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Session 121 - Molecular Clouds/Star Formation.
Oral session, Saturday, January 10
Georgetown,
We have used the VLA to map the Zeeman effect in absorption lines of H I and OH (both 1665 and 1667 MHz) toward NGC 2024 (Orion B). The Stokes V signal does not come from the entire line profile but from a sub-component at v \approx 10.2 km s^-1. The signal-to-noise ratios of the V signals are very high; the fields derived from the 1665 and 1667 MHz lines agree well, and the morphologies of the line-of-sight components of the magnetic field B_los determined from OH and H I also agree. These agreements add confidence to the interpretation of the observed Stokes V polarization signals as the Zeeman effect. For OH B_los \approx +80 \pm 4 \muG at the peak field position. There is considerable small-scale structure in B_los; it peaks to the southwest of a molecular gas clump with v \approx 10.2 km s^-1 which we can identify in the C^18O molecular line. This structure may simply be due to B being more nearly in the plane of the sky at the position of maximum molecular column density rather than to real variations in the magnitude of B. Linear polarization maps (by other observers) show that the field in the plane of the sky is mainly oriented east-west, perpendicular to the north-south distribution of molecular gas toward NGC 2024. These results are consistent with the magnetic field playing a significant role in support of the gas.
