Magnetic fields in shocked regions: Very large array observations of H ₂O masers

We present VLA observations of the Zeeman effect in 22 GHz H₂O masers in several high-mass star-forming regions. These masers are believed to arise from collisional pumping in postshock environments. Therefore, the Zeeman effect data provide the most direct measurements of magnetic field strengths in high-density (n ≳ 10⁸ cm^-3) postshock gas, where the field energy determines other physical conditions. Our observations yield significant magnetic field detections in W3 IRS 5, W3(OH), W49 N, and OH 43.8-0.1. In these sources, we detect line-of-sight field strengths ranging from 13 to 49 mG. For some regions, the detected fields provide a 2-3 point sampling of the magnetic field, indicating the nature of field variations on arcsecond scales. These field strengths are consistent with a shock-driven maser model having relatively low speed (20 km s^-1), C-type shocks. We examine the balance between magnetic field energy and turbulent kinetic energy in the masing regions. These energies appear close to equilibrium.