Abstract
We discuss the significance of previously published OH Zeeman effect measurements of the magnetic field in the ρ 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||| ≤ 10 μG. The ρ Oph cloud is significantly higher in column density than other clouds studied, and recent H I Zeeman effect observations have revealed B|| ≈ +10 μG in the H I self-absorption region in front of the cloud. The low values of B derived from the OH Zeeman effect may indicate that OH poorly samples the dense gas in the ρ Oph cloud, or else that the cloud is in a state of supercritical collapse. The TMC-1C cloud core may have formed via ambipolar diffusion in a manner similar to recent modeling of dark cloud Barnard 1.
Highlights
It has long been realized that many aspects of selfgravitating molecular clouds are difficult to understand in the absence of magnetic Ðelds
We discuss two of these positions toward the o Oph cloud (L1688), and we present new OH Zeeman e†ect data obtained with the Arecibo telescope for the dark cloud core TMC-1C
Observations of the 18 cm OH Zeeman e†ect toward the o Oph cloud are reported in Paper I as positions o Oph
Summary
It is not clear how these clouds can be supported against gravity in the absence of magnetic Ðelds nor how they can rid themselves of angular momentum as they form new stars Some of these issues are outlined in reviews by Mouschovias (1987, 1991a, 1991b), Shu, Adams, & Lizano (1987), and McKee et al. Some of these issues are outlined in reviews by Mouschovias (1987, 1991a, 1991b), Shu, Adams, & Lizano (1987), and McKee et al With these considerations in mind, we have carried out an extensive program of Zeeman e†ect observations in the 18 cm (1665 and 1667 MHz) OH emission lines from dark clouds (Crutcher et al 1993, hereafter Paper I). Preliminary evidence reported by Gu sten & Fiebig (1990) suggested a magnetic Ðeld of order 100 kG might exist in this core
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
