritical initial cloud. Possible reasons for the null result on the C₄H Zeeman effect are that (1) in the case of the quasi-static models, TMC-1 CP has not evolved yet to the onset of dynamic collapse, which follows the much longer AD phase, and (2) there is a weak|B| field throughout the Taurus complex and it is directed close to the plane of the sky, whereas the Zeeman effect is sensitive only to line-of-sight (circular) polarization. In the supersonic turbulence models, the subsonic nature observed for the C₄H emission lines argues that the energetic turbulence that formed TMC-1 CP has passed by TMC-1 CP at least a Myr ago (the crossing time for the energetic turbulence to traverse the clump).