The magnetic fields in the Ophiuchus and Taurus molecular clouds

We discuss the significance of previously published OH Zeeman effect measurements of the magnetic field in the rho Oph cloud (L1688) and also a new OH Zeeman effect measurement for the dark cloud core TMC-1C, the latter obtained with the Arecibo telescope. Results for both of these clouds, like previously published OH Zeeman results for other dark clouds, show that the line-of-sight field strength /B-parallel to/ less than or equal to 10 mu G. The rho Oph cloud is significantly higher in column density than other clouds studied, and recent H I Zeeman effect observations have revealed B-parallel to approximate to +10 mu G in the H I self-absorption region in front of the II derived from the OH Zeeman effect may indicate that OH poorly samples cloud. The low values of B-parallel to derived from the OH Zeeman effect may indicate that OH poorly samples the dense gas in the rho Oph cloud, or else that the cloud is in a state of supercritical collapse. The TMC-IC cloud core may have formed via ambipolar diffusion in a manner similar to recent modeling of dark cloud Barnard 1.